[TowerTalk] Understanding Coax Loss Measurements

[Kirk Kleinschmidt]

Well, this is interesting and valuable.
I didn't know that the "power meter method" wouldn't work with 75-ohm coax. I figured that it would be "close enough for government work," but apparently not. (I guess I could build a power meter with a 75-ohm sampling head?)

I didn't have time (before the onset of ugly WX) to figure out how to calibrate my VNA for 75 ohms (if it's possible), or to figure out how to set the Rigol SA for 75 ohm measurements (probably possible). But I will. And when I do, the AC6LA stuff will be quite helpful.
I don't necessarily like learning stuff the hard way -- but I do like learning it. :)
Thanks,
--Kirk, NT0Z



My book, "Stealth Amateur Radio," is now available from www.stealthamateur.com and on the Amazon Kindle (soon) 

    On Monday, November 30, 2020, 4:16:28 PM CST, jimlux <jimlux at earthlink.net> wrote:  
 
 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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