On Mon, 29 Aug 2005 15:17:01 GMT, Paul Hinman
wrote:

I came across an interesting antenna the other day on the web. It
called for four radials for each band that one would want to use it on.
In order to protect the lawn mower I would like to bury the radials. If
I use insulated wires for the radials can I bury multiple radials for
different frequencies in the same trench. I would rent a small trencher
like they use for underground power and utility feeds, make four
trenches and drop in the radials, then cover them.


Your subject line raises thoughts of many people separated by a common
language.

The usual reason for using four radials of certain length is that they
are elevated and resonant. Multiple radials of different length are
often operated in parallel because it is thought that there is then a
set that is optimum for each band.

Anyone who has tried to operate multiple dipoles in parallel will know
that without heroic effort this is a false premise, nevertheless, the
practice continues.

When you bury them, the radials are no longer resonant (at least where
you think they are) and trying to use multiple sets is an even less
good idea. You should use lay as many radials as possible. A rule of
thumb would be to make them at least as long as the radiator is high.

Anyone who has tried to operate multiple dipoles in parallel will know
that without heroic effort this is a false premise, nevertheless, the
practice continues.

Huh? Never had a problem here. But I don't "stack" mine like
some people do. That's what causes the coupling problems.
If you parallel feed two dipoles at right angles, there is very little
interaction at all. I spread mine apart as far as possible.
Problems are rare, and easily cured by just moving a wire
a few feet if it happens. It's the ones who tightly stack them with
short spreaders, that have the nightmare tuning problems.

Multiple radials of different length are
often operated in parallel because it is thought that there is then a
set that is optimum for each band.

Well, there is a set that is tuned for a certain band. They won't work
worth a hoot if they aren't resonant. But this applies only to
elevated
radials. On the ground, I agree...measuring is a waste of time.
Doesn't matter if the wire is insulated or not. Will act much the same
either way.
MK