[Moon-net] Dish antenna Feed Questions

[Pat Barthelow]

Folks:

We just completed a visit to the Jamesburg Dish in Carmel Valley.  We are 
working on setting it up for 1296 EME, and I, being a rookie EMEer, have 
some questions:

The dish is a 97 ft dia cassegrain feed.  The secondary reflector is 10 ft 
in diameter, and is suspended 27.4 ft above the bottom , vertex of the dish. 
  The Dish drawing I had access to shows the "Phase Center" as a point that 
is 61 inches above the bottom (Vertex)  of the dish. That leaves 17.83 ft 
betwee the phase center and the bottom, lowest point of 10' hyperbolic 
secondary reflector.
At the "phase center" is a corragated circular horn that is the COMSAT 
original feed horn

The secondary reflector subtends about 30 degrees from there.  If a feed 
antenna's beam pattern was 30 degrees wide, it would fit well to the 
subreflector.  (that would be about a  16 db yagi )
My question is, can I build a 1296 mhz, 16 db yagi, or loop qad and simply 
mount it near the phase center and point it straight up  to the secondary 
reflector?

Is the focal length of the antenna, twice the distance from the phase center 
to the subreflector?

Is there a way to defocus the needle point beam created by this antenna?  
The beam is less than 1 degree and we want to make that pattern broader for 
easier tracking.

73, DX, de Pat AA6EG aa6eg at hotmail.com;
Skype: Sparky599