[TowerTalk] Measuring Coax Loss

[Richard (Rick) Karlquist]

Gary Schafer wrote:
> Hi Rick,
> 
> I don't think that I have used the method that you describe. What happens
> when the cable length approaches a quarter wave length? Or is this only
> valid at a small fraction of a wave length?
> 
> 73
> Gary  K4FMX

This is intended for low frequencies where the coax is much less than
a quarter wave.

If you can measure a quarter wave of coax, then the thing to do is
measure the Q of it as a resonator, from which the loss follows.
The Q can be measured very accurately by various techniques.
Which one is best depends on various factors.  Hard to give
a one size fits all answer.

I recently measured the loss of 10 feet of 7/8" hardline that I
got free by measuring the Q of the 1/4 wave resonance with my
HP 4815 vector impedance meter.  Measured the frequency difference
between -45 degrees and +45 degrees and divide into the center
frequency.  This measurement also gives the velocity factor.
I needed that so I could determine how long my 3000 foot piece
was without unrolling it, hi!

Rick N6RK