[TowerTalk] Coax Losses on 160 and 75?

[Richard (Rick) Karlquist]

On 8/5/2016 10:00 PM, Jim Thomson wrote:

> ##  I like the wattmeter method, then I know it’s the ....real deal.  Just install a 50 ohm dummy load on the
> far end..just after the  2nd wattmeter.   Then you can test from 160- UHF.   Then you get real results.

No, this isn't the real deal.  A 2 port vector network analyzer
with full 2 port calibration is the real deal.  The wattmeter
approach has many error effects that are difficult to account
for.  The fact that the meters read identical when cascaded
does not mean the subsequent measurement is perfect.
The dummy load may have a mismatch.  The characteristic
impedance of the line is not exactly 50 ohms and is actually
complex in general.  Etc.

For additional reading on the pitfalls of wattmeters, see:

https://www.fars.k6ya.org/docs/K6OIK-A_Transmission_Line_Power_Paradox_and_Its_Resolution.pdf

Rick N6RK


> This assumes the pair of wattmeters are calibrated for the  freqS  to be tested.   They dont even have to be
> calibrated, they just have to read identical...when both are wired nose to tail, in series, at the xcvr /amp.
> I wire two in series 1st.....then test to ss how close they are  across  XXX bands /freqs.    Ok, then remove one of em
> and install at far end of coax.

> Jim   VE7RF