RadioBanter - View Single Post - Multi-band dipole question

Roy Lewallen · April 19th 07, 09:06 PM posted to rec.radio.amateur.antenna

wrote:
Hello,

Just joined the group and I have a question about constructing a multi-
band dipole.

I plan to construct a 80/40/20/15 band multi-element dipole using
separate dipole wires for each band,
but a single feedpoint. My question is: how important is the spacing
BETWEEN the
individual band wires. I understand that they can't/shouldn't touch,
but is there any distance
that is optimal? or is just keeping them from touching enough?

That's a good question, and here's the answer:

If you fan them apart at a fairly large angle, they don't interact much,
so on each band the antenna acts pretty much like the other elements
aren't there. But as you decrease the spacing, the interaction
increases, with several noticeable consequences. First, the resonant
length of each element is affected by all others. Mostly, the long
elements affect shorter ones. So tuning the antenna requires adjusting
the longest element first, then the next shorter one, and so forth. The
lengths you end up with are somewhat different than you would if the
elements were independent. Second, the antenna becomes more narrowbanded
on each band, except perhaps the lowest frequency band which uses the
longest element. Third, the feedpoint impedance at resonant frequencies
will be somewhat different than it would for an independent dipole. All
these effects increase as the spacing between elements decreases. I
haven't done a study, but believe that coupling will alter the patterns
of the higher frequency antennas somewhat when spacing is close.
Efficiency should be just fine for any spacing, though, so you'll be
radiating essentially all power you feed it.

Having 15 and 40 meter elements in the same antenna might make things
more difficult, because on 15 meters, the 40 meter element will be near
resonance and will get a fair amount of the power. It'll still radiate,
but you might find an unusual feedpoint impedance. You might consider
using the 40 meter element for both 40 and 15 -- you can probably tune
it to be reasonably close to resonance on both bands. On 15 you'll get a
cloverleaf pattern, but if you bend the two wires each 30 degrees in the
same direction to make a vee with 120 degree included angle, you'll get
a fairly narrow bidirectional pattern.

Roy Lewallen, W7EL