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Re: S-Band Antenna Comparisons, etc.





"Frederick M. Spinner" wrote:

> Ok, can the idea of facing the dish with aluminum window screening (yes, I
> know that windloading will be heavier) solve this problem somewhat?  I know
> there will be the
> issue with the dishes being 2ft in one axis and 3 ft in the other, but
> otherwise I imagine that that effect would
> be countered in this way.
>
> Fred W0FMS
>
> >From: "Gil Kowols" To: "Bob Snyder" , "Jon Ogden" CC: "NS1Z" , Subject: Re:
> >[amsat-bb] S-Band Antenna Comparisons, etc. Date: Mon, 8 Jan 2001 13:01:20
> >-0600
> >
> >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" To: "Jon Ogden" Cc: "NS1Z"
> >; 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." | ---- | Via the amsat-bb
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