[TowerTalk] 40-2cd

[D. Rodman, MD]

Antenna interaction can be modeled with computer simulation.  My analysis
shows it becomes problematic when one closes the gap to less than 10 feet
between antenna.  Typically electrostatic shielding causes the higher HF
frequency antenna to suffer more.  The first noticable difference would be
a drop in F/B ratio.  This would be followed by reduction in forward gain.
Finally, there is a lowering of feed point impedance, mostly on the higher
frequency antenna, again. The problem becomes more acute as the
antennas get closer together.  Finally, when they are on the same boom,
placement of the elements becomes highly critical.  One may lessen the
interactions by positioning the elements orthogonal or increasing the
separation distance as much as possible.  Many hams tend to reproduce what
they have seen because it imperically looks good, but there is theory
behind it.  Your separation of 9' would seem to be appropriate, based on
my modeling experiments.  Perhaps if you gave more detailed information on
the process by which you got to the point you are at, it might lead to a
better answer for you.

David J. Rodman, MD 
Assistant Clinical Professor 
Department of Ophthalmology 
State University of New York at Buffalo

FAX 716-859-4565 
Office 716-854-1137

e-mail: rodman@acsu.buffalo.edu


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