🚀 go-pugleaf

In article <726175452snz@chrism.demon.co.uk> chris@chrism.demon.co.uk writes:

>Is the HGA of no use at all unless completely open?  Couldn't a partially
>open antenna be used at all - or does the fact that it's not a parabola
>(or whatever) make it totally useless?
>

The gain of a parabolic reflector decreases rather rapidly as its shape 
deviates from an ideal paraboloid :-( 
 
Of course, if only a portion of the paraboloid is damaged, say the outer
rim or an angular sector, and the rest is in prsitine condition, then
the net gain is simply proportional to the portion which is intact.
 
If the surface is quite irregular, the Ruze relation provides a method
for estimating the reduction in gain due to random deviations from
perfection:
 
                k = cos^2 4*pi*delta/lambda
 
where k is the gain-loss factor (0 < k < 1), delta is the rms deviation from
perfection, and lambda is the operating wavelength.  

The Ruze relation is used to estimate the manufacturing tolerances for a
particular reflector.  A reflector suitable for use at 6 GHz may be rather  
disappointing when used at 12 GHZ, for example.
 
As a footnote, the proposal to upgrade Canada's Algonquin Radiotelescope
for use at millimetre wavelengths for studies of molecular clouds involved
reducing the rms deviation of the inner tw0-thirds of the dish and
replacing the feed.  The outer third of the dish would be so "rough" at
100 GHz that it would simply not contribute.
 
This doesn't directly answer your question but I hope it gives you a bit
more insight...
 
--
Dave Michelson
davem@ee.ubc.ca