----- Original Message ----- From: "Pete Norris, K1HZU" k1hzu@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