I think the recent discussion of using elevated radials to improve the
performance of a vertical antenna is moot. It seems to me that a better
solution is to increase the electrical length of the radiator to near
3/8 wave so it is essentially ground independent in terms of efficiency.
The length can be due to actual physical height or a combination of
physical height and loading. My 160m shunt-fed radiator consists of
126' of tower and mast, loaded by a 3L40 and stacked 5L20s. Its' effective
height is about .35 wave on 160m.
My grounding system consists of an 8' copper rod at each tower leg.
Each ground rod is connected by a loop of #4 AWG to which 8 buried radials
are soldered. The grass under the tower covers heavy-gage welded ranch
fencing out to 10' which is bonded to the wire loop. I "stapled" the fencing
down with bent pieces of stiff wire so the grass would grow over it. This
antenna seemed to work about as well with just the 3 ground rods alone so
I'm not certain the radials and screen are adding much to performance; the
main reason I added them was for lightning protection.
Since a 3/8 wave vertical radiator should have a real part of impedance
of near 200 ohms, even a rather poor ground will not degrade its efficiency
very much compared to the more common 1/4 antenna. If you don't have a big
tower loaded with big antennas, a 3/8 wave inverted L should give similar
results.
If you're using a trap vertical then by all means stick it on a 10' mast and
go for elevated radials!
73 de Bill, N6CQ/3 (n6cq@paonline.com)
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