[amsat-bb] Re HELIX REFLECTOR?

[i8cvs]

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" <jbelstner at yahoo.com>
To: "Clare Fowler" <clarefowler at rogers.com>
Cc: "amsat-bb" <amsat-bb at amsat.org>
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