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*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. ---- Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA. To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org

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