I'd like to second Gary's thoughts. A vertical at the ground, just like
a vertical 10 feet off the ground, induces (or tries to) a lot of
current in the ground. Dirt does not support the flow of that current
back to the feed-point very well. Air is even worse. LOTS of radials
on the ground will intercept enough of the field directly or through
close-coupling to the ground to return a significant portion of the
field impinging the earth back to the feed point as current. Elevated
radials will provide a counter-poise, but will not shield the ground
from the antenna nor return ground current to the feed point.
Un-returned ground current equals wasted power.
The WWV antennas, which work very well, are similar to what Gary
described as an elevated vertical that will work well. They are
elevated quarter wave monopoles, with the base 1/2 wavelength off the
ground, with nine quarter wave radials sloping down at 45 degrees. The
radiator is at the ideal height to NOT induce ground current and the 9
radials provide a very adequate counterpoise. Most of us can't get the
base of our 160M verticals up 270 feet!
Verticals look so simple....and are so complex...
Hal
-----Original Message-----
From: secc-bounces at contesting.com [mailto:secc-bounces at contesting.com]
On Behalf Of K9AY
Sent: Friday, August 24, 2007 1:04 PM
To: secc at contesting.com
Subject: [SECC] More on verticals and radials
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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