Changes in electrical length of bent coax cables

[Cursitor Doom]

Gentlemen,

Is it true that the electrical length of a coax patch lead changes if
it's bent through a radius in order to interconnect two different
pieces of test equipment and if so, which kind of coax is least
susceptible to the phase/mag effects of bending? I'm specifically
interested in frequencies of around 1Ghz.
Thanks.

-CD

 

[Jeroen Belleman]

On 2020-03-01 19:20, Cursitor Doom wrote:

Gentlemen,

Is it true that the electrical length of a coax patch lead changes if
it's bent through a radius in order to interconnect two different
pieces of test equipment and if so, which kind of coax is least
susceptible to the phase/mag effects of bending? I'm specifically
interested in frequencies of around 1Ghz.
Thanks.

-CD

Good quality cables do not change length so much when bent.
Cheap cables may. You can buy cables specified for phase
stability, the better, the more expensive. How much change
can you tolerate? For what angle and bending radius? Once or
repeatedly?

On second thought, I wonder if you are equipped to even notice
any change. Do you have a VNA?

Jeroen Belleman

 

[Phil Hobbs]

On 2020-03-01 13:38, Jeroen Belleman wrote:

On 2020-03-01 19:20, Cursitor Doom wrote:
Gentlemen,

Is it true that the electrical length of a coax patch lead changes if
it's bent through a radius in order to interconnect two different
pieces of test equipment and if so, which kind of coax is least
susceptible to the phase/mag effects of bending? I'm specifically
interested in frequencies of around 1Ghz.
Thanks.

-CD


Good quality cables do not change length so much when bent.
Cheap cables may. You can buy cables specified for phase
stability, the better, the more expensive. How much change
can you tolerate? For what angle and bending radius? Once or
repeatedly?

On second thought, I wonder if you are equipped to even notice
any change. Do you have a VNA?

Jeroen Belleman

Ordinary RG-58 is fairly bad for this. BITD when I was a grad student,
I built a 12-bit amplitude / 13-bit phase digitizer that worked at 60
MHz. The phase LSB was 16.7 ns/8192, or 2.0 ps. Jiggling a cable even
a little bit was enough to make many, many LSBs of phase perturbation.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote in
news:3e6298e9-b492-2361-221b-ab5a02403cc8@electrooptical.net:

On 2020-03-01 13:38, Jeroen Belleman wrote:
On 2020-03-01 19:20, Cursitor Doom wrote:
Gentlemen,

Is it true that the electrical length of a coax patch lead
changes if it's bent through a radius in order to interconnect
two different pieces of test equipment and if so, which kind of
coax is least susceptible to the phase/mag effects of bending?
I'm specifically interested in frequencies of around 1Ghz.
Thanks.

-CD


Good quality cables do not change length so much when bent.
Cheap cables may. You can buy cables specified for phase
stability, the better, the more expensive. How much change
can you tolerate? For what angle and bending radius? Once or
repeatedly?

On second thought, I wonder if you are equipped to even notice
any change. Do you have a VNA?

Jeroen Belleman



Ordinary RG-58 is fairly bad for this. BITD when I was a grad
student, I built a 12-bit amplitude / 13-bit phase digitizer that
worked at 60 MHz. The phase LSB was 16.7 ns/8192, or 2.0 ps.
Jiggling a cable even a little bit was enough to make many, many
LSBs of phase perturbation.

Cheers

Phil Hobbs

Have the guy use semi-rigid? Shape only once. Try a few routed
(shaped)differently. SEE if there is a SIGNificant difference for
your application.

 

[Cursitor Doom]

On Sun, 01 Mar 2020 19:38:06 +0100, Jeroen Belleman
<jeroen@nospam.please> wrote:

Good quality cables do not change length so much when bent.
Cheap cables may. You can buy cables specified for phase
stability, the better, the more expensive. How much change
can you tolerate? For what angle and bending radius? Once or
repeatedly?

If it was a one-off bend, I'd use rigid metal coaxial tx line. But I
need some flexibility on a repeated basis. Not outrageously so, but
within reason.

On second thought, I wonder if you are equipped to even notice
any change. Do you have a VNA?

Yes, I do. And yes I do.

Interconnects for VNAs tend to be made from very thick coax. I'm
currently using RG 213 but I've also made patch leads up from RG400
and some other thick mil-spec coax I can't recall the RG number of
off-hand.
So basically, I'm guessing the thick high quality brand-name cables
with N-types or TNCs are the way to go. And the more expensive the
better, no doubt!
But let's not stray off the subject here: electrical length changes
due to bending: fact or myth?

 

[Jeff Liebermann]

On Sun, 01 Mar 2020 18:20:37 +0000, Cursitor Doom
<curd@notformail.com> wrote:

Is it true that the electrical length of a coax patch lead changes if
it's bent through a radius in order to interconnect two different
pieces of test equipment and if so, which kind of coax is least
susceptible to the phase/mag effects of bending? I'm specifically
interested in frequencies of around 1Ghz.

Short answer: It depends on everything that you didn't mention such
as type of coax, type of connector, phase tolerance required, losses
tolerated, mechanical construction, temperature, etc.

Cheap solution: Semi-rigid coax such as:
<https://www.pasternack.com/t-Semi-Rigid-Cables.aspx> See item #4.
<https://www.timesmicrowave.com/DataSheets/Literature/Current%20innovations%20in%20phase%20stable%20coaxial%20cable.pdf>

Convince yourself: Buy or build a TDR (time domain reflectometer)
<https://www.ebay.com/sch/i.html?_nkw=time+domain+reflectometer>
<https://www.ebay.com/itm/152587007700>
and test whatever cable you find in your junk bin. You probably wont
see any changes unless your connectors are falling apart. However, if
you want something to worry about, try measuring the effective length
of the cable at varying temperatures.
<https://www.everythingrf.com/community/what-is-the-ptfe-knee>
You can also use a 1GHz scope and a Lissajous pattern or a DBM (double
balance mixer) phase detector, to compare the phase stability of your
patch cable against a know good and stiff reference cable. These type
of phase meters not very sensitive but will be sufficient to
demonstrate that braided cable and foam dielectrics are not very phase
stable.



--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

 

[Cursitor Doom]

On Sun, 01 Mar 2020 12:14:56 -0800, Jeff Liebermann <jeffl@cruzio.com>
wrote:

Short answer: It depends on everything that you didn't mention such
as type of coax, type of connector, phase tolerance required, losses
tolerated, mechanical construction, temperature, etc.

I purposely wanted to avoid specifics to keep it general. The core
question I'm trying not to get diverteed from is: does bending coax
change its electrical length and (if so) are certain types of coax
better than others in minimising/eliminating this issue?

I'll check out the links you kindly provided, Jeffrey. They're
probably way OTT for my purposes, but we'll see. A 1Ghz scope? TDR? I
wish! :-D

 

[Jeroen Belleman]

On 2020-03-01 20:26, Cursitor Doom wrote:

On Sun, 01 Mar 2020 19:38:06 +0100, Jeroen Belleman
jeroen@nospam.please> wrote:

Good quality cables do not change length so much when bent.
Cheap cables may. You can buy cables specified for phase
stability, the better, the more expensive. How much change
can you tolerate? For what angle and bending radius? Once or
repeatedly?

If it was a one-off bend, I'd use rigid metal coaxial tx line. But I
need some flexibility on a repeated basis. Not outrageously so, but
within reason.

On second thought, I wonder if you are equipped to even notice
any change. Do you have a VNA?

Yes, I do. And yes I do.

Interconnects for VNAs tend to be made from very thick coax. I'm
currently using RG 213 but I've also made patch leads up from RG400
and some other thick mil-spec coax I can't recall the RG number of
off-hand.
So basically, I'm guessing the thick high quality brand-name cables
with N-types or TNCs are the way to go. And the more expensive the
better, no doubt!
But let's not stray off the subject here: electrical length changes
due to bending: fact or myth?

Why fact, of course. Phase-stable cables are _designed_
to be phase-stable. They need to have the right materials
and the right construction and most often an extra layer
of protection. Why bother if it came for free?

I wouldn't trust N or TNC connectors to give repeatable
phase values. Properly torqued SMA should be much better,
but APC connectors are the real thing. I find that even
with SMA connectors, the phase vs. frequency deviations
exceed the phase variations due to bends.

If you have a VNA, it should be easy enough to get some
measurements of random cables, just to get an idea. TDRs,
unless with single-digit ps resolution, aren't the best
way to go about this. I get much better results from my
6GHz HP8753 VNA than from my 10GHz Tektronix TDR setup.

Again, what change can you tolerate?

Jeroen Belleman

 

[Gerhard Hoffmann]

Am 01.03.20 um 20:22 schrieb DecadentLinuxUserNumeroUno@decadence.org:

Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote in


Ordinary RG-58 is fairly bad for this. BITD when I was a grad
student, I built a 12-bit amplitude / 13-bit phase digitizer that
worked at 60 MHz. The phase LSB was 16.7 ns/8192, or 2.0 ps.
Jiggling a cable even a little bit was enough to make many, many
LSBs of phase perturbation.

Cheers

Phil Hobbs


Have the guy use semi-rigid? Shape only once. Try a few routed
(shaped)differently. SEE if there is a SIGNificant difference for
your application.

I once measured the phase stability of a distribution amplifier
with a 5475?A 50 GHz / 9ps scope plug in and 3.5 mm semi rigid cable.
(Nexans Quickform, which is no real copper tubing outside but some
dense mesh soaked with tin, for short runs)

When I laid my wrist on the cable, I could clearly see my heart
beat on the x-position.

Gerhard

 

[Cursitor Doom]

On Sun, 01 Mar 2020 23:40:12 +0100, Jeroen Belleman
<jeroen@nospam.please> wrote:

On 2020-03-01 20:26, Cursitor Doom wrote:
On Sun, 01 Mar 2020 19:38:06 +0100, Jeroen Belleman
jeroen@nospam.please> wrote:

Good quality cables do not change length so much when bent.
Cheap cables may. You can buy cables specified for phase
stability, the better, the more expensive. How much change
can you tolerate? For what angle and bending radius? Once or
repeatedly?

If it was a one-off bend, I'd use rigid metal coaxial tx line. But I
need some flexibility on a repeated basis. Not outrageously so, but
within reason.

On second thought, I wonder if you are equipped to even notice
any change. Do you have a VNA?

Yes, I do. And yes I do.

Interconnects for VNAs tend to be made from very thick coax. I'm
currently using RG 213 but I've also made patch leads up from RG400
and some other thick mil-spec coax I can't recall the RG number of
off-hand.
So basically, I'm guessing the thick high quality brand-name cables
with N-types or TNCs are the way to go. And the more expensive the
better, no doubt!
But let's not stray off the subject here: electrical length changes
due to bending: fact or myth?


Why fact, of course. Phase-stable cables are _designed_
to be phase-stable. They need to have the right materials
and the right construction and most often an extra layer
of protection. Why bother if it came for free?

I wouldn't trust N or TNC connectors to give repeatable
phase values. Properly torqued SMA should be much better,
but APC connectors are the real thing. I find that even
with SMA connectors, the phase vs. frequency deviations
exceed the phase variations due to bends.

If you have a VNA, it should be easy enough to get some
measurements of random cables, just to get an idea. TDRs,
unless with single-digit ps resolution, aren't the best
way to go about this. I get much better results from my
6GHz HP8753 VNA than from my 10GHz Tektronix TDR setup.

Again, what change can you tolerate?

Sigh... Let's say 2 degrees or better at 1300Mhz.

 

[Robert Baer]

Cursitor Doom wrote:

Gentlemen,

Is it true that the electrical length of a coax patch lead changes if
it's bent through a radius in order to interconnect two different
pieces of test equipment and if so, which kind of coax is least
susceptible to the phase/mag effects of bending? I'm specifically
interested in frequencies of around 1Ghz.
Thanks.

-CD

The rule i was given over 60 years ago was to have the bend radius
larger than ten times the cable diameter.

 

[boB]

On Sun, 01 Mar 2020 21:36:58 +0000, Cursitor Doom
<curd@notformail.com> wrote:

On Sun, 01 Mar 2020 12:14:56 -0800, Jeff Liebermann <jeffl@cruzio.com
wrote:

Short answer: It depends on everything that you didn't mention such
as type of coax, type of connector, phase tolerance required, losses
tolerated, mechanical construction, temperature, etc.

I purposely wanted to avoid specifics to keep it general. The core
question I'm trying not to get diverteed from is: does bending coax
change its electrical length and (if so) are certain types of coax
better than others in minimising/eliminating this issue?

I'll check out the links you kindly provided, Jeffrey. They're
probably way OTT for my purposes, but we'll see. A 1Ghz scope? TDR? I
wish! :-D

I would think that the more you bend the coax, the more is is like a
discontinuity or pinching of the coax which would cause the impedance
to change at that pinch point. That point will cause signal
refletions so might not be catastrophic for the application but is not
wanted. I'm thinking an extreme bending here but if not rigid coax,
might be enough to cause a problem in certain cases

 

[Jeroen Belleman]

Cursitor Doom wrote:

On Sun, 01 Mar 2020 23:40:12 +0100, Jeroen Belleman
jeroen@nospam.please> wrote:

On 2020-03-01 20:26, Cursitor Doom wrote:
On Sun, 01 Mar 2020 19:38:06 +0100, Jeroen Belleman
jeroen@nospam.please> wrote:

Good quality cables do not change length so much when bent.
Cheap cables may. You can buy cables specified for phase
stability, the better, the more expensive. How much change
can you tolerate? For what angle and bending radius? Once or
repeatedly?
If it was a one-off bend, I'd use rigid metal coaxial tx line. But I
need some flexibility on a repeated basis. Not outrageously so, but
within reason.

On second thought, I wonder if you are equipped to even notice
any change. Do you have a VNA?
Yes, I do. And yes I do.

Interconnects for VNAs tend to be made from very thick coax. I'm
currently using RG 213 but I've also made patch leads up from RG400
and some other thick mil-spec coax I can't recall the RG number of
off-hand.
So basically, I'm guessing the thick high quality brand-name cables
with N-types or TNCs are the way to go. And the more expensive the
better, no doubt!
But let's not stray off the subject here: electrical length changes
due to bending: fact or myth?

Why fact, of course. Phase-stable cables are _designed_
to be phase-stable. They need to have the right materials
and the right construction and most often an extra layer
of protection. Why bother if it came for free?

I wouldn't trust N or TNC connectors to give repeatable
phase values. Properly torqued SMA should be much better,
but APC connectors are the real thing. I find that even
with SMA connectors, the phase vs. frequency deviations
exceed the phase variations due to bends.

If you have a VNA, it should be easy enough to get some
measurements of random cables, just to get an idea. TDRs,
unless with single-digit ps resolution, aren't the best
way to go about this. I get much better results from my
6GHz HP8753 VNA than from my 10GHz Tektronix TDR setup.

Again, what change can you tolerate?

Sigh... Let's say 2 degrees or better at 1300Mhz.

OK. I tried a 3ns piece of Huber+Suhner GX-03272 cable,
much like RG58. It changed several degrees @ 1.3GHz when
bent through 180 degrees with a radius of 5cm. There's no
point in being more accurate. It's never the same twice
anyway. This cable is not good enough to guarantee less
than 2 degrees at 1.3GHz.

It's a fact: Electrical length changes when a cable is
bent.

Jeroen Belleman

 

[Jeff Liebermann]

On Sun, 01 Mar 2020 21:36:58 +0000, Cursitor Doom
<curd@notformail.com> wrote:

On Sun, 01 Mar 2020 12:14:56 -0800, Jeff Liebermann <jeffl@cruzio.com
wrote:

Short answer: It depends on everything that you didn't mention such
as type of coax, type of connector, phase tolerance required, losses
tolerated, mechanical construction, temperature, etc.

I purposely wanted to avoid specifics to keep it general.

Sure. If it's not specific, it must therefore be general. Perhaps
you meant vague or lacking in sufficient information to answer your
non-specific question? For example, the phase change in degrees
increases with the frequency of operation. A minor change in cable
length due to temperature, bending, or mechanical stress is
insignificant at 10MHz but could be a seriously huge at 1GHz and
above.

The core
question I'm trying not to get diverteed from is: does bending coax
change its electrical length

Yes, but with your probably quite short patch cable, the changes will
probably be measured in picoseconds. See section on "phase
stability":
<https://www.highfrequencyelectronics.com/index.php?view=article&id=1809>

and (if so) are certain types of coax
better than others in minimising/eliminating this issue?

Yes. Semi rigid coax cable with Telfon or silicon dioxide dielectric
are known for their stable dielectric constant. Copper, some copper
alloy are used for the center conductor and shielding. The connector
is soldered to the coax for maximum mechanical rigidity. See:
Understanding Phase Stability in RF Test Cables
<https://www.minicircuits.com/app/AN46-004.pdf>
Don't forget about temperature stability (although you probably won't
like the price tag):
<https://www.minicircuits.com/WebStore/dashboard.html?model=TMP40-3FT-KMKM%2B>

I'll check out the links you kindly provided, Jeffrey. They're
probably way OTT for my purposes, but we'll see.

What's OTT mean?

>A 1Ghz scope? TDR? I wish! :-D

Sorry. You're only allowed one wish.

I provided an eBay link to a $15 TDR that I suspect even you can
afford to purchase. I have 2 or 3 of them (one accidentally stolen
from a client). I've built a few others with somewhat better
performance. They're not the best performers and you won't be looking
at picosecond pulses or GHz responses with one, but such a simple TDR
and a common 100 MHz oscilloscope might possibly be suitable for your
unspecified application.

Incidentally, your phase and/or amplitude stability problem might be
the result of triboelectric effects caused by flexing and compressing
the dielectric.
<https://www.google.com/search?q=coaxial+cable+tribolectric>


--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

 

[Cursitor Doom]

On Sun, 01 Mar 2020 23:34:02 -0800, Jeff Liebermann <jeffl@cruzio.com>
wrote:

I provided an eBay link to a $15 TDR that I suspect even you can
afford to purchase. I have 2 or 3 of them (one accidentally stolen
from a client). I've built a few others with somewhat better
performance. They're not the best performers and you won't be looking
at picosecond pulses or GHz responses with one, but such a simple TDR
and a common 100 MHz oscilloscope might possibly be suitable for your
unspecified application.

Jeff, all it arose from was an argument with a chum of mine in the pub
the other evening. He said "bending coax changes its electrical
length" which I felt was a blanket statement worthy of deeper
investigation; that's all. It had *nothing* to do with any real world
situation or problem!!

 

[Cursitor Doom]

On Mon, 02 Mar 2020 08:35:03 +0100, Jeroen Belleman
<jeroen@nospam.please> wrote:

It's a fact: Electrical length changes when a cable is
bent.

There we go, Jeroen, that didn't hurt much did it?

 

[Tom Gardner]

It would have been courteous to everybody else if you had
stated that in your post that started the thread.

As it is you have encouraged several people to waste their
precious time giving you detailed answers.



On 02/03/20 10:20, Cursitor Doom wrote:

On Sun, 01 Mar 2020 23:34:02 -0800, Jeff Liebermann <jeffl@cruzio.com
wrote:

I provided an eBay link to a $15 TDR that I suspect even you can
afford to purchase. I have 2 or 3 of them (one accidentally stolen
from a client). I've built a few others with somewhat better
performance. They're not the best performers and you won't be looking
at picosecond pulses or GHz responses with one, but such a simple TDR
and a common 100 MHz oscilloscope might possibly be suitable for your
unspecified application.

Jeff, all it arose from was an argument with a chum of mine in the pub
the other evening. He said "bending coax changes its electrical
length" which I felt was a blanket statement worthy of deeper
investigation; that's all. It had *nothing* to do with any real world
situation or problem!!

 

[Jeroen Belleman]

Cursitor Doom wrote:

On Mon, 02 Mar 2020 08:35:03 +0100, Jeroen Belleman
jeroen@nospam.please> wrote:

It's a fact: Electrical length changes when a cable is
bent.

There we go, Jeroen, that didn't hurt much did it?

No, not much. I did have to say it twice though.

I have a few applications where this matters. I make
beam position measurement instruments for accelerators.
This involves taking the difference of RF signals from
pairs of electrodes exposed to the field of the passing
bunched beam. If the phase match is off, it looks as if
the beam wiggles, which is obviously not good.

Cheers,
Jeroen Belleman

 

[Jeff Liebermann]

On Mon, 02 Mar 2020 10:20:58 +0000, Cursitor Doom
<curd@notformail.com> wrote:

On Sun, 01 Mar 2020 23:34:02 -0800, Jeff Liebermann <jeffl@cruzio.com
wrote:
Jeff, all it arose from was an argument with a chum of mine in the pub
the other evening. He said "bending coax changes its electrical
length" which I felt was a blanket statement worthy of deeper
investigation; that's all. It had *nothing* to do with any real world
situation or problem!!

If you made a bet with your drinking accomplice, what is my share of
your winnings for providing you with answers, entertainment, and
insults? In the future, could you please be more specific as to what
you plan to do with the gathered information? That should help me
decide if it's worth my time trying to answer your question. It
should also help distinguish between whether you are trying to solve a
problem or create one.

I just realized that your question, "bending coax changes its
electrical length" is easily misinterpreted. Changing the electrical
length of the coax, without changing its physical length, implies a
change in velocity factor. Bending the coax doesn't do that as long
as the dielectric remains intact. Please send my share to the address
below.


--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

 

[sdy]

On Sunday, March 1, 2020 at 1:20:46 PM UTC-5, Cursitor Doom wrote:

Gentlemen,

Is it true that the electrical length of a coax patch lead changes if
it's bent through a radius in order to interconnect two different
pieces of test equipment and if so, which kind of coax is least
susceptible to the phase/mag effects of bending? I'm specifically
interested in frequencies of around 1Ghz.
Thanks.

-CD

Yes it does. I measure the electrical length of certain assemblies on a regular basis. Bend a coax and it will change a few degrees. I use phase stable cables, about $600 each.