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Re: IC-910 / 3731



At 10:01 PM 11/13/2003 -0700, Scott Townley wrote:

>True, I did skip a step.
>This is all from direct integration of the aperture distribution, 
>neglecting edge diffraction and an 11.5dB edge taper:
>The 4/24 case (4" dia feed blockage on a 24" dia dish) shows only a 
>0.084dB loss in efficiency.  Interestingly enough this is very close to 
>the simple geometrical method I alluded to (10*log(1-0.03)=-0.132dB).  I 
>don't claim the geometrical method to be accurate for larger cases!
>Where did you get the 0.7-0.8dB figure?

Hi Scott,

Doing the integration, you should get more not less loss
from the blockage. If you get .132 dB loss from simple
geometry (i.e. as if a uniform apperture distribution)
then you have to get a higher loss for the cos^2 case.
Was the .084 dB a typo (did you mean .84 dB?)

Anyway, I assumed a symmetrical, cos^2 distribution and I used
the actual dimensions of the G3RUH patch which is 120mm or
about 4.7 inch over a 60cm dish. You COULD integrate over
the apperture to find the loss but I am WAY to lazy to do
that unless I have to.

I used a little apperture analysis program called "SABOR"
from the Polytechnical University of Madrid.
I would guess this was a grad student's project and
you can download it for free from:
http://www.gr.ssr.upm.es/sabor.htm
It is very limited in what it can do but for what it handles,
it is very convenient and I have found it gives pretty
realistic results.

I think Paul Wade (W1GHZ) has something similar on his
web site that will calculate the feed blockage for you
(HDL_ANT) or something like that.

Ok, so fess-up now, how do you get no blockage from a 60 cm
prime focus dish at 2.4 GHz?


Cheers,
Tony AA2TX



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