[Date Prev][Date Next][Thread Prev][Thread Next] - [Date Index][Thread Index][Author Index]

RE: Patch Antennas



Ah, theory vs. reality....

The thing that probably saves the patch fed offset fed dish crowd is one of 
the reasons that offset fed dishes exist at all. (They give better G/T for 
space comms than prime focus dishes assuming a proper feed.)  Because of the 
20 plus degree "slant" of where the parabola slice points, the overspill 
points up and not at the ground (Yah, they could still point at a tree or 
building though), and the sidelobes point up into space somewhat and not 
along the horizon.  So even though overillumination and sidelobes hurt, they 
don't hurt as much as they do on a prime focus dish.

Another interesting possibility would be to try to do a phased array of, 
say, four patches at the feedpoint of an offset fed.  The trick, of course 
is both the physical size and the phasing, not something you can whip up in 
a half hour like a air-dielectric patch with a choke ring.  (I literally did 
that last night, iteration three of my patch feed)

Probably a 5-7 turn with a choke ring (cavity?  I think a choke ring is a 
more accurate discription) is the best a homebrewer can do at this point for 
an offset fed effective f/d 0.7 dish.  A horn feed, however has proven to be 
efficient according to N1GHZ and has some merit for experimentation on 
offset fed dishes.  The horn would be physically rather large for a 18" DSS 
dish @ 2.4 GHz, though.

Some day I'll have to try an offset fed dish, but I've been QUITE happy with 
a series of homebrew patches with my prime focus dishes.

Fred W0FMS



>From: "Howard Long" <howard@howardlong.com>
>To: <keith@gtonline.net>, <amsat-bb@AMSAT.Org>
>Subject: RE: [amsat-bb] Patch Antennas
>Date: Thu, 15 Nov 2001 13:54:40 -0000
>
>Hello Keith
>
> > Can a patch antenna be used to feed an offset dish?
> > Or must it be a prime focus dish.
>
>Generally I'd say 'not recommended' because the illumination angle for an
>offset fed dish is quite small, so the feed would overspill and thus 
>degrade
>your signal as it picks up extra ground noise from the overspill.
>
>However one must always put this in the context of the real world where a
>number of hams have been very successful doing just this.
>
>73 Howard G6LVB
>
>----
>Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
>To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org


_________________________________________________________________
Get your FREE download of MSN Explorer at http://explorer.msn.com/intl.asp

----
Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org



AMSAT Top AMSAT Home