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R: Antenna Separation



Hi   David

Any antenna has around of  it a capture area .The EM field inside of this
area is captured and  sent to the receiver.
Outside of this capture area the antenna cannot get any EM field energy.
The capture area is directly proportional to the antenna gain,ie ,more the
gain  and biger the area
The capture area of an antenna must not include parts or elements of another
antenna otherwise part of the
capture area of both is in common and both gain decreases
The capture area A in square meters can be computed by:

                                         A= G x lambda ^2 / 4 x 3,14

were:
G= gain in power factor and not dB (  as an example 13 dBi of gain
correspond to 20 time the gain of an
       isotropic antenna )
lambda= freespace wavelenght

If you connect in phase two antennas for 2 meters in the orizontal plane and
if the gain of each antenna is
13 dBi ,the computed area A =6,36 square meters
This area can roughly be seen as a circle of  1,42 meter radius surrounding
each antenna and so for maximum
gain the antennas should be separated by 1,42 x 2= 2,84 meters or better 10%
less to attenuate the side lobes.
This mean that for maximum performance inside this A area no other antenna
should  exists including the
dish.
The capture area A of a dish is equivalent to the area of is mouth time the
efficiency of the dish generally 0,5 or 50% for our purposes.
As no one will put a yagi antenna inside of a dish,as well no one should put
a dish inside the capture area of a yagi.
With all of this in mind it is possible to roughly compute the spacing
between different antennas not including obviously all necessary refinements

73 de i8CVS Domenico



----- Original Message -----
From: David Rush <ky0dr@arrl.net>
To: <amsat-bb@AMSAT.Org>
Sent: Tuesday, May 15, 2001 4:47 PM
Subject: [amsat-bb] Antenna Separation


> Hi folks.  What kind of separation is typically necessary for a pair of
crossed yagis and a dish on the same az/el rotor?
>
> I plan to build a cheapo hand-crank elevation rotor (hinges, wood,
all-thread rod, 3/8 socket speed wrench, etc.), and am leaning towards
> buying the cheapo Rat Shack TV rotor for azimuth.  On top of that will be
a wooden horizontal boom holding a CP Yagi for 2m, a CP Yagi for
> 70cm, and a 60cm dish for 13cm.
>
> Yes, this is way more than the Rat Shack rotor is intended to hold, and
it's a pretty sloppy rotor (especially with all the junk we plan to put on
it),
> but it's cheap (about $70) and this is just a Field Day install.  If we
get high winds, we'll run for cover. =:o  If anyone has any good ideas for a
> cheaper way to accomplish azimuth rotation at the top of 10-15 feet of
pole, please share it with me.  It doesn't need to be fancy, but it does
> need to work, and stay more-or-less where pointed for a while.  Extra
credit is given for designs that require little or no welding and use
> materials I can find at a good hardware store.
>
> Anyway, I've seen pictures of a cross-boom with 2m at one end, 70cm at the
other, and a dish in/near the center.  How far apart do I need to
> make them to avoid interaction?  I don't think we'll be shooting for
AO-10, so our output power shouldn't be too high for the other birds, I
don't
> think (AO-40, FO-20, FO-29, maybe the RS birds).  I'm thinking about
putting the Yagis at least 2m apart (1 wavelength), with the dish in
> between.
>
> David, ky0dr
>
>
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>


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