[TowerTalk] What Happens When Beams Are Stacked?

[w8ji.tom]

Hi David,

"Capture area" is mostly an amateur radio myth used to create an advantage
where none exists!!!
I think it was derived from the useful term "effective aperture" which is a
function of directivity and efficiency for a given frequency.

For example, any antenna (no matter how small or large) that has the same
efficiency and more directivity than another will have more effective
aperture ("capture area") than the other antenna.

A properly constructed 4 element yagi has more effective aperture than a
longwire with four times the physical area!

> If I properly stack two matched tri-banders and if the match perfect, I
> understand that I have twice the capture area on RX, but what is
happening
> on TX?

I have to go without looking this up (no time now), so I'm going by memory
of something I hardly use. But here is what I recall. It will only have
TWICE the "capture area" if it has 3 dB more gain than the single antenna!
Physical size is absolutely meaningless for signal capture or transmission.

> 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?
 
Each antenna gets about half the power and probably has about the same
beamwidth it had without the other antenna active, so that would mean
effective aperture is about half that of a single antenna alone. The less
directive you make the array for a given efficiency, the less "capture
area" the system has.

Like radiation resistance or the impedance of space, capture area will get
you in trouble quickly by causing sideways thinking. Such terms are
commonly misused to invent fictitious pathological science antennas that
rob people of money or mislead them, and should be banned from common
usage.  

73 Tom

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