>> Further more beams have a vertical directive pattern that points
>> straight ahead. Therefore the pattern component pointing down at
guy
>>wires
>> is greatly attenuated and any reflected RF is attenuated again back
>>toward the
>> beam.
>> The higher the gain of the beam the more isolated the beam is from
>> surrounding objects to the side/back and below. It just doesn't see
>> them.
>OK so far.
Actually, NOT ok.
The typical image we form in our mind of a beams pattern is a FAR FIELD
pattern, that is to say when we are far enough away from the beam that
the sum of the radiations from the beam to the observer has
significantly converged toward a point source in appearance.
The really destructive interactions are very much in the NEAR FIELD. The
intuitive pattern image does not apply. In order to figure out what is
going on, you have to consider such things as guy wires or any close
metal AS PART OF THE ANTENNA. Rather simple modeling applies well here,
just make sure that the model contains all the metal. None of the issues
that make it tricky are in force. Just a bunch of wires interacting with
each other.
For each individual wire, the model will figure out the interaction with
EVERY OTHER wire and then add up the resultant vectors. Most of the
antenna modelers will either display or report the resultant currents on
each of the wires in the model. That's when you find out a bunch of
NON-INTUITIVE aspects about guy wires underneath a beam.
There is an intuitive way to remember that near field stuff is
different. Ken's idea above incorrectly depended on all of the
interactions between driven element, reflectors and directors having
ALREADY been formed before the guy wire came into play. This obviously
is true in the far field. HOWEVER, energy from the yagi DE is going to
reach the guy wire in roughly the same or less time as it does the other
elements, and CERTAINLY before RE-radiation from the other elements has
reached the guy wire. The guy wire has PRIMARY radiation from the DE and
must be considered as part of the antenna.
Far field considerations are a SUBSET (remnant) of the rules in the near
field. The published ARRL non-resonant lengths were invented without
benefit of computer based complete near field analysis, and have
perpetuated a myth ever since.
73
-----------------
Guy Olinger
Apex, NC, USA
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