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Re: R: 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.)

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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