[TowerTalk] RG400 Measured Data

[Jim Brown]

One that's easy to make is TDR using a device with high time resolution. 
The DG8SAQ VNWA3e implements TDR with an inverse FFT of a sweep, and can 
sweep to 500 MHz with full dynamic range and to 1.3 GHz with reduced 
dynamic range. It's done by post processing, and five different 
windowing functions can be selected.  I've done that fairly often with 
cables whose life history or pedigree I don't know, and with new spools 
that I buy. With most spools that I've bought I can access the inside 
end to provide a short, and in rare cases to attach a connector (I did 
that with a spool of flooded Commscope RG6 once).

On 3/16/2018 12:58 PM, jimlux wrote:
> Is there some "doable by a ham with simple test equipment (which today 
> means an inexpensive 1/2 port VNA)" test that can distinguish between 
> "good" and "less good" coax?
>
> Preferably with a one port measurement (so you don't have to have 
> access to the inside of the spool or other end of the cable). 

> The CATV business has measurements and criteria that attempt to 
> characterize shielding effectiveness (not in an idealized modeled 
> sense, but as in "what you measure in a standard test fixture"), and 
> I'm sure there's some way to measure IMD from cable/connectors.

  There's another descriptor for shield effectiveness called the 
transfer impedance, which is the ratio of differential voltage induced 
in the cable by common mode current on the shield. I first learned of 
this from UK colleagues on the AES Standards Committee working group on 
EMC. Obviously, a small value is better. Henry Ott notes that the lower 
limit is the resistance of the shield the frequency of interest. Shield 
density and uniformity also contribute.

73, Jim K9YC