Dale: Yeah, I think that you're basically right on all points, but I'm relying on 35-year-old memories from my AP physics textbook, the one that went end-over-end down the median of I-15 with me and my '65 VW Beetle when we tried to defy Newton's 2nd Law of Motion one very cold December morning in 1971. Or was it the 1st Law? I've always been grateful to UDOT for modifying the design of the steel guardrails the previous summer so that I vaulted over the railing rather than getting impaled on it when our respective molecules collided. But I digress... In the days of the six- or seven-foot satellite TV dishes I wondered why it wouldn't work to build a cylindrical parabaloid (again, simplicity and cost) rather than pay the hundreds or thousands that the dish makers were charging. And, do those dishes, that I assume are now dinosaurs, have any use in amateur radio astronomy, or are they "tuned" (wrong terminology?) to the wrong wavelength? Do you, Dale, or does anyone else know the answers? I don't have enough hobbies and I've been thinking about building that aborted radio telescope that I never completed in high school. ;-) Maybe I'll even try to locate my old Beetle and restore it, too. The physics text could even still be inside. Who knows, I might even find that old stash of...never mind. Kim ----- Original Message ----- From: "Dale Hooper" <Dale.Hooper@sdl.usu.edu> To: "Utah Astronomy" <utah-astronomy@mailman.xmission.com> Sent: Thursday, September 08, 2005 4:27 PM Subject: RE: [Utah-astronomy] cylindrical parabolic antenna [was: amateurradio astronomy ] | Hi Kim, | | I was thinking a bit more about the cylindrical parabaloid and a few | synapses finally started to fire... | | I believe at the focus of the parabola you have basically a single | dipole antenna? My understanding is that the dipole needs to be an odd | number of half-wavelengths in size. | | This would basically be a hybrid and I think would have some of the | advantages/disadvantages of the other types. | | Advantages: | Aperture - as you mentioned you could build a good sized antenna using | some plywood and mesh or screen | Price - Very cheap to build and you could use scraps from other projects | | Disadvantages: | Noise rejection - Since your feed is a single dipole I think you would | get a lot of off axis noise. | Bulkiness - It would probably get fairly heavy if the length was much | more than a meter. | | I remember now that back in high school there was a kid at the regional | science fair (at Weber State) that built a cylindrical parabolic radio | telescope. (He probably stole your idea since this was 1977 <g>). He | covered the parabolic surface with aluminum foil. He said it worked | pretty well but I didn't know enough to ask any good questions. As I | recall it was obviously movable but was probably more semi-portable. | | Clear skies, | Dale. | | > ...snip... | >I finally determined to build a Yagi | > array, but not before I realized that a cylindrical parabaloid would | have | > also been very easy to build and with very little skill or exotic | > materials | > required. I don't recall why I settled on the Yagi array, I have | simply | > forgotten too much of the physics involved. And that finally brings me | to | > a | > question I have for you and others: What would be the | > advantages/disadvantages of one type of antenna over another? I would | > still | > rule out the conventional parabolic dish because of the relative | > complexity | > of its construction for a basement tinkerer like me. However, the | > cylindrical parabaloid is still quite easy to construct with some | plywood, | > a | > simple plot of a parabola on a piece of paper for a pattern, and some | > hardware fabric or insect screen to cover the surface. Can anyone | > enlighten | > me? Remember, you need to explain in terms my few remaining viable | brain | > cells can comprehend. | > | > Thanks all. | | | _______________________________________________ | Utah-Astronomy mailing list | Utah-Astronomy@mailman.xmission.com | http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy | Visit the Photo Gallery: http://www.utahastronomy.com | | ______________________________________________________________________ | This e-mail has been scanned by Cut.Net Managed Email Content Service, using Skeptic(tm) technology powered by MessageLabs. For more information on Cut.Nets Content Service, visit http://www.cut.net | ______________________________________________________________________ | |