Jim M0BMU,
Thanks for publishing the impedance measurements on your LF antenna. You
have done a more thorough job, and over a wider band, than I did some years
back on my LF antenna, but your findings are very similar to what I found.
Firstly, I found that resistance increased with decreasing frequency. I
used an RC bridge directly on the antenna feedpoint (no loading coil). My
results are shown in a table in the ARRL Antenna Book, which are:
Frequency Resistance Capacitance
100 kHz 14 ohms 790 pF
165 11 800
190 10 805
250 9 810
300 8 815
The increase in capacitance with frequency is not unexpected, as
transmission line theory for a monopole predicts a cotangent relationship.
Some stray capacitance would be in parallel around the feedpoint for a
practical antenna (with loading coil and possibly some horizontal component
to connections or up-wire).
In another experiment, I checked the impedance of each part of my top loaded
vertical, starting with the up-wire, then with each side of the top loading
alone, then the whole system. I tested at one frequency only. The
impedance results were difficult to interpret, but once I converted them to
admittance, there was a clear result that connecting top loading was very
similar to connecting lossy capacitors in parallel. More top loading,
covering a fresh ground area, gives lower net resistance. Regarding your
comment:
..... I think this offers support for
the theory that the losses in amateur LF antennas are normally
dominated by dielectric losses. ...
is supported by me. I believe it applies even for amateur antennas in
fairly clear environments. Moist soil is a lossy dielectric, always has
been, and always will be. Antenna tuning also depends on the soil
dielectric factors, and changes somewhat with recent rainfall and seasons.
Trees and buildings in the near field could also be considered to be lossy
dielectric factors.
Most of the textbook information on "broadcast verticals" is for a virtual
copper plated ground plane (hundreds of radials, and quite long) and is
definitely not typical of amateur LF antennas.
73, Bob ZL2CA
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