[amsat-bb] Re: HELIX REFLECTOR?

[i8cvs]

----- Original Message -----
From: "Pete Norris, K1HZU" <k1hzu at yahoo.com>
To: <amsat-bb at 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