Phasing towers and even detuning structures in AM broadcast radio
systems is not without a multitude of variables which as Tom points out
require so much testing, a good calibrate FSM with reliable check
points, cable measurement with expensive equipment, on and on. not
taking these steps may be a mistake to assume any benefit. Just phasing
two exsiting metal structures in your back yard may make you feel good
but not produce the theoretical values by the book of model. So I have
suggestion of an old remedy that works well. It is three phased
verticals without any phasing coax and switches. it is the classic
Bobtailed Curtain which resembles a elongated letter M consisting of
three 1/4 wave drop wires from a full wave flat top which is fed by a
center drop wire at the ground with a matching network or Hi-Z to Low Z
transformer or taped line section. This antenna has a bi-directional
broadside pattern with cancellation of high angle radiation from the
flat top. The predicted gain is about 6db. Wooden poles are recommended
unless of course you could have two insulated steel towers properly
spaced for the support. The base voltage at the bottom of the drop wires
is very high and not many tower insulators unless kept very clean would
be exempt from flashover with high power.
Years ago I had one of these hung from tress on 40 meters and it always
out performed a two element commercial yagi at 75 feet on 40. You might
want to try one on 10 Mhz from a couple of trees or wooden posts and see
what the classic Bob-Tailed curtain will do for you.
Herb Schoenbohm, KV4FZ
On 5/31/2013 10:12 AM, Tom W8JI wrote:
I have a system with two gamma matched towers that works pretty well. I
did all I could to make them appear to be similar. The gamma dimensions
and capacitance are identical - and I used a loading wire on top of
one of
them - making it longer and longer until the match was similar.>>>
Results are heavily dependent on the particular gamma match system and
tower resonance, because the gamma acts not only as a transformer, but
as a transmission line. Sometimes, through luck, it works. Sometimes
it does not work at all.
The key in all of this are current phase and current levels in the
towers.
All good topband ops know how to put up a beverage at night.
_________________
Topband Reflector
All good topband ops know how to put up a beverage at night.
_________________
Topband Reflector
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