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



Hi Charles,and Greg

I cannot simulate the effect of a shorted quarter wave stub at 2401 MHz
connected across the feed point of the 2401 MHz helix  and so i cannot
simulate its ability to reject the incoming  power at 435 MHz  from the S
band converter input.

Unfortunately i have not a software for such type of simulation

The only think i can see on the Smith Chart is that a piece of 1/4
electrical wavelenght long  of 50 ohm transmission line at 2401 MHz
shorted at one end show an inductive reactance of  about 15 ohm
at 435 MHz where it is only 0,0452 wavelenght long at this frequency.

An inductive reactance of 15 ohm at 435 MHz is provided by an inductance
of about 5,5 nH (nanohenry)

An inductance of 5,5 nanohenry can be built using a very small square loop
made with 0,8 mm copper wire in wich the lenght of each leg is 9 mm long
internally separated by a distance of 4 mm

I did  and measured this inductance to day and it is about big like
this  |_| shaped jumper.

In a few minutes it is possible to build this inductance and solder it
directly across the connector central pin and the flange of it without
adding any other lenght of wire.

Now it is necessary to remove the S band converter and connect the
 2401 MHz helix to a 435 MHz RF generator or TX and measure the SWR
of the system at 435 MHz

If the SWR is very high it means that the system return loss is very low and
the feed reject back part of  the 435 MHz  incident power.

As soon we have the return loss it is possible to made many other
calculations.

Alternatively,for a more accurate estimation  it is possible to build the
23,5 mm long shorted stub using the UT 141 semirigid coax cable,made
the same SWR measurements,connect again the S band converter to
the helix,the 435 MHz TX to its antenna and see the results receiving
AO40 while using the 70 cm uplink

At the end of this measurement dont forget to remove the 5,5 nH inductance
from the helix !

I am very curious to  know the SWR of the system at 435 MHz with a jumper
like that connected across the helix connector but unfortunately i cannot
work on the roof  now days because it is very cold  here !

Can you try to do this experiment ?

73 de i8CVS Domenico

----- Original Message -----
From: Charles Suckling <charles.suckling@ntlworld.com>
To: i8cvs <domenico.i8cvs@tin.it>; Greg D. <ko6th_greg@hotmail.com>;
AMSAT-BB <amsat-bb@AMSAT.Org>
Sent: Monday, January 07, 2002 10:50 AM
Subject: Re: R: [amsat-bb] 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).
> > > >
> > >
> > >
> > > _________________________________________________________________
> > > MSN Photos is the easiest way to share and print your photos:
> > > http://photos.msn.com/support/worldwide.aspx
> > >
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> >
> >
> >
> >
> >
> >
> >
> >
> >
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