[Mep-dev] Dish Defocusing Addressed at MUD2009

[Michelle]

Hi everyone!

Defocusing an amateur radio microwave dish has been proposed as a way to widen the beamwidth in order to make it easier to find other stations. We've talked about using this method to help achieve station discovery. Paul Wade W1GHZ took up the topic at the 2009 Microwave Update in his talk and article “Parabolic Dish Focus, Zoom, and Tilt”.

Article available here:
http://www.w1ghz.org/antbook/conf/Parabolic_Dish_Focus_Zoom_and_Tilt.pdf

In the proceedings, on page 44, he introduces the topic by explaining, “It is occasionally suggested that one can adjust the beamwidth of a dish by moving the feed in and out, to act like a zoom lens on a camera. This would presumably make it easier to find other stations.”

Paul Wade’s findings were that the 3-dB beamwidth of a prime-focus dish does not increase significantly until the feed has moved about one wavelength. At one wavelength, the gain has fallen by 10dB. There is no “gradual zoom”. As Paul puts in in the proceedings, “The sharp three-dB beamwidth hardly changes until the feed is moved a full wavelength, but the gain decreases as the feed is moved from the focus. So what we have is not a zoom control, but a gain control.”

I think these results show a drawback to using dish defocus as a station discovery method. The offset Gregorian feed, which Paul mentions as a successful zoom antenna feed, are more complicated than the single-feed, single-reflector antennas most amateurs are using.

A Gregorian antenna uses a concave sub-reflector. A sub-reflector is, quite often, shaped like a second much-smaller dish. From the feed, the energy is transmitted towards the sub-reflector, which reflects the energy towards the reflector (main dish). 

Here is an article with some nice photos of large Gregorian sub-reflector type antennas. 
http://www.cv.nrao.edu/course/astr534/RadioTelescopes.html

Cassegrain sub-reflectors are also featured on this page. As you can see, the difference is that the sub-reflector is convex.

Another reflecting element means more work. 

I’m therefore leaning towards dropping the defocusing the dish method from station discovery. What do you all think?

Michelle W5NYV
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