[TowerTalk] Phasing Help

[W4EF@comcast.net]

----- Original Message ----- 
From: "Bill Tippett" <btippett at alum.mit.edu>

> 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).
>

Don't forget that there are ground reflection "images" of both antennas
that are each 180 degrees out of phase with the respective real antennas.
In essence this is a 4 element broadside array so by adding a phase
offset to the bottom real antenna you are really adding a phase offset to
the middle two elements of the 4 element broadside array. The top antenna
and its ground reflection image dominate the very low angle behavior,
so it is not surprising that the relative phase of the two antennas doesn't
do much to the position of the main lobe. In fact if you look at the beam
squint of the array formed by the ground reflection image of the real
antennas, it will be in the opposite direction as the squint in the array
formed by the real antennas (one points up and the other points down),
so the squint effect gets cancelled out. In a free space array where their
is no ground reflection, you would see the 3 degree squint from the
10 degree phase offset.

BTW, I agree with Jim's assesment of the o'scope method for phasing
disimilar antennas. It probably has more error associated with it than
the actual phase center difference between disimilar yagis. Modeling
the two antennas is a much better proposition since you can control
all the extraneous stuff (terrain irregularities, scattering from buildings,
mutual coupling, etc). The only source of error with this method will
be model fidelity (e.g. how closely does the antenna model match the
real antenna). With monobanders model fidelity should be very high
(just a matter of making accurate mechanical measurements of the
two antennas).

73 de Mike, W4EF...............................

>         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
>
>