----- Original Message -----
From: "Bill Tippett" <btippett@alum.mit.edu>
To: <towertalk@contesting.com>
Sent: Wednesday, May 11, 2005 5:16 AM
Subject: [TowerTalk] Phasing Help
> NM5G:
> >Apparently I need to model the two antennas to
> find the difference in phase angle in degrees, then add the appropriate
> amount of coax to offset the difference.
>
> Stacking patterns are not adversely affected
> by small phase differences.
>
> http://dayton.akorn.net/pipermail/towertalk/2002-August/050063.html
>
> "P.S. For example, assuming phase shifts of 0, +10, and +20 degrees
> for the 3 antennas going up my tower, gain only changes by a negligible
> -0.08 dB and vertical beamwidth decreases by a negligible 0.1 degrees."
>
> 73, Bill W4ZV
Yes, indeed, the gain (or vertical beamwidth, which is related to the
gain/directivity) won't change very much for pretty significant phase
errors.
However, the direction that the beam points WILL change quite a bit. Say
you've got two20m antennas spaced 30 ft apart (call it 10 meters, a rough
half wavelength : 180 deg of phase). If you have an extra 10 degrees in one
of the antennas, the beam will be shifted by arctan(10/180) = about 3
degrees. (same effect if you have one antenna where the phase center is
shifted a couple feet).
A three degree change in vertical take off angle might have a significant
gain because you've moved into a different part of the antenna pattern,
especially for takeoff angles close to horizontal, where the gain changes
rapidly.
Mutual coupling, mechanical differences, the fact that one antenna is closer
to the ground, etc., could easily result in a 10 degree phase shift.
Whether this is significant in most ham installations is another question,
and one that you'd have to answer for yourself.
Jim, W6RMK
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