[TowerTalk] Aperture and stacking distance

[Jim Smith]

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.  
It involves working out the "radius of effective aperture" of each beam 
and adjusting the distance between any 2 beams to be equal to or greater 
than the sum of the radius of effective aperture for the 2 beams.

The following are for units of feet, not metres.

First one calculates the "Effective Aperture" (A) for a beam from

A = Wavelength squared X the antenna gain relative to a dipole (ratio 
not dB) and divided by 4 Pi

Then one calculates the radius of the effective aperture (R) from

R = A divided by [(H + 2) X Pi]

Where H is the height of the antenna in feet.  Lord only knows what the 
2 is doing in there.

So, for a 40m beam operating at 7150 with a 4.1 dBd gain at 70 ft

A = 137.7^2 X 2.57 / (4 X 3.14) = 3878 sq ft

R = 3875 / (72 X 3.14) = 17.15 ft

Similarly a 6m beam operating at 51 MHz with a 7.8 dBd gain at 86 ft

A = 179 sq ft

R = 0.65 ft

 From which the minimum desirable vertical spacing between a 40m and a 
6m beam is:

17.15 + 0.65 = 17.8 ft

So one would, in principle, move the 6m beam up to the 88 ft level and 
recalculate.

Similarly, two 6m beams should be spaced about 1.3 ft.  Sounds a little 
close to me but I don't know much about this stuff.

Is this method actually useful or is it all hogwash? 

73, Jim   VE7FO