[TowerTalk] What Happens When Beams Are Stacked?

[Bill Hider, N3RR]

Clarification:

If the stacked antennas (both fed) are pointing within a 3 dB beamwidth but not
within the 1 dB beamwidth (or so) there will be a problem.  In other words, use the
stack as a stack (pointing in the same direction) or be careful when the antennas
are pointing "close" to each other's  heading. If they are pointing 90 degs - 180
degs from each other, you'll be ok.  It's only when they get close, but not
"right-on"  that you'll have a problem.

Sorry for the confusion.

Bill

Bill Hider, N3RR wrote:

> You're rignt - Here we go again!  Let me try this one:
>
> DavidC wrote:
>
> > "Here I go again" (quote from Ronald Reagan, don't want to be a Mike
> > Barnacle) ...
> >
> > If I properly stack two matched tri-banders and if the match perfect, I
> > understand that I have twice the capture area on RX,
>
> This isn't exactly how I'd describe it.  In general:
>
> The gain of a stack in the first (lowest angle) lobe ranges from 1 to 3 dB more
> than either the lower or the upper antenna alone. That gain is dependent on the
> spacing of the antennas (above the ground and between antennas).
>
> The 1st lobe of a stack is also broader in the elevation plane than either
> antenna alone.  This lobe also is at a  lower elevation (takeoff) angle than
> the lower antenna alone and  at a higher elevation (takeoff) angle than the
> upper alone.
>
> > but what is happening
> > on TX?
> >
>
> Same as above on TX.
>
> > Example:  If I TX 1000 watts and one beam is pointed South and the other
> > West, do I radiate roughly equal signals in both directions (correcting for
> > differences in height and propagation)?  Or is the power roughly divided in
> > half in the phasing system?
> >
>
> Hummm two questions? or one?  You will have to model this to get an approximate
> answer. But IN GENERAL, there will be plenty of interaction between the two
> antennas and the resulting pattern will look like a "twin peakes" with a high
> valley between them from South to West.  "Equality" of the beams is nigh
> impossible since one antenna is high and the other is low (presumably).
>
> I've done plenty of modelling of this situation and it ain't pretty!
>
> BTW, if you operate the stack by feeding both antennas and the azmiuth angle
> between the antennas is one 3dB beamwidth OR LESS, you will have very
> unpredicictable results with nulls where you thought you were getting gain!
> Model this and see!!
>
> When planning stacks, you need to do a requirements analysis first. (Don't you
> do this for everything you buy/build/acquire/install/etc??)  If you don't, you
> have no way of knowing whether what you install will do the job you want it
> to.   For example, you should analyze the areas of the country (world) you wish
> to communicate with on your band of interest and determine the elevation
> angle(s) you need to achieve that goal, then plan your stack accordingly by
> modelling it and iterating the design until you achieve (on paper) your desired
> goal.  If you don't do this, you may wind up with a situation that would be
> unacceptable to you when a little more planning would yield the desired result
> with the same resources (antenna/tower) only installed in a different manner.I
> really need to add this subject to my web site.  It's crutial to good amateur
> design practice.
>
> 73
>
> Bill, N3RR
> http://www.erols.com/n3rr
>
> > Thanks & 73, DavidC  AA1FA
> >
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