There have been some good comments posted, but let me add a bit of
background on a couple issues that have been mentioned...
1. Some of the first modeling studies and measurements of broadcast stations
with elevated radials appeared to show that 8 elevated radials at about 10
feet were equivalent to a full 120-radial in-ground system. These studies
were later proved to be erroneous -- including one test system that had
elevated radials over the old full ground system, ignoring the fact that it
was still there, even if "disconnected" from the antenna. There was probably
some "wishful thinking" in analysis as well. These errors have been
corrected but, unfortunately, these early studies started the myth that a
few elevated radials is just as good as a full-blown buried counterpoise.
2. One of the less-well-known studies involved a Medium Wave transmitting
antenna with the feedpoint up about 1/8 wave and a "skirt" of 8 quarter-wave
radials sloping downward from that point -- like the classic VHF ground
plane. Ground-wave measurements showed that this antenna was nearly
identical a typical 1/4-wave with a 120-radial ground system. Of course, it
was 3/8 wave overall height, which is a signficant expense at that
frequency. However, it was one of very few experiments that demonstrated how
well elevated radials can work. W4WA's 160M vertical is another good test
case -- A quarter-wave (+ a little), above a feedpoint at 30 feet, with
close to 30 elevated radials that are 15-20 feet high at the far ends. (At
least that's how it was last time I operated there in 2002.) The
effectiveness of this antenna is well documented in contest results.
73, Gary
K9AY
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