[TowerTalk] Stacks (optimizing gain at takeoff angle)

[n4kg@juno.com]

Long ago, some people thought there might be an advantage
to mounting the boom of an antenna at the angle corresponding
to the desired takeoff angle.

My mentor, N4NO, analyzed this and came to the conclusion
that such an installation actually produced LESS signal at
the desired angle.

Let us consider a high takeoff angle of 30 degrees which
will emphasize the variations to make them more visible
to the naked eye or immagination.

Picture the antenna pattern in the vertical plane.  As the 
boom is rotated up, there is a small increase in gain from
30 degrees to the axis of the boom (0 degrees).

BUT, as the boom is rotated up, there is a LARGER DECREASE
in gain at the ground reflection angle where the angle to the boom
varies from 30 degrees to 60 degrees.  

The COMBINED signal from the incident wave plus the ground
reflection will be LESS for a boom in-line with the desired
angle than for a horizontal boom which illuminates the 
incident angle and the ground reflection angle equally.

de  Tom  N4KG



On Sun, 13 Aug 2000 alsopb <alsopb@gloryroad.net> writes:
> 
> Guys,
> 
> Anybody out there doing work with altering the phasing of the 
> stacked
> antennas.
> 
> Right now they the designs call for being fed in phase (whether they
> are in practice is another story!).  Is there a benefit in adjusting
> the phasing to put the angle of radiation where you want it, rather
> than just feeding in phase and getting what results.
> 
> It would seem that rather than having a stack of 3 or 4 one could 
> get
> by with a stack of 2 (and perhaps save some $) and a few switched in
> pieces of coax to alter the phase for most probable angles.
> 
> Assuming the antennas are resonant and "pure resistive", this ought 
> to
> work.  Getting the proper power sharing to the antennas may be a 
> real
> headache if the impedances are complex.
> 
> Any real world experience in ham-type applications?
> 
> Brian/K3KO
> 
>

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