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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