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Re: S-Band Antenna Comparisons, etc.



I can assure you that the grid reflector (which you
call a bar-b-que grill) MUST be aligned with the feed
polarization to be effective.  My previous employer's
productline for over 25 years depended on this.  (I
raised my family on knowing this I might add.)

In order to be a reflector , the grid spacing must be
considered a "waveguide beyond cut-off", otherwise the
feed radiation will 'leak thru' and you blow your front
to back.  I have a patent on increasing the
effectiveness of this grid by changing the geometry of
the grids.

If you place the feed cross to the grids, you really
blow the pattern and have virtually no reflector.

One of the features of the product was the increased
rejection of the cross polarized signals, allowing the
antennas to reejct interference better.  This is
demonstrated by the antenna patterns (which I still
have around here somewhere or go tot he new owners of
the old Mark Products antenna line .  see
http://www.tripointglobal.com/ ).

Icing on the reflector DOES increase the weight and
wind loading but the wind loading is no greater than a
solid reflector.  In fact, one option of commercial
antennas was the installation of heating wire behind
the grid to heat the grids above the ice forming
temperature.

As someone mentioned, the grid reflector behind a helix
feed will act as a filter and literally convert the
operation to a linear antenna and lose 3 dB of the
reflector illumination.



gil, w9bub
http://members.home.net/gil9/


----- Original Message -----
From: "Bob Snyder" <kd1vv@mediaone.net>
To: "Jon Ogden" <na9d@mindspring.com>
Cc: "NS1Z" <ns1z@arrl.net>; <amsat-bb@AMSAT.Org>
Sent: Monday, January 08, 2001 7:24 AM
Subject: Re: [amsat-bb] S-Band Antenna Comparisons,
etc.


| Not to belabor the point, but the reflector
absolutely DOES influence
| the polarization of the signal received (or
transmitted) by the feed, if
| it's not solid or fine mesh.  Reflectors reflect by
having currents
| induced on them.  That's why their surfaces must be
conductive.
| Circular polarization induces circular currents,
which can't flow on a
| reflector with just parallel wires.  Think of the
bar-b-que reflector as
| a polarization filter.  Try to illuminate it with a
circularly polarized
| feed, and out comes linear polarized signals. Only
the vertical (i.e.
| parallel to the reflector wires) polarization
component is reflected;
| the horizontal component goes right through.
|
| Re-read Joe's (K0VTY) post earlier this morning.  He
cites Kraus to
| verify this point.  It's also mentioned in Johnson's
"Antenna
| Engineering Handbook" (3rd ed. pg.30-19).
|
| Bob
|
|
| Jon Ogden wrote:
| >
| > on 1/7/01 4:08 PM, NS1Z at ns1z@arrl.net wrote:
| >
| > > Hmmmm?? Is the spacing of the framework that bad?
Usually the design is such
| > > that it approximates a full metal back but is
ribbed to lighten it and
| > > reduce (!) wind resistance.... Something about
waveguide beyond cutoff?????
| >
| > The reflector is just that -> it reflects RF power
to the feed.  So it
| > matters not which way the reflector grill runs.  As
Dave Tipton pointed out,
| > what matters is the feed.
| >
| > Your comment about reducing wind resistance or wind
loading is interesting.
| > This is the conventional wisdom behind these.
However, I've talked to at
| > least one cellular/PCS operator up in my area about
these.  He hates them
| > and uses solid dishes.  He said solid dishes have
LESS windloading in
| > conditions of heavy ice and snow.  He said ice
build up on the BBQ antennas
| > creates a tremendous load on it.  So he didn't use
them.  Good point.  I
| > suppose that those of you down south who don't have
regular ice or snow
| > build up wouldn't find that a problem.  But to
anyone who has a regular
| > winter like we are having in Chicago, think twice
about getting a BBQ dish.
| > In fact, its even worse for folks in say, St. Louis
where you get more ice
| > storms than we do.  You get ice built up on that
sucker and it will get
| > HEAVY and you gain in windloading surface area as
well.
| >
| > 73,
| >
| > Jon
| > NA9D
| >
| > -------------------------------------
| > Jon Ogden
| > NA9D (ex: KE9NA)
| >
| > Member:  ARRL, AMSAT, DXCC, NRA
| >
| > http://www.qsl.net/ke9na
| >
| > "A life lived in fear is a life half lived."
| ----
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of AMSAT-NA.
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