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NM5K[_4_] · May 3rd 12, 07:41 AM posted to rec.radio.amateur.antenna

On 5/1/2012 12:43 PM, wrote:

FYI I spent an afternoon with EZNEC exploring fan dipoles.

I've used those for many years.. Probably my #1 most used antenna.
I have them here at the house, and also up in the country at
the recreational living center.

What I found was the farther apart the wires, the less the interaction,
which is what one would expect from common sense.

Yep, and if at right angles, there is virtually no interaction at all.
I've had cases with dipoles at right angles where one leg broke off,
and the SWR for the unbroken intact dipole did not change.

From my limited runs it appears that somewhere around 20 degrees is about
where the minimum separation needs to be for practical length adjustments.

At angles less than that be prepared to spend a lot of time pruning.

yep, and it can get weird.. IE: the tuning appears to work backwards
from the norm.. It will almost always be one of the higher bands that
is effected. Rarely the lower band. And in many cases, you will end
up adding wire to the higher band dipole, to come up the band. A bit
peculiar, but I've seen it many times.

It doesn't matter if the wires are separated vertically or horizontally
other than horizontal separatation means you need lots of support points.

If the bands are too close together, i.e. 20-17, 17-15, 12-10, you can
never get both bands to "work", i.e. a decent match.

Not sure if I agree with that. I would think it *should* work if the
two dipoles are at right angles. And likely good nuff for gov work,
even if at a closer angle.
But it's not something I've really tested, as I generally don't worry
about the higher bands, and tend to load the lower band wires on the
higher bands. I rarely ever string dipoles for 17,15,12,or 10..
The 40 meter legs pretty much cover 15m for instance.. I've never added
15m legs, as I always have 40 legs.. I have often added 20m legs though,
as the 40m as a half wave on 20 is the pits when coax fed..

That appears to be also true for trap antennas from some limited EZNEC
modeling.

Of course, there is always the alternative of throwing technology and
money at the problem and just put up whatever wire you can and put an
autotuner up.

Yep.. But I've always like the parallel fed dipoles.
It's almost a perfect system. When tuned and operated correctly, the
dipoles function pretty much the same as single band dipoles, with
no tuner needed, and pretty much the same efficiency as a single band
dipole fed with coax. For multi band use, it's more efficient than
most any other scheme I can think of as far as the bands that have
dipoles, or have a good match for other reasons. IE: 3/4 wave resonance.
The one here at the house is 80/40/20. The one up in the country is
80 and 40. I did also have 160m legs on that one at one time, but a
storm whacked it out, and when I rebuilt it, I didn't bother with the
160m legs.
If I want to use a higher band, I use a tuner and accept the small
loss.. I actually don't get on the higher bands much.. I'm on 40 in
the day, and 80 at night most all of the time. Also 160 at night if
I have it, but that is a bit seasonal.. You won't see me on 160 much
in the summer.. So much noise, you really need an amp, and none of the
amps I have work 160m.