Re: [amsat-bb] Positioning a Helix feed

["Bill Magnusson" <williamb@xxxxxxxxxxx>]

Hi Greg,

> 3) Cover the dish temporarily with tin foil, and shine
> a light on it (or use the sun).  

No.3 is the way to go. 
For my 6ft dish I made a test setup consisting of a 6 inch disc
made from tracing paper (translucent) on the end of a piece 
of broomstick. I polished the dish surface and used the Sun 
as a source. Keep the shadow of the disc in the centre of the dish. 
Dish geometry is based on parallel rays from the source and there 
is a small difference if you use a close source. 
(not so small if the source is very close). 
This method works well and also gives you a "feel" for the degree 
of accuracy required in this type of exercise. My dish threw a "spot" 
about 2 inches minimum diameter and in fact the focal point was not
critical within a half inch or so and it coincided exactly with the 
calculated point. I also use a helix feed and I took the end of the
first turn as being the epi-centre of the helix.

Good luck,

Bill...vk3jt
Milawa
Australia


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