On 5/23/2011 8:55 AM, Al Williams wrote:
> There have been many, many postings on how to set up the radials but never a
> discussion on why or how radials actually effect the radials as it seems to
> be "beyond the scope....".
There are excellent discussions of this in the ARRL publications like
The Antenna Book, the Handbook, and the ON4UN book. Al Christman (K3LC)
and Rudy Severns (N6LF) have written really good stuff on this. Google
to find it. All you have to do is study them.
The executive summary is that a vertical radiator fed at the base needs
a return path for antenna current, and the fields created by the antenna
need to cause current flow in that return path. The earth is a lossy
path, so it burns transmitter power. We use radials to shield the earth
from that field so that current flows in low resistance copper rather
than high resistance soil
Looking at it from a simple equivalent circuit point of view, the return
path (either the earth or the radials or a counterpoise) show up in
series with the radiation resistance. The antenna current flows through
that series circuit, which forms a voltage divider. The radiation
resistance depends on the length of the antenna (as a fraction of a
wavelength), and is about 35 ohms for a quarter wave. Lossy earth
typically is on the order of 30-100 ohms. If it were 35 ohms it would
burn half the TX power. A good radial system (30 radials on the ground)
would reduce that to 10 ohms or so, reducing the loss to a dB or so.
Short verticals have much lower radiation resistance (typically 10-15
ohms for 1/8 wavelength), so resistance in the earth/radial system burns
more TX power.
73, Jim K9YC
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