Another issue with the "steel and aluminum jungle" is detuning unwanted
re-radiating structures. For example, a few years back, I had to somewhat
regularly de-tune towers in the vicinity of AM broadcast stations that were
distorting the radiation pattern. In one case, a single tower nearly two
away caused a full 3 dB of signal reduction to an otherwise omni-directional
Note that guy wires were far more often the culprit than the tower itself.
several levels of guys, the variety of lengths almost gurantees that
will be close to resonance. So, insulate and break up your guy wires on all
towers in a multi-tower station to reduce the chance of interaction.
In a more practical case, I helped a fellow ham de-tune his shunt-fed tower
when in the receive mode, to keep it from coupling to his receive antennas.
We added a T/R relay and a variable capacitor at the feedpoint, tuned for
minimum noise while listening on the receive antennas.
This approach could be useful for Bill's 4-square, making the center tower
become "invisible" at the operating frequency (3.5 MHz I think in his case).
Unfortunately, I have not had the need to model a structure for minimum
re-radiation. Do some of you broadcaster-types have any experience with
modeling gamma wires or other means of detuning? Does the close-spacing
limitation of NEC prevent this from being done accurately?
Snips of the original message:
From: Bill Fisher - W4AN <firstname.lastname@example.org>
To: email@example.com <firstname.lastname@example.org>
Date: Tuesday, December 22, 1998 2:16 PM
Subject: [antennaware] Station Modelling
>>With the few towers I have modelled so far, it is interesting to see how a
>nearby structure can totally screw up a great pattern from an optimized
>antenna. I now have to wonder about these guys who stick up a ton of
>antennas on one mast or tower.
>One thing I have learned is that my 4-square which surrounds a 115' Rohn
>45 tower is definatley affected by the tower in the middle. The F/B goes
>from a 25DB in the perfect model to 18DB or 19DB with the tower in the
>center of the square. In my case, it's the only place I can put the
>verticals. But it explains a few observations.
>Has anyone else done any complex station modelling to see the affects of
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