[TowerTalk] Aperture and stacking distance

[Jim Lux]

At 12:36 AM 12/4/2005, Jim Smith wrote:
>I ran across something which appears to give a quantitative answer to
>how far apart to stack beams, whether for the same or different bands.
><snip>



>Is this method actually useful or is it all hogwash?


Well.. not totally hogwash, but confuses two entirely different 
problems.  Effective Aperture is a way to represent antenna gain and/or 
directivity (depending on where you keep track of efficiency).  To a first 
order, if you have two antennas each with effective aperture X and gain Y, 
then the composite antenna might have aperture 2X and gain 2Y.  You might 
also use effective aperture calculations when looking at things like 
performance of a radar phased array.

The stacking distance issue is more one of near field interactions, which 
is more determined by the volume around the antenna within which there's 
significant stored energy.  There's a technical definition for the near 
field/far field boundary (where the radiated energy and stored energy 
crossing are equal in magnitude), but even that's not particularly useful 
for antenna interaction analysis.

For very simple antennas (resonant dipoles and some other relatively low 
gain configurations of dipoles), the two just happen to be relatively close.

These days, with numerical modeling being so easy and fast and free, 
there's really no reason not to just analyze it.  Even a rudimentary model 
will give better results than a "rule of thumb", and given the hours you'll 
spend climbing a tower (or working to buy or build that antenna), spending 
an hour to model it seems a good tradeoff.

A suggested modeling strategy would be to just model the antennas as simple 
arrangements of wires (don't sweat the tapering, etc.).  Don't agonize 
about the exact gain, but run the model with different spacings and see how 
far apart the antennas are when things stop changing.

the other way to analyze it is to drive only one antenna in the model, and 
look at the current in the other antenna elements.  If they're less than, 
say, 1% of the currents in the driven antenna, then you can figure they 
have negligible effect. (1% of current corresponds to 40dB down)