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

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
> 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
> 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
> >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
> >2.4 gig desired. It's a neat trick...and you can avoid cutting by using
> >1/8 wave line with a small tuning cap across the end and then tune for
> >null. (I think it was an 1/8 wave ...it's been a long time since I did
> >
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