[amsat-bb] Phasing lines for 2-m Yagi circular polarized project
tosca005 at umn.edu
Fri Feb 10 18:00:26 UTC 2017
I presume that you know that to get circular polarization, one set of
elements needs to be electrically 1/4 wavelength ahead or behind the other
set of elements. So if one line was 1/2 wavelength long, the other line
would be 1/4 wavelength or 3/4 wavelength long, if the horizontal and
vertical elements were at exactly the same spot on the boom. The more
common method is to set the vertical and horizontal elements at 1/4
wavelength different locations on the boom, and then you only need two
feedlines that are exactly the same length. The advantages there include
the fact that phasing lines' lengths need to take account of the velocity
factor of the coax, whereas placing elements at different boom locations is
equivalent to a velocity factor of 1.0 (air).
So how to get the two feedlines exactly equal? Besides lining them up
side-by-side when cutting them, you can connect one end of the cables to a
2-port power divider (or a simple T connector if you aren't worried about
an impedance bump to your signal source), and the other ends of the cables
to the X and Y inputs of an oscilloscope that has X-Y plotting capability.
For two equal lengths, you should get a perfect diagonal line, and if one
cable is a bit longer than the other, you will get a long, slender,
diagonal ellipse. If the cables differ by a full 1/4 wavelength (or 3/4
wavelength), you would get a perfect circle. If the cables differ in length
by exactly 1/2 wavelength, you would get a perfect diagonal line with the
opposite tilt direction as the zero-difference tracing. Googling "Lissajous
Figures" for more information on that gave me the two pertinent links below:
On Fri, Feb 10, 2017 at 8:30 AM, Dave Mann <cwo4mann at comcast.net> wrote:
> I have prepared my 2-m yagis. Now ready to set them up to use on the
> birds. I'm thinking setting one horizontally and the other vertically,
> spaced about 1 ft apart on a boom. Now, how to measure the coax and
> connect them for circular polarization?
> FYI, when all set up, I'll put the project details on my Dropbox with a
> Request ideas, guidance, advice, comments, please.
> Dave N4CVX
> Sent from my iPad
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