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Re: Re: AIDC 3733 3' BBQ Dish



>From: "John / NS1Z" <ns1z@arrl.net>
>Along the same lines: How does one figure the focal point of the offset fed
>dishes?
>
>----- Original Message -----
>From: Ronald Long <rlong3@columbus.rr.com>
>>
>> The formula is:
>>
>> f/D = D/(16*d)
>>
>> or f = (D*D)/(16*d)

John,

That is quite the problem since, unlike a "normal" circular dish {or
prime-feed, center-feed, take your choice of expressions}, the offset dish
is not symmetrical in shape.  This is because it is a segment of a larger
circular dish.  You may have heard them described as cut out from a larger
circular dish.  Think of cutting with a round "cookie cutter where the edge
of the cutter cuts at the center and edge of the normal dish.  This is how
most of the offset-feed, satellite-TV dishes are designed.

Here's another analogy:  Think of shining a flashlight onto a normal dish
so that the beam hits only one quarter of the surface; think of adjusting
the light beam edges so they hit at the center of the dish and at the outer
edge.  If you cut on the line drawn by the light beam's edge you would get
an offset dish of one-half the diameter and 1/4 the gain.  The focal point
is the same physical point, only it now is offset from the "cut out" dish.  

The original focal point is a function of the curvature of the parabolic
surface which we usually describe using f/d.  Of course the diameter of the
offset dish is half, so f/d doubles.  Most offset dishes are derived from
f/d 0.35 dishes, so their f/d(offset) = 2*f/d(normal) = 0.70.  This is why
a helix designed for a center-feed dish cannot be used for an offset dish.
You need a feed beam that is half as wide.

So how do you measure it?  I have no formula for you.  The best solution is
to get the feed support arm that came with the dish, look at {measure} one
assembled with the TV feedhorn, and assume the focal point is about where
the front surface of that feedhorn is, and mount your own feed so its phase
center is positioned at that same point {hint: if your feed is a helix this
is the first turn from the reflector; if a circular horn feed use the mouth
of the horn; a pyramidal horn is a little bit inside the mouth}.

The best reference is to read Paul Wade's Microwave Handbook:
http://www.w1ghz.cx/antbook/preface.htm

Chose the "Contents page" link and click on chapter-5 "Offset Parabolic
Dishes".  The subchapter describing the 18 inch DSS dish is the best info I
can point to.  Most oval {elliptical} shaped offset dishes can be assumed
to be similar.  The wide "Primestar" dish is a different critter {you
better get the feed arm for this one}.

With extensive mathematical analysis, a set of formula probably can be
derived for determining the focal point and gain.  I have never encountered
them in any text {and Paul doesn't give any, either}.  He does offer a
unique manner of finding the focus, though {read the article}.

Ed
Short of this, you can always write the dish manufacturer for the
information...good luck! 

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