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Re: R: false signals-images-harmonics-whatever? FIXED !!



Dom, Greg

I have simulated the effect of a quarter wave open circuit stub at 435MHz 
and 2401MHz.  The stub at 435MHz should provide approx 40dB of notch at 
435MHz and as such should be quite effective at removing  interference 
caused by a 435MHz uplink.  As Dom notes, the stub has an inductive 
reactance at 2401MHz, and the simulation shows that it will  introduce a 
VSWR in the region of 2:1 at 2401MHz, resulting in a "loss" of about 1dB at 
this frequency (in a 50 ohm system).  This could be tuned out if desired 
with a shunt capacitor to ground at the junction of the stub to the antenna 
feedpoint.  The theoretical value for the capacitor is 1.2pF, but some 
experimentation with the value would be needed to allow for inductance in 
the capacitor and any leads.

I have also simulated the effect of a shorted quarter wave stub at 2401MHz 
across the feedpoint of the antenna.  It looks like this will be a much 
less effective interference remover, showing only 6dB rejection at 435 and 
0.85dB at 1269MHz.

73

Charlie G3WDG

   At 04:28 07/01/02 +0100, i8cvs wrote:
>Hi Greg,
>
>An electrical 1/4 wavelenght long open stub at 435 MHz represent a short
>circuit for 435 MHz at the other end connected to the helix feed  for 2401
>MHz because at 435 MHz the impedance of the stub in the point connected to
>the helix is Z = 0 + j0 ohm
>
>By the way a shorted stub 1/4 electrical wavelenght long at 2401 MHz
>represents a very high impedance at the open end connected to the
>helix feed.
>
>This impedance is theoretically infinite at the open end and so its presence
>is not seen as a load by the helix for 2401 MHz wich is not disturbed very
>much by the shorted stub.
>
>Following your idea and using the Smith Chart i have simulated a shorted
>stub 1/4 wavelenght long at 2401 MHz made with a piece of semirigid
>UT 141, Zo=50 ohm coax cable made with PTFE insulation and a copper
>tubing  as the outer conductor.
>
>The velocity factor for the UT 141 is Vf = 0,75 and so the electrical lenght
>of the 1/4 wavelenght shorted stub for 2401 MHz is 23,5 mm wich represents
>  0,25 electrical wavelenght at 2401 MHz
>
>The same lenght of 23,5 mm represents 0,0452  electrical wavelenght at 435
>MHz and 0,132 electrical wavelength at 1270 MHz
>
>If you put this wavelengts in to the Smith Chart you will find that the
>impedance of the shorted stub at the open end connected to the helix is
>Z= 0 + j15 ohm  at 435 MHz and Z= 0 + j55 ohm at 1270 MHz
>
>The shorted stub for 2401 MHz has a very small  inductive reactance at the
>open end of 15 ohm at 435 MHz and 55 ohm at 1270 MHz  and so it can be
>considered a short circuit at 435 MHz and 1270 MHz while an open circuit
>at 2401 MHz
>
>I believe that it works fine to short out  the unwanted uplink signals  that
>compresses the gain of your converter without disturbing to much the wanted
>downlink signals at 2401 MHz
>
>Since the UT 141 has a copper tubing as an outer conductor it is possible to
>solder it to the reflector for the entire lenght of  23,5 mm  and the inner
>conductor to the central pin of the N connector for the helix.
>
>The UT 141 coax cable is very small in diameter,only about 4 mm,it
>is easy to solder and to find.
>
>I  hope this help
>
>73 de i8CVS Domenico
>
>
>----- Original Message -----
>From: Greg D. <ko6th_greg@hotmail.com>
>To: <amsat-bb@AMSAT.Org>
>Sent: Sunday, January 06, 2002 12:02 AM
>Subject: Re: [amsat-bb] false signals-images-harmonics-whatever? FIXED !!
>
>
> > Hi folks,
> >
> > The alternative to a 1/4 wave open stub at 435 would be a 1/4 wave shorted
> > stub at 2401, right?  If so, wouldn't that be easy to build into a helix
> > feed?  I'm thinking of a short piece of wire parallel to the ground plane,
> > with
> > one end attached to the feed point (N-connector), and the other end
>soldered
> > to
> > the ground plane.  234/2401*12 = 1.17 inches long?
> >
> > My thought here is that being a shorted stub, it would put the feed coil
>at
> > ground
> > potential for most frequencies - lightning protection, 432, etc., and
> > probably
> > do a fair job at 1.2 ghz.
> >
> > Im I close?  Is there a reason to use a 1/4 open stub on 435 instead?
> >
> > Greg KO6TH
> >
> >
> >
> > >From: hasan schiers <schiers@netins.net>
> > >To: Jens Schmidt <j.schmidt@paradise.net.nz>, Tracy <k7kcs@attbi.com>
> > >CC: amsat-bb@AMSAT.Org
> > >Subject: Re: [amsat-bb] false signals-images-harmonics-whatever? FIXED !!
> > >Date: Sat, 5 Jan 2002 06:57:16 -0600
> > >
> >
> > >From the foggy depths of my morning mind, I think it is a quarter-wave
>open
> > >stub, acting like a short circuit. At 70cm this calculates out to 5.4
> > >inches,
> > >with a velocity factor of .80
> > >
> > >The idea is to "notch" the 435 mhz offending signal without screwing up
>the
> > >2.4 gig desired. It's a neat trick...and you can avoid cutting by using
>an
> > >1/8 wave line with a small tuning cap across the end and then tune for
>the
> > >null. (I think it was an 1/8 wave ...it's been a long time since I did
>it).
> > >
> >
> >
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>
>
>
>
>
>
>
>
>
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