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.
73, Joe
K2XX
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