[BC] Satellite Dish Illumination...

[Burt I. Weiner]

It's general practice to allow the feedhorn to see only about 80% of 
the dish.  This is adjusted by the spacing of the Scalar rings 
relative to the opening at the front of the feedhorn.  Seeing to the 
edge of the dish or beyond it can increase the likelihood of TI.

Picture this: Sicily, 1923.  If you were to take a light in a dark 
room and put it inside a paper towel cardboard tube. Point the tube 
towards a wall.   As you move the light towards the end of the tube 
the diameter of the light on the wall will increase and as you pull 
it back, the diameter will decrease.  The scalar rings and the 
feedhorn work essentially the same way.  Do not confuse this with the 
focal point of the dish.  Note that if you take a light into a dark 
room it's no longer a dark room unless the light isn't working, which 
will screw up the whole experiment.  You get the idea.

Burt

At 07:46 PM 3/21/2009, you wrote:
>From: "r.j.carpenter" <rcarpen at comcast.net>
>Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>
>I don't believe anyone has mentioned that the dish is probably not
>uniformly illuminated by the feed.  They don't want to illuminate the
>earth behind the dish and get 300-deg K noise from it instead of sky
>noise temp. I think this means that the outer portions of the dish
>contribute less to the gain that their area would suggest. Obscuring s
>bit of the outer area of the dish shouldn't decrease the gain a lot, but
>might increase the noise temperature.
>
>Or am I as stupid as usual?