Re: OT: High Frequency coax?? Copper tubing??

Reginald Beardsley
 

Ian,

I put together some quick examples for you as I have everything sitting next to me. Have a look at this:

https://www.eevblog.com/forum/rf-microwave/testing-rf-connectors-and-cables/msg2768098/#msg2768098

The Tek 11801/SD-24/SD-26 setup I used has a frequency response to 20 GHz. In the top photo you're seeing the reflection responses for the 3 open cables. The SD-24 produces a <19 ps rise time 100 kHz square wave. The low frequency of the square wave allows testing very long cables. The fast rise time allows testing to 20 GHz.

The lower scope photo shows both the reflection and the thru. Reflection is the shorter step delay and thru the longer step delay.

My first encounter with SMA connectors was an HP 8601A. Because they are small, cheap and have lots of BW they have become the default choice for lots of Chinese modules on ebay.

If you look through the thread you'll see various adapter and connector combinations. The wider the reflection bumps, the lower the frequency that unit can pass. The SMA-F/N-F connector comparisons shows that my ebay Chinese versions are only good up to 1 GHz. But both my rigs only go up to the 70 cm ham band at 430 MHz, so they will do just fine for antenna switching using Radiall ST6P SMA relays.

A bump downwards is capacitive reactance and a bump upwards is inductive reactance. The width of the bump shows the approximate cutoff frequency. At best BNC is usable to 3-4 GHz, but only if they are very good quality and are fairly new. The marvelous thing about TDR, is that every discontinuity appears in sequence from left to right. So you can tell if it's the cable to connector or the connector to connector interface.

Cycle limit on SMA connectors is typically 500 connections. Pretty much all test gear has used SMA internally for connections to the exterior bulkhead connectors for many years. It makes replacing worn bulkhead connectors easy. The exception would be things that go too high to use SMA. At which point the connectors get *very* expensive.

Certainly in my view the question is appropriate here. It's a really fine group of people most of whom also follow the HPAK list. I just wish I could fish an R&S SMS out of the trash. :-)

BTW There is an R&S group in the process of moving from yahoo to groups.io.

Have Fun!
Reg

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