Re: [amsat-bb] mounting crossed yagis

["Jon Ogden" <jono@xxxxxxxxxxxx>]

> At 09:20 PM 8/6/2000 -0500, Jon Ogden wrote:
> >However, if you use a metal mast (likely),  a metal crossboom (possible,
> >but not recommended), or you have any other vertical or horizontal metal
> >nearby, potential problems could occur when mounting them in the +
> >configuration.  Mounting in the X configuration helps to ensure that you
> >are decoupled from any other nearby metal objects.
>
> I don't believe this.

You never want to have a metal object in the same plane as your antenna
elements.  That's why for a vertically polarized yagi, you don't want to use
a metalic mast unless it is behind the reflectors (at the very end of the
antenna).  Now, I don't know what effect it has on a circularly polarized
antenna, but I would think it would have some.  However, by putting the
elements at 45 degrees to the boom, I think you minimize any coupling.
Ideal decoupling is to have the metal object orthogonal to the antenna
element, but in a CP antenna that's not possible, so you pick the option
that minimizes the damage.
>
> I believe that the metal crossboom screws up the polarization whether or
> not the crossed yagis are mounted in the + or X configuration.  Rotation
by
> 45 degrees doesn't fix the boom problem.

I'd agree that a metal crossboom is not a good idea regardless of the
position of the elements.  And like I said, I don't know if it matters since
the polarization is CP, but I would think that a nearby metal object in the
same plane as one set of elements could couple and distort your pattern.  I
was thinking of the mast more than the crossboom when I said this.

73,

Jon
NA9D


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