On 26 Sep 2006 10:31:14 -0700, "Zack" 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

Hi Zack,

You may be confusing (or have been confused with the content of this
thread) antenna effeciency with system efficiency. The page makes the
point of there being a feed "appropriateness." I would suspect the
method of feed makes the difference (and those issues that lie beyond
that include method of detection, noise, and so on).

As for putting it to the antenna's merit, the roughness is far more
significant to shorter wavelengths (roughness is on order of eight
wave). This in itself produces problems of phase control, and phase
control is the name of the game in directivity.

73's
Richard Clark, KB7QHC