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.
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