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Re: HELP please AO40 RHC Crossed Yagi connections



Darren Rainer wrote:
 
> Hi all,I am currently setting up for operation on AO40 but am a bit
> unsure how to connect up a F9FT Tonna 435MHz 2 X 19ele crossed Yagi
> for RHC polarisation.  Now both yagis on the boom are 50 ohms. The
> horizontal Yagi is positioned 20cm in front of the vertical Yagi on
> the boom.

To get circular polarization, the horizontal and vertical elements
need to be 1/4 wavelength out of phase.  At 435 MHz, 1/4 wavelength
is about 17.25 cm.  So your elements are already offset to produce
an elliptical pattern, but seem to not be spaced quite optimally for
true circularity at 435 MHz.  A spacing of 20cm should give a circular
pattern at 375 MHz.  Are you sure the spacing is 20cm, or was that just
an approximate guess?

To match the impedances, there are two basic choices.  Transform
the impedance of each 50 ohm antenna to 100 ohms, and then put the
two 100 ohm loads in parallel to make 50 ohms, -or- connect the two
50 ohm antennas in parallell to make 25 ohms and transform the 25 ohm
impedance up to 50 ohms.

The first method is what you are describing.  Odd quarter-wavelengths
of 75 ohm coax will transform 50 ohms to approximately 100 ohms.  To
compute the exact values, Zcoax = SQRT(Zin x Zout), so the ideal coax
impedance would be SQRT(5000) = 70.71 ohms.  Most people would probably
use RG-59 and PL-259 "UHF connectors" to a SO-239 TEE, but if you can
find 75 ohm cable to fit N connectors, an N TEE might be better.  The
TEE should be of the 50 ohm variety, since that is the impedance of the
feedline connected to the common port of the TEE (going to the radio).
Oh, and the quarter-wavelengths of coax must take into account the
velocity factor of the coax, so they won't be exactly a quarter
wavelength long.

The second method is arguably better.  You use only standard 50 ohm
cables and connectors, and use a 2-port power divider to do the
impedance transformation between 25 ohms (antenna side) and 50 ohms
(radio side).  The coax jumpers from each antenna to the 2-port power
divider can be any length, so long as they are EQUAL in length.  And
if you need to make one of them longer to compensate for the fact that
your elements are not quite at the right spacing, you just add in the
extra length.  No need to worry about 1/4 wavelength matching, or
velocity factor, etc.

The power divider looks like an aluminum pipe (round or square in
cross-section) with a single N connector on one end, and two N
connectors on the other end.  Inside the pipe is a piece of tubing
(usually brass or copper) of optimal diameter, which is connected
to the center pins of the three N connectors.  It functions as a
quarter-wavelength air-dialectric coaxial transmission line whose
impedance is a function of the diameters of the inside (brass) and
outside (aluminum) tubes.  To transform Zin=25 ohms to Zout=50 ohms
you want the power divider to have an impedance of SQRT(25 x 50) =
SQRT(1250) = 35.4 ohms.  If the outer sleeve is square aluminum tube
(easy to mount N connectors to), and the innter sleeve is round brass
or copper tube (easy to find, easy to solder to), and D is the inner
width of the outer tube, and d is the outer diameter of the inner tube,
the formula for impedance is Z = 138 log(1.08 x (D/d)).  If the outer
tube has D = 7/8" (0.875 inches) and the inner tube has d = 1/2" (0.5
inches), the impedance would be 138 log(1.08 * 0.875 / 0.5) =
138 log(1.89) = 1.38 x 0.276 = 38.15, which is pretty close to the
desired 35.4 ohms.

If you aren't handy building such a device, they are available
commercially:
  http://www.downeastmicrowave.com/Antennas.htm
    Part number 70-2PD ....................................... $58 US

  http://www.directivesystems.com/pricelst.htm
    Part number 70-2PD ....................................... $58 US

  http://www.m2inc.com/products/acess/pwrdivs.html
    Part 70cm 2 Port Power Divider ........................... $65 US
    http://aesham.com
    Part 70CM2PORTPD ......................................... $66 US

Hope that helps.
John (KB0ZEV)
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