Kristinn,
you certainly have a nice GND plane for your vertical. I looked into
several ways to make a simple, mechanically sound antenna using your 20
ft pieces of tubing. The 20 ft tube could be top loaded or the 40 foot
could be used with base loading. Here are the assumptions I made for
the modeling: GND loss = 5 ohms, coil Q=300. The 40 foot base-loaded
"whip" has an input R of 10 ohms, with a radiation resistance of 2.4
ohms. Efficiency is 24% and the 3:1 VSWR bandwidth is 24 kHz. It can
be fed by taping the 68 uH base coil or using a 1:4 balun. The 20 foot
top-loaded uses a 210uH coil at the top with four spokes 4 feet long.
Input R is 17 ohms with a radiation R of 2.2 ohms. Efficiency is 15%
while the 3:1 VSWR bandwidth is 12 kHz. This can be fed with a 1:2 or
4:1 balun. The best thing electrically would be to top, or center load,
the 40 foot mast with a top hat. This is quite a job mechancically
though. My last 160m antenna was a top loaded 22 foot mast GND mounted.
Efficiency was only 10% but I had good results on the air.
Dave , WX7G
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