[TowerTalk] Stacking

[Eric Scace K3NA]

Careful!  The answer is not as simple as you might think!

If you are not careful, twisting one antenna into a different direction,
while feeding both antennas, can produce unanticipated and significant holes
in your elevation and azimuth patterns.

My understanding of your situation is that you are feeding two yagis in
phase and with equal power.  When I looked at this in detail for one band at
one station (W3LPL 20m yagis at 50 and 100 feet), twisting the antennas to
different azimuths worked more or less as expected -- actually, less.  One
has to 'overtwist' in order to get the pattern broadening that you are
anticipating at the -3 dB points.  Of course, the elevation patterns will be
different at different azimuths as well.  But, for the stack that I studied,
the transition was smooth.

This was NOT the case for two yagis which were on the same tower but not
part of a stack; e.g., high 20m yagi at 200 feet fed together with a stack
at 100/50 feet.  Bad things happened to the patterns (e.g., -15 dB holes
into certain parts of Europe), which could be corrected by proper phasing
and power feeding.

Life gets complicated when antennas for the same band are on different
towers, too.

Bottom line: run a computer model with all the antennas present, and
pointing in the various directions you are considering.  Then look carefully
at the pattern in 3-dimensions in detail.  (This requires post-processing of
the output of YO or other programs to get at the details, but can be done
with a spreadsheet and some charting skills.)  This will give you some
insight into potential problems to watch for.  (No guarantee the model will
correspond closely with reality, depending on other metal, ground effects,
etc -- usual modelling qualification statement goes here.)

-- Eric K3NA

-----Original Message-----
From: owner-towertalk@contesting.com
[mailto:owner-towertalk@contesting.com]On Behalf Of nn4t@bellsouth.net
Sent: 2000 Jul 04 Tue 16:54
To: towertalk@contesting.com
Subject: [TowerTalk] Stacking



I apologize if this topic has been discussed before, but I don't
remember it coming up. I have modest stacks of monobanders for 10, 15
and 20 on separate towers. On each the top antenna rotates while the
lower is fixed on Europe. When running Europe I always try to align the
antennas as best I can. My assumption is the better aligned the
antennas, the better the pattern. My question is, how critical is this
alignment? Is there a point where some cancellation/degradation of the
pattern begins to take place? Or would having one antenna at 30 degrees
and the other at 55 give you broader coverage to the various countries
of Europe? Thank you. Steve, NN4T.


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