[TowerTalk] Coax Losses on 160 and 75?

[Jim Thomson]

Date: Fri, 5 Aug 2016 23:03:14 -0700
From: "Richard (Rick) Karlquist" <richard at karlquist.com>
To: Jim Thomson <jim.thom at telus.net>, towertalk at contesting.com
Subject: Re: [TowerTalk] Coax Losses on 160 and 75?

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

##  I read through the pdf.    My dummy load does not have a mismatch, coax is 50 ohms, swr is  1:1
at each end of the coax, and there is no reflected power..at either end.  The 7-16 din connectors are installed
correctly.  I use rack mount bird wattmeters, coaxial dynamics wattmeters and  array solutions powermaster
digital wattemteters and various HF couplers for different power levels.   The array solutions gear is not affected by temperature.

##  as long as there is zero reflected power and swr is 1:1 at both ends, where are the possible measurement errors coming from ? 
Id suggest that the results are plenty good enough for typ ham use...for evaluating different coax cable types.   The only thing I use here
is LMR 400 /600 /1200  and also RG-393...and some short lengths of 213-U.  

##  If installing coaxial dynamics /bird sections at each end of 50 ohm coax is good enough for commercial FM broadcast use, its
good enough for my purposes.  Im suggesting to folks who have really long coax runs, esp > 10 mhz, that if you installed a wattmeter
at the top of the tower, or far end of the coax,  and apply 1 kw cxr through a flat swr, that you will be in for a surprise. 

Jim   VE7RF