Pete, this is precisely the same issue I raised a couple of months ago,
but I didn't get any satisfactory explanations. I've pasted my post below.
"I have used the MFJ-269 to measure coax loss at HF frequencies for
several years and my experiences have been generally good: losses
measured for new coax very closely agreed with the published specs. I
recently was checking a 100' piece of previously used low loss coax on
144 MHz and noticed that the loss values seemed to oscillate up and down
as I varied the frequency and the differences were not trivial: 2 db +
or - 1 dB within a frequency range of 1.4 MHz. In other words as the
frequency increased, the loss did not increase smoothly with frequency
as expected.
I then went back and checked the behavior on the low HF bands and the
same type of pattern was evident but the variability was much smaller: +
or - 0.1 dB when the loss was about 0.5-1 dB at 3.5-10 MHz. As the
frequency was increased the amount of variability increased.
In order to exaggerate the effect, I then checked a 400' run of low loss
coax from 114-150 MHz at 1 MHz intervals. The losses varied from a low
of 2.9 dB to a high of 14! When the data are plotted, a very rough
cyclical wave shape is obtained with a period of about 9-10 MHz. The
loss peaks did not necessarily increase with frequency but seemed very
erratic.
Has anyone else observed similar behavior? Are we saying that the loss
data obtained with this unit are only really reliable in the low HF and
it is really unusable above, say, 30 MHz?
I should mention that I have never tried to calibrate the unit and it is
about four years old. It does give reasonable SWR, R and X data when
checked into a dummy load at up to 144 MHz."
I suspect that the unit becomes less reliable at higher frequency or
that, based on some info on the W8JI site, there is a bias problem with
unit's oscillator. I'm at least glad to see that I'm not the only one
making these observations and would like to know if there's any
reasonable explanation for the behavior.
73, Joe
K2XX
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
> The World HF Contest Station Database
> Full details on 3300 contest stations
> Updated 5/5/07 http://www.pvrc.org/WCSD/WCSDsearch.htm
>
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