To get an orthogonal polarisation (near circular), two identical yagi share the same boom with a 90° polarisation angle (H/V), each of the two yagis are physical 90° apart on the bom (this will enable the use of a symmetrical phasing harness). Each of the yagi must have a 50 ohm feed point.

Then a power divider is needed to connect both antennas to the transmitter.

To my knowledge there is no easy formula to get a yagi with a direct 50ohm feed point. The only way is simulation.

A parametric model in CST will be very convenient :

The goals for this design :

• gain of the antenna should be at least 12dB
• front to back ratio should be at least 10dB
• RL at feed point should be under -15dB

Parameters to adjust to get this result :

• reflector length (max : 120mm, min : 70mm)
• reflector spacing (max : 34mm, min : 15mm)

Obviously to lower the reflector spacing is the key to success. Unfortunately to get a decent front to back radio we need to adjust the reflector length.

Here is a result that match the criterias :

• reflector spacing : 20mm
• reflector length : 99.98mm

This is not perfect. The side lobes are still strongs, but to get something better we need to have a very different feed, so for now it will be enought.