In communication and radar systems, duplexing is one of the essential functions that allows a single antenna to perform the transmission and receiving tasks at same time. Both circulators and orthomode transducers (OMT) can play this role. The question is: Which device will the designers pick?
The circulator is a ferrite based non-reciprocal device. The basic configuration is single junction type. The schematic is shown below. The single junction circulator can only provide 20 dB typical port isolation when the ports are well matched. In order to increase the port isolation, dual junction or three junction circulators are offered as the solutions.
The three-junction circulator can offer isolation up to 35 dB in perfect port matching conditions. However, the port isolation can be deteriorated dramatically if ports are mis-matched. Let us look at the single junction circulator schematic as an example. In general, Port 1 is used as the TX port, Port 2 is the antenna port, and Port 3 is the RX port. The isolation between Port 1 and Port 3 is essential for healthy RX operation. Unfortunately, it can be easily disturbed if Port 2 is mismatched due to an obstacle in the front of the antenna. This is the drawback of using a circulator as a duplexer.
As a contrast, the orthomode transducers (OMT) do not have the same issues as the circulator. The OMT realizes port isolation via polarization. The pictures shown below indicate how the OMT is used as a duplexer.
If the vertical port is used as the TX port then the horizontal port is used as RX port and the cross polarization at the antenna port (square port) will be around 40 dB. In addition, the isolation between the vertical port and horizontal port is also around 40 dB. The OMT antenna port being mismatched has minimal effects on the port isolation due to its inherent mode structure. Therefore, the OMT is a better device when used as duplexer in communication systems if one can use vertical and horizontal linear polarized waveforms for TX and RX signal carriers separately. The additional advantage of using the OMT in any radar system is that receiver can always receive reflected signals from the objects regardless of the polarization; this can prevent the possibilities of signal detection loss (blinding). Furthermore, the OMT offers broader bandwidths that exceed the waveguide operation frequency range.
In conclusion, the OMT is a better device for duplexing applications in general.