[TowerTalk] Measuring coax loss

[Pete Smith <n4zr@contesting.com>]

My HF antennas are fed with a composite feedline consisting of about 50 
feet of RG-213, a run of 75-ohm CATV hardline, and a pigtail of RG-11 at 
the antenna end.  The hardline was calculated to be a half-wave at 1750 Hz, 
and was then measured at 28 MHz to get as close to the half-wave multiple 
measurement as possible.  This feedline arrangement has been very 
successful for 9 years, posing no problems feedling antennas from 160 thru 
10 meters.  I have periodically measured the minimum R from the shack with 
the far end of the feedline open, and it has always run in the area of 7-9 
ohms at resonance in the 10-meter band.  When I had my Autek RF-1, I used 
to calculate the equivalent feedline loss in dB using a formula in the RF-1 
manual, and think the value was on the order of .7 dB.

Recently, I got an MFJ-259B, and have been trying to use the Coax Loss 
function to measure the same parameter.  I find I am getting wildly varying 
numbers, depending on the frequency,  which in all cases seem to be higher 
than they should be, from what I remember.  The reported loss is highest at 
the frequencies where the line is resonant (~2.0 dB at a near-30 MHz 
resonance, down to 1.5 dB at one near 28 MHz).  Between resonances, it goes 
as low as 1.3 dB, which still seems high unless the line(s) are faulty.

I presume that the mixed impedance of this line is such that the built-in 
loss calculation provided by the 259B is not reliable.  I also wonder about 
the accuracy of the equation from the RF-1 manual in this case-- can anyone 
refresh my memory on it?  Is it really necessary to measure only the 75-ohm 
portion of the line with the 259B, then the 50-ohm portion, and add the 
power ratios in order to compute the total loss?  Alternatively, is there 
any way I can  do it with the line in situ?  Sorry, but my theory just 
isn't strong enough to answer the question for myself.

73, Pete N4ZR
The World HF Contest Station Database
was updated 13 December 2004
2843 contest stations at
www.pvrc.org/WCSD/WCSDsearch.htm  

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