A coax cable for extremely low temperatures

[Andrey_irk]

Hello!
I am looking for a 0.086'' semi-rigid or rigid coax cable which will be mounted in a box filled with liquid nitrogen. So, it must withstand temperature as low as ~ -200C. It will be mounted in a fixed position, but the temperature can rise up to the room temperature.
The signal will be low power and 9 to 10 GHz. And I don't have special requirements for losses, phase stability etc. It doesn't have to be of a very low VSWR either.
Usually the lower operating temperature in the specs  is for the case where the cable can be bent and moved, which is not my case.

Does anybody know of such cable? Or does someone have experience in using coaxes in such environment?

[Kire Pûdsje]

Normal PTFE semi rigid will work. I did not experience any problems with them in an R&D setting.
Don't know for a production environment what the reliability is. Probably the biggest problem is the thermal expansion of the outer shield and the connectors.
If you have the money you could always buy these special cryogenic cables.

[Andrey_irk]

Kire Pûdsje,
How many cycles of freezing do your cables usually survive?

[sixtimesseven]

I would recommend you contact: http://www.gigalink-mce.net/RF_Products_ULL.html
I know that they have a non-catalog silicone hull coax cable for ultra low, dynamicly stressed cables.

[Hawke]

 I have about 60 metres of a similar cable on my roof. It was described as a hardline coax/heliax and was developed for use on the space shuttle and things in space I presume. It has an aluminium outer core with some type of UV resistant black PVC over it. The core inside of the aluminium has a dielectric space for Nitrogen and the centre OFC copper sits inside the Teflon looking core. The aluminium core is like a 2mm walled pipe. It came with connectors already attached and a valve for filling it with Nitrogen. It has NASA stamped on it as well as a type number etc. Its 50 to 52 ohm from memory I use it for UHF. From memory again the insertion loss was incredibly low per 100 metres it was in the range of .005% over the 100 metres. I bought it at a surplus sale in Australia from a seller that brought tons of cable over from the states. Its not very flexible due to the aluminium tubing inside of it. I.m going up on my roof in the next few days to do some antenna work and if the numbers have survived anywhere I will pass them on to you. Its been up there since 1993 so the white stamping may have faded. I'm pretty sure you would be able to find this type easily now. It was rare in 1993 but probably not so much now. Its great stuff I can honestly see it being up there another 25 years. It may or may not be the type you  are after but thought I would mention it just in case. Cheers 73's

[Kire Pûdsje]

As for the number of cycles, in the order of 50 times. limited by the experiments, never seen the semi-rigid failing. (Hence the word probably)
As for Gigalink, mentioned by sixtimesseven, they indeed have some very nice cable. A representative showed me some nice silicone samples, that were supposed to stay flexible at 77K. Never used them.
BTW in general loss does not matter as much at these temperatures, as the added noise due to the loss is only about a quarter of what is would be at room temperature assuming the loss is the same at both temperatures (Bosma's theorem).

[Andrey_irk]

Thank you all for replying!

The suggestion of using an ordinary PTFE cable looks promising. Although it is always better to have an assurance from a manufacturer that it won't fail, in this case it won't be cheap. Something to think about.

Actually, I've never heard of the Gigalink, but I'll definitely contact them.

Hawke,
It would be interesting to have a look at this cable.