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
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