Making Crossed Polarized Yagi

As all you know for successful communication via Amateur radio satellites the antenna system should have both polarization capability. That’s for very simple reason – Faraday effect. Faraday effect is a rotation of the plane of polarization of electromagnetic waves in certain substances in a magnetic field. So for all moving and rotating objects that transmit/receive radiowaves this is very undesired effect. Anyways to make a story short let’s get directly to the subject.

I’ve used crossed polarized yagis for satellite communication all the time. It’s good when you can buy, but what if you can make by yourself from what you already have. And this is was my approach. I have two identical 435MHz 10el yagi Diamond Antenna A430S10. They are very inexpensive and sell for $60. Having two of them you can make a good crossed polarized single boom yagi. You do not earn gain from crossed polarized yagi, gain will remain same as a single antenna.

So my choice was Diamond Antenna A430S10

Product Specifications:
Power Rating: 100 watts
Frequency: 430 – 440 MHz
Gain: 13dBi
Length: 43″

From theory we know that antennas on same boom shall be separated by at least lambda/4 which is for 435MHz is 172mm. In my case I separated antenna over full wavelength.

But separating antennas it’s not the only one thing you need to do. How to match to 50 ohm antennas to 50 ohm cable. For this we going to use a power splitter/divider which is in case of two 50 ohm antennas will be two pieces of 75 ohm cables lambda/4*e(coefficient) as seen on the figure above and below.

Thanks to SV1BSX pictures and article. I just used in for convenience. For more details please check this link http://sv1bsx.50webs.com/antenna-pol/polarization.html

“Power splitting” between the two antennas is now equal, as the two “Matching Stubs” (2 Coaxial pieces 75 Ohms ) in the splitting have precisely equal length and therefore give identical transformation of antenna’s impedance. So, the basic disadvantage of crossed Yagis with a Coaxial’s “Phasing Stub” now has been completely eliminated!

So after all of that I just built the simple coaxial splitter shown on picture below.

RG-59 cable used for this splitter.

Now it’s the time for mechanical work which was obviously the easiest part of this effort. I like Diamond antennas because it’s easy to work with them. Just keep the distances between each element as in original antenna, drill mounting holes and that’s it. Since antennas are 90deg to each other there should be no change of performance for each polarization. After all mechanical work this is how antenna looked now.

Now it’s time to install a power splitter and run cable to it. The cable length for each antenna must be identical otherwise we’ll have a problem with antenna phasing. BTW if you desire to create a circular polarization (Right or Left) one piece of 50 ohm cable with lambda/4*e(coefficient) length shall be used in series with one of two antennas. Please refer to figure below:

In my case I didn’t want to do this and kept linear polarizations. In picture below this is how antenna looks after all modifications and 2 hours of work.

I already made number satellite QSOs using this antenna via linear and FM birds. No magic but only expected performance. Now it’s the time to modify my 145MHz antenna in same way.

Some technical notes:

For 435.6 MHz splitter 750hms cables should have length:

lambda/4 * coefficient = 0.688/4*0.66=0.113m(113mm)

0.66 is a coefficient for RG59 cable

So length for each cable connected to the antenna is 113mm. In this case both 50 ohms antennas will be matched to 50ohm cable after 3dB splitting loss.

73,

Alex K6VHF

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