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R: R: Antenna and rotor gallery



Hi  Greg,

The metod i have used to match the helix impedance to 50 ohm coax
transmission line is based on a formula wich i found in the following
reference:
"A 50 ohm input impedance for Helical Beam Antenna "by John Kraus W8JK,
IEEE, Transactions on the Antennas and Propagation Vol. AP-25, Nov 1977,
Page 913

The input impedance of the helix antenna,nominally 140 ohm,can be easily
transformed to Z=50 ohm  provided

                                           Z   = 138 log     (4h /d )
                                                                 10

where:

Z =50 ohm =impedance of transmission line

h = high in mm taken from the center of conductor of the helix  and the
       reflector plane and  measured  from the point in wich the helix is
       soldered to the central pin of connector.

d = diameter in mm of the helix conductor

To answere your question,obviously  (h) is the most critical parameter and
if  (h) is to great the resulting return loss is to low and not adjustable
at all.

In order to make (h) variable without soldering  and unsoldering the helix
by cut and try,it is best to cut the 1/4 turn maching strip in order to form
many fins on both side of the wire ( 7 fins in my case )

 Obviously the first and second fins beginnig from the connector pin are the
most sensitive to VSWR variations and this is why they  where bent a little
bit up and down from the reflector while matching and adjusting  for
maximum return loss.

For return loss measurements i have used a 2401 MHz generator,in my case a
1.5 watt transverter along with a directional coupler and a TPM-4 power
meter from SSB Electronic.

At first i got a return loss of  16.5 dB or VSWR=1,35 but finally,after some
refinements i got a return loss of about 20 dB or VSWR=1,22 as measured
 on the bank.

I  have repeated this measurement with the helix  on the dish but,as
expected, the VSWR do not change significantly because the helix
 radiate LHCP to the dish and the dish reflects RHCP to the helix and
so the polarization mismatch  strongly reduce the amount of energy
absorbed by the feed wich in this case is less sensitive to VSWR
variations than a linearly polarized feed measured first on the bank
and than on the dish.

I have included  for you a picture taken during  the above mentioned VSWR
measurement on the dish.

Best 73 de i8CVS Domenico



----- Original Message -----
From: Greg D. <ko6th_greg@hotmail.com>
To: <domenico.i8cvs@tin.it>
Sent: Saturday, June 16, 2001 7:39 AM
Subject: Re: R: [amsat-bb] Antenna and rotor gallery


> Hello Domenico,
>
> Thank you very much for your excellent pictures.  I notice that you
> have bent the segments of the 1/4 turn matching strip up over most
> of its length, and down near the N connector.  What is the purpose
> of the bends, and how did you determine just how much to bend them?
> Or, does it not make a difference?
>
> 73s,
>
> Greg  KO6TH
>
>
> >From: "i8cvs" <domenico.i8cvs@tin.it>
> >To: "Greg D." <ko6th_greg@hotmail.com>
> >Subject: R: [amsat-bb] Antenna and rotor gallery
> >Date: Fri, 15 Jun 2001 23:28:55 +0200
> >
> >Hello Greg
> >
> >Please find here included two pictures of  3 turns helix feed for my
prime
> >focus 1,2 meter dish F/D=0,45
> >maybe they are useful for you.
> >
> >Best 73 de i8CVS Domenico
> >
> >----- Original Message -----
> >From: Greg D. <ko6th_greg@hotmail.com>
> >To: <amsat-bb@AMSAT.Org>
> >Sent: Friday, June 15, 2001 9:16 AM
> >Subject: [amsat-bb] Antenna and rotor gallery
> >
> >
> > > Hi folks,
> > >
> > > Thanks to all for your responses.  It sounds like I came close to the
> > > "right" way to build a helical feed for the 2.4 ghz AO-40 downlink,
> > > but there is room for improvement at the connector.
> > >
> > > I've uploaded a bunch of pictures of my setup, if you're curious.
> > > The thumbnail photos haven't been resized yet, so the page loads
> > > very slowly - I'll fix that tomorrow.  Any comments or suggestions
> > > on the antenna details, now that you can see what I've done, would
> > > be appreciated.  I'm heading back to the roof on Saturday for the
> > > next set of tweeks.
> > >
> > > The site is at: http://www.jps.net/gregd/hamsite.htm
> > >
> > > 73s,
> > >
> > > Greg KO6TH
> > >
> > > _________________________________________________________________
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> > >
> > > ----
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> ><< i8CVS5.jpg >>
> ><< i8CVS6.jpg >>
>
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>







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