To: <towertalk@contesting.com>
> Date: Sun, 18 Oct 1998 18:24:38 -0600
> From: "Josh Logan" <jlogan@bewellnet.com>
> Subject: [TowerTalk] N6AW on 30m BIG GUN SIGNAL
>
> The other night I heard Jan N6AW working
> Europeans and Russians on 30m
> Most of the DX I could barely hear !!
> He said that he was using a Rhombic 350 meters
> in over- all length aiming Europe.
> HE was putting out an absolutely awesome signal !! with 200 watts!
> getting 599 from ZS6QU off the side.
>
> Can Rhombics be effective with Metal supports?
> I always here about the tall telephone poles supporting Rhombics?
>
> Josh N7XM in Colorado
I was just doing some rhombic modeling yesterday. I'm not
sure what "350m in overall length" means, whether tip to tip
or length of wire. If tip to tip, you are looking at a leg
length of 200m or so, depending on angle. This is over 6 wavelengths.
When you get to this size, you have a 3 db beamwidth of less than 10
degrees in both azimuth and elevation. The gain, if the station you
are working is lucky enough to be on target, is indeed around 20 dBi.
This is 10 dB better than a tribander, but hardly the difference between
inaudibility and 599. The reason why the rhombic has such a baseball
bat pattern is that it wastes a lot of energy with sidelobes. It is
entirely possible to work DX off of one of these sizable lobes, again
if the DX station happens to get hit by one.
Having heard a lot of hype such as this and reading W6AM's blurb sheet
on a web site dedicated to rhombics, I was all set to put one up, but
after modeling, it looks like side by side and/or stacked Yagi's would
get the same gain over a larger angle, plus be rotatable (try that
with a rhombic). In my modeling I tried a bunch of different angles
lengths and heights, so I don't think I missed anything.
I've often wondered about the telephone pole deal myself. I see
no reason why you shouldn't be able to use metal supports, since
it is horizontally polarized. The one nice thing about wood is
that you can run the open wire line close to it, which is what
W6AM did. BTW, you ever wonder why its OK for the utility company
to stick a wood pole in the ground with no engineering and no
concrete base, but if you want to put up a metal tower, you have
to analyze the soil, put in concrete and rebar, etc and have UBC
calcs for the tower.
I would be interested in hearing opinions about the pros and cons
of rhombics, or the hype vs reality.
One other comment: I don't know where N6AW is, but from my location
in central Calfornia, there seems to be a pipeline to ZS6, in terms
of propagation. If he is near me, that may have been a factor.
Rick Karlquist N6RK
rick@karlquist.com or rick@area.com
www.karlquist.com
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