A Brolly Dish for AO-40
Build a portable 1.2m parabolic dish antenna for AO-40 satellite communication using an umbrella frame.
Description
Constructing a portable, high-gain antenna for AO-40 satellite operations presents unique challenges, particularly regarding mechanical stability and parabolic accuracy. This resource details the build of a 1.2-meter "brolly dish" antenna, utilizing a non-conducting fiberglass umbrella frame as its foundation. The project outlines a method for achieving a parabolic shape using stressed aluminum fly screen mesh, guided by practical geometry and a temporary dowel template. Key steps include selecting an appropriate umbrella with a suitable f/D ratio (ideally >0.25), removing the original fabric, and precisely cutting and attaching eight segments of fly screen to the struts to form the reflective surface.
The construction process, which took approximately five hours for the author, G6LVB, resulted in a dish with an f/D of 0.27 (depth=270mm, diameter=1160mm, f=310mm). The article also describes a modification to a TransSystem AIDC feed, incorporating a PCB reflector behind the dipole for easier mounting. Performance tests at a squint angle of 15 deg and a range of 50,000km yielded a signal-to-noise ratio of 33dB on the S2 beacon and 23dB for SSB signals, indicating strong reception. The author notes that the modified umbrella may not close fully without risking surface disfigurement.