John,
4 radials isn't anywhere near enough (unless they're elevated). SWR doesn't
tell you anything about antenna efficiency. Ideally, you want to keep
adding radials until your feedpoint impedance levels off. A good antenna
analyzer (like the AIM 4170B) can be a big help.
The easiest way I've found to optimize your radials is to walk around the
area where your antenna is, and place a small marker every 0.015 wavelengths
(2.5 meters for an antenna on 160), up to about 0.25 wavelengths away from
the antenna. Then run radials to those markers, so that the tips are 0.015
wavelengths apart. Space the radials out evenly if you would rather do the
work a little at a time. If you're limited by the amount of wire you have,
make the radials shorter, but keep the tip spacing at 0.015 wavelengths.
As you add radials, ground resistance will go down, antenna efficiency will
go up, and SWR will probably also go up. Use an L-network (or variations)
or a tuner to match the antenna to your amp/rig.
73, Rick ZL2HAM
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