Coax/Twinax cable
Mike Saunders
networks) is compatible with domestic coax cable as used for TV's etc.
I have quite a bit of this wired into the structure of the house and it
would be very convenient if I could use this for wiring up the TV's
Many thanks
Mike
Tim S
Do you mean 10base2 (Thin black stuff as opposed to fat yellow stuff)?
Some of that was referred to as twinax, but there's another twinax used
for gig networking (1000baseCX).
TV aerials have a characteristic impedance of 75 ohms - and 10base2 was
IIRC 50 ohms. So it may not work that well unless you correct with a balun
at each end, as to whether that would work, I've not idea.
Are you in a position to test a bit out of curiousity?
Tim
Andy Hall
wrote:
If you really mean Twinax cable as was used for hooking up terminals
on IBM System 3x and AS400s then it is its own thing and not Ethernet.
There are two versions of Twinax - 78ohm and 95 ohm. 78ohm would
be reasonably close to the required 75 ohm for TV distribution, but 90
ohn would not. Added to this, the cable has a poor high frequency
response, so for UHF TV is likely to be poor.
There was also a twin coax used for 10base2 ethernet together with
special connectors from Amphenol. The concept was that you could
unplug the connector from the wall and the network bus would be
maintained. These were horrendously unreliable and if one failed, the
whole segment would go down. The cable, like all 10base2 was 50ohm.
The short answer is that you counld try this because the runs are
likely to be short, but don't be surprised if the results are poor,
especially on high numbered TV channels.
.andy
To email, substitute .nospam with .gl
T i m
wrote:
Nice answer ;-)
T i m
Mike Saunders
> On Mon, 30 Aug 2004 12:43:09 -0500, Mike Saunders wrote:
>
> > Does anyone know if Twinax cable (as used in older ethernet computer
> > networks) is compatible with domestic coax cable as used for TV's
> > etc. I have quite a bit of this wired into the structure of the
> > house and it would be very convenient if I could use this for
> > wiring up the TV's
> >
> > Many thanks
> >
> > Mike
>
> Do you mean 10base2 (Thin black stuff as opposed to fat yellow stuff)?
> Some of that was referred to as twinax, but there's another twinax
> used for gig networking (1000baseCX).
Yes the thin black stuff known as 10base2
>
> TV aerials have a characteristic impedance of 75 ohms - and 10base2
> was IIRC 50 ohms. So it may not work that well unless you correct
> with a balun at each end, as to whether that would work, I've not
> idea.
>
Could you please expand on what a balun is and where these are to be
found It might be worth experimenting if there is a reasonable chance
of success
> Are you in a position to test a bit out of curiousity?
Yes but I wanted to know first if it was worth the effort
Mike
Tim S
> Tim S wrote:
>
>> Do you mean 10base2 (Thin black stuff as opposed to fat yellow stuff)?
>> Some of that was referred to as twinax, but there's another twinax used
>> for gig networking (1000baseCX).
>
> Yes the thin black stuff known as 10base2
OK - then I'm familiar with this.
>>
>> TV aerials have a characteristic impedance of 75 ohms - and 10base2 was
>> IIRC 50 ohms. So it may not work that well unless you correct with a
>> balun at each end, as to whether that would work, I've not idea.
>>
>>
> Could you please expand on what a balun is and where these are to be found
> It might be worth experimenting if there is a reasonable chance of success
A balun is an impedance balancing transformer. I'm not an RF expert but I
did do physics. A TV arial is designed assuming a 75 ohm circuit. If you
wack a bit of cable on that is rated to a difference impedance you will
get reflections (exactly as light reflects off water - that's an impedance
mismatch) and the system won't resonate at the desired frequencies.
In short it's sub optimal and you get a crap signal down the other end.
You can get baluns from Maplin etc. but I'm not sure if 75-50 ohm is a
normal type or not.
>
>> Are you in a position to test a bit out of curiousity?
>
Well - if you had a strong signal to start with it might work. Or might
not. It's an inherently suboptimal system - but it might be good enough.
I've never tried it so I can't tell you whether it will just work or not -
sorry. Need an RF bod or someone who's actually tried it for that.
Sorry to sound unhelpful but I honestly don't know. If you can experiment,
then that's the answer. If you have to commit a lot of time, then first
see if you can find a pair of 50-75 ohm baluns becaus eyou'll probably be
able to save the day with them.
Cheers
Timbo
PS Sorry for any typos - I'm a bit p*ssed.
tony sayer
<t...@dionic.net> writes
At the end of the day I'd just buy a reel of RG6 or CT100 which is the
right stuff for domestic aerial installation. I'm an RF engineer and I
wouldn't use the system what you're discussing!.....
--
Tony Sayer
Dave Plowman (News)
Mike Saunders <mi...@folleytech.co.uk> wrote:
> Could you please expand on what a balun is and where these are to be
> found It might be worth experimenting if there is a reasonable chance
> of success
They're a balance to unbalance trasformer. Check the usual component
suppliers. Might even be possible to make them, given the frequencies
involved - but that's a guess.
But to be honest, I'd just use video coax for baseband - it's cheap enough.
Using transformers to drive composite down a balanced line is bound to
introduce losses, so you'd be into some form of amp to sort it out as well.
--
*Why do we say something is out of whack? What is a whack?
Dave Plowman da...@davenoise.co.uk London SW
To e-mail, change noise into sound.
The Natural Philosopher
Better than a piece of wet string, but not a patch on stuff desined for
teh job.
invalid
Philosopher <a@b.c> writes
The impedance is in the terminators which are plugged into either end of
the network, and not in the cable itself.
Apart from that, I couldn't say whether it would be suitable for TV.
Hth
Bill
Dave Liquorice
>> Could you please expand on what a balun is ...
>
> They're a balance to unbalance trasformer. Check the usual component
> suppliers. Might even be possible to make them, given the
> frequencies involved - but that's a guess.
"balun" is also (mis)used to refer to an impedance transformer. In
this context it's possible to make something using cut lengths of the
50 and 75 ohm coax. I expect digging about on the web would find many
references.
> But to be honest, I'd just use video coax for baseband - it's cheap
> enough.
I think you have your threads crossed Dave. This one is about using
old coax based network cable to carry RF, not the one about wiring a
place with new Cat5 and using that for "TV" (not sure if that is
baseband or RF...)
> Using transformers to drive composite down a balanced line is bound
> to introduce losses, so you'd be into some form of amp to sort it
> out as well.
Wander over to the Canford site, 675m is the distance quoted for those
baluns before "significant degradation of the signal" occurs. As
Canford are broadcast suppliers I expect that quoted phrase to mean
that there is very little ringing or HF loss until you get a fair bit
further away than 675m. Not played with a pair though...
--
Cheers new...@howhill.com
Dave. pam is missing e-mail
fred
<bi...@invalid.com> writes
>>Its wrong imedance - 50 ohms IIRC.
>>
>>Better than a piece of wet string, but not a patch on stuff desined for
>>teh job.
>>
>
>The impedance is in the terminators which are plugged into either end of
>the network, and not in the cable itself.
>
the combination of its inductance and capacitance. At the sort of
frequencies involved in TV, satelite (& computer networks) a mismatch
between the source and/or cable and/or receiver termination is likely to
cause significant problems. As many have said, this cable is not the right
stuff for TV.
--
fred
Andy Hall
No that isn't correct. The cable has a characteristic impedance and
if terminators are used they should match thst impedance.
So if 50 ohm cable is used for TV, there will be a mismatch with
consequent deleterious effects.
invalid
<an...@hall.nospam> writes
Hmmm, just went out to the garage and pulled out some cable and
terminators.
The cable is "Thin net 50 Ohm 10-base2" it measures 0 Ohms as near as
dammit. The terminator measures 50 Ohms as near as dammit. Admittedly
there is no signal being passed down it, and to determine what the
impedance would be with a sine wave (video, data etc) passed down that
same piece of cable is a different matter. The impedance will vary
according to the frequency of the signal passed down it.
The quoted impedance is based on it being used for a specific technology
or application, and will be different under different applications.
I have no idea whether this would be an issue for television signals,
and over what sort of distances.
I would suggest the OP plugs in an aerial/tv and tries it.
Regards
Bill
Dave Stanton
> The cable is "Thin net 50 Ohm 10-base2" it measures 0 Ohms as near as
> dammit. The terminator measures 50 Ohms as near as dammit. Admittedly
> there is no signal being passed down it, and to determine what the
> impedance would be with a sine wave (video, data etc) passed down that
> same piece of cable is a different matter. The impedance will vary
> according to the frequency of the signal passed down it.
>
> The quoted impedance is based on it being used for a specific technology
> or application, and will be different under different applications.
>
> I have no idea whether this would be an issue for television signals,
> and over what sort of distances.
>
> I would suggest the OP plugs in an aerial/tv and tries it.
>
> Regards
> Bill
You cant measure impendance with a normal dmm. Its not resistance in the
normal sense, its made up of capacitance, some resistance and inductance.
Take a look at a good amateur radio book, it should show you a
theoretical circuit of coax using lumped C AND L.
Dave
--
Some people use windows, others have a life.
Andy Hall
>>>
>>>The impedance is in the terminators which are plugged into either end of
>>>the network, and not in the cable itself.
>>>
>>>Apart from that, I couldn't say whether it would be suitable for TV.
>>>
>>>Hth
>>>Bill
>>
>>
>>
>>No that isn't correct. The cable has a characteristic impedance and
>>if terminators are used they should match thst impedance.
>>
>>So if 50 ohm cable is used for TV, there will be a mismatch with
>>consequent deleterious effects.
>>
>>
>>
>>.andy
>>
>
>Hmmm, just went out to the garage and pulled out some cable and
>terminators.
>
>The cable is "Thin net 50 Ohm 10-base2" it measures 0 Ohms as near as
>dammit. The terminator measures 50 Ohms as near as dammit.
Yes, but presumably this is with a DC meter. It will measure 50ohms
for the terminators and zero along the conductors.
>Admittedly
>there is no signal being passed down it, and to determine what the
>impedance would be with a sine wave (video, data etc) passed down that
>same piece of cable is a different matter. The impedance will vary
>according to the frequency of the signal passed down it.
>
>The quoted impedance is based on it being used for a specific technology
>or application, and will be different under different applications.
>
>I have no idea whether this would be an issue for television signals,
>and over what sort of distances.
There is the impedance mismatch and also that this type of cable will
show high losses at higher frequencies.
The combination of these may produce quite uneven results across the
range.
Dave Liquorice
> You cant measure impendance with a normal dmm.
Or impedance even... B-)
> Its not resistance in the normal sense, its made up of capacitance,
> some resistance and inductance.
And as the capacitance and inductance are influenced by the physical
cable dimensions you can tell if you have 50 or 75 ohm cable by
looking at the end. Fairly sure that for same screen diameter, 50 ohm
has a larger dia core and also fairly sure that frequency doesn't have
anything to do with the characteristic impedance but it's a very long
time since I did AC Theory...
invalid
Liquorice <new...@howhill.com> writes
>On Tue, 31 Aug 2004 16:27:14 +0100, Dave Stanton wrote:
>
>> You cant measure impendance with a normal dmm.
>
>Or impedance even... B-)
>
>> Its not resistance in the normal sense, its made up of capacitance,
>> some resistance and inductance.
>
>And as the capacitance and inductance are influenced by the physical
>cable dimensions you can tell if you have 50 or 75 ohm cable by
>looking at the end. Fairly sure that for same screen diameter, 50 ohm
>has a larger dia core
No, it depends on many things - including the core conductor, the
quality of the cable, whether it is solid core or multi core etc.
>and also fairly sure that frequency doesn't have
>anything to do with the characteristic impedance but it's a very long
>time since I did AC Theory...
The frequency influences the impedances, info about it here...
http://www.tpub.com/neets/book2/4a.htm
Regards
Bill
Andy Wade
> The frequency influences the impedances, info about it here...
> http://www.tpub.com/neets/book2/4a.htm
I sense you're a bit out of your depth here... That page is just about
the reactance of an inductor - merely part of the story.
The characteristic impedance of a cable is what you'd measure between
the wires with your multimeter if you could connect it to an infinite
length of the cable. Its value is the square root of the ratio of the
inductance and capacitance per unit length of the wires: Zo =
sqrt(L/C)). To a very good approximation, in most cases, it *doesn't*
vary with the frequency, except at very low frequencies where the
resistance of the wires becomes significant compared to the inductive
reactance.
This is a more relevant URL:
http://www.allaboutcircuits.com/vol_2/chpt_13/3.html, or you could delve
further from http://en.wikipedia.org/wiki/Characteristic_impedance.
--
Andy
raden
<bi...@invalid.com> writes
>In message <ch1dnv$ktj$1$8302...@news.demon.co.uk>, The Natural
>Philosopher <a@b.c> writes
>>Mike Saunders wrote:
>>> Does anyone know if Twinax cable (as used in older ethernet computer
>>> networks) is compatible with domestic coax cable as used for TV's etc.
>>> I have quite a bit of this wired into the structure of the house and it
>>> would be very convenient if I could use this for wiring up the TV's
>>> Many thanks
>>> Mike
>>>
>>Its wrong imedance - 50 ohms IIRC.
>>
>>Better than a piece of wet string, but not a patch on stuff desined
>>for teh job.
>>
>
>The impedance is in the terminators which are plugged into either end
>of the network, and not in the cable itself.
No it's not, the cable has a characteristic impedance which depends on
the geometry and dielectric of the cable.
>
>Apart from that, I couldn't say whether it would be suitable for TV.
>
>Hth
>Bill
--
geoff
raden
<bi...@invalid.com> writes
>
>Hmmm, just went out to the garage and pulled out some cable and
>terminators.
>
>The cable is "Thin net 50 Ohm 10-base2" it measures 0 Ohms as near as
>dammit.
At what frequency ?
At DC, of course you'll get a short circuit. At the working frequency,
it will be a completely different matter. Inductive effects only occur
with AC not DC (no flames please)
--
geoff
raden
>
>> You cant measure impendance with a normal dmm.
>
>Or impedance even... B-)
>
>> Its not resistance in the normal sense, its made up of capacitance,
>> some resistance and inductance.
>
>And as the capacitance and inductance are influenced by the physical
>cable dimensions you can tell if you have 50 or 75 ohm cable by
>looking at the end. Fairly sure that for same screen diameter, 50 ohm
>has a larger dia core
>
--
geoff
fred
<spamb...@ajwade.clara.co.uk> writes
Sorry, not you Andy, that other bloke
--
fred
invalid
writes
You should be more careful with your reading and/or snipping. That is
exactly what I said.... the paragraph following the one you quoted (ie
where you snipped) said....
"Admittedly there is no signal being passed down it, and to determine
what the impedance would be with a sine wave (video, data etc) passed
down that same piece of cable is a different matter. The impedance will
vary according to the frequency of the signal passed down it."
It will be there on google for any one to check.
I'll post you a sensible reply, when you stop arguing the fact against
paragraphs you have snipped.
Bill
invalid
No troll, but I have noticed a tendency in some groups for anyone who
disagrees with frequent posters to be labelled as such.
I note that while you're referring to me as a troll (what for, my
views?) you do not refer to geoff as a troll too. That's strange,
because even though I have discounted his post for gratuitous (sp)
snipping, he seems to agree with my view about a sine wave inducing
impedance (although he does seem to have missed the fact that DC can be
sinusoidal).
It seems I'll have to post some references back to google groups in this
thread in order to support my disdain at your unhelpful/non constructive
post.
Bill
invalid
<spamb...@ajwade.clara.co.uk> writes
Hi Andy,
Sorry, this thread seems to have been interrupted by some people
throwing insults rather than views.
It's getting late - I have only looked at the second link you posted.
However, the link does not appear to be appropriate for this scenario.
One of the points about RG58 used in networking is that the very reason
for the existence of a terminator is to avoid reflections back up the
line... your link says ...
"A uniform line terminated in its characteristic impedance will have no
standing waves, no reflections from the end, and a constant ratio of
voltage to current at a given frequency at every point on the line."
Note the part about "constant [...] at a given frequency".
Regarding "no reflections", that is actually a requirement of RG58 and
the purpose of the terminator. I would like to read further regarding
the term "characteristic impedance" as I am not familiar with it. I
would be inclined to suggest that (as I said in an earlier post) it
relates to the designed/intended use of the cable and this seems to be
supported by your link.
This last statement appears to support my view and that of geoff (the
surreptitious snipper) regarding impedance being influenced by frequency
of a sinusoidal wave.
It seems our disagreement of opinion/fact/whatever is being hijacked by
people throwing personal insults. I would suggest we return to the
original point over whether TV Antennae signal could be satisfactorily
passed down RG58 thin net cable.
Of that, I still have no idea (as stated in earlier posts) and still
recommend the OP tries it himself (I might even go and buy some m/f
antenna connectors tomorrow and try it *myself* :-)
If I make it into town to do so, I'll post the findings regardless of
the results... even though they could be snipped up and a "troll" label
attached too ;-)
Regards
Bill
Andy Dingley
<new...@howhill.com> wrote:
>fairly sure that frequency doesn't have
>anything to do with the characteristic impedance
It has a small effect. The impedance depends on both the geometry and
the behaviour of the dielectric, and this can have a small variation
in its properties with frequency.
But for two solid core polyethylene coax cables, yes you can eyeball
which is the 50 and which the 75 ohm.
(for bonus points, why is it 93 ohms ? 8-) )
--
Smert' spamionam
Andy Hall
Probably because IBM wanted to create a market for something
incompatible with anything else. They did it with pretty much
everything else, although less so now.
Andy Wade
93 ohm coax - RG62 - was around long before IBM used it for networking.
I've only ever used in situations where I've simply wanted to minimise
shunt capacitance.
So go on Andy D: why 93 ohms? I can explain the logic behind 50 ohms
and 75 ohms, but the reason for the existence of 93 ohm coax has always
been something of a mystery.
--
Andy
Andy Wade
> [big snip]
> "A uniform line terminated in its characteristic impedance will have no
> standing waves, no reflections from the end, and a constant ratio of
> voltage to current at a given frequency at every point on the line."
>
> Note the part about "constant [...] at a given frequency".
The ratio of voltage and current on the line will be constant over a
wide range of frequency if the line is properly terminated.
> Regarding "no reflections", that is actually a requirement of RG58 and
> the purpose of the terminator. I would like to read further regarding
> the term "characteristic impedance" as I am not familiar with it.
A Google search for "characteristic impedance" "transmission line" gives
13,600 hits. Happy reading. Otherwise, for books giving a simple
introduction, try something like Scroggie's "Foundations of Wireless" or
the RSGB or ARRL handbooks.
> [...] I would suggest we return to the
> original point over whether TV Antennae signal could be satisfactorily
> passed down RG58 thin net cable.
FSVO "satisfactorily". The loss (attenuation) of that cable at UHF will
be too high for anything other than very short runs, unless you can
afford to throw away a lot of signal. And using a 50 ohm cable in what
would normally be a 75 ohm system isn't good practice, although it won't
lead to any great disaster.
--
Andy
Andy Luckman (AJL Electronics)
>
> The impedance is in the terminators which are plugged into either end of
> the network, and not in the cable itself.
Are you sure of this? Why do you think that cable is available in a variety
of impedance values?
--
AJL Electronics (G6FGO) Ltd : Satellite and TV aerial systems
http://www.classicmicrocars.co.uk : http://www.ajlelectronics.co.uk
Andy Luckman (AJL Electronics)
<URL:mailto:bi...@invalid.com> wrote:
> The cable is "Thin net 50 Ohm 10-base2" it measures 0 Ohms as near as
> dammit. The terminator measures 50 Ohms as near as dammit.
Impedance is NOT the same as resistance. A piece of coaxial cable can be
open circuit at DC, but a short circuit at RF.
Andy Luckman (AJL Electronics)
<URL:mailto:spamb...@ajwade.clara.co.uk> wrote:
And using a 50 ohm cable in what
> would normally be a 75 ohm system isn't good practice, although it won't
> lead to any great disaster.
The tilt characteristics of sub standard cable will lead to some interesting
signal levels!
We have a problem with a new development that we have trunked. It seems that
the dropleads were pre-installed using RG59. Although it offends all my
sensibilities, it could be OK(ish), except for the fact that the digital
Muxes are at C/D and the analogue down at group A. The dropcable runs are
quite long too, so there are some flats that will never have full
performance, without spending more developers money. You can imagine that
Porcine Aviation will launch first!
Dave Plowman (News)
Andy Wade <spamb...@ajwade.clara.co.uk> wrote:
> > [...] I would suggest we return to the
> > original point over whether TV Antennae signal could be satisfactorily
> > passed down RG58 thin net cable.
> FSVO "satisfactorily". The loss (attenuation) of that cable at UHF will
> be too high for anything other than very short runs, unless you can
> afford to throw away a lot of signal. And using a 50 ohm cable in what
> would normally be a 75 ohm system isn't good practice, although it won't
> lead to any great disaster.
From RS, the attenuation of RG58 is 7.6dB per 10 metres at 1000MHz.
El cheapo UHF aerial cable is about half that - before any mismatch is
taken into consideration.
CT 100 is approx 2 db per 10 metres at 1000MHz.
--
*Caution: I drive like you do.
Dave Plowman da...@davenoise.co.uk London SW
To e-mail, change noise into sound.
invalid
(AJL Electronics)" <m...@privacy.net> writes
>In article <4gDcPxjL...@spamnothing.co.uk>, invalid
><URL:mailto:bi...@invalid.com> wrote:
>>
>> The impedance is in the terminators which are plugged into either end of
>> the network, and not in the cable itself.
>
>Are you sure of this? Why do you think that cable is available in a variety
>of impedance values?
>
Isn't that impedance at a given frequency (I believe I discussed this in
an earlier post). "Intended/designed use".
Bill
invalid
<spamb...@ajwade.clara.co.uk> writes
Andy,
Try this link with two reasonable descriptions of
impedance/resistance/reactance etc. For the shortened version, check the
very last paragraph which reads...
"The short answer is -- impedance includes reactance, and reactance
includes effects which vary with frequency due to inductance and
capacitance." Ted Pavlic, Electrical Engineering Undergrad Student, Ohio
St.
http://www.physlink.com/Education/AskExperts/ae517.cfm
I'm afraid the continual mis-quoting and devious snipping (not from you)
isn't being constructive to this thread so I'll leave it here. I guess
you and I will continue to disagree and other posters will continue to
misquote me.
Regards
Bill
invalid
(AJL Electronics)" <m...@privacy.net> writes
><URL:mailto:bi...@invalid.com> wrote:
>
>> The cable is "Thin net 50 Ohm 10-base2" it measures 0 Ohms as near as
>> dammit. The terminator measures 50 Ohms as near as dammit.
>
>Impedance is NOT the same as resistance. A piece of coaxial cable can be
>open circuit at DC, but a short circuit at RF.
>
>
I'm getting a bit fed up of this surreptitious snipping. The very next
sentence in the paragraph where you have snipped reads...
"Admittedly there is no signal being passed down it, and to determine
what the impedance would be with a sine wave (video, data etc) passed
down that same piece of cable is a different matter. The impedance will
vary according to the frequency of the signal passed down it."
Bill
Peter Parry
wrote:
>Probably because IBM wanted to create a market for something
>incompatible with anything else.
93 Ohms is still a cable standard and its use in instrumentation
predates networks by some years. For once IBM is not guilty :-).
Cable attenuation is minimum at 77 ohms; the breakdown voltage is
maximum at 60 ohms and the power-carrying capacity is maximum at 30
ohms. 50 Ohms is the best compromise between voltage/power handling,
75 Ohms gives the lowest loss (but lower power handling) and 93 Ohms
gives the lowest capacitance which reduced loading and allowed for
longer cable runs.
(75 Ohm rather than 77 Ohm because the diameter ratios could be more
easily made with existing tooling for standard cable sizes).
--
Peter Parry.
http://www.wpp.ltd.uk/
Ian White
>In message <ant01081...@office.ajlelectronics.co.uk>, "Andy
>Luckman (AJL Electronics)" <m...@privacy.net> writes
>>In article <yPvuEmr5...@spamnothing.co.uk>, invalid
>><URL:mailto:bi...@invalid.com> wrote:
>>
>>> The cable is "Thin net 50 Ohm 10-base2" it measures 0 Ohms as near as
>>> dammit. The terminator measures 50 Ohms as near as dammit.
>>
>>Impedance is NOT the same as resistance. A piece of coaxial cable can be
>>open circuit at DC, but a short circuit at RF.
>>
>>
>
>I'm getting a bit fed up of this surreptitious snipping. The very next
>sentence in the paragraph where you have snipped reads...
>
You have fallen into almost the same trap - you are reading snippets of
information in isolation, when you don't understand the background.
And whatever the subject may be, you can always find a web page written
by someone who doesn't understand it either:
>"Admittedly there is no signal being passed down it, and to determine
>what the impedance would be with a sine wave (video, data etc) passed
>down that same piece of cable is a different matter. The impedance will
>vary according to the frequency of the signal passed down it."
That last sentence is just plain wrong! As Andy is trying to tell you:
the characteristic impedance of a transmission line - of which coaxial
cable is one example - is in fact *constant* over a very wide range of
frequencies.
Do not confuse this with the impedance of an inductor or a capacitor,
both of which do vary with frequency, in opposite directions (inversely
to each other).
You may have read that a transmission line contains distributed
inductance and capacitance; but the salient feature of RF transmission
lines is that it contains *both* of these things, and that they combine
to produce a characteristic impedance that does *not* vary with
frequency.
(Behind that simple basic fact, there are some fine details that do lead
to small deviations away from a constant value at very low frequencies
and at microwaves; but those really are small details.)
--
Ian White
Abingdon, England
fred
<bi...@invalid.com> writes
Hi Bill,
I think you are trolling, because you are posting information in an
authoritative tone when it is clear you don't have any real knowledge on the
subject.
Whatever your knowledge in other fields or even in the field of electronics,
you do not appear to have enough knowledge in the area of transmission
line theory and characteristic impedance to make a useful input into this
thread.
If you don't know the answer, please don't guess.
The original poster's question and his follow-up were answered
comprehensively in the first 10 replies to this thread, before your first post,
but here we are some 30 replies later and still going. The misinformation in
your first post was politely corrected by 2 people, me included, but you
continue to post like you are an expert in the subject, which you are not,
and which I'm sure will be very confusing for subsequent googlers on this
subject.
If you were/are trolling, you have a result, congratulations.
You are IMM and I claim my five pounds . . . . .
Love & Hugs,
--
fred
The Natural Philosopher
The impedance is independent of frequency, and is characteristic of the
cable.
The terminators merely 'look' like an infinitely long bit of cable
tacked on the end. So they match the cable, but do not define it.
The Natural Philosopher
Which just goes to show how a little knowledge identifies a prat.
The whole point about a cable is that it has distrubited capacitance and
inductance, and together they end up looking like an almost perefect
resustance over a wide range of freqencies.
Which is precisely why we use it.
Impress the girlfriend: Google 'transmission line theory'
>
> http://www.physlink.com/Education/AskExperts/ae517.cfm
>
> I'm afraid the continual mis-quoting and devious snipping (not from you)
> isn't being constructive to this thread so I'll leave it here. I guess
> you and I will continue to disagree and other posters will continue to
> misquote me.
no need. You misquote other enough aleady.,
>
> Regards
> Bill
Dave Stanton
> On Tue, 31 Aug 2004 16:27:14 +0100, Dave Stanton wrote:
>
>> You cant measure impendance with a normal dmm.
>
> Or impedance even... B-)
>
Correct, full marks that person.
Dave
--
Some people use windows, others have a life.
raden
<bi...@invalid.com> writes
>
>No troll, but I have noticed a tendency in some groups for anyone who
>disagrees with frequent posters to be labelled as such.
>
>I note that while you're referring to me as a troll (what for, my
>views?) you do not refer to geoff as a troll too.
>That's strange, because even though I have discounted his post for
>gratuitous (sp) snipping,
I snip what isn't relevant to what I'm replying to (sometimes)
>he seems to agree with my view about a sine wave inducing impedance
Where did I mention sine waves? and what exactly do you mean by
"sinewaves inducing impedance "?
>(although he does seem to have missed the fact that DC can be
>sinusoidal).
I presume you are talking about AC with a DC offset, it's not really
relevant to the argument is it?
--
geoff
raden
<bi...@invalid.com> writes
>
>
>"The short answer is -- impedance includes reactance, and reactance
>includes effects which vary with frequency due to inductance and
>capacitance.
keep learning grasshopper
>
>I'm afraid the continual mis-quoting and devious snipping
Do fuck off
--
geoff
raden
<m...@privacy.net> writes
>On Tue, 31 Aug 2004 17:46:03 +0100, Dave Liquorice wrote:
>
>> On Tue, 31 Aug 2004 16:27:14 +0100, Dave Stanton wrote:
>>
>>> You cant measure impendance with a normal dmm.
>>
>> Or impedance even... B-)
>>
>
>Correct, full marks that person.
>
--
geoff
Andy Hall
I've got one.
When somebody posts an intelligent question and we wait for the
impending doom of an IMM reply.
Impendance would be the measurement of time between the original post
and the entry into Wonderland.
I suppose we could also apply special meanings of conductance,
reluctance etc. in this context.
fred
<an...@hall.nospam> writes
>>I like impendance, all we need to do now is to find a meaning for it
>
>I've got one.
>
>When somebody posts an intelligent question and we wait for the
>impending doom of an IMM reply.
>Impendance would be the measurement of time between the original post
>and the entry into Wonderland.
>
>I suppose we could also apply special meanings of conductance,
>reluctance etc. in this context.
--
fred
Dave Plowman (News)
raden <ra...@ntlworld.com> wrote:
> >>> You cant measure impendance with a normal dmm.
> >>
> >> Or impedance even... B-)
> >>
> >
> >Correct, full marks that person.
> >
> I like impendance, all we need to do now is to find a meaning for it
I like inpedance and outpedance myself. Less tongue twisting than input
impedance and output impedance.
--
*Why is it that doctors call what they do "practice"?
Andy Wade
> Cable attenuation is minimum at 77 ohms;
Only for an air dielectric cable.
> [...] 50 Ohms is the best compromise between voltage/power handling
It also happens to be the Zo for minimum loss when epsilon_r = 2.25,
i.e. solid polyethylene dielectric.
> [...] and 93 Ohms gives the lowest capacitance
Compared to what? The higher the Zo the less the capacitance will be
(all other things remaining constant). I suspect 93 ohms was another
practical compromise - FWIW I notice it corresponds fairly closely with
an outer/inner diameter ratio of 10:1 with a polyethylene dielectric.
> (75 Ohm rather than 77 Ohm because the diameter ratios could be more
> easily made with existing tooling for standard cable sizes).
Not heard that one before. At one time you could get 70, 72, 75 and 80
ohm coaxes from different manufacturers; then they converged and
standardised on 75 ohm.
--
Andy
The Natural Philosopher
Its obviously the quality of something about to become
As in 'the impendance of her pregnancy militated an accelerated transit
process towards the most proximate maternal unit' etc. etc.
Definitely Pseud Speak for 'shit, I'm popping, get me to the hospital'
Peter Parry
<spamb...@ajwade.clara.co.uk> wrote:
>Peter Parry wrote:
>
>> Cable attenuation is minimum at 77 ohms;
>
>Only for an air dielectric cable.
I was writing the condensed version :-).
>> [...] 50 Ohms is the best compromise between voltage/power handling
>
>It also happens to be the Zo for minimum loss when epsilon_r = 2.25,
>i.e. solid polyethylene dielectric.
>
>> [...] and 93 Ohms gives the lowest capacitance
>
>Compared to what? The higher the Zo the less the capacitance will be
>(all other things remaining constant). I suspect 93 ohms was another
>practical compromise -
Indeed, it represented the top end impedance value which was
practically achievable with co-ax cable. Obviously open wire feeders
can exhibit higher impedances.
>... At one time you could get 70, 72, 75 and 80
>ohm coaxes from different manufacturers; then they converged and
>standardised on 75 ohm.
You could get almost anything - somewhere in the garage is an
impedance and size matching device for joining 49 Ohm to 53 Ohm
airspaced rigid coax from an old radar (it is co-ax - not waveguide).
The convergence on 50 and later 75 ohms was initially driven by the
wartime imperative to minimise stock holdings and maximise
interoperability. At one point a single Destroyer would have to
carry something like 15 cables of impedances which differed only
slightly and similar (but not identical) size and role. Each of
these required different connectors.
raden
Philosopher <a@b.c> writes
and the maternity unit, and is a measure of contractions/red light.
--
geoff