# patch feed and proper dish

• Subject: [amsat-bb] patch feed and proper dish
• From: Ronald Long <rlong3@xxxxxxxxxxxxxxx>
• Date: Sun, 14 Oct 2001 09:35:04 -0400

```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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