There are at least three problems associated with using a 1/8 wavelength
vertical on 160 meters. Those are feedpoint impedance, bandwidth, and
coil dissipation.
To give you an example, for a 20 meter base loaded vertical with ground
loss of 8 ohms, and assuming you build a high Q coil with Air Dux type
construction, the coil should have a low frequency inductance of about
50 uH, a distributed capacitance of about 4 pf, and a loss resistance of
about 1.5 ohms on 160 meters. Putting that data into EZNEC, yields a
resonant frequency of 1.83 MHz, a feedpoint impedance of 17 ohms, and a
2 to 1 SWR bandwidth of 28 kHz, and for 1500 watts continuous input, the
coil dissipation is 137 watts. Total power lost in the coil plus the
ground is 3.5 dB. Radiation resistance is about 7 ohms.
So you need another element in the matching network to transform the 17
ohms to 50 ohms, and the 28 kHz bandwidth may be too narrow for you so
you may need a solution to that, and the coil dissipation is very high,
so if you run high power you need a way to get the heat out of the
coil. I don't think it will be possible to wind a much higher Q coil,
and even if you could do that, the bandwidth will get even narrower.
There is an additional problem if you do any switching in the matching
network, and that is voltage across the coil. For 1500 watts it is
about 5500 volts RMS (7800 volts peak), so you need vacuum relays and a
way to keep every thing dry.
Since the voltage on the coil is so large and has to be kept dry any
insulation you put around the coil will likely add some stray
capacitance around the coil. This will increase the circulating current
in the coil and therefore increase the coil dissipation. For the base
loaded example above each additional 1 pf across the coil increases the
coil dissipation by 1.8 watts. From this information you can also see
why using a poor coil form will increase coil dissipation.
A solution using a loading coil further up the antenna is not easy
either. That coil will have to be higher inductance. It will be
larger, its loss resistance will be higher even though the coil
dissipation may not be a lot different, its distributed capacitance will
be higher, and the voltage across it will be much higher. A trap will
have similar problems.
So you can see the problems get very difficult if the antenna is very
short. It is always better to increase the radiation resistance of the
antenna, usually by top loading.
Jerry, K4SAV
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