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Re: Dish depth question



This is going to seem like a bizarre question, but is the dish REALLY REALLY 
deep?  I wonder if the dish is a spherical (or toroidal) dish rather than a 
parabolic dish.  A parabolic dish uses a point feed like we know and love a 
spherical typically uses a different type of feed entirely.

I got that crazy idea when it was mentioned that the focal point is very 
broad.  That's assuming that you are near the primary focal point on the 
dish at all.  The comments here to calculate f/D to find the focal length is 
a good idea.

Fred W0FMS


>From: Scott Townley <nx7u@arrl.net>
>To: Phil <phil@spiderweb.com.au>, amsat-bb@AMSAT.Org
>Subject: Re: [amsat-bb] Dish depth question
>Date: Tue, 10 Jun 2003 19:36:47 -0700
>
>Hi Phil.
>Dish depth doesn't have anything to do with frequency, but everything to do 
>with the feed you use.
>What kind of feed antenna did you use?  If the dish is deeper you will want 
>a less directive feed.  It is likely that your "old" feed was too 
>directive, so didn't realize all the benefits of the size of the dish.
>Likely the focal point (where the feed needs to be place) is different from 
>where you put the feed...you don't mention recalculating the new focal 
>length for this dish...did you?
>
>At 18:53 2003-06-10, Phil wrote:
>>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
>>
>>----
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>
>Scott Townley NX7U
>Gilbert, AZ  DM43di
>http://members.cox.net/nx7u
>
>----
>Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
>Not an AMSAT member? Join now to support the amateur satellite program!
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