[amsat-bb] Re: Re HELIX REFLECTOR?
andres at msu.edu
Sat Jun 5 14:10:57 PDT 2010
john, whats a good rule for height of the antennas to avoid large
back lobes reflecting, ie how many wavelengths up should the
antennas be? Thanks...
On Saturday 05 June 2010 17:07:15 John Belstner wrote:
> 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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