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Re: S-Band Antenna Comparisons, etc.




If I remember rightly the holes in the mesh should be no greater than one
tenth wavelength at freq of operation.

73 Simon GM4PLM






"Gil Kowols" <gil9@home.com>@AMSAT.Org on 09/01/2001 03:46:17

Sent by:  owner-AMSAT-BB@AMSAT.Org


To:   "Frederick M. Spinner" <fspinner@hotmail.com>, <amsat-bb@AMSAT.Org>
cc:

Subject:  Re: [amsat-bb] S-Band Antenna Comparisons, etc.



Yes, that will work fine.

There was an engineer for NW Bell who took one of our
15 ft grid dishes that was only good to 900 MHz (4"
spacing between grides made of 3/4" tubing), covered it
with aluminum screening , and used it as a TVRO antenna
at 3.9 GHz for both polarizations.  Worked great.  Once
you eliminate that grid spacing problem, the only other
problem is accuracy of the parabolic shape.



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


| 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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