Hi John,
I agree with you.
I have in my hands the book "RADIO ASTRONOMY" by John Kraus ISBN 07-035392-1
This is the text of page-200
"An example of a partially steerable (meridian transit) array antenna is presented in Fig.6-41 This antenna,built in 1952 at the Ohio State University radio observatory, consists of an array of 96 helical-beam antennas, each of 11 turns, mounted on a tiltable steel grounded ground plane 160 ft long (east-west) by 22 ft wide. At a wavelenght of 1.2 meters the beam width measured 1 degree in right ascension by 8 degrees in declination."
My comment:
As seen from the photograph 6.41 the tiltable steel ground plane seems to be mounted at no more than 10 to 12 ft from the ground so that when the reflector is very large it seems that the high of it from the ground is not very important both for gain and front to back ratio.
In this array the tiltable steel ground plane is 160 ft long and 22 ft wide with 24 helices in the longer side and 4 line of helices in the wide side (24 x 4 = 96 helices) so that the total ground plane area is 160 x 22 = 3520 square foot and each helix reflector takes 3520 / 96 = 37 square foot or about a square surface of 6 x 6 foot or a round area of 3.4 square meters with a diameter of 2.08 meters.
Since the operating wavelenght of the radiotelescope is 1.2 meters the reflector diameter for each helix antenna has been made large 2.08 / 1.2 = 1.73 wavelenght and probably this is why a tiltable steel ground plane made so large can be mounted very close to the ground surface without affecting gain, front to back ratio and without to take too much noise at 290 kelvin from the ground.
73" de
i8CVS Domenico
----- Original Message ----- From: "John Belstner" [email protected] To: "Clare Fowler" [email protected] Cc: "amsat-bb" [email protected] Sent: Saturday, June 05, 2010 11:07 PM Subject: [amsat-bb] Re: Re HELIX REFLECTOR?
Just another $0.02 to add.
You will find that the size and shape of the reflector will not affect the forward gain as much as it does the F/B ratio. It depends on what is important to you and (of course) how high you are above the ground. Even for satellite operation pointing up, large back lobes reflecting off the ground can adversely affect the forward pattern when the antenna is mounted only 6-8 feet above ground.
On Jun 4, 2010, at 10:13 PM, Clare Fowler wrote:
To add to the discussion the July/Aug 2007 Amsat Journal has an article covering some gain comparisonmeasurements I made between four 13 turn (2.88 wavelengths) 13cm antennas with different square solid aluminum reflectors. The sizes were 0.56 wavelengths, 0.84 wavelengths, 1.0 wavelength and 1.4 wavelengths. There was no difference between the 0.84, 1.0 and 1.4 wavelengths but the antenna with the0.56 wavelength reflector had 1.5 db less gain.
However for my 70cm helix antennas I followed the Satellite Handbook minimum size of 0.6 wavelengthsor slightly over 16 inches. I used 1/2 inch hardware cloth mesh to keep the weight and windloading down. These antennas have performed well however it appears that they would be a bit better with a somewhat larger reflector.
A brief description and picture of the 70 cm reflector is in the November/December 2005 Amsat Journal article on The Development of a Quarter Wave Match for helical antennas.
Clare VE3NPC
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