"Zack" wrote in message
ups.com...

Harold E. Johnson wrote:
Does anyone know why the efficiency of the Stanford Big Dish (150 feet)
is only 35% on 1420MHz, compared to 55% on 150 and 400MHz?

http://www-star.stanford.edu/rsg/bigdish.php

--Zack Lau W1VT

More than likely, mesh in the reflector is too big and parabolic
perfection
is poorer at the higher frequency

According to my interpretation of material written by Dick Knadle,
K2RIW published in the ARRL Antenna Book, a reflector error on the
order of 1 inch peak to peak results in a gain deterioration of 0.3 dB
on 1420MHz. I doubt the mesh adds more than another 0.2 dB of loss.
There is still another 1.5 dB of loss to account for the lower
efficiency. Could the dish be optimized for receiving, sacrificing
some gain for a better gain to temperature ratio?

Zack Lau W1VT

W4ZCB

Please be a bit more careful where you plan your responses Zack, I wasn't
the one that posed that question above.

I suppose that they could be under-illuminating the dish in order to
suppress the "hot" ground behind it. For a dish that size though, one inch
is awfully tight. Why don't you ask them?

W4ZCB