Date sent: Thu, 11 Mar 1999 20:32:47 -0500 (EST)
From: Bill Fisher - W4AN <w4an@contesting.com>
> I have modelled my tower and 4-square and if you believe the model, I lose
> 5DB of F/B with the tower in the center. This year I plan to detune the
> tower and see if it makes much difference.
The amount they hurt F/B depends on the phase and amplitude of
re-radiation, which depends on your luck. The deterioration can be
a whole bunch, or not very much.
The center support in my 4 square is a 200 ft insulated tower. If I
measure F/null ratio, I can go from over 40 dB (I don't use 90 180
shift, I use 110/220 shift) to very little null depth by tuning the
tower's resonant frequency.
I can't think of a simple way to measure problems. The easiest way is
to measure null depth when the system is correctly installed.
If you don't get the expected front-to-null depth (on groundwave)
and you are positive everything else is right, start looking at re-
radiation from other structures. I have a 80/40 meter dipole hanging
in the center of my foursquare, and it requires a large feedline
choke to prevent messing up the pattern of the 4 square. My tall
tower, 400-500 feet away from the four square center, puts a 6-7
dB null to the west unless it is detuned!
I'm adding another tower 300 feet away stacked with yagis. I figure
I'll be an expert on detuning by the end of the summer, or I'll have
moved the four-square far away.
73, Tom W8JI
w8ji@contesting.com
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