[BC] Dish accuracy...

[Burt I. Weiner]

I would recommend that the "string test" be made with the dish in its 
normal operating position in order to get any meaningful 
information.  You can sometimes get a pretty good idea by simply 
walking around a dish that's in its normal operating position and 
look across it from the sides and bottom.  The opposing sides should 
precisely line up with each other.  A dish laying on the ground 
facing up may not show a problem.

Sag will distort the "optics" of the dish and therefore its ability 
to focus accurately on a very small point in the sky.  Not only can a 
distorted dish begin to have less rejection to a neighboring 
satellite, but the ability to get a good cross-polarization null 
adjustment will diminish.  Think of a dish as the reflector mirror in 
a telescope and you should be able to visualize how the dish geometry 
comes into play.

All of this is not to say that this particular Comtech dish won't 
perform well for you, but depending on it's condition, it may not 
perform as well as a sturdier dish of the same size.  If it's really 
distorted, it may serve better as a duck pond.

Burt

>From: Mike McCarthy <towers at mre.com>
>
>One important question is whether the dish was laid flat or supported in
>the middle. When you go to string check, measure the depth of the dish and
>confirm the dish is still the correct parabolic curve/shape along equal
>points of both axi and at 45 deg. offsets (total of 8 radials).  More than
>1/4" deviation from expected and it's possible the dish is no longer round
>or true from sitting in an unsupoprted position.
>
>MM
>
> > Burt,
> >
> > That's interesting to know.  Has anyone else had similar problems with
> > these?   Does the sag degrade its directivity in the azimuth plane, or
> > just cause an elevation angle shift which can be corrected by cranking
> > the bottom jack?
> >
> > We recently acquired a used Comtech 3.8 at an unbeatable price -- the
> > cost of picking it up.  Surplus from a consolidation project, it had
> > been disassembled, was stored indoors and appears to be in good
> > condition, but before we spend money pouring the foundation, I would
> > feel more comfortable if I knew that it hasn't sagged appreciably.
> > At least I'm planning to put the reflector back together and pull two
> > strings across the rim just to see how close they meet before we
> > proceed.
> >
> > If we invert the mounting 180 degrees, might this help the sag to
> > "cancel out" over time?
> >
> > Mark