Changes in electrical length of bent coax cables

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

Phil Hobbs wrote in news: snipped-for-privacy@electrooptical.net:

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.

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.

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?

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: See item #4.

Convince yourself: Buy or build a TDR (time domain reflectometer) 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. 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.

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

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

Again, what change can you tolerate?

Jeroen Belleman

Am 01.03.20 um 20:22 schrieb snipped-for-privacy@decadence.org:

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

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

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

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

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.

Yes, but with your probably quite short patch cable, the changes will probably be measured in picoseconds. See section on "phase stability":

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 Don't forget about temperature stability (although you probably won't like the price tag):

What's OTT mean?

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.

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

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

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!!

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.

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

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.

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.