ORIGINAL MESSAGE:
On Fri, 05 Oct 2007 17:43:25 -0400, Pete Smith <n4zr@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
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