[amsat-bb] Re: big dish

Jason White jason at jason.white.name
Wed Oct 18 06:36:09 PDT 2006

If you get creative and mount the feed in such a way that you can slide 
it in towards the dish, you can select your beam width and effectively 
gain at will by adjusting the illumination of the dish. (If you only 
illuminate 4 feet of your 12 foot reflector, you should have the same 
gain and beamwidth as the four foot dish, just with a longer focal 
length) I used an acrylic rod about two or three feet long on a C-Band 
TVRO dish, and it saved a lot of repointing when the feed was moved 
closer to the dish. It was also interesting to see the increase in 
signal strength as the feed moved closer to the focal point.

The only real advantage your four foot dish has over the 12 is being 
smaller and lighter, so it's easier to point and has less wind loading. 
If you can mount and maneuver the 12, do that. I used a 16' dish for a 
while on AO-40 with excellent results.. but mounting and positioning it 
was not a trivial task (for me at least)...Especially in my case since 
it was installed on the west coast of Florida in very sandy soil. I 
could dismantle it for hurricanes, but I still had to plan for any 
unexpected storms when mounting it.  In the end, I found a 90cm dish 
preferable because I could position it by hand with minimal effort, not 
spend all my time repositioning, and it was also easily removed and 
stored during hurricanes. I doubt that will be as much of a problem in 

The only thing to remember is that some TVRO dishes have a mesh that 
isn't very good above C band, particularly the older fiberglass ones, 
but you should be fine if you intend to use it for L/S.

73s, Jason N1XBP

P.S. - I wouldn't have gotten anywhere with my big dish if it wasn't for 
W0LMD's webpage:


w7lrd at comcast.net wrote:
> Hello
> I have this 12 foot dish waiting for "something".  I want a dual L/S dish system.  Given the beamwidth of a 12 footer would be small.  What would happen if the feed were placed at a spot other than the focal point?  I realize  I would be  not using the dish to it's full potential.  Would that broaden the beam width?  I also have a four footer which would be more applicable.  I am planning for the next HEO and retirement in six months.
> 73 Bob W7LRD
> Seattle
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