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Re: S-Band Antenna Comparisons, etc.



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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