R: R: [amsat-bb] phase shift with coax

["i8cvs" <domenico.i8cvs@xxxxxx>]

----- Original Message -----
From: John P. Toscano <tosca005@tc.umn.edu>
To: <Stan@Vandiver.com>
Cc: i8cvs <domenico.i8cvs@tin.it>; amsat-bb <amsat-bb@AMSAT.Org>
Sent: Wednesday, July 11, 2001 1:36 AM
Subject: Re: R: [amsat-bb] phase shift with coax


> Stan Vandiver wrote:
> >
> > Hi Domenico and all,
> >
> > On 10 Jul 2001, at 17:16, i8cvs wrote:
> >
> > > As you realize this coax lines uses crimped PL connectors and i  see
> > > that your 10-5/8" measurement refers only to the lengh of the outside
> > > black jacket of the cable that you see between the crimped  ends  of
> > > the PL connector and not to the entire line leght considering in it
> > > both lenghts of the connector itself .
> >
> > These indeed seem to be factory crimped pieces that I have... but
> > the length that I gave is from tip-to-tip, not just the jacketing
> > between connectors.  Another friend has the same 10-5/8"
> > dimension tip-to-tip too.  So its interesting that your cables are
> > different.
> >
> > Well, I think that I have established that everyone uses the free
> > space formula for calculating wavelength on this cabling question.
> > If I have time, I will cut a "long version" (by the formula) and try to
> > test it out.... or if not, I will just cut to what seems to be the
factory
> > dimensions and run with it.  I'm intending to use this antenna
> > around the end of July for
>
> Another pseudo-random thought:  You could build a 2-port power
> divider to do the impedance transformation and not have to deal
> with the 75 ohm cables.  They are a bit expensive to buy, but
> surprisingly easy to build out of square aluminum tubing for the
> outside, cut a couple of inches longer than a quarter wavelength,
> round brass tubing for the inside, cut to exactly a quarter
> wavelength (velocity factor = 1.0 for air dialectric), three
> N connectors and a little solder, and some mounting screws for
> the panel-mount N females.  Add a hacksaw, ruler, drill, measuring
> tape, and soldering iron, and away you go.  The ratio of the inside
> diameter of the square tubing to the outside diameter of the round
> tubing determines the impedance of the transformer, so the ratio
> changes depending on whether you're building a 2-port, 3-port,
> 4-port, etc. power divider.
>
> For a 2-port divider, using 1-inch outside width square aluminum
> stock that is 1/16 of an inch wall thickness, the inside width
> is 7/8 of an inch.  A 1/2 inch outside diameter brass tubing
> section gives the proper ratio.
>
> Of course, since you're in a hurry, you may want to go with the
> coaxial cable transformer and save the power divider project for
> another day.  (The power divider method still requires you to
> make two 50-ohm feedlines, one 1/4 wavelength longer than the
> other, to shift the phase for circular polarity.)
>

Hello John,

If the impedance of each antenna is 50 ohm and if i use two 50 ohm
feedlines of any lenght but one 1/4 wavelenght longer than the other i get
only the phase shift for circular polarization but not the impedance
matching.

Infact connecting both lines in parallel to a "T" connector i get an
impedance of 25 ohm.

In order to match 25 ohm to a 50 ohm feed line i need a transformer made by
a line 1/4 electrical wavelengt long wich impedance is SQR ( 25 x 50 )= 35
ohm and i have to made it with tubing or using two 75 ohm lines each
1/4 electrical  wavelenght long connected in parallel.

This is more complicated than to match the impedance using  two 75 ohm lines
each 1/4 electrical wavelenght long as used with the original phasing
harness on the A144-20 T antennas.

In this case infact if the antenna impedance is 50 ohm and the 1/4
wavelength transformer line is 75 ohm the transformed impedance emerging to
the open end of this line is 100 ohm and two 100 ohms impedances connected
in parallel to a "T" connector make a direct 50 ohm mach to the RX/TX 50 ohm
line using a very simple and economical  soluction.

73 de i8CVS Domenico


> Feeding a low-level 146 MHz signal into the whole power divider
> plus phase shifter assembly, with the two antenna connectors
> attached to the X and Y inputs of an oscilloscope should give
> a circular trace if the lengths of the phase shift part of the
> cabling are correct.  If the lengths are a little bit off, the
> pattern looks like a diagonal ellipse.  (Which is why it is
> called elliptical polarity.)  And if the lengths were incorrect
> by a full quarter wavelength, you'd see that ellipse shrink all
> the way down to a diagonal line.  Performing the same test with
> only the impedance transformer part of the coax assembly should
> give the diagonal line if the two 1/4 wavelength sections are
> the exact same length, though it can't tell you if both sections
> are too long or too short by the same amount. . .
>
> 73 de KB0ZEV
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