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R: Dish depth question



Hello Phil,

Stretch a rope across the mouth of your dish passing  over the center of it
and measure the distance from the rope to the deep of  the dish and
call  C the above measured distance.

D is the diameter of your dish

Calculate first the focus/diameter ratio F/D of the dish using the formula:

F/D= D/ (16 x C )

Calculate the focal distance F of the dish using the formula F= (F/D) x D

If the new dish is deeper you will find that its F/D ratio and its focal
distance F are both smaller than in the previous one

As a first step to try put your feed in the new calculated focal distance F
but remember that smaller is the F/D ratio of the dish and broader must
be the radiation  pattern of your feed.

It means that the same feed is not suitable for every F/D ratios since we
need to illuminate the edge of the dish at a level of -10 dB or  less
respective to the center for best performance.

The deep of the dish has nothing to do with its operating frequency and in
general deeper the dish and less the F/D ratio make the focal point F closer
to the mouth.

In this conditions the "spillower" is less because the feed cannot see
directly to much of the noisy ground at 290 kelvin on the back of the dish
and so the feed get less noise to the receiving system but the feed is more
difficult to designe.

For more detailed informations on this matter look please at the ARRL
Antenna Book  15th Ed., 1988, Chapters 18, 19

73" de

i8CVS Domenico

----- Original Message -----
From: Phil <phil@spiderweb.com.au>
To: <amsat-bb@AMSAT.Org>
Sent: Wednesday, June 11, 2003 3:53 AM
Subject: [amsat-bb] Dish depth question


> Hello All,
>
> I got hold of another 1.8 metre dish a few days ago (it never rains but it
> pours). This dish is solid aluminium alloy and is quite a bit deeper, and
> heavier, than my other steel petal dish.
>
> I was initially quite excited about finding an aluminium dish to replace
my
> rusted and slightly deformed steel dish. After a 300 km trip I was keen to
> test it and after building some support arms for the down-converter my
> excitement soon turned to disappointment.
>
> The focal point is very broad and the beacon was only just audible above
the
> noise. The deeper dish is obviously not intended for satellite TV use and
so
> I wonder what frequency it might have been built for? A lower frequency, I
> suspect.
>
> Another question, is the squint angle predicted by Predict accurate?  I
> thought that the squint angle was suppose to be closer to zero around
about
> now.
>
> --
> Regards,
> Phil
> phil@spiderweb.com.au
>
> ----
> Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
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