Single Dish Radio Astronomy Inefficiencies

[Sanjay87]

Hi,

What is the cause of inefficiencies in single dish radio astronomy? I think it's related to the efficiency of the dish itself ...that the effective area of the dish is less than its total area. Is that right? If so, why is that the case?

Thanks,
San

 

[Pumblechook]

It is easier to think of a dish transmitting a signal rather than receiving one but the theory is that same.

The effective area is less than the theoretical because it is imposible to illuminate the dish uniformly. You want the same Watts/sq cm over the entire reflector surface and it to fall to zero at the edges. It can't be done. It won't be uniform and either energy from the feed extends beyond the edge where it is wasted or the dish is under-illuminated..not much energy near the edge.

I might be easy to consider a light source and parabolic mirror.

Also the feed itself and its mounting (horn for high frequencies or 2 or 3 element yagi for lower frequencies (like TV Aerial)) blocks some of the signal. This can be eliminated by using an offset feed dish which is the case in most domestic satellite TV dishes.

http://www.qsl.net/n1bwt/chap5.pdf

Cassegrain feed may be a bit more efficient..not sure.


http://www.radio-electronics.com/info/antennas/parabolic/parabolic_reflector.php