Some mention has been made of using SWR changes to look for
interaction.
By the time you see the slightest SWR change, you could have
totally "cooked" the antenna's pattern. As ZS6EZ pointed out, it
only takes a tiny bit of re-radiation to screw up a null. You'd never
see the effect on SWR, unless you made a drastic change in the
main lobe of the antenna!
> > 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.
That assumes the far field pattern is the same as the nearfield
pattern. That generally is true ONLY for the null area of the yagi
antenna a reasonable distance behind the antenna, and directly off
the element ends a small distance away from the antenna.
Above and below the yagi, broadside to the elements, there is
considerable energy. That's true even at a fairly large distance.
It only takes a tiny bit of re-radiation (signals don't "bounce", the
initial radiation causes charges to move and that movement causes
new radiation) to produce new EM fields that will destroy an
antenna's pattern.
73, Tom W8JI
W8JI@contesting.com
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