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
> >
> > --
> > FAQ on WWW: http://www.contesting.com/towertalkfaq.html
> > Submissions: towertalk@contesting.com
> > Administrative requests: towertalk-REQUEST@contesting.com
> > Problems: owner-towertalk@contesting.com
> > Search: http://www.contesting.com/km9p/search.htm
>
> --
> FAQ on WWW: http://www.contesting.com/towertalkfaq.html
> Submissions: towertalk@contesting.com
> Administrative requests: towertalk-REQUEST@contesting.com
> Problems: owner-towertalk@contesting.com
> Search: http://www.contesting.com/km9p/search.htm
--
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|