[amsat-bb] patch feed and proper dish

[Ronald Long <rlong3@xxxxxxxxxxxxxxx>]

Steve KA1LM asked a question about the G3RUH/ON6UG patch feed and a 
suitable dish. I wrote him and decided to copy to amsat-bb.

The URL for the feed under discussion is:
http://www.jrmiller.demon.co.uk/products/patch.html

To get best results with this CP feed you need to have a matching dish.
First, yes since this is a CP feed you need a solid dish or one covered 
with mesh, not the barbecue grill type. It is generally accepted that a 
dish feed should have a pattern such that it is 10dB down at the edge of 
the dish. The G3RUH/ON6UG patch has a beamwidth of 125 degrees at the 10dB 
points. So you should have a dish with a subtended angle from the focal 
point of 125 degrees. This corresponds to a dish with an f/D of 
approximately 0.4.

Another thing I think this patch feed has a rotationally symmetric pattern. 
The barbecue dishes are really just a circular dish with a diameter of the 
longer dimension with two slices cut off to make the short dimension. They 
can do this because the linear dipole feed has a pattern which is broader 
in one plane so that it properly illuminates the larger dimension but does 
not overilluminate the shorter dimension. So even with mesh covering the 
barbecue the patch feed would not work well - even if the f/D was a proper 0.4.

How do you compute the f/D of a dish? The formula is f/D = D/16d where
D is the diameter and d is the depth. For your barbecue lay a straight edge 
across the LONG dimension and measure the depth. I have one of the old 
original so-called Myers barbecues. My long dimension is 32 inches and the 
depth is 5.75 inches. This computes to an f/D of 0.35 or a focal length of 
approx. 11.25 inches.

Finally how do you compute the subtended angle of the dish edges from the 
focal point?
The formula is
      tan(angle/2) = (D/2)/(f-D)
As an example I have a solid round dish with D = 62.4cm. d = 9.5cm
Using these formulas I compute f/D = 0.41 and subtended angle = 125 
degrees, a perfect match for the G3RUH patch feed.

Hope this helps.
ron w8gus.

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