All,
I think I have found a workable solution. My initial plan to add a delay
line to the C-4XL will likely eliminate the phase difference. The delay
line will be 70 inches (about 6 feet), which is 83 inches (horizontal
difference between feedpoints) times 84% velocity factor of the 9913F7.
This is possible in my situation because both antennas have identical feed
systems and element spacing.
Once the stack is installed (this weekend, wx permitting), I will
double-check the results using a time domain reflectometer borrowed from a
friend.
73, Keith NM5G
-----Original Message-----
From: towertalk-bounces@contesting.com
[mailto:towertalk-bounces@contesting.com] On Behalf Of Bill Tippett
Sent: Wednesday, May 11, 2005 9:20 AM
To: Jim Lux; towertalk@contesting.com
Subject: Re: [TowerTalk] Phasing Help
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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_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
_______________________________________________
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