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Re: helix location vs focus

Ok, I've been holding off on saying this, but... I do have one piece of 
advice from much experience with dishes.  That is you can guess/calculate 
the focal point first if you desire, but if you are
designing a mount for the feed, make it adjustable (in and out).  Careful 
tweaking of the focal point often can make a few to several dB of 

Once you acquire the signal, it should be obvious what the best point for 
your feed/dish/construction errors combination is with tweaking.

So, please make the focal length on your dishes adjustable.  You'll be glad 
you did.

Fred W0FMS

>The focal length is 10",
>with a top-to-bottom length of 32", so that matches.  The
>feed is central, not offset.
>So, it sounds like I may have an odd-ball canned helix.  I
>re-measured the position of the feed, and 10" gets you from
>the dish to the lip of the 2" deep 4" diameter can.  But, I
>didn't re-experiment with the feed position when I removed
>the support stick.  Perhaps I should...
>>From: cbuttsch@slonet.org
>>Reply-To: cbuttsch@slonet.org
>>To: nx7u@arrl.net
>>CC: "Greg D." <ko6th_greg@hotmail.com>, amsat-bb@AMSAT.Org
>>Subject: Re: [amsat-bb] helix location vs focus
>>Date: Mon, 10 Jun 2002 16:52:19 +0000
>>I've been experimenting with the focus location as well.  The length
>>of the can does make a difference and I have yet to determine just how
>>long the edges of the can should be.  When a helix is used, two inches
>>seems about right.  When a patch is used, then an inch and a half seems
>>better as the patch is so much closer to the ground plane.  The helix
>>phase center does seem to be one turn in from the end closest to the
>>reflector, while the patch appears to be at the surface of the patch.
>>      Scott and Greg, what F/D are you using and are your dishes center
>>fed or offset fed?  The goal of this is to obtain the minimum amount of
>>terrestrial interference which in my view is the limitation of mode S.
>>Cliff K7RR
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