Re: [amsat-bb] Dish

[John Franke <j.m.franke@xxxxxxxxxxxxx>]

Hi,

I am one of the writers of the NASA Tech Brief.  The design was for 
about 11 GHz where one wavelength is one inch.  Hence the ridges 
could befabricated from stock quarter-inch thick aluminum.

John  WA4WDL


>Actually, there are several versions of "flat plate" antenna that have
>existed commercially. A few years ago I saw a TVRO antenna that was actually
>a Fresnel lense. This flat plate antenna still required a feed horn at he
>focus point. Original prototypes of this antenna had the lense in front of
>the feed, focusing the energy into the feed. Later work by NASA showed that
>because the Fesnel lense has a focus point on both sides of the antenna,
>maximum efficiency was not achieved with the lense in front of the feed. The
>NASA variation put a ground plane reflector at a precise point behind the
>lense and placing the feed in front of the lense at the focal point. Energy
>reflected from the ground plane was refocused back toward the feed. This was
>written up in NASA Tech briefs 6 or 7 years ago.
>
>Another form of flat antenna simulates a parabolic reflector using
>unterminated dipole elements strategically arranged on a flat plate. The
>length and position of the dipoles cause them to act as lossless
>re-radiators that shift the phase of the re-radiated signal. By simulating
>the phase shift of the wavefront across a parabola, the energy is focused at
>a focal point, just as it would be with a physical parabola. A feed at the
>focal point is used just like in a conventional reflector. This application
>is patented by Malibu Research in California. Their website is
>www.maliburesearch.com .
>
>Just about every Inmarsat terminal shipped these days employs a flat panel
>antenna. These are all patch arrays fed in phase. No feed is required since
>these are not lense antenna's. The majority of Inmarsat antenna's come from
>one manufacturer in South Africa, Omnipless, www.omnipless.com . In my
>experience, a well designed patch array can't be beat for long term
>performance in a transportable environment. On the other hand, it's hard to
>beat the dB/$ ratio of a piece of formed aluminum! Every type of antenna has
>advantages and dis-advantages. It all comes down to applications and what's
>important to YOU.
>
>Howie, AB2S
>
>----- Original Message -----
>From: "laura halliday" <marsgal42@hotmail.com>
>To: <amsat-bb@amsat.org>
>Sent: Wednesday, July 24, 2002 5:23 PM
>Subject: Re: [amsat-bb] Dish
>
>
>  > "Ronald A. Stunden" <ve7yc@shaw.ca> wrote:
>  >
>  > >Out of curiosity has anyone tried a flat plate dish?? I remember that we
>  > >went to af flat dish for the aircraft radar, But I can't really remember
>  > >why. Ron VE7YC
>  >
>  > Huh? A flat plate won't focus anything, so it would make
>  > a lousy antenna. Not even worth trying.
>  >
>  > Maybe you had some other kind of antenna in mind?
>  > Most of the "flat plate" antennas are actually phased
>  > arrays.
>  >
>  > Laura Halliday VE7LDH     "Que les nuages soient notre
>  > Grid: CN89lg                    pied a terre..."
>  > ICBM: 49 16.57 N 123 0.24 W        - Hospital/Shafte
>  >
>  >
>  > _________________________________________________________________
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