Tom .... I think everything is real quiet while a bunch of folks are busy
looking at manuals and crunching numbers with their analyzers.
Wes,
The problem Jim suggests is real, but the numbers are so small at HF for
normal good cables they are meaningless. Until we get a really small cable,
or have some effect like getting down into the steel core of a copper clad
steel wire, it is unimportant. A 100 ft long LMR-400 sample measures zero
reactance crossings at the following:
4.013
8.019
12.035
16.059
20.089
24.131
28.175
32.194
If we look at that, we have an error of 8*4.013MHz = 32.104 MHz -32.194 , or
around 0.3% away from the expected zero reactance crossing on the 8th
harmonic. This isn't much change, and it is easy to have a measurement error
that large.
An F6 type cable sample produced the following:
5.199
10.347
15.549
20.739
Looking at the 4th harmonic, 20.739/ 20.796 = .27% error in harmonic
relationship.
Now going to a very small conductor lossy cable like RG174 we have 1.538 MHz
for first minimum and 17.513 for 9th. That's 1.45*9 = 13.05 MHz for what
should be the 9th harmonic if we have no dispersion. Clearly we have a
problem cable here. The nulls are:
1.538
3.457
5.441
7.436
9.453
11.487
13.468
15.499
17.513
The cable has 1.919 MHz between the lowest nulls, and then quickly goes up
to around 2 MHz spacing between nulls. I'm sure if we looked at the
conductor we would see loss resistance being a major part of series
impedance below 3 MHz, or some other funny business going on. I'm not even
sure if the cable is crushed or wet up in the roll. The large change in
spread from the first null to the second is a concern.
I'm not sure what Jim used to measure his cable or the cable condition, but
unless we have a very lossy small cable or start getting down into a steel
core on a CCS center, or get really low in frequency, I can't imagine anyone
needing to worry about Vf changes with frequency. Even the really small
cable I measured looks very smooth above 2-3 MHz.
I'd worry a lot more about measurement and instrument errors at HF and
higher unless I had some junk cable.
The nice thing about this is where the errors become large the cable length
in wavelengths becomes short, unless we run a telephone company or have some
pretty bad cable.
73 Tom
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UR RST IS ... ... ..9 QSB QSB - hw? BK
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