----- Original Message ----- From: "Pete Norris, K1HZU" <k1...@yahoo.com> To: <amsat-bb@amsat.org> Sent: Friday, June 04, 2010 2:51 AM Subject: [amsat-bb] HELIX REFLECTOR?
> Hi All: > I am rebuilding a 440 MHZ Helix that I built several years ago . It worked very well, but I would like to reduce the size of the reflector to a more manageable size than I had before. The only reference to reflector size I can find is, "minimum 20" ". I may be looking in the wrong places. I would appreciate it, if someone would steer me in the right direction. > Thanks, > Pete, K1HZU > Hi Pete, K1HZU My 15 turns RHCP 70 cm Helix Antenna was built at first with a round aluminum sheet perforated reflector with a diameter of 460 mm ( 0.67 wavelenght) and it worked very well but after enlarging the diameter of reflector to 690 mm (about 1 wavelenght) overlapping to it a perforated aluminum mesh I realized that the gain increases by about 2 dB and the front to back ratio was much better than before. The Helix is made with a non annealed wiredrawn aluminum rod 8 mm in diameter and the boom is made with a very hard plastic pipe 42 mm outside diameter and 31 mm inside diameter originally used for hight pressure oil ducts. Following "ANTENNAS" from John Kraus the lenght of a turn has been made 1 wavelenght long into free space and the pitch angle between turns is about 13.8 degrees while the calculated half-power beam width is about 28 degrees. The matching system between the 150 ohm impedance at the feed point and a 50 ohm coax cable is made using a 1/4 electrical wavelenght impedance transformer with Zo = 86 ohm made with two coaxial tubing. For better performance and not to distort the pattern the antenna is fastened to the rear of reflector and the weight is balanced with a counterweight made with few lead disks. The picture of the above 15 turns helix antenna is visible at i8CVS in QRZ.com I have built two Helix Antennas the first one is a 10 turns with 0.67 wavelenght round reflector used beginning from OSCAR-7 to actually FO-29 and HO-68 and it works very well. The second one is a 15 turns helix with a 1 wavelenght in diameter round reflector and it was used for the uplink from OSCAR-10 to AO40 as can be seen at i8CVS in QRZ.com but unfortunately I cannot use it for LEO satellites because the AZ/EL mount is slow because it was designed for HEO satellites and this is why I pull for P3E ! If someone is interested to built the above antenna for 10 or 15 turns I can send a zipped file with all the electrical and mechanical sized drawings of it. Best 73" de i8CVS Domenico _______________________________________________ Sent via amsat...@amsat.org. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
