[TowerTalk] Measuring coax loss with MFJ-259B

[Bill Turner]

ORIGINAL MESSAGE:

On Fri, 05 Oct 2007 17:43:25 -0400, Pete Smith <n4zr at contesting.com>
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.

------------ REPLY FOLLOWS ------------

I suspect your MFJ-259 is reporting accurately - that the loss really
does vary markedly with small changes in frequency. I would further
suspect this is caused by having two different pieces of coax in line,
one new low-loss hardline and one old lossy coax. Each piece has
separate loss vs frequency curves and how they combine is anyone's
guess.

Just a theory, I can't prove it.

Bill W6WRT