I tried optimizing a loaded dipole for 3.525 and 7.025 MHz. I used an
RLC load to properly represent a self-resonant coil. After optimization,
the load wound up out 40.5' on a 53.9' wire (one side of dipole). Load
loss for a 3" coil of #12 copper wire was < 0.1 dB on both bands. The
free-space impedance was about 60 ohms on each band with negligible
reactance. SWR = 2 bandwidth was 100 kHz on 80 and 210 kHz on 40. The
antenna looks quite useful as a dual-band CW antenna.
I then optimized the same model using an RL load with no shunt
capacitance. The idea was to show how the RLC model was essential for
the operation of this antenna. The wire lengths changed slightly, but
the RL model worked just fine. The shunt capacitance has nothing to do
with antenna operation!
This image shows the wire current at 7.025 MHz:
https://i.postimg.cc/ydh1DDc5/dipole.gif
The red dot is the feed point. The green dot is the coil. The yellow
trace is phasor current. Current changes phase just before the coil. At
7.025 MHz you can think of this antenna as a shortened, resonant,
3/2-wavelength dipole.
The longer I stare at that explanation, the less sure I am that it
offers any insight. But it's the best I can come up with after being
humiliated by the RL model.
Brian
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