[TowerTalk] Understanding Coax Loss Measurements

[jimlux]

On 11/30/20 2:00 PM, Jim Brown wrote:
> On 11/30/2020 12:44 PM, Kirk Kleinschmidt via TowerTalk wrote:
>> I may have improperly used my power ratio to decibels calculator.:)
>> With exactly 10 W going into the cables, I measured
>> 8 W out (RG-11); 7.2 W (RG-213); and 7 W (RG-6) at 30 MHz.
>> Upon reflection (pun intended), my 2.8 dB figure can't be correct...
> 
> We can't use a 50 ohm wattmeter to measure loss in 75 ohm cable, thanks 
> to the standing waves resulting from the mismatch. A study of how 
> transmission lines work is advised. See the ARRL Handbook and Antenna Book.
> 

Hmm, if you were to use exactly a wavelength (or multiple) of the 75 ohm 
cable, would the wattmeter approach work?




> What DOES work is the method I outlined earlier with AC6LA's spreadsheet 
> and a properly calibrated Vector Analyzer.
> 
Fully agree in general


There is a niche edge case, one could have a VNA calibrated for some 
other impedance (75 ohms) - and then it should work fine without needing 
the spreadsheet when measuring 75 ohm cable.  You'd need the spreadsheet 
(or equivalent) if you measured 50 ohm cable.