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Re: S-Band Antenna Comparisons, etc.



Also look at the installation instructions for any commercial point to point
grid pack antenna - the grids MUST run in the direction of the polarisation.

Rex  VK8RH

----- Original Message -----
From: "Bob Snyder" <kd1vv@mediaone.net>
To: "Jon Ogden" <na9d@mindspring.com>
Cc: "NS1Z" <ns1z@arrl.net>; <amsat-bb@AMSAT.Org>
Sent: 8 January 2001 22:54
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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