Polarization switch

Hi everyone,

@Acinonyx and I are designing a circular polarization switch for a Cross Yagi Antenna for the 70 cm band. We have a question and we would like to ask the community.

First of all I’m going to summarize the main concepts needed for understanding our design:

A Cross Yagi Antenna is formed by a couple of Linear Yagi Antennas, one placed perpendicular to the second. In this kind of antenna, a circular polarization (RHCP or LHCP) is obtained when the energy radiated in both Linear Yagi antennas has a difference of phase of ±π/2.

In our design, this difference of phase is reached by switching between transmission lines with different electric length. The following schema shows the main work of the switch.

If the path 1 (marked in red) is selected, then the Yagi 1 have a phase +π/2 (λ/4) with respect of the Yagi2. However, if the selected path is the number 2, then the difference of phase between the Yagi 1 and Yagi 2 is –π/2 (-λ/4).

The element called WPD is a Wilkinson Power Divider. Its function is to split the energy, which comes from the transmitter (in TX), into two equal parts, and combine the energy from the couple of linear Yagi into a common port (in RX).

With an ideal behaviour of the switches (without return losses and insertion losses), the simulation of our design gives a balanced design and the energy in both ports is almost the same. This can be seen in the following image.

The problems arise (and here is where we need your advises :D) when I model the losses of the switch. I have selected a commercial SPDT for low power applications from Analog Devices, the AD919. The following image is a capture from its datasheet.

In this case, as you may be wondering, we have the Yagi antennas with unbalanced losses, and therefore this is going to affect the circular polarization. As you can see in the last image, the difference is about 1dB, but we do not know if this is a small and assumable difference, or on the contrary, it is enough for ruining our purpose.

What do you think? Could someone give us some advice or any tips in order to improve our design, before we build the first prototype?

Thanks in advance!


Hi again!

I have calculated the theoretical loss due to polarization mismatch provoked by the polarization switch.

This 1 dB of difference between the two Yagis in our Cross Yagi Antenna is a quantity too small to affect significantly the communication with another circular-polarization Antenna. The total loss I have calculated is about -0.06 dB.

You can look my conclusions in this document.

I will be grateful if someone finds any mistake or have anything to add to this issue :smile: