[TowerTalk] Measuring coax loss with MFJ-259B

Steve Maki steve at oakcom.com
Fri Oct 5 18:50:36 EDT 2007


This is perfectly normal behavior - I sweep feedlines on a regular basis 
  with Anritsu Sitemasters, and usually use (highest peak + lowest 
valley)/2 as my cable loss value. A short (instead of an open) at the 
far end gives slightly smoother readings for some reason, but it still 
always looks like a roller coaster. Impedance bumps in the line (UHF 
connectors, etc.) cause the effect to get much worse. I'll leave it to 
Jim Lux and/or others to give you the formulas, but it's related to line 
lengths and impedance translations on imperfect lines.

In order to see increasing loss with frequency, you have to sweep over a 
much larger frequency range.

Steve K8LX

Pete Smith wrote:
> 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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