The effect of large antenna close to a smaller one is always HUGE. Try to
model it and you will see. The effect can be eliminated by changing the
sizes of the given antennas, the results are often unpredictable. In some
cases the larger structure might be more influenced by the smaller one in
others it might be the opposite and in some rare cases there might be
virtually no influence. In any case for instance to have large 6 el. Yagi
for 15m and 20m closer than 2 meters is completely destroying provided you
will not change the design, if you change it, it might live together happily
on one boom. See NW3Z.contesting.com or a good example are all the "more"
banders of F12. If you try to model each of the antennas alone you get poor
functionality - as it is on one boom and the design is modified you get
CLOSE TO ALMOST monoband performance from the modified - compared to free
space - design.
73!
Jiri
OK1RI
-----Original Message-----
From: owner-towertalk@contesting.com
[mailto:owner-towertalk@contesting.com]On Behalf Of Tom Rauch
Sent: Monday, January 29, 2001 9:39 PM
To: towertalk@contesting.com; Dave Hachadorian
Subject: Re: [TowerTalk] Shadowing of small antennas by larger ones?
> Since the far-field pattern is made up of equal parts of direct
> ray and ground-reflected ray, it seems to me that it wouldn't
> matter which antenna is on top.
That implies ground loss is zero and the near field effect of a large
structure near the antenna will have no effect.
We can be certain there is some ground loss and that a large
antenna mounted near the antenna being used will always affect
the pattern some amount.
Contrary to what is popularly thought, an adjacent conductor does
not need to be resonant nor in front of the antenna to have an major
effect on pattern. I'd certainly model the antenna combination,
booms and all, before reaching any conclusions!
73, Tom W8JI
w8ji@contesting.com
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