[TowerTalk] Measuring coax loss with MFJ-259B

[Pete Smith]

I recently replaced my two main 190-foot feedlines to the tower with 
LDF4-50A hardline.  On the shack end, there is about 50 feet of fairly old 
RG-213 (12 years or so).  Today I used my 259B to measure the combined loss 
at 30 MHz, with the far end open-circuited.  I got 1.3 dB on one cable and 
1.4 on the other, which correlates very nicely with the rated attenuation 
of the cables.  However, there was one strange thing - strange to me, 
anyway.  with the 259B in coax loss mode, I found that relatively small 
frequency excursions would cause a relatively large change in the reported 
loss - going, for example, from 1.3 dB at 30 MHz to 1.6 at 29.6 MHz - and 
of course, if anything, you'd expect the attenuation to go down with 
decreasiong frequency.  The variation in reported loss is also cyclical as 
I tune further down.

The manual for the 259B doesn't say anything about this sort of frequency 
sensitivity, but I remember that with my old Autek it was necessary to tune 
for minimum R and then apply a formula to determine loss.  Is this just an 
omission in the MFJ manual, or something I don't understand, or maybe a 
problem with the unit itself?

73, Pete N4ZR
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