[amsat-bb] Re: Optimal Distance

Edward Cole kl7uw at acsalaska.net
Mon May 25 07:38:04 PDT 2009

At 02:39 AM 5/25/2009, Norman W Osborne wrote:
>Hello everyone,
>I am looking for suggestions for optimal distance between X yagi's
>on a boom, so that there will be none or very little interaction with their
>respective radiation fields.
>Also I will be adding a 24" dish between the two yagi's
>on the same boom with the 70cm and 2 meter X yagi.
>Thanks to all in advance


Since I have not seen any response to you on amsat-bb, I will.

First we need to know the aperture of the antennas.  This will have 
to be calculated from the gain.  Unfortunately, most antenna 
manufacturers are optimistic about the gain of their antenna.  But if 
you have the gains in dBd:
1. Convert to dBi = 2.1 + dBd
2. Convert gain to a ratio G = 10^(dBi/10)
2. Use this formula G = 4*pi*Ae/lambda^2 to derive Ae
3. Ae = G*lambda^2/(4*pi) = G*4.33/12.56 = 3.45*G
4. Ae for x-yagi is assumed a circle:  Ae = pi*R^2
5. R = sqrt(Ae/3.14),  R is the width of your aperture so half that 
would be used for stacking distance.
6. The distance between 2m and 70cm antenna = 1/2 (R(2m) + R(70cm))

The dish will probably sit inside the 2m aperture but be outside the 
aperture of the 70cm antenna.  I mounted my 436CP42 and 2m antennas 
five feet apart with the 33-inch dish set on the UHF side of the 
B5400 az-el rotator.  This is probably closer than using the 
apertures for stacking.

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