How to check for a 50 or 75 ohm connector
George Shaw
Using the simplest equipment possible (multimeter?) HOW do I check if
the connector is for 50 Ohm or 75 Ohm coax connection. There are no
external markings.
Rene Tschaggelar
Rene
--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net
Don Pearce
wrote:
Look inside the end of the connector. The 75 ohm will have the pin,
surrounded by just the slotted metal sleeve. On the 50 ohm connector,
the metal sleeve will have a layer of plastic insulation just inside
it, forming a second sleeve.
d
_____________________________
Telecommunications consultant
http://www.pearce.uk.com
Alan Melia
good connectors anyway.....get new ones.
Cheers Alan G3NYK
"George Shaw" <georg...@ukf.net> wrote in message
news:thm90vof9eov5lgk2...@4ax.com...
Gareth Rowlands
George Shaw wrote:
> I have a large box of mixed BNC connectors/tees etc.
The 50 ohm plugs have slightly larger diameter pins than the 75's.
Sometimes too, the end of a pin on 75 ohm BNC plug is a sharp point and
then there is a gradual chamfer back to the maximum diameter of the pin,
whereas a 50 ohm pin is more likely to be a rounded 'nose' and a
constant diameter along the rest of the shaft.
The sockets are best tested comparitively by looking at the centre
socket diameters.
Forget the multimeter, size really does matter in this case.
Cheers !
Gareth.
Chris Carlen
I find it very difficult to accept the suggestions that 50R vs. 75R
connectors have different diameter pins, as that would imply that the
"BNC" connector has different mechanical standards for different
impedances. If this is truly the case, then I will be eager to learn
that it is so.
But until I'm convinced, I would hesitate to make any certain judgements
about the connector impedances. Also, the appearance or presence of an
insulating material is of little value in determining the impedance as well.
I would instead rely on a TDR (time domain reflectometry) type of
experiment.
Take two pieces of 50 ohm cable, and put one of your connectors in the
middle, joining the two. Set up a signal source to drive one end,
preferably with output impedance equal to that of the cable and put a 50
ohm T on that end. Terminate the end of the cables with a 50 ohm
terminator. Now probe the open T at the generator with a proper high
impedance RF scope probe.
Feed square waves into the cable and look at what's on the scope. A
slight reflection from the termination may be visible from a slight
mismatch at the end. Also, a small blip from the connector in the
middle may be visible. If you swap known 50 ohm vs. 75 ohm connectors
in the middle position, you should see the size of the reflection blip
from the middle connector change in size and character.
When you know the way a 50 ohm connector looks vs. a 75 ohm, then sort
all your connectors.
This is my untested idea. I'm not an expert on the subject, but it
follows from what I have learned from transmission line theory that this
should work.
Good day.
--
_____________________
Christopher R. Carlen
cr...@earthlink.net
Suse 8.1 Linux 2.4.19
Dave Holford
Chris Carlen wrote:
>
> George Shaw wrote:
> > I have a large box of mixed BNC connectors/tees etc.
> >
> > Using the simplest equipment possible (multimeter?) HOW do I check if
> > the connector is for 50 Ohm or 75 Ohm coax connection. There are no
> > external markings.
>
> I find it very difficult to accept the suggestions that 50R vs. 75R
> connectors have different diameter pins, as that would imply that the
> "BNC" connector has different mechanical standards for different
> impedances. If this is truly the case, then I will be eager to learn
> that it is so.
>
Although I have never noticed any obvious difference in the dimension of
BNC connectors, other than the cable size, I have always understood that
the difference between the 50 and 75 Ohm "N" connectors was the pin size
and that attempting to interconnect them could result in damage to the
connector if attempting to insert a large pin, I forget which was which,
into a small diameter socket.
If the connectors are made to either standard I would certainly expect
them to have a code stamped on somewhere which would identify which one
they are. If they are not identified then I would doubt that they meet
any standard.
Dave
Ian Walker
<don...@pearce.uk.com> writes
>On Sat, 21 Dec 2002 22:13:43 +0000, George Shaw <georg...@ukf.net>
>wrote:
>
>>I have a large box of mixed BNC connectors/tees etc.
>>
>>Using the simplest equipment possible (multimeter?) HOW do I check if
>>the connector is for 50 Ohm or 75 Ohm coax connection. There are no
>>external markings.
>>
>>
>Look inside the end of the connector. The 75 ohm will have the pin,
>surrounded by just the slotted metal sleeve. On the 50 ohm connector,
>the metal sleeve will have a layer of plastic insulation just inside
>it, forming a second sleeve.
That is almost correct for BNC; except there are two styles of 75 ohm
BNC, one as you describe and the other has no insulating sleeve around
the female receptacle and thin sleeve around the inside of the outer
conductor of the male compared to 50 ohm connector. Both types of 75 ohm
and the 50 ohm BNC can be mated non-destructively but the impedance
discontinuity could cause problems. The difference with N type is much
simpler, the 50 ohm has the same diameters as a BNC, the 75 ohm has a
smaller centre pin. The two types can not be mated, a 50 ohm plug
inserted into a 75 ohm socket will damage the socket, a 75 ohm plug
inserted into a 50 ohm socket will fail to provide a reliable centre
connection. The observant will have noticed that you can push a 50 ohm
N onto either impedance BNC without harm, although this is not
recommended for power or where you need to measure amplitude.
--
Ian G8ILZ
Tony Williams
Dave Holford <hol...@sympatico.ca> wrote:
> Although I have never noticed any obvious difference in the dimension of
> BNC connectors, other than the cable size, I have always understood that
> the difference between the 50 and 75 Ohm "N" connectors was the pin size
> and that attempting to interconnect them could result in damage to the
> connector if attempting to insert a large pin, I forget which was which,
> into a small diameter socket.
AFAIR the 50 Ohm pin is significantly larger than the 75 Ohm.
--
Tony Williams.
Ian Walker
<gar...@lightfox.demon.co.uk> writes
>In article <thm90vof9eov5lgk2...@4ax.com>,
> George Shaw wrote:
>
>> I have a large box of mixed BNC connectors/tees etc.
>
>The 50 ohm plugs have slightly larger diameter pins than the 75's.
Only if they are N type, the centre pin of the male is often as you
state below.
>Sometimes too, the end of a pin on 75 ohm BNC plug is a sharp point and
>then there is a gradual chamfer back to the maximum diameter of the pin,
>whereas a 50 ohm pin is more likely to be a rounded 'nose' and a
>constant diameter along the rest of the shaft.
>
>The sockets are best tested comparitively by looking at the centre
>socket diameters.
Again only for N type, not for BNC.
--
Ian G8ILZ
Ian Walker
>
>
>Chris Carlen wrote:
>>
>> George Shaw wrote:
>> > I have a large box of mixed BNC connectors/tees etc.
>> >
>> > Using the simplest equipment possible (multimeter?) HOW do I check if
>> > the connector is for 50 Ohm or 75 Ohm coax connection. There are no
>> > external markings.
>>
>> I find it very difficult to accept the suggestions that 50R vs. 75R
>> connectors have different diameter pins, as that would imply that the
>> "BNC" connector has different mechanical standards for different
>> impedances. If this is truly the case, then I will be eager to learn
>> that it is so.
>
>Although I have never noticed any obvious difference in the dimension of
>BNC connectors, other than the cable size, I have always understood that
>the difference between the 50 and 75 Ohm "N" connectors was the pin size
>and that attempting to interconnect them could result in damage to the
>connector if attempting to insert a large pin, I forget which was which,
>into a small diameter socket.
It is an urban myth that the centre pins of 50 and75 ohm BNC connectors
are different diameters, probably brought about by an optical illusion
caused by the taper of the pin along with the knowledge that the inner
to outer ratio must be greater for higher impedances. This overlooks
that using a different dielectric will change the impedance, i.e. more
air as in the case of 75 ohm BNC connectors.
>If the connectors are made to either standard I would certainly expect
>them to have a code stamped on somewhere which would identify which one
>they are. If they are not identified then I would doubt that they meet
>any standard.
They do not have to be stamped, although reputable manufactures will
generally want to put their name and part number on them (OEM not
withstanding).
--
Ian G8ILZ
mike
etc.
Using the simplest equipment possible
(multimeter?) HOW do I check if
the connector is for 50 Ohm or 75 Ohm coax
connection. There are no
external markings.
____________________________
At the risk of oversimplifying things....
If you can't tell the difference it don't matter.
Stated another way, if a multimeter is the best
you can do, it's
unlikely that you'll be able to tell the
difference in any application
that you have the instrumentation to measure.
It'll take some doin'
to create a situation where the connector
impedance matters.
Any application where a centimeter of
discontinuity from 50 to 75 ohms
matters, probably shouldn't be using a BNC anyway.
For most applications, reach into the box, pull
one out, and use it.
If you think that random hunk of coax is really
50.0 ohms, you also have
a surprise coming. Six feet of 52+/- ohms will
have way more effect than
1cm of 75 ohms in most 50 ohm systems.
Having said all that, if you have access to a high
resolution computer monitor
with high quality BNC input, you can install one
of your unknown connectors and
look for ringing on vertical lines. Use the green
input, cause it has the most
visibility. It's like a 75 ohm TDR with higher
resoluton than the
scope that most of us can afford...but still not
sure you'll be able to tell the
diff between connectors.
I got curious and put a 50 ohm elbow connector on
the 75 ohm green input.
at 1280x1024 resolution, I can't see ANY
difference with a 25X microscope.
mike
--
Bunch of stuff For Sale and Wanted at the link
below.
Tek 2465, TFT projector, ham radio, 30pS pulser
Tektronix Concept Books...
http://www.geocities.com/SiliconValley/Monitor/4710/
pk
> it is matter of diameter ratios, not measurable Ohms.
cant say any better than that.
suggest, you try and get a definite example of a 50 or 75 ohm one
and compare.
harrogate
"Ian Walker" <Jun...@127.0.0.1> wrote in message
news:ICsB0ChK...@newbrain.demon.co.uk...
Also look at the hole in the pin - 75R is significantly smaller that 50R as
the core is thinner.
--
Woody
John Woodgate
<cr...@BOGUS.earthlink.net> wrote (in <Qe9N9.297$ka5....@newsread1.pro
d.itd.earthlink.net>) about 'How to check for a 50 or 75 ohm connector',
on Sun, 22 Dec 2002:
>I find it very difficult to accept the suggestions that 50R vs. 75R
>connectors have different diameter pins, as that would imply that the
>"BNC" connector has different mechanical standards for different
>impedances. If this is truly the case, then I will be eager to learn
>that it is so.
It IS so. It's basically the ratio of pin diameter to sleeve diameter
that determines the characteristic impedance. The required ratio is
affected by the presence of insulating material.
>
>But until I'm convinced, I would hesitate to make any certain judgements
>about the connector impedances.
Well, it can be proved mathematically that what I say above is true, but
I can't reproduce the proof here. Try a textbook.
> Also, the appearance or presence of an
>insulating material is of little value in determining the impedance as well.
By increasing the capacitance between inner and outer, insulating
material tends to lower the characteristic impedance.
--
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
Interested in professional sound reinforcement and distribution? Then go to
http://www.isce.org.uk
PLEASE do NOT copy news posts to me by E-MAIL!
Don Pearce
No, the pin diameters are identical - the 50 ohm connector has an
insulation layer between the pin and the outer that the 75 ohm lacks.
Unlike N types, 50 and 75 ohm BNCs are fully inter-mateable, without
damage.
Don Pearce
OK, I have just looked up the technical spec for BNC plugs, and the
pin diameter for BOTH 50 and 75 ohm devices is given as 1.32 to 1.37
mm.
Ian Walker
<harr...@ntlworld.com> writes
With N type that is correct; but for BNC you are wrong, Wrong, WRONG!
How many times must I state the B******g fact!!
--
Ian G8ILZ
Don Pearce
<DarkM...@thebarattheendoftheuniverse.org> wrote:
>On Sun, 22 Dec 2002 02:15:12 GMT, Chris Carlen
><cr...@BOGUS.earthlink.net> Gave us:
>
>>George Shaw wrote:
>>> I have a large box of mixed BNC connectors/tees etc.
>>>
>>> Using the simplest equipment possible (multimeter?) HOW do I check if
>>> the connector is for 50 Ohm or 75 Ohm coax connection. There are no
>>> external markings.
>
> For George: It is based on the cable that the connector is meant to
>be used for. You cannot electrically measure it, you must physically
>measure the cable side of the connector, and match those particulars
>up with your coax. Basically, the center conductor's insulator
>diameter changes, as well as the material, and the overall cable
>diameter changes. In fact, THAT is how the impedance is determined.
>
You are getting to different issues hopelessly confused here. The
cable end of the connector is available in many different styles, to
suit all sorts of different cable types. That is true. The mating end
of the connector is where you will find the specification for 50 or 75
ohms. They are different in design.
A connector will normally have a part number stamped onto it. That
part number will determine the appropriate cable type that will fit
it, and somewhere in that part number will be a 50 or a 75 to tell you
the impedance of the mating surface.
>>
>>I find it very difficult to accept the suggestions that 50R vs. 75R
>>connectors have different diameter pins, as that would imply that the
>>"BNC" connector has different mechanical standards for different
>>impedances. If this is truly the case, then I will be eager to learn
>>that it is so.
>
> The connector does not possess the impedance. That reference is
>really about the transmission cable that gets used. Connectors can
>differ between them, but the cable is what really determines the
>impedance.
>
The connector does indeed contain the impedance. The design is such
that the correct impedance is maintained as far much as possible
throughout the mating connector pair. BNC isn't terribly good in this
regard, but the connectors certainly do contain the impedance.
>>
>>But until I'm convinced, I would hesitate to make any certain judgements
>>about the connector impedances. Also, the appearance or presence of an
>>insulating material is of little value in determining the impedance as well.
>>
>>I would instead rely on a TDR (time domain reflectometry) type of
>>experiment.
>
The two impedance connectors are of quite different design visually at
the mating end, and there is no need to resort to test gear to
separate them.
> You would get NOTHING from a connector alone. It would require a
>CABLE, terminated properly at both ends with no kinks, and THAT is
>what the meter would test. The connectors themselves are shaped and
>sized merely to keep someone from mismatching connection cabling.
>>
>>Take two pieces of 50 ohm cable, and put one of your connectors in the
>>middle, joining the two. Set up a signal source to drive one end,
>>preferably with output impedance equal to that of the cable and put a 50
>>ohm T on that end. Terminate the end of the cables with a 50 ohm
>>terminator. Now probe the open T at the generator with a proper high
>>impedance RF scope probe.
>>
>>Feed square waves into the cable and look at what's on the scope. A
>>slight reflection from the termination may be visible from a slight
>>mismatch at the end. Also, a small blip from the connector in the
>>middle may be visible. If you swap known 50 ohm vs. 75 ohm connectors
>>in the middle position, you should see the size of the reflection blip
>>from the middle connector change in size and character.
>>
>>When you know the way a 50 ohm connector looks vs. a 75 ohm, then sort
>>all your connectors.
>
> As far as I know, BNC is BNC is BNC, and the ONLY differences are
>the size of the rear portion of the connector for different sized
>cable. THAT would determine the proper cable for the connector, and
>*that* cable would determine the impedance. So, by default of
>application, the connectors differ. On the front, connection side of
>them, they are all the same, however. There are "hybrids" that will
>claim some super duty or used gold plated parts, but BNC nomenclature
>is the same. It has to be able to be hooked directly in to ANY BNC
>female ANYWHERE. So they are certainly ALL the same on the connectors
>side.
>
Quite wrong. You have been misinformed.
> The cabling determines the impedance. RG-59 is 75 ohm, RG-58 is 50
>ohm. Then, there is RG-6 (75ohm), RG-174, etc. ALL have BNC
>connector versions.
>
Well, the system design determines the impedance. You then use cabling
and connectors of appropriate impedance.
>>
>>This is my untested idea. I'm not an expert on the subject, but it
>>follows from what I have learned from transmission line theory that this
>>should work.
>
> Sure it would. At several GHz... maybe. At Mhz frequencies the
>connector contributes little to the overall impedance of the cable
>run. That is determined practically solely on the cable run itself.
>
This is true. Long lengths of cable certainly have much greater
effects than short lengths of connector.
> What can be determined from different connectors is how well they
>contain what signals they carry. That would be RFI leakage at the
>terminations. That is why the connectors are "shrouded" in nature by
>grounded elements.
Trevor Day
<DarkM...@thebarattheendoftheuniverse.org> writes
> Most folks that work with them on a daily basis can tell by
>examination of the rear, cable insertion end of the device in
>question. RG-58, RG-59, RG-62, RG-6, RG-174... etc. ALL have BNC
>iterations, and all are different sizes. Hell, RG-174 is like an
>eighth inch in diameter.
I have always believed that there is a diameter difference between the
two connectors (BNC50 and BNC75). This is re-inforced by the fact that
I own two Inter-series ex MoD connector kits which are identical
*except* one contains 75 ohm adaptors, the other 50. In this case they
can be easily told apart as one set has bright red insulation (75 ohms)
whilst the other (50 ohms) is white.
Looking at the pins, they appear to be pretty much the same size when
viewed from the mating end; this applies to both the 'male' pins in the
plugs and the 'female' pins in the sockets. There is a noticeable
difference however in the diameter of both male and female pins when
viewed from the rear. The 75 ohm pins are significantly slimmer. Both
50 and 75 ohm plugs and sockets will mate happily enough however, the
internal connectors will not. My conclusions are:
1. There is a diameter difference between the two that is difficult
to check by merely looking at the mating ends of the pins.
2. The MoD (and therefore industry who supply) believe there is a
significant difference between 50 and 75 ohm BNCs.
3. The above information is not much help unless you are using
Inter series connectors :-)
Trev G3ZYY
(Who by the way, is willing to put my name where my mouth is!)
--
Trevor Day
Ian Walker
<to...@ledelec.demon.co.uk> writes
For N type that is correct, for BNC it is incorrect.
P.S. Both impedances of BNC and N type are 7mm connectors, but for
measurements I use APC7.
--
Ian G8ILZ
Tony Williams
Don Pearce <don...@pearce.uk.com> wrote:
Has that always been true Don? I'm sure that,
<mumble,mumble> years ago, there were even printed
warnings in the manuals of some equipment.
--
Tony Williams.
Don Pearce
<to...@ledelec.demon.co.uk> wrote:
>In article <qb5b0vg66m4jo1tat...@4ax.com>,
> Don Pearce <don...@pearce.uk.com> wrote:
>
>> On Sun, 22 Dec 2002 08:25:29 +0000 (GMT), Tony Williams
>> <to...@ledelec.demon.co.uk> wrote:
>> > AFAIR the 50 Ohm pin is significantly larger than the 75 Ohm.
>
>> No, the pin diameters are identical - the 50 ohm connector has an
>> insulation layer between the pin and the outer that the 75 ohm lacks.
>> Unlike N types, 50 and 75 ohm BNCs are fully inter-mateable, without
>> damage.
>
> Has that always been true Don? I'm sure that,
> <mumble,mumble> years ago, there were even printed
> warnings in the manuals of some equipment.
The warnings are only for N Type - they use air dielectric on both 50
and 75 ohms, and change impedance by pin diameter.
Whether there was ever a time when 50 and 75 ohm BNC were not
interchangeable, I'm not sure. But I have seen some very old kit that
has BNCs up to the current dimensions.
John Woodgate
wrote (in <3keudKjA...@newbrain.demon.co.uk>) about 'How to check
for a 50 or 75 ohm connector', on Sun, 22 Dec 2002:
>It is an urban myth that the centre pins of 50 and75 ohm BNC connectors
>are different diameters, probably brought about by an optical illusion
>caused by the taper of the pin along with the knowledge that the inner
>to outer ratio must be greater for higher impedances. This overlooks
>that using a different dielectric will change the impedance, i.e. more
>air as in the case of 75 ohm BNC connectors.
No urban myth. I have both sorts of connector, marked with their
impedances, and the centre pins ARE of different diameters. It MAY be
possible to use the same size pin and different insulators, but I don't
have any examples of that.
Don Pearce
<j...@jmwa.demon.contraspam.yuk> wrote:
>I read in sci.electronics.design that Ian Walker <Jun...@127.0.0.1>
>wrote (in <3keudKjA...@newbrain.demon.co.uk>) about 'How to check
>for a 50 or 75 ohm connector', on Sun, 22 Dec 2002:
>
>>It is an urban myth that the centre pins of 50 and75 ohm BNC connectors
>>are different diameters, probably brought about by an optical illusion
>>caused by the taper of the pin along with the knowledge that the inner
>>to outer ratio must be greater for higher impedances. This overlooks
>>that using a different dielectric will change the impedance, i.e. more
>>air as in the case of 75 ohm BNC connectors.
>
>No urban myth. I have both sorts of connector, marked with their
>impedances, and the centre pins ARE of different diameters. It MAY be
>possible to use the same size pin and different insulators, but I don't
>have any examples of that.
Of course, now I think about it - the diameter of the pin doesn't
actually do anything for the impedance of the connector when it is
mated. It is the outer diameter of the female jack that forms the
impedance-determining dimension in the connector.
J.
wrote:
>it is matter of diameter ratios, not measurable Ohms.
I heard it is a matter of the length of the centre
pin in relation to its insulator or the same sort
of comparison with regard to the female receptacle
which the pin fits into.
Unfortunately I can't find a reference, but no doubt someone
else will know exactly what I mean.
--
___________________
Posted by J.
West Bank
Tweeksbury
* Season's Greeting to all our Readers *
J.
>On Sat, 21 Dec 2002 22:13:43 +0000, George Shaw <georg...@ukf.net>
>wrote:
>
>>I have a large box of mixed BNC connectors/tees etc.
>>
>>Using the simplest equipment possible (multimeter?) HOW do I check if
>>the connector is for 50 Ohm or 75 Ohm coax connection. There are no
>>external markings.
>>
>>
>Look inside the end of the connector. The 75 ohm will have the pin,
>surrounded by just the slotted metal sleeve. On the 50 ohm connector,
>the metal sleeve will have a layer of plastic insulation just inside
>it, forming a second sleeve.
SNAP!
Ian Walker
<DarkM...@thebarattheendoftheuniverse.org> writes
>On Sun, 22 Dec 2002 08:22:24 +0000, Ian Walker <Jun...@127.0.0.1> Gave
>us:
>polyvinyl foam or teflon or other insulator as the dielectric sheath
>over the center wire. ALL BNC pin diameters are the same. The
>difference is that connectors are made to fit their rear ends into
>different cable shapes. THAT is where they differ. The plug side is
>always the same.
Wrong, I use both 50 and 75 ohm BNC connectors, they are different;
although the diameters of the inner and outer conductors are the same
the amount of dielectric between them ,and therefore the amount of air,
is different.
-----------Cut------------
> Most folks that work with them on a daily basis can tell by
>examination of the rear, cable insertion end of the device in
>question. RG-58, RG-59, RG-62, RG-6, RG-174... etc. ALL have BNC
>iterations, and all are different sizes. Hell, RG-174 is like an
>eighth inch in diameter.
I have seen N types made for RG-174 & 6, I find the cable side difficult
to judge since there are many different cables and looking at the outer
diameter as indicated by the rear of the connector tells you nothing
about the inner diameter or the dielectric of the cable it is made for.
Only looking at the front of the connector tells you its design
impedance.
--
Ian G8ILZ
Ian Walker
<DarkM...@thebarattheendoftheuniverse.org> writes
> ALL BNC connectors are the same shape on the connector side. ALL "f"
>type connectors are the same shape on the connector side. ALL "RCA"
>type connectors are the same shape on the connector side. Starting to
>make sense yet?
>
> Since one can have both 75 and 50 ohm versions of each cable type yet
>still retain the moniker for the connector, the cable end of the
>connectors mentioned are the only things about them that change as said
>cables change. The connector side is standardized, however. The world
>would be a huge mess otherwise.
Wrong! Whilst 50 and 70 ohm N (BNC) type connectors are both N (BNC)
type they are incompletely described by calling them N (or BNC) type.
You must also specify the impedance for correct electrical (and
mechanical in the case of N type) performance, the type of cable
providing it is suitable for the connector is irrelevant (i.e. you can
have RG-174 on one side of an N system and RG-203 on the other).
--
Ian G8ILZ
Ian Walker
<Tr...@secornwall.removethis.com> writes
The last is correct, the specification of Inter-Series adapters is
propriety to Greenpar and is in no way indicative of the internationally
agreed standards for BNC and N connectors.
--
Ian G8ILZ
Rene Tschaggelar
> On Sun, 22 Dec 2002 02:15:12 GMT, Chris Carlen
> <cr...@BOGUS.earthlink.net> Gave us:
>
>
>>George Shaw wrote:
>>
> The connector does not possess the impedance. That reference is
> really about the transmission cable that gets used. Connectors can
> differ between them, but the cable is what really determines the
> impedance.
You're a DC worker, I assume.
It is true that you won't notice the difference between a 50 and 75
Ohms connector at a few MHz, but there you also could use those 4mm
Banana plugs. BNC is specified for up too 4GHz @ 50 Ohms and
1GHz @ 75 Ohms. And N is specified for 18GHz @ 50 Ohms and
1.5GHz @ 75 Ohms.
Longer antenna cables are preferably made with 75 Ohms as the voltage
is higher and the losses therefore lower for a given signal power,
especially at the upper end.
Rene
--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net
Ian Walker
<don...@pearce.uk.com> writes
between the inner and outer contacts.
--
Ian G8ILZ
Ian Walker
<j...@jmwa.demon.contraspam.yuk> writes
>I read in sci.electronics.design that Ian Walker <Jun...@127.0.0.1>
>wrote (in <3keudKjA...@newbrain.demon.co.uk>) about 'How to check
>for a 50 or 75 ohm connector', on Sun, 22 Dec 2002:
>
>>It is an urban myth that the centre pins of 50 and75 ohm BNC connectors
>>are different diameters, probably brought about by an optical illusion
>>caused by the taper of the pin along with the knowledge that the inner
>>to outer ratio must be greater for higher impedances. This overlooks
>>that using a different dielectric will change the impedance, i.e. more
>>air as in the case of 75 ohm BNC connectors.
>
>No urban myth. I have both sorts of connector, marked with their
>impedances, and the centre pins ARE of different diameters. It MAY be
>possible to use the same size pin and different insulators, but I don't
>have any examples of that.
They will only be different diameters if they are N type, if they are
BNC then they will both be 1.32-1.37mm diameter (the same as 50 ohm N
type plugs), the difference for BNC is in the amount air verses plastic
between the inner and outer forming the dielectric. I use both type of
BNC, along with 93 ohm BNC. I also use: 2.4, K, APC3.5, APC7, N and
7/16.
--
Ian G8ILZ
Ian Walker
<nordl...@hotmail.com> writes
>On Sat, 21 Dec 2002 23:17:31 +0100, Rene Tschaggelar <tscha...@dplanet.ch>
>wrote:
>
>>it is matter of diameter ratios, not measurable Ohms.
>
>I heard it is a matter of the length of the centre
>pin in relation to its insulator or the same sort
>of comparison with regard to the female receptacle
>which the pin fits into.
Then what you have heard is totally wrong.
>Unfortunately I can't find a reference, but no doubt someone
>else will know exactly what I mean.
You won't, and no one will!
--
Ian G8ILZ
James Meyer
>
>
>OK, I have just looked up the technical spec for BNC plugs, and the
>pin diameter for BOTH 50 and 75 ohm devices is given as 1.32 to 1.37
>mm.
>
>d
>
It's about time someone actually looked at the technical drawings for
those BNC connectors instead of pulling half-baked theories from dark places.
8-)
OK, now look at the diameter of the "shank" of the pin that has the hole
in it for the center conductor of the coax. The pin diameter IS the same, but
the shank diameter will be larger for the 50 Ohm connector. You will find the
shank diameter on both the pin and recepticle the same for the same impedance
but different for 50 and 75 ohms.
It's the ratio of inner to outer conductor diameter in coax or coax
connectors that determine their impedance as long as the dielectric constant of
the insulator stays the same.
Jim
James Meyer
>
> The connector does not possess the impedance. That reference is
>really about the transmission cable that gets used. Connectors can
>differ between them, but the cable is what really determines the
>impedance.
>
The connector DOES posses the impedance. If you look at a cross section
of a mated pair of connectors along with a bit of coax on each, you will find
that the ratios of the diameters of the conductors and the dielectric constansts
of the insulation for all the parts will be sized so that there is a constant
impedance both for the coax AND for the connectors if all the parts have the
same stated impedance.
That said, if you use 75 Ohm connectors on 50 Ohm cable, you have
inserted a mis-match into the system. However, the length of the line comprised
of the connectors is so short that you would have to take very careful
measurements at very high frequencies to even begin to be able to quantify the
effects of the mis-match.
Jim
John Woodgate
wrote (in <0r5b0v818kme0f0qc...@4ax.com>) about 'How to
check for a 50 or 75 ohm connector', on Sun, 22 Dec 2002:
>OK, I have just looked up the technical spec for BNC plugs, and the
>pin diameter for BOTH 50 and 75 ohm devices is given as 1.32 to 1.37
>mm.
What 'technical specs'?
Brian Reay
news:eb0c0v0gbmm9v13kb...@4ax.com...
> On Sun, 22 Dec 2002 15:07:35 +0000, Ian Walker <Jun...@127.0.0.1> Gave
>
> This is because a BNC connector has a physical spec on the mating
> side that is impedance independent.
How is this achieved? The mating part is coaxial so how is it made
"impedance independent"
You can see the mismatch if you have a 75 ohm connector in a 50 ohm system
with a good network analyser-- even if the cable is correct.
As regards the original question, I'd suspect the bag to be full of 50 ohm
parts as many are coming onto the surplus market as LANs are changed to Cat5
(or 6) from the old coax based systems.
73
Brian
Don Pearce
<j...@jmwa.demon.contraspam.yuk> wrote:
>I read in sci.electronics.design that Don Pearce <don...@pearce.uk.com>
>wrote (in <0r5b0v818kme0f0qc...@4ax.com>) about 'How to
>check for a 50 or 75 ohm connector', on Sun, 22 Dec 2002:
>
>>OK, I have just looked up the technical spec for BNC plugs, and the
>>pin diameter for BOTH 50 and 75 ohm devices is given as 1.32 to 1.37
>>mm.
>
>What 'technical specs'?
I looked here - grab the pfd's for 50 and 75, compare and contrast.
http://www.hubersuhner.de/p-rf/p-rf-con/p-rf-con-overview/p-rf-con-bnc75.htm
Don Pearce
<DarkM...@thebarattheendoftheuniverse.org> wrote:
> The cable is the SOLE determining factor.
>
> Try again. Review your data, and make information for yourself out
>of it. Unless disseminated properly, it is mere data. It is not
>INFORMATION until it gets examined, and absorbed properly.
I won't argue this any further - go and look here at the 50 and 75 ohm
pdf files. You will see the difference in the connector (mating) side
of the connectors. Look inside the connector at the design of the
insulation (or lack of insulation in the case of 75 ohms).
http://www.hubersuhner.de/p-rf/p-rf-con/p-rf-con-overview/p-rf-con-bnc75.htm
Gareth Rowlands
Don Pearce wrote:
> of the connectors. Look inside the connector at the design of the
> insulation (or lack of insulation in the case of 75 ohms).
That's the manufacturer's choosing. Lately, I have noticed that there
are 75 ohm BNC plugs coming on the market that do not have plastic
dielectric material running up inside the inner flange to the surface.
I thought this was to help reduce loss.
I've got an honest to goodness 75 ohm BNC termination here as I write
this (I suspect it's a Greenpar, but not 100%) that has the white
plastic dielectric all the way to the end, and the chamfered pin too.
But it's an older connector dating from the last century !
Cheers,
Gareth.
Dave Holford
http://www.amphenolrf.com/products/bnc.asp
Amphenol, and others, manufacture 50 and 75 Ohm constant impedance
connectors. Acually Amphenol makes two types of 75 and one 50 Ohm.
Although the connectors preserve the nominal impedance they all mate:
"50 Ohm and 75 Ohm connectors are intermateable to ensure
non-destructive mating" - they learned a lesson from the "N" design!
So; 50 and 75 Ohm BNC connectors are different to preserve the
impedance, but they are physically designed to mate without damage -
very useful given their extensive use on test equipment.
There, that was pretty simple, wasn't it?
Dave
Michael A. Terrell
>
> On Sun, 22 Dec 2002 15:59:30 +0000, Ian Walker <Jun...@127.0.0.1> Gave
> us:
>
> >
> >The last is correct, the specification of Inter-Series adapters is
> >propriety to Greenpar and is in no way indicative of the internationally
> >agreed standards for BNC and N connectors.
> >
>
> By the way, if you actually intend on receiving e-mail at the
> address you post in your headers, you will be disappointed.
> 127.0.0.1 is an INTERNAL LAN IP addy only. You will never receive
> mail on that IP. You would have to publish the IP address that your
> cable, dsl, or ppp connection issues.
That is a lame attempt at a spam trap.
--
Michael A. Terrell
Central Florida
John Woodgate
endoftheuniverse.org> wrote (in <u1vb0vg3ptp1smc9320vg7viond96s7e0o@4ax.
com>) about 'How to check for a 50 or 75 ohm connector', on Sun, 22 Dec
2002:
> One can purchase the simple "consumer electronics" "F" style
>connector in a number of cable termination sizes. Depending on WHICH
>cable gets used, the impedance will differ. The PLUG side, however is
>ALWAYS absolutely identical.
Since the 'centre pin' of an F-connector plug is the inner conductor of
the cable, I don't see how it can always be the same size!
Ian Walker
<DarkM...@thebarattheendoftheuniverse.org> writes
>On Sun, 22 Dec 2002 15:07:35 +0000, Ian Walker <Jun...@127.0.0.1> Gave
>us:
>
>>Wrong, I use both 50 and 75 ohm BNC connectors, they are different;
>>although the diameters of the inner and outer conductors are the same
>>the amount of dielectric between them ,and therefore the amount of air,
>>is different.
>
>
>JACK side is ALWAYS the same.
Wrong, the thickness of the dielectric is DIFFERENT, this along with the
internal construction determines the impedance. The internal
construction must also provide an effective constant impedance
transition to whatever diameter cable is being used. If you interconnect
50 and 75 ohm BNCs you might not notice the effect, and if you are using
equipment which can you probably should not be using BNC.
--
Ian G8ILZ
Ian Walker
<DarkM...@thebarattheendoftheuniverse.org> writes
>On Sun, 22 Dec 2002 15:59:30 +0000, Ian Walker <Jun...@127.0.0.1> Gave
>us:
>
>>
>>The last is correct, the specification of Inter-Series adapters is
>>propriety to Greenpar and is in no way indicative of the internationally
>>agreed standards for BNC and N connectors.
>>
>
> By the way, if you actually intend on receiving e-mail at the
>address you post in your headers, you will be disappointed.
>127.0.0.1 is an INTERNAL LAN IP addy only. You will never receive
>mail on that IP. You would have to publish the IP address that your
>cable, dsl, or ppp connection issues.
My "Reply-To:" is valid, my "Sender" and "From" are not; this an
anti-spam measure.
--
Ian G8ILZ
Alun Palmer
news:jo2c0v8t39utmckth...@4ax.com:
The centre pins on 75 Ohm BNC plugs are very slightly larger, so that
inserting them in a 50 Ohm BNC socket causes damage by enlarging the hole.
I used to work in a lab where we had both.
I'm not sure how F connectors got into this thread, but they are nasty
rubbish, worse even than Belling-Lee connectors. Only use F connectors if
you absolutely have to. They make UHF connectors look like an engineering
masterpiece by comparison, which they're not.
F connectors leak RF in and out like a sieve. One of my old colleagues did
extensive tests of 'transfer impedance' (this has nothing to do with
characteristic impedance, but is a measure of screening) and could find
nothing under the sun that had worse screening than an F connector.
dmi...@spamblock.demon.co.uk
> The warnings are only for N Type - they use air dielectric on both 50
> and 75 ohms, and change impedance by pin diameter.
> Whether there was ever a time when 50 and 75 ohm BNC were not
> interchangeable, I'm not sure. But I have seen some very old kit that
> has BNCs up to the current dimensions.
There was a version of the "BNC" made by one manufacturer which had different
size centre pins and obviously failed to meet the real BNC spec, but it has
not been on the market for quite a few years this I suspect is where the
rumor started (ref. a long article about this in the back of the Canford
catalog).
Note that (before digital video) it was common practice to use 50ohm
connectors in 75ohm systems because they were felt to be better mechanically
and the length of the connector was so short compared to the wavelength of
the highest frequency to be passed (about 5Mhz), this is no longer viable
with the advent of 270mb/s digital video....
New facitities should use the connector appropriate to the impeadence in use
but BNC is fully cross pluggable between 50 and 75 ohm systems.
Regards, Dan.
--
** The email address *IS* valid, do NOT remove the spamblock
And on the evening of the first day the lord said...........
.... LX 1, GO!; and there was light.
dmi...@spamblock.demon.co.uk
> OK, I have just looked up the technical spec for BNC plugs, and the
> pin diameter for BOTH 50 and 75 ohm devices is given as 1.32 to 1.37
> mm.
Yes, the difference is in the dielectric geometry.
Now if we could just standardise the crimp tools for different
manufacturers BNC connectors.......
dmi...@spamblock.demon.co.uk
> On Sun, 22 Dec 2002 02:15:12 GMT, Chris Carlen
> <cr...@BOGUS.earthlink.net> Gave us:
<Snip>
> The connector does not possess the impedance. That reference is
> really about the transmission cable that gets used. Connectors can
> differ between them, but the cable is what really determines the
> impedance.
Well the connector is in line with the cable so if the connector is a
signifigant part of a wavelength long at the operating frequency then
it has to be viewed as a transmission line in itself and thus will have
a line impeadence in its own right.....
<Snip>
> You would get NOTHING from a connector alone. It would require a
> CABLE, terminated properly at both ends with no kinks, and THAT is
> what the meter would test. The connectors themselves are shaped and
> sized merely to keep someone from mismatching connection cabling.
No... It doesn't matter at low frequencies but by the time you are passing
signals with components up above 0.5Ghz (I am thinking SDV here) it becomes
measurable. The dialectric geometry is different in 75ohm vs 50ohm BNC
and this changes the charicteristic impeadence of the connector, it is
orthognal to the issue of the cable.
Consider that a bulkhead through connector with a BNC plugged into both
ends is probably about 5cm long and that 5cm (at VF=0.6) is a wavelength
at 3.6Ghz. Which means that for any frequency greater then about 360Mhz
we need to view this BNC-> through panel adaptor -> BNC as a transmission
line in its own right.....
> As far as I know, BNC is BNC is BNC, and the ONLY differences are
> the size of the rear portion of the connector for different sized
> cable. THAT would determine the proper cable for the connector, and
> *that* cable would determine the impedance. So, by default of
> application, the connectors differ. On the front, connection side of
> them, they are all the same, however. There are "hybrids" that will
> claim some super duty or used gold plated parts, but BNC nomenclature
> is the same. It has to be able to be hooked directly in to ANY BNC
> female ANYWHERE. So they are certainly ALL the same on the connectors
> side.
No. They have to be able to mate between 50R and 75R systems but that just
means the pins have to have the same geometry, it says nothing about the
dialectric which is what changes between the two connectors.
> The cabling determines the impedance. RG-59 is 75 ohm, RG-58 is 50
> ohm. Then, there is RG-6 (75ohm), RG-174, etc. ALL have BNC
> connector versions.
The cable determines the impeadence of the cable (obviously) but to avoid
discontinuities at the ends any connector used at a frequency where the
connector is a reasonable part of a wavelength in electrical length must
also be considered to be a transmission line with all that that implies.
> Sure it would. At several GHz... maybe. At Mhz frequencies the
> connector contributes little to the overall impedance of the cable
> run. That is determined practically solely on the cable run itself.
Obviously, at low frequencies the connector is short compared to the
wavelength and in this case the differences do not much matter, but we
do not all use connectors solely at low frequencies, and in any case for
a not uncommon BNC setup the threashold is only a few hundred Mhz.
Ian Walker
<DarkM...@thebarattheendoftheuniverse.org> writes
>On Sun, 22 Dec 2002 15:15:52 +0000, Ian Walker <Jun...@127.0.0.1> Gave
>us:
>
>>Wrong! Whilst 50 and 70 ohm N (BNC) type connectors are both N (BNC)
>>type they are incompletely described by calling them N (or BNC) type.
>>You must also specify the impedance for correct electrical (and
>>mechanical in the case of N type) performance, the type of cable
>>providing it is suitable for the connector is irrelevant (i.e. you can
>>have RG-174 on one side of an N system and RG-203 on the other).
>
>the determining factor.
Only in that the properties of the system determine the type of cable
that must used (RG-174, RG-58, RG-203, Su-104, etc.), both the cable and
the system determine the connector that must be used (BNC, N, C, K,
PC3.5, PC7, 2.4, 1.0, etc.). If the connector side is not designed to
have the same impedance as the cable then you will get a reflection from
that discontinuity.
If you have only got a bag of unidentified plugs you will have to
measure the inner and outer diameters and, along with a guess at the
dielectric calculate the impedance.
--
Ian G8ILZ
Ian Walker
<DarkM...@thebarattheendoftheuniverse.org> writes
>On Sun, 22 Dec 2002 15:28:05 -0500, "Michael A. Terrell"
><ter...@mfi.net> Gave us:
>
>>DarkMatter wrote:
>>>
>>> By the way, if you actually intend on receiving e-mail at the
>>> address you post in your headers, you will be disappointed.
>>> 127.0.0.1 is an INTERNAL LAN IP addy only. You will never receive
>>> mail on that IP. You would have to publish the IP address that your
>>> cable, dsl, or ppp connection issues.
>>
>> That is a lame attempt at a spam trap.
>
But if you click on reply it will be sent to my reply-to address; the
reply-to is not in the information harvested by spammers, they will only
get the local machine address.
--
Ian G8ILZ
Ian Walker
<hol...@sympatico.ca> writes
>Read, for example,
>http://www.amphenolrf.com/products/bnc.asp
>
>Amphenol, and others, manufacture 50 and 75 Ohm constant impedance
>connectors. Acually Amphenol makes two types of 75 and one 50 Ohm.
>
>Although the connectors preserve the nominal impedance they all mate:
>"50 Ohm and 75 Ohm connectors are intermateable to ensure
>non-destructive mating" - they learned a lesson from the "N" design!
Just as an aside, I have seen the result of some ???? pushing a PL259
into a 75 ohm N, and they did it again after we repaired the instrument!
--
Ian G8ILZ
MattD..
> Coax hasn't been used in ethernet networks since the 2 Mb/s days, or
> obscure topologies like TCNS.
Um, no quite true old boy. 10base2 was 10Mbps. Throughput was slower
(compared to a switch based network rather than a hub based one) due to all
cards sharing the same bandwidth, but I assure you that the bandwidth was
10Mbps. The biggest drawback was some clod removing the terminator or t
piece from his card.
Brian is correct. The bag, since it contains loads of t pieces, is most
likely full of ex-LAN bits. A dead giveaway would be a couple of 50R
terminators (look just like BNCs with a plastic end cap where the cable
would go) in the bag with them. These you _could_ bell out with an Ohmmeter
to ascertain their impedance.
George Shaw
I have looked at known 50 and 75 BNCs I have here from a reliable
source and can see NO difference in the sizes.
If a measuring equipment was available, what would be used to test the
BNC ????
MattD..
> Yes, and as the 10baseT was also generally available on the NICs,
> twisted pair was generally used. Also, on trying to get 10base2 to
> actually work at 10Mb/s. The collision levels were horrendous.
Absolutely right. I have a whole stack of Etherlink IIIs that back you up
on that statement. Cat3/5 was/is so much easier to work with, too...
>
> Trust me. Most systems were RJ-45 right after the jump from 2Mb/s to
> the availability of it. Most skipped right over 10Base2, because
> RJ-45 was actually easier to terminate, and cheaper to implement.
> It was electrically better as well.
Oh, I do. I do remember some comapnies sticking with their already hugely
expensive coax nets because of the investment they had already made, but
some months later, after 20 or 30 calls for support due to cable trapped in
doors, high-heeled shoes and floor runs or just some looney pulling the
terminator they came around... Especially given the £85 call-out charge :o)
>
> Arcnet was fun though. What was that? 360kb/s? Hahahah....
> Those were the days... Essentially it was Tandy Corp, not Billy boy,
> that put us all where we are today. Connected. Regardless of OS.
>
Remember twinax? And TDRs? Ah, the nostalgia (or psychotic interludes,
depending on your position in the food chain in those days...)
MattD..
> A time domain reflectometer. Fluke and many others makes cable
> testers for LANs and other communication systems, but I do not know if
> this class is covered, and the tester is very pricey.
You _had_ to mention TDRs, didn't you?
/me runs screaming from the tape measure...
Ian Walker
<DarkM...@thebarattheendoftheuniverse.org> writes
>On Sun, 22 Dec 2002 21:31:08 +0000, Ian Walker <Jun...@127.0.0.1> Gave
>us:
>
>>In article <9j0c0vk06vfcjbbdq...@4ax.com>, DarkMatter
>><DarkM...@thebarattheendoftheuniverse.org> writes
>>>On Sun, 22 Dec 2002 15:15:52 +0000, Ian Walker <Jun...@127.0.0.1> Gave
>>>us:
>>>
>>>>Wrong! Whilst 50 and 70 ohm N (BNC) type connectors are both N (BNC)
>>>>type they are incompletely described by calling them N (or BNC) type.
>>>>You must also specify the impedance for correct electrical (and
>>>>mechanical in the case of N type) performance, the type of cable
>>>>providing it is suitable for the connector is irrelevant (i.e. you can
>>>>have RG-174 on one side of an N system and RG-203 on the other).
>>>
>>> You prove in your own statement that it IS indeed the cable that is
>>>the determining factor.
>>
>>Only in that the properties of the system determine the type of cable
>>that must used (RG-174, RG-58, RG-203, Su-104, etc.), both the cable and
>>the system determine the connector that must be used (BNC, N, C, K,
>>PC3.5, PC7, 2.4, 1.0, etc.). If the connector side is not designed to
>>have the same impedance as the cable then you will get a reflection from
>>that discontinuity.
>
>the grounding shroud, and the center pin. That media is usually air.
>Some use a hard dielectric.
They might be mostly air as in the case of precession low loss N type,
but they must have some hard dielectric otherwise you would not be able
to maintain concentricity with a consequent degradation in return loss
and an increased risk of damage which would further degrade
performance.
>The differences in losses or impedance
>will only show up at high GHz level frequencies and are all but
>negligible in many if not most commercial applications for such
>connections.
Perfectly true that the effect is generally negligible in many
commercial applications; but not in all.
> Note that the current industry uses newer, smaller connectors for
>today's modern Ghz applications as BNC is too bulky in today's
>miniaturized world.
Not only are the smaller connectors less bulky but they provide a better
performance above 1GHz.
>>If you have only got a bag of unidentified plugs you will have to
>>measure the inner and outer diameters and, along with a guess at the
>>dielectric calculate the impedance.
>
> All you have to do is determine the cable type that it was meant to
>be terminated with. That cable's impedance determines the impedance
>of the jumper, and the correct connector to use for it. The connector
>itself MUST be identical on the PLUG/JACK side so that anyone can plug
>into any jack when both are declared as BNC. Sheesh, this is so
>simple, I can't believe you don't get it.
There are too many types of cable to make this practical, it would even
be possible to have cables with identical inner and outer diameters but
different impedances by choice of dielectric. I have not looked for
cables of this type, but then again that is how 50 and 75 ohm BNC
connectors are implemented. Is it sooo difficult to understand, if the
inner and outer must be the same diameter to allow non-destructive
mating then the only way of making the impedance different is to change
the dielectric between them. I can not believe you do not understand
this!
> IT IS the CABLE that determines the jumper's impedance, and the
>connector is of very little sway in that regard. That FACT that one
>cannot use a RG-6 BNC connector on an RG-174 coax proves that. You
>can make either impedance jumper, and BOTH will still plug into any
>BNC connector. If someone wants to claim "Oh no. It won't plug into
>an "N" type BNC connector." then the idiot is too blind to grasp the
>fact that he is referring to a different connector entirely.
>
> BNC is BNC is BNC. Impedance is cable specific. Pretty simple.
>
> Another proof is in simple numbers. A 100 foot length of cable is a
>capacitor, for sure. A half inch connector is a negligible amount of
>that run length. If you remember ANY of your training, you will know
>that such tiny additions are typically rounded off.
We are not talking about capacitors but transmission lines. If I connect
two 50 ohm BNC cables via a 75 ohm coupler it will exhibit a worse
return loss than if I use a 50 ohm coupler. If I really feel bored and
have nothing useful to do tomorrow then I might perform this experiment
with an SNA, I won't confuse you with a VNA and a sliding termination.
--
Ian G8ILZ
Gareth Rowlands
"dmills" wrote:
> There was a version of the "BNC" made by one manufacturer which had
> different size centre pins ... (ref. a long article about this in the
> back of the Canford catalog).
> Note that (before digital video) it was common practice to use 50ohm
> connectors in 75ohm systems because they were felt to be better
> mechanically .... this is no longer viable with the advent of 270mb/s
> digital video....
The above fits in with my experiences, although I never saw the article
in the Canford catalogue :-(
Richard Kaulfuss
<DarkM...@thebarattheendoftheuniverse.org> wrote:
>
> Impedance is a reactive term. That means capacitance and
>inductance. Guess which coaxial cable exhibit that determines their
>impedance the most?
>
> Of course we are talking about capacitance. Have you ever seen an
>HV transmission coax? It looks JUST LIKE a large diameter coax for an
>old CB radio set.
>
> When one applies the juice through it, however, the thing jumps.
>That is a physical contraction due to capacitive charging.
>
> We have always been talking about a capacitor. If one looks at a
>circuit where there is a jumper between two circuit element, and that
>jumper is coaxial, it appears as a capacitor to the circuit.
>Every time. Unless used to pass DC, of course. Even in that case, it
>does charge up. I can make various HV capacitive loads for testing HV
>supplies by simply changing the cable length.
>
> It IS basic electronics. Capacitive reactance is called impedance.
>
I'm beginning to suspect you don't actually understand the concepts
of transmission lines and characteristic impedance.
>>If I connect
>>two 50 ohm BNC cables via a 75 ohm coupler it will exhibit a worse
>>return loss than if I use a 50 ohm coupler. If I really feel bored and
>>have nothing useful to do tomorrow then I might perform this experiment
>>with an SNA, I won't confuse you with a VNA and a sliding termination.
>
> Hahahah... whatever.
Hmm...thought as much.
--
Dick
Boris Mohar
<DarkM...@thebarattheendoftheuniverse.org> wrote:
>On Sun, 22 Dec 2002 23:13:11 +0000, George Shaw <georg...@ukf.net>
>Gave us:
> A time domain reflectometer. Fluke and many others makes cable
>testers for LANs and other communication systems, but I do not know if
>this class is covered, and the tester is very pricey.
>
> You could lease one from an equipment vendor for a month, but it the
>application that critical? You could have had a fedex shipment by now
>of the type you need, and would still have your hair, AND would be
>able to compare upon receipt thereof.
>
> Normal test gear, such as meters and scopes will yield no
>discernable differences. If you are not doing Ghz applications, you
>only need to worry about matching up the physical side. The cable.
Little do you know. You are pulling this out of your but without any
direct knowledge or experience. My first intro to the TDR was about
thirty five years ago when the instructor hooked up one up to a peace of
cabling that consisted of various lengths that were interconnected with
various connectors. He proceeded to demonstrate the importance of good
impedance match between the cable and the connector as each of the splices
showed up on the TDR. He than pinched a cable slightly and that slight
deformation showed up also. Every connector represents a impedance
discontinuity. The whole point is to minimize that. Often you work with
signals of low enough frequency and cables that are short enough so that
these things are not obvious but believe me they are there and they
sometimes matter. I work on ground penetrating radar where we send a 1kV
2ns rise time pulse to antenna. Any little discontinuity on the line
shows up as false echo and really messes up the return signal.
You can make a reasonable TDS out of a fast pulse generator and a decent
scope. Go convince yourself.
Boris Mohar
Ian Walker
<DarkM...@thebarattheendoftheuniverse.org> writes
>On Sun, 22 Dec 2002 21:33:01 +0000, Ian Walker <Jun...@127.0.0.1> Gave
>us:
>
>connector, I fail to see any relevance. At all.
My comment was intended to show that if users can not even see that a
PL259 and a 75 ohm N socket are not intended to mate then there is no
hope they will avoid mixing 50 and 75 N. Hence the importance of making
50 and 75 ohm BNC mechanically interchangeable and determining the
impedance by selection of dielectric.
There is no such thing as a 'BNC "N"' connector, what makes you think
there is, certainly not anything I typed above, perhaps this imaginary
connector of yours (or your inability to read what is typed) explains
your poor understanding of achieving different impedance BNC connectors.
--
Ian G8ILZ
Ian Walker
<georg...@ukf.net> writes
>Ok thanks to ALL that have replied so far, interesting information.
>
>I have looked at known 50 and 75 BNCs I have here from a reliable
>source and can see NO difference in the sizes.
At last a good practical and reproducible experiment. Would you also
report on the size and nature of any insulation between the inner and
outer conductors.
>If a measuring equipment was available, what would be used to test the
>BNC ????
Any of the following: scalar network analyser (SNA), vector network
analyser (VNA), time domain reflectometer (TDR), micrometer and a
dielectric measurement bridge. I have used the first three frequently
and once had to lash-up a TDR using a fast rise time scope by connecting
the A gate out via a attenuator to a T piece on the Y1 input and then
connecting the suspect system to it. I found a 1m length of 75 ohm
cable and a badly fitted BNC plug in the system. I could also see
discontinuities at some of the T pieces on the LAN.
--
Ian G8ILZ
Ian Walker
<DarkM...@thebarattheendoftheuniverse.org> writes
>On Mon, 23 Dec 2002 01:05:15 +0000, Richard Kaulfuss
><dka...@boehme.demon.co.uk> Gave us:
>
>
>> I'm beginning to suspect you don't actually understand the concepts
>> of transmission lines and characteristic impedance.
>
>don't have to.
>
>>
>>>>If I connect
>>>>two 50 ohm BNC cables via a 75 ohm coupler it will exhibit a worse
>>>>return loss than if I use a 50 ohm coupler. If I really feel bored and
>>>>have nothing useful to do tomorrow then I might perform this experiment
>>>>with an SNA, I won't confuse you with a VNA and a sliding termination.
>>>
>>> Hahahah... whatever.
>>
>> Hmm...thought as much.
>
>connector physical nomenclature, NOT SNA, or VNA, or any other
>bullshit.
Do you know what VNA and SNA are? They are instruments (scalar network
analyser and vector network analyser) with which you measure some of the
physical properties of electronic circuits, i.e. the effects of LCR in
the transmission line presented by a BNC connector. Presumably you don't
know as you are so dismissive of their use in showing the very real
impedance difference between 50 and 75 ohm BNC connectors!
--
Ian G8ILZ
John Woodgate
(in <uvm4ua...@spamblock.demon.co.uk>) about 'How to check for a 50
or 75 ohm connector', on Sun, 22 Dec 2002:
>There was a version of the "BNC" made by one manufacturer which had different
>size centre pins and obviously failed to meet the real BNC spec, but it has
>not been on the market for quite a few years this I suspect is where the
>rumor started (ref. a long article about this in the back of the Canford
>catalog).
That may well be the explanation for the disagreement. The 75 ohm
connectors that I have are over 30 years old, and they don't make
reliable contact in a 50 ohm socket.
I'm not at all sure that there WAS a 'real BNC spec' for 75 ohm parts
that long ago. AIUI, the connector was conceived and designed as a 50
ohm part and no 75 ohm version was originally contemplated.
I wonder if any German manufacturer made a 60 ohm part.
John Woodgate
wrote (in <Xns92ECA393B88D...@130.133.1.4>) about 'How to
check for a 50 or 75 ohm connector', on Sun, 22 Dec 2002:
>The centre pins on 75 Ohm BNC plugs are very slightly larger, so that
>inserting them in a 50 Ohm BNC socket causes damage by enlarging the hole.
>I used to work in a lab where we had both.
That cannot possibly be so. If anything, the 75 ohm pin MUST be smaller.
Alun Palmer
news:40gc0vo3h3737pnl7...@4ax.com:
> On 22 Dec 2002 21:05:18 GMT, Alun Palmer <elek...@yahoo.com> Gave us:
>
>
>>
>>The centre pins on 75 Ohm BNC plugs are very slightly larger, so that
>>inserting them in a 50 Ohm BNC socket causes damage by enlarging the
>>hole. I used to work in a lab where we had both.
>>
>
> Absolutely incorrect.
I speak from actual observation, not from reading a spec sheet. If this
means that the 75 Ohm types didn't meet the spec, then that is a slightly
different issue.
>
>>I'm not sure how F connectors got into this thread, but they are nasty
>>rubbish, worse even than Belling-Lee connectors. Only use F connectors
>>if you absolutely have to. They make UHF connectors look like an
>>engineering masterpiece by comparison, which they're not.
>
> Whatever. Your opinions are probably as good as your
> mis"information" is. Which is nil.
F connectors are cheap mass-market junk.
> The center pins are the SAME
> diameter. The connector you refer to as causing damage is NOT of the
> BNC type.
Technically it may not be, but it looks enough like a BNC to be a problem
> In order to be called BNC legally
Lawyers don't attach much meaning to the word 'legally' - what do you mean
by it here?
> , it has to be able to
> plug into a BNC connector jack. Period.
They did plug in and work, but...
> And that would be ANY BNC
> jack, and that would be without subsequent damage.
You can't change the impedance and keep the same dimensions. If there is a
standard, I'm assuming you're saying it's 50 Ohm only, and therefore the
75 Ohm version isn't a BNC. The trouble is it looks like one, and it
damages the 50 Ohm socket. What is the standard number, then?
>
>>
>>F connectors leak RF in and out like a sieve. One of my old colleagues
>>did extensive tests of 'transfer impedance' (this has nothing to do
>>with characteristic impedance, but is a measure of screening) and could
>>find nothing under the sun that had worse screening than an F
>>connector.
>
> The shield level of the cable used is a determining factor in that
> test as well.
Not if the test is done properly. It is possible to get independent
numbers for connectors and cables. Since the F doesn't have a centre pin,
though, I'll admit that does present a unique problem.
>
> Also, the assembler has to know what the hell is going on in order
> to fashion the things right as well.
>
He had a PhD, so he wasn't exactly a dummy. He designed his own jigs for
measuring transfer impedance, which gave repeatable figures up to 2 GHz,
as far as I recall. His last name was Smithers, and I am pretty sure his
work was published. For the record, I am an electrical engineer (only a
BSc, not PhD).
Alun Palmer
Brian Reay
news:Xns92ED1C8B253E...@130.133.1.4...
> DarkMatter <DarkM...@thebarattheendoftheuniverse.org> wrote in
> news:40gc0vo3h3737pnl7...@4ax.com:
>
>
> standard,
Yes you can.
With an air dielectric the formula for the impedance for a coaxial pair of
conductors is:
138 log (D/d) where D is inside dia of outer and d is outside dia of
inner.
If there is a different dielectric the above formula is modified by a fact
of
1 / (k ^1/2) where k is the dielectric constant of the insulator.
73
Brian
Ed Price
"DarkMatter" <DarkM...@thebarattheendoftheuniverse.org> wrote in message
news:haac0vkaj7s3cr8uv...@4ax.com...
> On Sun, 22 Dec 2002 19:53:14 +0000, John Woodgate
> <j...@jmwa.demon.contraspam.yuk> Gave us:
>
> >I read in sci.electronics.design that DarkMatter <DarkMatter@thebaratthe
> >endoftheuniverse.org> wrote (in <u1vb0vg3ptp1smc9320vg7viond96s7e0o@4ax.
> >com>) about 'How to check for a 50 or 75 ohm connector', on Sun, 22 Dec
> >2002:
> >
> >> One can purchase the simple "consumer electronics" "F" style
> >>connector in a number of cable termination sizes. Depending on WHICH
> >>cable gets used, the impedance will differ. The PLUG side, however is
> >>ALWAYS absolutely identical.
> >
> >Since the 'centre pin' of an F-connector plug is the inner conductor of
> >the cable, I don't see how it can always be the same size!
>
>
> pin of the cable itself yes. I knew that, but missed it in my
> statements. You are correct. It is, however, proven more so by that
> remark that the impedance is determined by the cabling, not the
> connector.
>
> This would also be the reason why "F" type connectors have such wide
> spread ability on the female center conductor grasping tines.
>
> RG-6 center wires are a lot bigger than an RG-174. An RG-174 "F"
> type connector, however is one of the few that do NOT use the center
> wire as the "stinger", they have solder cups and hard mounted center
> pins. A much more expensive termination.
>
> You caught me with my QA reviews of my posts down. :-]
The F connector is a system; that is, a connection requires both a male and
female connector. Most of the impedance discontinuities occur in the female
half of the system, due to those "grasping tines".
Ed
Mike W
>
<big snip>
>Most of the impedance discontinuities occur in the female
>half of the system, due to those "grasping tines".
>
This brings something else to mind ;-)
Seasons wotzits, Mike W
Alun Palmer
news:Imz3PHBd...@jmwa.demon.co.uk:
> I read in sci.electronics.design that Alun Palmer <elek...@yahoo.com>
> wrote (in <Xns92ECA393B88D...@130.133.1.4>) about 'How to
> check for a 50 or 75 ohm connector', on Sun, 22 Dec 2002:
>
>>The centre pins on 75 Ohm BNC plugs are very slightly larger, so that
>>inserting them in a 50 Ohm BNC socket causes damage by enlarging the
>>hole. I used to work in a lab where we had both.
>
> That cannot possibly be so. If anything, the 75 ohm pin MUST be
> smaller.
Come to think of it, you may be right. The point is, they were not the
same size, notwithstanding other posts.
Alun Palmer
news:au6hhf$4p8j6$1...@ID-140397.news.dfncis.de:
Well, yes, but they did have the same dielectric
Brian Reay
> >
>
> Well, yes, but they did have the same dielectric
You mean it looked the same. Also, was it the same thickness ie was the
overall dielectric constant (the air plus the insulator) the same. Not all
PTFEs are equal ;-)
73
Brian
Ian Walker
<DarkM...@thebarattheendoftheuniverse.org> writes
>On Sun, 22 Dec 2002 23:48:54 +0000, Ian Walker <Jun...@127.0.0.1> Gave
From concentric // adj. (often foll. by with)
(esp. of circles) having a common centre
>Of what? The grounding shroud?
Yes, the centre inner must be on the line formed by the centre of the
outer in order to produce a non-reactive transmission line.
> Are you joking?
> AGAIN, we are NOT talking about "N" type BNCs, we are talking about
>BNCs only.
Again you introduce some mythical connector called '"N" type BNC' maybe
you are getting confused with "C" type which is ay least a bayonet like
the BNC but it is not called '"N" type BNC'. Am regret to say this only
typifies your ignorance of co-ax connectors
>>>The differences in losses or impedance
>>>will only show up at high GHz level frequencies and are all but
>>>negligible in many if not most commercial applications for such
>>>connections.
>>
>>Perfectly true that the effect is generally negligible in many
>>commercial applications; but not in all.
>>
>>> Note that the current industry uses newer, smaller connectors for
>>>today's modern Ghz applications as BNC is too bulky in today's
>>>miniaturized world.
>>
>>Not only are the smaller connectors less bulky but they provide a better
>>performance above 1GHz.
>
> Which makes your previous statement all but moot.
There are several factors determining a designers choice of cable:
voltage stress, I squared R heating, loss due to voltage drop, cut-off
frequency, flexibility, and last size constraints. The physics of the
materials determine the cable which must be used, the equipment must
accommodate the size of the cable.
>>>>If you have only got a bag of unidentified plugs you will have to
>>>>measure the inner and outer diameters and, along with a guess at the
>>>>dielectric calculate the impedance.
>>>
>>> All you have to do is determine the cable type that it was meant to
>>>be terminated with. That cable's impedance determines the impedance
>>>of the jumper, and the correct connector to use for it. The connector
>>>itself MUST be identical on the PLUG/JACK side so that anyone can plug
>>>into any jack when both are declared as BNC. Sheesh, this is so
>>>simple, I can't believe you don't get it.
>>
>>There are too many types of cable to make this practical,
>
> I'm sorry, but AMPHENOL lists over a hundred coax types that mate to
>BNC front ends.
Which only serves to confirm my point that there are too many to make
measuring the rear of the connector and then comparing against all the
possible cables practical. It is much easier to measure the front where
all you need do is measure the thickness of any solid dielectric between
the inner and outer conductors.
>> it would even
>>be possible to have cables with identical inner and outer diameters but
>>different impedances by choice of dielectric. I have not looked for
>>cables of this type, but then again that is how 50 and 75 ohm BNC
>>connectors are implemented. Is it sooo difficult to understand, if the
>>inner and outer must be the same diameter to allow non-destructive
>>mating then the only way of making the impedance different is to change
>>the dielectric between them. I can not believe you do not understand
>>this!
>
> What? That is what I said. The pin, and the shroud are the same
>for all. The media between is all that changes. That IS exactly what
>I said.
It is you who have insisted that the impedance is determined by the
cable side and not the mating side of the connector
>>> IT IS the CABLE that determines the jumper's impedance, and the
>>>connector is of very little sway in that regard. That FACT that one
>>>cannot use a RG-6 BNC connector on an RG-174 coax proves that. You
>>>can make either impedance jumper, and BOTH will still plug into any
>>>BNC connector. If someone wants to claim "Oh no. It won't plug into
>>>an "N" type BNC connector." then the idiot is too blind to grasp the
>>>fact that he is referring to a different connector entirely.
>>>
>>> BNC is BNC is BNC. Impedance is cable specific. Pretty simple.
>>>
>>> Another proof is in simple numbers. A 100 foot length of cable is a
>>>capacitor, for sure. A half inch connector is a negligible amount of
>>>that run length. If you remember ANY of your training, you will know
>>>that such tiny additions are typically rounded off.
>>
>>We are not talking about capacitors but transmission lines.
>
> Impedance is a reactive term. That means capacitance and
>inductance. Guess which coaxial cable exhibit that determines their
>impedance the most?
A transmission line is best represented as an infinite lumped LCR
network and can be drawn as follows
--L-R----L-R----L-R----L-R-- >
¦ ¦ ¦
C C C
¦ ¦ ¦
---------------------------- >
Where there are an infinite number of LCR elements in a finite length of
cable (you will need your calculus for this), XL = -XC (thus its
impedance is not frequency dependent) and R increases with frequency
(this is inevitable).
> Of course we are talking about capacitance. Have you ever seen an
>HV transmission coax? It looks JUST LIKE a large diameter coax for an
>old CB radio set.
It only looks capacitive at DC, not at AC where it is a transmission
line and we are talking about transmission lines. Even at DC the applied
step voltage requires that it be analysed as a transmission line.
> When one applies the juice through it, however, the thing jumps.
>That is a physical contraction due to capacitive charging.
The charge is not instant along the length of the cable but propagates
along it at the characteristic velocity (VP), until the charge reaches
the far end, and any reflections have died out it must be analysed as a
transmission line.
> We have always been talking about a capacitor.
We are talking about a transmission line
>If one looks at a
>circuit where there is a jumper between two circuit element, and that
>jumper is coaxial, it appears as a capacitor to the circuit.
It appears as a transmission line.
>Every time. Unless used to pass DC, of course. Even in that case, it
>does charge up. I can make various HV capacitive loads for testing HV
>supplies by simply changing the cable length.
Of course you can, but the normal use of a co-axial line, and the
function for which it was designed is as a transmission line
> It IS basic electronics. Capacitive reactance is called impedance.
As can inductive reactance, as also can any combination of inductance
and capacitance. They should always be written as either "R+jX" or "R
theta degrees" (of course you know that the two R in these expressions
re not identical but are related by converting between Cartesian and
polar co-ordinates). For the frequency at which XL = -XC the impedance
will be indistinguishable from a resistor and can be safely written as R
(you might have to specify the frequency). In the special case of a
transmission line XL = -XC for all frequencies of use.
It IS basic transmission line theory, which is perhaps a bit more
advanced than basic electronics; but then most electronics has to be
interconnected and whilst you might ignore transmission line theory at
Dc you can not do so at AC.
>>If I connect
>>two 50 ohm BNC cables via a 75 ohm coupler it will exhibit a worse
>>return loss than if I use a 50 ohm coupler. If I really feel bored and
>>have nothing useful to do tomorrow then I might perform this experiment
>>with an SNA, I won't confuse you with a VNA and a sliding termination.
>
> Hahahah... whatever.
I conducted the experiment using the following HP (Agilent) apparatus:
85032B, 8757A, 8350B, 83595A and 85027A. I also used the following
adapters: two off Suhner 33 BNC-N-50-1, a 50ohm BNC female coupler, and
one 75 ohm female BNC coupler as the item to be investigated.
I connected one of the 33 BNC-N-50-1 to the 85027A via an APC7 - N(m)
adapter from the 85032B and fitted a 909F from the 85032A to the second
33 BNC-N-50-1 and coupled it to the first 33 BNC-N-50-1 using the 50 ohm
BNC coupler. I then normalised this and replaced the 50 ohm coupler with
the 75 ohm and plotted the results. This produced a rising response from
~odB at 10MHz to +18dB at 1GHz. Since there was no cable in the
transmission line under test, only the coupler, this indicates that the
properties of the connectors is determined by more than the cable they
designed for.
Had I used an SNA I would have obtained readings in the form R+jX or R
theta degrees.
Using a step driven TDR produces the following:
B C
----------
¦ ¦
¦ ¦
----------- ----------- >
A B C D
Where:
A-B is the 50 ohm line from the step generator and any adapters
required to allow connecting the test line,
B-C is the 75 ohm test line
C-D is the 50 ohm termination after the test line, and in fact
continues to infinity.
If normalised to 50 ohm then the line B-C represents an SWR of 1.5:1.
--
Ian G8ILZ
Dave Holford
> F connectors leak RF in and out like a sieve. One of my old colleagues did
> extensive tests of 'transfer impedance' (this has nothing to do with
> characteristic impedance, but is a measure of screening) and could find
> nothing under the sun that had worse screening than an F connector.
While I am totally unimpressed by "F" connectors and would never use
them if almost anything else could be used, I have some difficulty with
the leakage stated above.
If that is true I find it unlikely that their widespread use would be
permitted in the vicinity of airports since cablevision systems commonly
utilize frequencies used for aviation communications and navigation
systems; and the prospect of all those connectors leaking RF at
frequencies used for Instrument Landing Systems would pose a very real
hazard to aviation safety. I can think of several locations where large
housing complexes and apartments lie within 1 mile of an ILS system and
no problems have ever been experienced with either localizer or glidpath
interference.
Also, I have at least 16 "F" connectors in my home and they are carrying
RF in the 144-148MHz band which is almost undetectable with several
sensitive receivers, unless the antennas are placed in close proximity
to one of the plastic cased devices attached to the system.
As to the continuing saga of the BNC. If the 50 and 75 Ohm connectors
were indeed incompatible I wonder why I have never encountered test
equipment with 50 or 75 Ohm restrictions? The organization for which I
worked for almost 30 years used vast quantities of test equipment in the
servicing and maintenance of 50 Ohm communications and 75 Ohm video
systems and I never recall ever seeing any piece of test equipment
restricted to communications or video systems - even in our research
labs. Just did a quick search of a couple of oscilloscope manuals and
can find NO warning about using the 'wrong' type of BNC plugs.
Dave
Ian Walker
<j...@jmwa.demon.contraspam.yuk> writes
>I read in sci.electronics.design that Alun Palmer <elek...@yahoo.com>
>wrote (in <Xns92ECA393B88D...@130.133.1.4>) about 'How to
>check for a 50 or 75 ohm connector', on Sun, 22 Dec 2002:
>
>>The centre pins on 75 Ohm BNC plugs are very slightly larger, so that
>>inserting them in a 50 Ohm BNC socket causes damage by enlarging the hole.
>>I used to work in a lab where we had both.
>
>That cannot possibly be so. If anything, the 75 ohm pin MUST be smaller.
It depends on the thickness of PTFE between inner and out, increasing it
would necessitate reduction the ratio of inner to outer. That said I
have not seen a non-standard BNC. There are "BNC" like connectors where
the inner contacts are transposed to prevent mating with BNC, I would be
inclined to user TNC rather than a connector with only one supplier.
--
Ian G8ILZ
John Woodgate
wrote (in <$SRMgh+9...@newbrain.demon.co.uk>) about 'How to check
for a 50 or 75 ohm connector', on Mon, 23 Dec 2002:
>In the special case of a
>transmission line XL = -XC for all frequencies of use.
The mind boggles.
Richard Kaulfuss
<hol...@sympatico.ca> wrote:
>As to the continuing saga of the BNC. If the 50 and 75 Ohm connectors
>were indeed incompatible I wonder why I have never encountered test
>equipment with 50 or 75 Ohm restrictions? The organization for which I
>worked for almost 30 years used vast quantities of test equipment in the
>servicing and maintenance of 50 Ohm communications and 75 Ohm video
>systems and I never recall ever seeing any piece of test equipment
>restricted to communications or video systems - even in our research
>labs. Just did a quick search of a couple of oscilloscope manuals and
>can find NO warning about using the 'wrong' type of BNC plugs.
>
I know of at least one major test equipment manufacturer who
routinely uses 50 ohm connectors on their 75 ohm inputs/outputs.
The dielectric gives the centre pin added mechanical protection
and the mismatch is insignificant below 100MHz.
--
Dick
Alun Palmer
news:lv3f0vgbeicbgm0cm...@4ax.com:
If the mismatch in size were the 'right' way around no damage would
result, and the difference was very small. From reading the various posts
in this thread it seems that some more recent 75 Ohm BNCs have the same
dimensions as the 50 Ohm variety anyway, and that the problem was with
older ones.
Alun Palmer
Well, Brian. I can't actually prove that the dielectric was the same, of
course, but I did observe that the centre pins were different sizes. This
may have been due to the lack of a 75 Ohm standard, and may have been
overcome by later manufacturers playing with the dielectric to compensate.
Graham Sharp
<hol...@sympatico.ca> wrote:
>
>> F connectors leak RF in and out like a sieve. One of my old colleagues did
>> extensive tests of 'transfer impedance' (this has nothing to do with
>> characteristic impedance, but is a measure of screening) and could find
>> nothing under the sun that had worse screening than an F connector.
>
>
>
>While I am totally unimpressed by "F" connectors and would never use
>them if almost anything else could be used, I have some difficulty with
>the leakage stated above.
Agreed, but I found that to get leakage down to decent levels, the
connectors have to be 'nipped up' by about 1/4 turn with a 7/16"
spanner. Finger-tight is not enough. (This was with crimped
connecters, not the nasty screw-on type.)
>
>If that is true I find it unlikely that their widespread use would be
>permitted in the vicinity of airports since cablevision systems commonly
>utilize frequencies used for aviation communications and navigation
>systems; and the prospect of all those connectors leaking RF at
>frequencies used for Instrument Landing Systems would pose a very real
>hazard to aviation safety. I can think of several locations where large
>housing complexes and apartments lie within 1 mile of an ILS system and
>no problems have ever been experienced with either localizer or glidpath
>interference.
I have used "F" connectors <inside> several airports with no adverse
effects!
Graham
Graham Sharp
<j...@jmwa.demon.contraspam.yuk> wrote:
>I read in sci.electronics.design that Alun Palmer <elek...@yahoo.com>
>wrote (in <Xns92ECA393B88D...@130.133.1.4>) about 'How to
>check for a 50 or 75 ohm connector', on Sun, 22 Dec 2002:
>
>>The centre pins on 75 Ohm BNC plugs are very slightly larger, so that
>>inserting them in a 50 Ohm BNC socket causes damage by enlarging the hole.
>>I used to work in a lab where we had both.
>
>That cannot possibly be so. If anything, the 75 ohm pin MUST be smaller.
I don't know if these were 'certified' BNC, but I can recall that on
equipment I worked with about 30 years ago, the pins of 75 Ohm plugs
were smaller than 50 Ohm, probably to allow for the fact that the
split sockets that they mated with had smaller OD. Repeated insertion
of 50 Ohm plugs into 75 Ohm sockets on a certain type of equipment
eventually resulted in one of the four segments of the socket breaking
off. . .
Graham
Ian Walker
<j...@jmwa.demon.contraspam.yuk> writes
>I read in sci.electronics.design that Ian Walker <Jun...@127.0.0.1>
>wrote (in <$SRMgh+9...@newbrain.demon.co.uk>) about 'How to check
>for a 50 or 75 ohm connector', on Mon, 23 Dec 2002:
>
>>In the special case of a
>>transmission line XL = -XC for all frequencies of use.
>
>The mind boggles.
Why? Have you done your calculus for this?
--
Ian G8ILZ
John Woodgate
for a 50 or 75 ohm connector', on Tue, 24 Dec 2002:
Yes, but I'm not sure that differential calculus is necessary to explain
the boggle. A finite element method seems adequate.
A uniform transmission line has a fixed inductance L per unit length
(which can be as short a length as you like) and a fixed capacitance C
per the same unit length. XL = wL, XC = 1/(wC) where w is the angular
frequency. So how can XL = -XC for all w?
John Woodgate
wrote (in <1l5h0v8lg3h8k7h7j...@4ax.com>) about 'How to
check for a 50 or 75 ohm connector', on Tue, 24 Dec 2002:
parts had a smaller pin, but now they don't.
Ian Walker
>I read in sci.electronics.design that Ian Walker <Jun...@127.0.0.1>
>wrote (in <OKPAG6HL...@newbrain.demon.co.uk>) about 'How to check
>for a 50 or 75 ohm connector', on Tue, 24 Dec 2002:
>>In article <I6z3I$AdB4B...@jmwa.demon.co.uk>, John Woodgate
>><j...@jmwa.demon.contraspam.yuk> writes
>>>I read in sci.electronics.design that Ian Walker <Jun...@127.0.0.1>
>>>wrote (in <$SRMgh+9...@newbrain.demon.co.uk>) about 'How to check
>>>for a 50 or 75 ohm connector', on Mon, 23 Dec 2002:
>>>
>>>>In the special case of a
>>>>transmission line XL = -XC for all frequencies of use.
>>>
>>>The mind boggles.
>>
>>
>>Why? Have you done your calculus for this?
>
>Yes, but I'm not sure that differential calculus is necessary to explain
>the boggle. A finite element method seems adequate.
>
>A uniform transmission line has a fixed inductance L per unit length
>(which can be as short a length as you like) and a fixed capacitance C
>per the same unit length. XL = wL, XC = 1/(wC) where w is the angular
>frequency. So how can XL = -XC for all w?
Try a little thought experiment (or a real one if you have a VNA)
connect a length of 50 ohm co-axial cable to a co-axial 50 ohm
termination and now measure the impedance presented to the open end of
the co-axial line; for all frequencies below cut-off you will obtain a
value of 50 +j0 ohm (or 50 +o degrees), if you perform the experiment
then the readings obtained will depend on the uncertainties in the
system. It can only present a non-reactive value if for all values of w
if XL = -XC. If it did not then it could only be used a spot frequency
with a specific length, if at all. Having demonstrated that finite
analysis fails I regret to advise you the you will have to use the
mathematics of the infinitely small.
--
Ian G8ILZ
John Woodgate
for a 50 or 75 ohm connector', on Tue, 24 Dec 2002:
>It can only present a non-reactive value if for all values of w
>if XL = -XC.
Well, you carry on thinking that. I hope it never gets you into trouble.
If you can't immediately see that it's impossible, I see no way to
convince you.
>If it did not then it could only be used a spot frequency
>with a specific length, if at all.
?????
>Having demonstrated that finite
>analysis fails I regret to advise you the you will have to use the
>mathematics of the infinitely small.
Note that I said that my finite element can be as small as you like.
Your contention fails however you do the calculations, if you do them
correctly.
Characteristic impedance Zo = sqrt{(R + jwL)/(G + jwC)}
For Zo to be independent of w, all that is required is that R and G are
negligible compared with wL and wC respectively. Then:
Zo = sqrt(jwL/jwC) = sqrt(L/C), and is also equal to sqrt(XL x XC).
Nothing requires XL = XC.
Tom MacIntyre
<j...@jmwa.demon.contraspam.yuk> wrote:
>I read in sci.electronics.design that Ian Walker <Jun...@127.0.0.1>
>wrote (in <oVj5BtM6$NC+...@newbrain.demon.co.uk>) about 'How to check
>for a 50 or 75 ohm connector', on Tue, 24 Dec 2002:
>
>>It can only present a non-reactive value if for all values of w
>>if XL = -XC.
>
>Well, you carry on thinking that. I hope it never gets you into trouble.
>If you can't immediately see that it's impossible, I see no way to
>convince you.
>
>>If it did not then it could only be used a spot frequency
>>with a specific length, if at all.
>
>?????
Q of infinity? I wonder how the Smith Chart handles this?
Tom
Ian Walker
<j...@jmwa.demon.contraspam.yuk> writes
>I read in sci.electronics.design that Ian Walker <Jun...@127.0.0.1>
>wrote (in <oVj5BtM6$NC+...@newbrain.demon.co.uk>) about 'How to check
>for a 50 or 75 ohm connector', on Tue, 24 Dec 2002:
>
>>It can only present a non-reactive value if for all values of w
>>if XL = -XC.
>
>Well, you carry on thinking that. I hope it never gets you into trouble.
>If you can't immediately see that it's impossible, I see no way to
>convince you.
>
>>If it did not then it could only be used a spot frequency
>>with a specific length, if at all.
>
>?????
>
>>Having demonstrated that finite
>>analysis fails I regret to advise you the you will have to use the
>>mathematics of the infinitely small.
>
>Note that I said that my finite element can be as small as you like.
>Your contention fails however you do the calculations, if you do them
>correctly.
>
>Characteristic impedance Zo = sqrt{(R + jwL)/(G + jwC)}
>
>For Zo to be independent of w, all that is required is that R and G are
>negligible compared with wL and wC respectively. Then:
>
>Zo = sqrt(jwL/jwC) = sqrt(L/C), and is also equal to sqrt(XL x XC).
>
>Nothing requires XL = XC.
I have to confess that I had remembered that there was a constant
equality from my transmission line principles of many years ago but not
the details.
--
Ian G8ILZ
Ian Walker
<j...@jmwa.demon.contraspam.yuk> writes
>I read in sci.electronics.design that Ian Walker <Jun...@127.0.0.1>
>wrote (in <oVj5BtM6$NC+...@newbrain.demon.co.uk>) about 'How to check
>for a 50 or 75 ohm connector', on Tue, 24 Dec 2002:
>
>>It can only present a non-reactive value if for all values of w
>>if XL = -XC.
>
>Well, you carry on thinking that. I hope it never gets you into trouble.
>If you can't immediately see that it's impossible, I see no way to
>convince you.
>
>>If it did not then it could only be used a spot frequency
>>with a specific length, if at all.
>
>?????
>
>>Having demonstrated that finite
>>analysis fails I regret to advise you the you will have to use the
>>mathematics of the infinitely small.
>
>Note that I said that my finite element can be as small as you like.
>Your contention fails however you do the calculations, if you do them
>correctly.
>
>Characteristic impedance Zo = sqrt{(R + jwL)/(G + jwC)}
>
>For Zo to be independent of w, all that is required is that R and G are
>negligible compared with wL and wC respectively. Then:
>
>Zo = sqrt(jwL/jwC) = sqrt(L/C), and is also equal to sqrt(XL x XC).
>
>Nothing requires XL = XC.
I have to confess that I had remembered that there was a constant
relationship and that it must be non-reactive from my transmission line
principles of many years ago.
--
Ian G8ILZ
John Woodgate
for a 50 or 75 ohm connector', on Wed, 25 Dec 2002:
>I have to confess that I had remembered that there was a constant
>relationship and that it must be non-reactive from my transmission line
>principles of many years ago.
OK. The person who never made a mistake never made anything.
Davey Boy
Difference between 50 Ohm and 75 Ohm BNC Connectors:
http://www.levitonvoicedata.com/support/technotes/tech_note_detail.asp?tnID=
175
50 Ohm BNC connector on a 75 Ohm cable (this could be applicable for
75 Ohm plug on 50 Ohm cable)
http://www.levitonvoicedata.com/support/technotes/tech_note_detail.asp?tnID=
176
And this one has pictures highlighting the differences between the two
different types:
http://www.townsendhillengineering.com/THEtechnicaltidbits.html
"George Shaw" <georg...@ukf.net> wrote in message
news:thm90vof9eov5lgk2...@4ax.com...
> I have a large box of mixed BNC connectors/tees etc.
>
> Using the simplest equipment possible (multimeter?) HOW do I check if
> the connector is for 50 Ohm or 75 Ohm coax connection. There are no
> external markings.
>
Found these links:
Difference between 50 Ohm and 75 Ohm BNC Connectors:
http://www.levitonvoicedata.com/support/technotes/tech_note_detail.asp?tnID=
175
50 Ohm BNC connector on a 75 Ohm cable (this could be applicable for
75 Ohm plug on 50 Ohm cable)
http://www.levitonvoicedata.com/support/technotes/tech_note_detail.asp?tnID=
176
And this one has pictures highlighting the differences between the two
different types (and the best explanation):
http://www.townsendhillengineering.com/THEtechnicaltidbits.html
HTH
Dave M3SKA
Andy Cowley
>
> At the risk of oversimplifying things....
> If you can't tell the difference it don't matter.
> Stated another way, if a multimeter is the best
> you can do, it's
> unlikely that you'll be able to tell the
> difference in any application
> that you have the instrumentation to measure.
> It'll take some doin'
> to create a situation where the connector
> impedance matters.
> Any application where a centimeter of
> discontinuity from 50 to 75 ohms
> matters, probably shouldn't be using a BNC anyway.
Try putting a 75 ohm T-piece into a 10base2 network and
you will soon be disabused of this incorrect assumption.
vy 73
Andy, M3ABC