When we received a parabolic reflector 'on loan' from a local cellphone
company, some thinking was in order. We were planning several other
antennas of this type for weather satellite tracking, so some time
to gather information was needed.
As a helix is easy to construct, and very tolerant of size errors, it's an excellent plaything.
According to literature, the minimum number of turns for a helix is between 3 and 5. Only after 3 turns, acceptable circularity is realized. Still, with the f/d ratio of the available reflector, we needed near 3 turns to illuminate it efficiently. So maybe this solution is a bit of a compromise.
Here's a page with images of a 3-turn helix
that was constructed for reception in the 1690 MHz Wefax band
(geostationary weather satelliteslike GOES).
Helical Antenna Design
Bob Atkins KA1GT, The ARRL UHF/Microwave Experimenter's Handbook,
ISBN 0-87259-312-6 (More info at Amazon.com)
Karl Rothammel Y21BK, Antennenbuch, Telekosmos Verlag, ISBN 3-440-04791-1
How to Make a Simple 2.425GHz Helical
Jason Hecker, http://www.air.net.au/~jason/heliaerials.html
|(c) John Coppens ON6JC/LW3HAZ|