Hi Peter,
I use a homebrew short vertical for 160 meters modeled with EZNEC software
prior to erection. I live in the big city on a very small lot, so space is
at a premium here as well. My antenna is about the same size as yours (20
Meters). It is top loaded with two wires (a sloping T) and also has a large
20 cm diameter coil of 0.5 cm aluminum tubing about 2 meters down from the
top. It is made of 2 meter telescoping sections Al tubing.
I designed a remote control L network to feed right at the base of the
antenna. The unit uses a larger Johnson roller inductor with vacuum
latching relay switched lumped inductance, as well a large vac variable cap
with vac latching relay selected Sangamo G3s to pad the cap. This method
allows a 50 ohm coax feed line to the tuner at the antenna's base and an
absolutely flat VSWR on the on the coax run. This prevents any voltage
hotspots from arcing internally inside the coax due to use of high power
with high VSWR. This is also why it is best to avoid the use of at the
shack side of the feed line run with coax.
You can use L as shunt to ground to allow static bleed off. Open wire line
is best suited for a balanced antenna using a double balanced L network to
transform the transmission line to the xmitter's impedance. T networks have
the unfortunate ability to make poor efficiency matches at an almost
infinite number of settings, but you must use a network analyzer to achieve
an efficient match with the T tuner reliably. An L network avoids the
efficiency problem almost entirely; if they are in resonance with a good
match, they are at their peak efficiency.
Another perfectly good approach is to use a W2FMI type UNUN to match at the
antenna's base. This is a more difficult approach to set up correctly for a
good match, but once done they have excellent efficiency if you don't
saturate the core and exceed their curie thermal limits. There are
multitapped versions available that offer a wide range of impedance
selection (CWS Bytemark manufactures them in the States). Once you know
your drive impedance for the various bands, you could just operate vacuum
latching relays remotely to switch bands quickly. I do this to feed my Hi-Q
antenna mounted on my truck (it operates 160 -10 meters mobile).
Good luck & 73
Paul WN7T
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