On Nov 2, 6:12 pm, Roy Lewallen wrote:
Here's the deal. If you put the wires close together you get a lot of
interaction. The manifestation of the interaction is that the
higher-frequency dipoles end up considerably shorter than normal, and
they'll have a narrower bandwidth than an isolated dipole. The longest
one will also be affected by the others, but not nearly so much. You'll
also find that small differences in spacing can have quite an effect on
the dipole resonant frequencies, which is why a cookbook approach
usually doesn't work unless the writer is very careful to document the
antenna accurately and you're extremely careful to exactly duplicate it.

I had read the points you make above in the antenna books... but did
not realize exactly how variable the effects are especially for the
cases where the elements are physically close. The examples in the
ARRL Antenna book are particularly heinous: they show elements
separated by a fraction of an inch (e.g. the twin-lead example, the
picture that shows the wires hanging from egg insulators) and these
examples are - from my experiments - the least likely to work at all.

The interaction decreases rapidly as you spread the dipoles apart. If
you can get them around 30 degrees apart, the interaction is minimal and
you can just about treat them like separate dipoles. A lot of
installations fall between these extremes, so the dipoles have some
interaction but it's not as severe as it is when they're very closely
spaced.

This is a very fundamental piece of wisdom, and a piece that deserves
more attention in the ARRL Antenna books. The current statement - "The
separation between the dipoles for the various frequencies does not
seem to be especially critical" is incredibly wrong for the close-
spaced exampls shown in the book.

Tim.