Owen Duffy wrote:
On Sun, 2 Jul 2006 18:43:35 -0000, "Dave" wrote:

you don't want to think 'loading coils' you want to think 'traps'. they are
two very different animals. loading coils are simply inductors that make
the wire look longer electrically so you don't need as much room. traps are
resonant circuits that exhibit a very large impedance at their tuned
frequency so that it cuts off part of the antenna. usually you use a trap
for multi-band dipoles, the first trap out from the center is tuned to cut
off the outside of the element at the highest frequency, and then at lower
frequencies it looks more like a small inductor so helps a bit as a loading
coil on the lower band.

Yes, but the trap (which is a parallel tuned circuit) doesn't need to
be resonant in one of the operating bands, and there are reasons for
choosing resonance other than at an operating frequency. For example,
you can design a 40/80 trapped antenna that uses a trap resonant
around 6MHz.

Such a configuration requires a bit more complex explanation than the
trap "cutting off" part of the antenna (which it doesn't do anyway).

I believe that the W3DZZ antenna uses this principle to get a reasonable
match on several bands with only one pair of "traps".

Roy Lewallen, W7EL