Hi Jim,
At 08:43 AM 5/11/05, Jim Lux wrote:
>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).
What do you mean "direction"??? All antennas are assumed
to be in the same azimuthal direction. The combined vertical "direction"
is hardly affected by small phase differences, e.g. the bottom two Yagis
in my 10m stack (70'/35') using Eznec over Average flat ground:
In-phase: 16.85 dBi @ 8 deg TOA, VBW 8.9 deg
10 deg: 16.83 dBi @ 8 deg TOA, VBW 9.0 deg
20 deg: 16.76 dBi @ 8 deg TOA, VBW 9.0 deg
30 deg: 16.63 dBi @ 8 deg TOA, VBW 9.1 deg
Phase increments were added to bottom antenna only. I didn't model
for the top one only but the results should not be significantly different.
Eznec does not give more resolution than 1 degree for the TOA gain
peak, but these are all clearly less than 1 degree...nowhere near 3.
>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.
Mutual coupling and proximity to ground are both
taken care of in Eznec. Not sure what you mean by "mechanical"
differences (feed system?), but they should also be taken care of
in a properly constructed model.
>Whether this is significant in most ham installations is another question,
>and one that you'd have to answer for yourself.
I've apparently missed your point because your comments
seem internally inconsistent. If you are talking horizontal
azimuthal skewing, that is a completely different case covered
by K3NA in his NCJ series.
73, Bill W4ZV
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