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Re: Prime Star Dishes etc.




----- Original Message -----
From: "Jon Ogden" <na9d@mindspring.com>
To: "Keeth Jim (Indy)" <KeethJ@tce.com>; <amsat-bb@AMSAT.Org>; "'Bruce
Bostwick'" <lihan@ccwf.cc.utexas.edu>
Sent: Thursday, January 04, 2001 2:36 PM
Subject: Re: [amsat-bb] Prime Star Dishes etc.


> on 1/3/01 3:02 PM, Keeth Jim (Indy) at KeethJ@tce.com wrote:
>
> > If you do the math, you find the image freuqency band is 9.8 to 10.3 GHz
...
> > pretty close to 10.451 GHz!  The problem is, there is a microstrip band
pass
> > filter which passes only the 12.2 to 12.7 band and provides pretty good
> > image rejection.  Someone who is very clever might find a way to cut
that
> > filter out and make it work for the 10.4 GHz band.
>
> Depending on how the circuit is laid out and the type of substrate used,
> it's probably pretty easy.  At 10 GHz, a microstrip filter will likely be
> several open stubs spaced certain distances apart.  The transmission line
> should be a solid line through all theses stubs.  If you go in with an
> Exacto knife or Dremel tool you can just remove the stubs and viola! no
more
> filter!
>
> If it is a stripline design (basically a microstrip with another layer of
> dielectric and then a ground plane on top of the microstrip), it won't be
so
> easy to modify (and may not be possible).

If it is the more common approach, that is several tuned lengths of
microstrip almost parrallel to each other but not touching, the technique I
have heard about but not tried yet is to solder 2mm x 2mm squares of brass
shim on the ends of these microstrips which adds capacitance and lowers the
frequency of the band pass filter. Amateurs in Europe have do this and
managed to move the band pass to 10.0 - 10.5 GHz. This technique works best
if you can sweep the band with a signal generator and watch the output.
Something like a toothpic can be used to slide the square arround to
optimise the filter. Unfortunately it is difficult to get the LNB to operate
any where near correctly with the "top" off but the band pass filter
shouldn't be that fussy and so if you can see something happening you might
be OK. When you put the "top" back on it should all work to specifications
again but with the band pass in the right place.

WARNING: I am not a expert in these techniques.

Maybe someone on this BB who has done this sucessfully might like to
comment.

Murray Peterson
VK2KGM
Sydney, NSW, Australia

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