[TowerTalk] Measuring coax loss

[Pete Smith]

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
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