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Re: Various antennas on a boom



At 03:36 PM 2/23/99 , Eduardo Jacob wrote:
>Is it possible to put a Myers Parabolic 2,4GHz Antenna and a 1,2 yagi
>antena on the central part of a maspro boom, without denerating th e gain
>of the U/V antennas? The boom is about 2 meters long. Would be possible
>with a longer boom?
>
>This would the situation:
>
>        X--+-|-O--x 

This is an excellent question.  I see a lot of people crowd Yagi antennas onto a boom, and I often wonder whether they realize that they are damaging the patterns of the high gain antennas when they do this.

With dishes, the situation is pretty simple, because you can see the aperture of a dish.  It is simply the dish's surface.  If you don't block that surface, you're in pretty good shape.  (Although nearby metal will cause scattering which degrades the pattern, especially by creating sidelobes, although this is a 2nd order effect.)

In the case of Yagi's, the situation is a little different.  The Yagi has an aperture too, but you can't immediately see it.  There are no signposts.  This thing is trying to collect energy out of an area, just like the dish, and if you put metal objects in that area it reduces the effectiveness of the antenna.  We can compute the effective area of the aperture, then presuming the aperture is a circular area, we can compute its radius.  This would be a good first order guess for how far we ought to keep things away from a Yagi if we want it to perform well.

A = lambda^2 G / (4 pi)    formula for effective area (aperture)

A = pi r^2                 formula for a circle

Throw them together, and you get

r = lambda * sqrt(G) / (2pi)

Notice that when the gain of a Yagi is high, its aperture is large, so the radius of the aperture is large too.  This is important, as in satellite work (well AO-10, AO-13 style satellite work anyway) we use high gain Yagis.  High gain Yagis need more empty space around them than low gain Yagis.
 
Some examples:
2 meter band, 10dB Yagi:  lambda =2m, G=10
     ==> r = 1.0m = 3.25 ft
70cm band, 20dB Yagi:  lambda=0.7m, G=100
     ==> r = 1.1m = 3.6 ft

These numbers are reasonably consistent with typical practice.  Many people use 5ft or 7ft crossbooms.  I use a 10ft crossboom, which provides plenty of room between the antennas and the rotor.  The downside is that the long crossboom tends to sag, misalligning the antennas when the satellite is high in elevation.

Of course there are many issues which affect the degree to which an antenna is degraded by something nearby.  A vertically polarized antenna, for example, is not affected much by metal aligned in the horizontal orientation and vice versa.  Unfortunately circularly polarized antennas are affected by just about anything.  How much a piece of metal affects an antenna's pattern depends on position in the third axis too.  (Exactly where that metal is located relative to distance measured along the Yagi's boom.)  Example: Someone presented some analysis awhile back that showed that it didn't do much harm to strap the feedline to the crossboom IF it was positioned in precisely the right location relative to the Yagi's elements.  I'm sure that if you get that wrong you have a major degradation.

I would really like to see someone do a careful study of antenna spacing.  Seems like it would be relatively easy to do with modern antenna analysis software.  One could model a high gain Yagi in free space, then add a model of a 2nd Yagi nearby (terminated, parasitically excited), and let the computer tell us how much gain, or circularity, or sidebands were affected at various distances between the antennas.

One could also model rotor, mast, etc.



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