[amsat-bb] Re: Re HELIX REFLECTOR?

[Edward R Cole]

For space oriented antenna (satellite, eme, radio astronomy) that are 
elevated to the sky, the height above ground should not matter since 
ground-gain is not a factor once a minimum of elevation occurs.  Many 
antennas in these services are mounted close to ground to achieve 
several objectives:
1) mechanical stability
2) lowered exposure to man-made RF interference
3) cost
4) maintenance access
5) lower physical exposure (wind area).

The response to radiation in other than the prime direction is very 
important to reduce both interference and thermal ground noise.  F/B 
is just one of these parameters.  Sidelobe level is another.  It is 
not primarily a pursuit of maximum gain that is important for maximum 
receive sensitivity.  The parameter that measures this is called G/T 
ratio  (gain/temperature ratio).  So often some gain is sacrificed to 
achieve lower (noise) temperature.

One should be sure that the E-M field of the driven element of the 
antenna system is not affected by ground or other nearby objects that 
can either de-tune of distort the pattern.  But other than that 
height is not a factor.  Only when the antenna elevation is near the 
horizon is this not true.  Typically eme'rs see ground-gain effects 
only below about 10-degrees elevation.

73, Ed

The Ohio State Big Ear Radiotelescope reflector was mounted with the 
lower edge on the ground.  The ground between the parabolic sector 
antenna and the tiltable reflector was a conducting metallic 
surface.  But this is diverges from the original topic.



At 05:21 PM 6/5/2010, i8cvs wrote:
>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
>
>
>
>
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73, Ed - KL7UW, WD2XSH/45
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