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What Shielded wire to use for RF Test Leads
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olds...@tubes.com
Mar 14, 2017, 1:46:24 AM3/14/17
to
Looking on the web, I have found numerous suggestions and no definite
answer.
To make a shielded test lead for use on a RF device, such as a Signal
generator, Oscilloscope, or other, I have seen recommendations for TV
coax (RG59), CB/Ham antenna wire (RG58) and microphone cable. Which is
best?
I know that the TV cable is stiff, with that solid wire in the center,
so that would not be the ideal cable to use. It's been years since I
used the CB/Ham cable. I know that stuff is stoffer too, but maybe not
as bad. As far as handling and softness, I know the Mic cable is
probably the best, but I read that it's probably the least desireable
because of impedance issues.
So what is normally used?
BTW:
* I might be wrong on those RG numbers. I'm just going on my failing
memory.
answer.
To make a shielded test lead for use on a RF device, such as a Signal
generator, Oscilloscope, or other, I have seen recommendations for TV
coax (RG59), CB/Ham antenna wire (RG58) and microphone cable. Which is
best?
I know that the TV cable is stiff, with that solid wire in the center,
so that would not be the ideal cable to use. It's been years since I
used the CB/Ham cable. I know that stuff is stoffer too, but maybe not
as bad. As far as handling and softness, I know the Mic cable is
probably the best, but I read that it's probably the least desireable
because of impedance issues.
So what is normally used?
BTW:
* I might be wrong on those RG numbers. I'm just going on my failing
memory.
Look165
Mar 14, 2017, 3:33:16 AM3/14/17
to
For RF, the best is the cheapest !
A tube of aspirine for instance with a hole on the top for the lead.
I use this for my scope probe, it is perfect.
olds...@tubes.com a écrit :
A tube of aspirine for instance with a hole on the top for the lead.
I use this for my scope probe, it is perfect.
olds...@tubes.com a écrit :
gregz
Mar 14, 2017, 4:43:41 AM3/14/17
to
Greg
Ralph Mowery
Mar 14, 2017, 10:02:58 AM3/14/17
to
In article <060fcchp00vc2d7rv...@4ax.com>,
olds...@tubes.com says...
All depends on the type of testing. For RF I usually use some rg-400.
It is slightly less than 1/4 of an inch in diameter ( same size as the
rg-58) That is double shielded silver plated and a stranded center wire.
But that is because I am often doing duplex work and need the isolation
of the double shielding. RG 58 is fine if you do not need the extra
shielding. Just get the kind with the stranded center conductor.
Usualy rg 174 for light weight leads. It is about 1/8 of an inch in
diameter. I am not sure of the number but there is a teflon and silver
cable made in the same size.
All that is for a 50 ohm system or audio work.
Most scopes will use a probe with a 10/1 on it. You can get them from
the China places off ebay for about 15 to 20 dollars per pair. I bought
a set and compaired them to some HP probes rated at 100 MHz and they
seem to be about the same. Not sure how long they will last, but at
that price should be good for a while if not mistreated. Without the
10/1 probe you will often load down a circuit with the scope leads. The
cable will have about 30 pf of capacitance per foot. I am thinking the
scope only has about a 1 meg ohm of impedance without the 10/1 probe.
olds...@tubes.com says...
It is slightly less than 1/4 of an inch in diameter ( same size as the
rg-58) That is double shielded silver plated and a stranded center wire.
But that is because I am often doing duplex work and need the isolation
of the double shielding. RG 58 is fine if you do not need the extra
shielding. Just get the kind with the stranded center conductor.
Usualy rg 174 for light weight leads. It is about 1/8 of an inch in
diameter. I am not sure of the number but there is a teflon and silver
cable made in the same size.
All that is for a 50 ohm system or audio work.
Most scopes will use a probe with a 10/1 on it. You can get them from
the China places off ebay for about 15 to 20 dollars per pair. I bought
a set and compaired them to some HP probes rated at 100 MHz and they
seem to be about the same. Not sure how long they will last, but at
that price should be good for a while if not mistreated. Without the
10/1 probe you will often load down a circuit with the scope leads. The
cable will have about 30 pf of capacitance per foot. I am thinking the
scope only has about a 1 meg ohm of impedance without the 10/1 probe.
Jeff Liebermann
Mar 14, 2017, 12:19:58 PM3/14/17
to
On Tue, 14 Mar 2017 00:45:43 -0500, olds...@tubes.com wrote:
>To make a shielded test lead for use on a RF device, such as a Signal
>generator, Oscilloscope, or other, I have seen recommendations for TV
>coax (RG59), CB/Ham antenna wire (RG58) and microphone cable. Which is
>best?
(...)
>To make a shielded test lead for use on a RF device, such as a Signal
>generator, Oscilloscope, or other, I have seen recommendations for TV
>coax (RG59), CB/Ham antenna wire (RG58) and microphone cable. Which is
>best?
>So what is normally used?
I use RG-58c/u for 50 ohm test leads. The stranded center wire is
more flexible than the rather stiff RG-58a/u.
For 75 ohm RF, I use RG-6/u double shielded CATV cable with F
connectors on both ends. If I need to plug into test equipment, I
have female F to Male BNC adapater.
--
Jeff Liebermann je...@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
olds...@tubes.com
Mar 14, 2017, 12:40:17 PM3/14/17
to
On Tue, 14 Mar 2017 09:19:52 -0700, Jeff Liebermann <je...@cruzio.com>
wrote:
>On Tue, 14 Mar 2017 00:45:43 -0500, olds...@tubes.com wrote:
>
>>To make a shielded test lead for use on a RF device, such as a Signal
>>generator, Oscilloscope, or other, I have seen recommendations for TV
>>coax (RG59), CB/Ham antenna wire (RG58) and microphone cable. Which is
>>best?
>(...)
>>So what is normally used?
>
>I use RG-58c/u for 50 ohm test leads. The stranded center wire is
>more flexible than the rather stiff RG-58a/u.
>
>For 75 ohm RF, I use RG-6/u double shielded CATV cable with F
>connectors on both ends. If I need to plug into test equipment, I
>have female F to Male BNC adapater.
I've never heard of RG58 C/U. Where would someone buy a small amount of
it (Online)? Looking on Ebay, I see the A/U type. And that stuff is
mostly pre-made cables with either Pl-259 , or BNC connectors on it, OR
large rolls of the bulk wire. (25 ft is the shortest).
I could buy a 12ft with BNCs, and cut it in half to make two leads, but
that's the A/U type.
I'm thinking this is something that might need to be ordered from a
store that sells Ham Radio stuff, but do they sell small lengths and do
they ship it?
wrote:
>On Tue, 14 Mar 2017 00:45:43 -0500, olds...@tubes.com wrote:
>
>>To make a shielded test lead for use on a RF device, such as a Signal
>>generator, Oscilloscope, or other, I have seen recommendations for TV
>>coax (RG59), CB/Ham antenna wire (RG58) and microphone cable. Which is
>>best?
>(...)
>>So what is normally used?
>
>I use RG-58c/u for 50 ohm test leads. The stranded center wire is
>more flexible than the rather stiff RG-58a/u.
>
>For 75 ohm RF, I use RG-6/u double shielded CATV cable with F
>connectors on both ends. If I need to plug into test equipment, I
>have female F to Male BNC adapater.
it (Online)? Looking on Ebay, I see the A/U type. And that stuff is
mostly pre-made cables with either Pl-259 , or BNC connectors on it, OR
large rolls of the bulk wire. (25 ft is the shortest).
I could buy a 12ft with BNCs, and cut it in half to make two leads, but
that's the A/U type.
I'm thinking this is something that might need to be ordered from a
store that sells Ham Radio stuff, but do they sell small lengths and do
they ship it?
Jeff Liebermann
Mar 14, 2017, 6:03:22 PM3/14/17
to
On Tue, 14 Mar 2017 11:39:39 -0500, olds...@tubes.com wrote:
>On Tue, 14 Mar 2017 09:19:52 -0700, Jeff Liebermann <je...@cruzio.com>
>wrote:
>
>>On Tue, 14 Mar 2017 00:45:43 -0500, olds...@tubes.com wrote:
>>
>>>To make a shielded test lead for use on a RF device, such as a Signal
>>>generator, Oscilloscope, or other, I have seen recommendations for TV
>>>coax (RG59), CB/Ham antenna wire (RG58) and microphone cable. Which is
>>>best?
>>(...)
>>>So what is normally used?
>>
>>I use RG-58c/u for 50 ohm test leads. The stranded center wire is
>>more flexible than the rather stiff RG-58a/u.
>>
>>For 75 ohm RF, I use RG-6/u double shielded CATV cable with F
>>connectors on both ends. If I need to plug into test equipment, I
>>have female F to Male BNC adapater.
>I've never heard of RG58 C/U.
<http://www.ebay.com/sch/i.html?_nkw=rg-58c%2Fu>
>On Tue, 14 Mar 2017 09:19:52 -0700, Jeff Liebermann <je...@cruzio.com>
>wrote:
>
>>On Tue, 14 Mar 2017 00:45:43 -0500, olds...@tubes.com wrote:
>>
>>>To make a shielded test lead for use on a RF device, such as a Signal
>>>generator, Oscilloscope, or other, I have seen recommendations for TV
>>>coax (RG59), CB/Ham antenna wire (RG58) and microphone cable. Which is
>>>best?
>>(...)
>>>So what is normally used?
>>
>>I use RG-58c/u for 50 ohm test leads. The stranded center wire is
>>more flexible than the rather stiff RG-58a/u.
>>
>>For 75 ohm RF, I use RG-6/u double shielded CATV cable with F
>>connectors on both ends. If I need to plug into test equipment, I
>>have female F to Male BNC adapater.
>I've never heard of RG58 C/U.
Also known as Belden 8262.
30ft for $36 including shipping and BNC connectors on the ends.
<http://www.ebay.com/itm/191993013419>
A little pricey but about what I would expect for a long test lead.
It's what's used on test leads and mobile antenna coax cables. Bend
the solid center conductor RG-58a/u a few times and it will break. Try
to get the type that has both foil and wire braid shields.
>Where would someone buy a small amount of
>it (Online)? Looking on Ebay, I see the A/U type. And that stuff is
>mostly pre-made cables with either Pl-259 , or BNC connectors on it, OR
>large rolls of the bulk wire. (25 ft is the shortest).
>I could buy a 12ft with BNCs, and cut it in half to make two leads, but
>that's the A/U type.
for you. How many feet in a "large roll of bulk wire"? eBay is a
good place to get short lengths, but not high quality cables.
>I'm thinking this is something that might need to be ordered from a
>store that sells Ham Radio stuff, but do they sell small lengths and do
>they ship it?
Also, you'll need mating BNC, UHF, whatever connectors. I avoid
solder type connectors because they like to fall apart. Crimp type
are stronger and more reliable. You'll need a crimper:
<http://802.11junk.com/jeffl/antennas/Misc/slides/crimpers.html>
100ft for $42 plus shipping:
<http://www.l-com.com/coaxial-rg58c-u-bulk-coaxial-cable-stranded-center-conductor-50-ohm-cable>
Ron D.
Mar 15, 2017, 10:33:51 AM3/15/17
to
Honestly, we havent; the foggiest of what you want to do.
The generator probably has a 50 ohm output. If it's an FM radio, the input is likely 75 ohms. So, you do need a matching transformer. So, there's generally BNC to BNC and an F connector at the receiver. You can buy adapters to adapt the BNC to F. or BNC to quick F.
RG-58 isn't terribly inflexible. If you want flexibility, then RG174 should work.
At one point, at work we had lots of crimped BNC cables, bought from a reputable manufacturer. Nearly all failed at one point. the broken ones were eventually fitted with clamp BNC plugs. They are not the easiest things to put together, but a resistance soldering tool made it a lot easier.
It also depends where you want to put the adapters.
The generator probably has a 50 ohm output. If it's an FM radio, the input is likely 75 ohms. So, you do need a matching transformer. So, there's generally BNC to BNC and an F connector at the receiver. You can buy adapters to adapt the BNC to F. or BNC to quick F.
RG-58 isn't terribly inflexible. If you want flexibility, then RG174 should work.
At one point, at work we had lots of crimped BNC cables, bought from a reputable manufacturer. Nearly all failed at one point. the broken ones were eventually fitted with clamp BNC plugs. They are not the easiest things to put together, but a resistance soldering tool made it a lot easier.
It also depends where you want to put the adapters.
Jeff Liebermann
Mar 15, 2017, 11:49:03 PM3/15/17
to
On Wed, 15 Mar 2017 07:33:45 -0700 (PDT), "Ron D."
<ron.d...@gmail.com> wrote:
>The generator probably has a 50 ohm output. If it's
>an FM radio, the input is likely 75 ohms. So, you
>do need a matching transformer.
Nope. Most of my test equipment is 50 ohms. However, I also work
<ron.d...@gmail.com> wrote:
>The generator probably has a 50 ohm output. If it's
>an FM radio, the input is likely 75 ohms. So, you
>do need a matching transformer.
with 75 ohm devices and antennas. Transformers do not have a very
flat frequency response. If I need a flat frequency response from DC
to many GHz, I use a minimum loss pad:
<https://www.maximintegrated.com/en/app-notes/index.mvp/id/972>
<https://www.minicircuits.com/WebStore/dashboard.html?model=BMP-5075R%2B>
<https://www.minicircuits.com/pdfs/BMP-5075R+.pdf>
<https://www.pasternack.com/matching-pads-category.aspx>
<http://www.comm-connect.com/8006.html>
etc... Figure on 5.71dB loss. The technique also works for 50 to
300, 50 to 600 ohms, and other impedances.
>It also depends where you want to put the adapters.
<http://802.11junk.com/jeffl/antennas/connector-loss/Adapter%20Colluge.jpg>
More:
<http://ham-radio.com/k6sti/match.htm>
<https://www.microwaves101.com/encyclopedias/l-pads#minloss>
Michael A. Terrell
Mar 16, 2017, 1:58:23 AM3/16/17
to
Jeff Liebermann wrote:
> On Wed, 15 Mar 2017 07:33:45 -0700 (PDT), "Ron D."
> <ron.d...@gmail.com> wrote:
>
>> The generator probably has a 50 ohm output. If it's
>> an FM radio, the input is likely 75 ohms. So, you
>> do need a matching transformer.
>
> Nope. Most of my test equipment is 50 ohms. However, I also work
> with 75 ohm devices and antennas. Transformers do not have a very
> flat frequency response. If I need a flat frequency response from DC
> to many GHz, I use a minimum loss pad:
> <https://www.maximintegrated.com/en/app-notes/index.mvp/id/972>
> <https://www.minicircuits.com/WebStore/dashboard.html?model=BMP-5075R%2B>
> <https://www.minicircuits.com/pdfs/BMP-5075R+.pdf>
> <https://www.pasternack.com/matching-pads-category.aspx>
> <http://www.comm-connect.com/8006.html>
> etc... Figure on 5.71dB loss. The technique also works for 50 to
> 300, 50 to 600 ohms, and other impedances.
>
>> It also depends where you want to put the adapters.
>
> Yep. Coax adapters are fun:
> <http://802.11junk.com/jeffl/antennas/connector-loss/Adapter%20Colluge.jpg>
>
> More:
> <http://ham-radio.com/k6sti/match.htm>
> <https://www.microwaves101.com/encyclopedias/l-pads#minloss>
I have used some of the six hole ferrite beads to make transformers.
> On Wed, 15 Mar 2017 07:33:45 -0700 (PDT), "Ron D."
> <ron.d...@gmail.com> wrote:
>
>> The generator probably has a 50 ohm output. If it's
>> an FM radio, the input is likely 75 ohms. So, you
>> do need a matching transformer.
>
> Nope. Most of my test equipment is 50 ohms. However, I also work
> with 75 ohm devices and antennas. Transformers do not have a very
> flat frequency response. If I need a flat frequency response from DC
> to many GHz, I use a minimum loss pad:
> <https://www.maximintegrated.com/en/app-notes/index.mvp/id/972>
> <https://www.minicircuits.com/WebStore/dashboard.html?model=BMP-5075R%2B>
> <https://www.minicircuits.com/pdfs/BMP-5075R+.pdf>
> <https://www.pasternack.com/matching-pads-category.aspx>
> <http://www.comm-connect.com/8006.html>
> etc... Figure on 5.71dB loss. The technique also works for 50 to
> 300, 50 to 600 ohms, and other impedances.
>
>> It also depends where you want to put the adapters.
>
> Yep. Coax adapters are fun:
> <http://802.11junk.com/jeffl/antennas/connector-loss/Adapter%20Colluge.jpg>
>
> More:
> <http://ham-radio.com/k6sti/match.htm>
> <https://www.microwaves101.com/encyclopedias/l-pads#minloss>
six turns, tapped at the fifth turn. The 1.2 turns ratio, squared is a
1.44:1 impedance ratio. These are the same beads used for CATV line taps
and broadband splitters.
They convert 75 ohms to 52 ohms. I put some into some old Blonder
Tongue inline attenuator housings with an F connect on the 75 ohm end
and a BNC connector on the 52 ohm end. (52.0833 Ohms) They were handy to
use a TV FSM as a 50 ohm RF millivoltmeter. Now, I have some precision
attenuators, and a Boonton 9200 RF Millivoltmeter ans different
feedthrough terminators.. :)
--
Never piss off an Engineer!
They don't get mad.
They don't get even.
They go for over unity! ;-)
Phil Allison
Mar 16, 2017, 3:25:55 AM3/16/17
to
Ron D. wrote:
> Honestly, we havent; the foggiest of what you want to do.
>
** The dopey OP has one of these:
> Honestly, we havent; the foggiest of what you want to do.
>
https://elektrotanya.com/PREVIEWS/38933510/23432455/eico/eico_320_150k-34mhz_am_signalgenerator_1956_sch.pdf_1.png
The output Z is anyone's guess.
He only wants to use it for old, tube AM radios.
IMO use no cable at all, just let it radiate via a length of wire.
.... Phil
gregz
Mar 16, 2017, 4:22:34 AM3/16/17
to
Jeff Liebermann <je...@cruzio.com> wrote:
> On Tue, 14 Mar 2017 11:39:39 -0500, olds...@tubes.com wrote:
>
>> On Tue, 14 Mar 2017 09:19:52 -0700, Jeff Liebermann <je...@cruzio.com>
>> wrote:
>>
>>> On Tue, 14 Mar 2017 00:45:43 -0500, olds...@tubes.com wrote:
>>>
>>>> To make a shielded test lead for use on a RF device, such as a Signal
>>>> generator, Oscilloscope, or other, I have seen recommendations for TV
>>>> coax (RG59), CB/Ham antenna wire (RG58) and microphone cable. Which is
>>>> best?
>>> (...)
>>>> So what is normally used?
>>>
>>> I use RG-58c/u for 50 ohm test leads. The stranded center wire is
>>> more flexible than the rather stiff RG-58a/u.
>>>
A/U is stranded center too. Rg58 is solid. Au cu difference seems to be
> On Tue, 14 Mar 2017 11:39:39 -0500, olds...@tubes.com wrote:
>
>> On Tue, 14 Mar 2017 09:19:52 -0700, Jeff Liebermann <je...@cruzio.com>
>> wrote:
>>
>>> On Tue, 14 Mar 2017 00:45:43 -0500, olds...@tubes.com wrote:
>>>
>>>> To make a shielded test lead for use on a RF device, such as a Signal
>>>> generator, Oscilloscope, or other, I have seen recommendations for TV
>>>> coax (RG59), CB/Ham antenna wire (RG58) and microphone cable. Which is
>>>> best?
>>> (...)
>>>> So what is normally used?
>>>
>>> I use RG-58c/u for 50 ohm test leads. The stranded center wire is
>>> more flexible than the rather stiff RG-58a/u.
>>>
outer covering difference.
Greg
Jeff Liebermann
Mar 16, 2017, 1:55:41 PM3/16/17
to
On Thu, 16 Mar 2017 08:19:39 -0000 (UTC), gregz <ze...@comcast.net>
wrote:
>Jeff Liebermann <je...@cruzio.com> wrote:
>>>> I use RG-58c/u for 50 ohm test leads. The stranded center wire is
>>>> more flexible than the rather stiff RG-58a/u.
>A/U is stranded center too. Rg58 is solid. Au cu difference seems to be
>outer covering difference.
>Greg
Oops, you're right. I stand corrected:
RG-58a/u (Belden 8259)
<http://www.belden.com/techdatas/metric/8259.pdf>
PVC jacket
UL temp temp rating = 75C
Attenuation at 1GHz = 70.5dB
RG-58c/u (Belden 8262)
<http://www.belden.com/techdatas/metric/8262.pdf>
Non-contaminating PVC jacket
UL temp temp rating = 85C
Attenuation at 1GHz = 74.8dB
So, different jackets, higher temperature rating, and less loss for
RG-58c/u. Also, RG-58c/u appears to have a military designation
(MIL-C-17, M17/155-00001), while RG-58a/u does not.
More:
<https://abrind.com/faq/>
Q: I see different jackets types advertised, what are
the differences, and why I should care?
A:Jacket Types:
PVC-I Polyvinyl Chloride (PVC), black (contaminating).
Contaminating jackets will breakdown from sunlight (Ultra-Violet
Resistant), and cannot be buried. The PVC-I is rated with a
shorter life cycle than PVC-IIA. Suitable for low grade jumpers.
PVC-IIA Polyvinyl Chloride (PVC), black (non-contaminating).
Non-Contaminating jackets will not breakdown from sunlight
(Ultra-Violet Resistant), and suitable for direct burial
without conduit. The PVC-IIA is rated with a twenty-year
life cycle. The best all-around jacket.
(etc...)
wrote:
>Jeff Liebermann <je...@cruzio.com> wrote:
>>>> I use RG-58c/u for 50 ohm test leads. The stranded center wire is
>>>> more flexible than the rather stiff RG-58a/u.
>A/U is stranded center too. Rg58 is solid. Au cu difference seems to be
>outer covering difference.
>Greg
RG-58a/u (Belden 8259)
<http://www.belden.com/techdatas/metric/8259.pdf>
PVC jacket
UL temp temp rating = 75C
Attenuation at 1GHz = 70.5dB
RG-58c/u (Belden 8262)
<http://www.belden.com/techdatas/metric/8262.pdf>
Non-contaminating PVC jacket
UL temp temp rating = 85C
Attenuation at 1GHz = 74.8dB
So, different jackets, higher temperature rating, and less loss for
RG-58c/u. Also, RG-58c/u appears to have a military designation
(MIL-C-17, M17/155-00001), while RG-58a/u does not.
More:
<https://abrind.com/faq/>
Q: I see different jackets types advertised, what are
the differences, and why I should care?
A:Jacket Types:
PVC-I Polyvinyl Chloride (PVC), black (contaminating).
Contaminating jackets will breakdown from sunlight (Ultra-Violet
Resistant), and cannot be buried. The PVC-I is rated with a
shorter life cycle than PVC-IIA. Suitable for low grade jumpers.
PVC-IIA Polyvinyl Chloride (PVC), black (non-contaminating).
Non-Contaminating jackets will not breakdown from sunlight
(Ultra-Violet Resistant), and suitable for direct burial
without conduit. The PVC-IIA is rated with a twenty-year
life cycle. The best all-around jacket.
(etc...)
Jeff Liebermann
Mar 16, 2017, 2:19:25 PM3/16/17
to
On Thu, 16 Mar 2017 01:58:16 -0400, "Michael A. Terrell"
<mike.t...@earthlink.net> wrote:
>Jeff Liebermann wrote:
>> Nope. Most of my test equipment is 50 ohms. However, I also work
>> with 75 ohm devices and antennas. Transformers do not have a very
>> flat frequency response. If I need a flat frequency response from DC
>> to many GHz, I use a minimum loss pad:
>> <https://www.maximintegrated.com/en/app-notes/index.mvp/id/972>
>> (...)
> I have used some of the six hole ferrite beads to make transformers.
>six turns, tapped at the fifth turn. The 1.2 turns ratio, squared is a
>1.44:1 impedance ratio. These are the same beads used for CATV line taps
>and broadband splitters.
>
> They convert 75 ohms to 52 ohms. I put some into some old Blonder
>Tongue inline attenuator housings with an F connect on the 75 ohm end
>and a BNC connector on the 52 ohm end. (52.0833 Ohms) They were handy to
>use a TV FSM as a 50 ohm RF millivoltmeter. Now, I have some precision
>attenuators, and a Boonton 9200 RF Millivoltmeter ans different
>feedthrough terminators.. :)
I initially did the same thing using various iron and ferrite toroid
cores. They worked, but not if I wanted a really flat frequency
response (+/-0.5dB) over many octaves of frequency range. I had a few
adapters that I optimized for frequencies of interest, but getting it
fairly flat from 1.5 to 30 MHz (marine bands at about 4.5 octaves) was
rather difficult. Instead of fighting the problem, I switched to a
minimum loss pad. The low frequency pads ended up inside Pomona
aluminum boxes, while the microwave stuff was on microstrip stuffed
into a box made from brass or unetched PCB material.
Actually, it wasn't quite a minimum loss pad. I worked out the
numbers for a 50 to 75 ohm -6.0dB pad instead of -5.71dB, which made
power and voltage scaling, measurements, and calculations easier.
One problem with minimum loss pads. They develop feet and walk away.
I wasted about a day of company time building and characterizing a
collection of about 20 attenuators for my own use. Within about 8
weeks, they were all gone, probably "borrowed" by my co-workers.
Hint: There's no such thing as a precision attenuator in a lab that
works with transmitters. Eventually, they all get cooked. The best I
could do was characterize what was left of the attenuator after
someone accidentally transmit into it, and compensate for the changes
in loss and impedance.
<mike.t...@earthlink.net> wrote:
>Jeff Liebermann wrote:
>> Nope. Most of my test equipment is 50 ohms. However, I also work
>> with 75 ohm devices and antennas. Transformers do not have a very
>> flat frequency response. If I need a flat frequency response from DC
>> to many GHz, I use a minimum loss pad:
>> <https://www.maximintegrated.com/en/app-notes/index.mvp/id/972>
> I have used some of the six hole ferrite beads to make transformers.
>six turns, tapped at the fifth turn. The 1.2 turns ratio, squared is a
>1.44:1 impedance ratio. These are the same beads used for CATV line taps
>and broadband splitters.
>
> They convert 75 ohms to 52 ohms. I put some into some old Blonder
>Tongue inline attenuator housings with an F connect on the 75 ohm end
>and a BNC connector on the 52 ohm end. (52.0833 Ohms) They were handy to
>use a TV FSM as a 50 ohm RF millivoltmeter. Now, I have some precision
>attenuators, and a Boonton 9200 RF Millivoltmeter ans different
>feedthrough terminators.. :)
cores. They worked, but not if I wanted a really flat frequency
response (+/-0.5dB) over many octaves of frequency range. I had a few
adapters that I optimized for frequencies of interest, but getting it
fairly flat from 1.5 to 30 MHz (marine bands at about 4.5 octaves) was
rather difficult. Instead of fighting the problem, I switched to a
minimum loss pad. The low frequency pads ended up inside Pomona
aluminum boxes, while the microwave stuff was on microstrip stuffed
into a box made from brass or unetched PCB material.
Actually, it wasn't quite a minimum loss pad. I worked out the
numbers for a 50 to 75 ohm -6.0dB pad instead of -5.71dB, which made
power and voltage scaling, measurements, and calculations easier.
One problem with minimum loss pads. They develop feet and walk away.
I wasted about a day of company time building and characterizing a
collection of about 20 attenuators for my own use. Within about 8
weeks, they were all gone, probably "borrowed" by my co-workers.
Hint: There's no such thing as a precision attenuator in a lab that
works with transmitters. Eventually, they all get cooked. The best I
could do was characterize what was left of the attenuator after
someone accidentally transmit into it, and compensate for the changes
in loss and impedance.
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