In many communication and radar systems, duplexing allows a single antenna to perform both the transmit and receive tasks at the same time. Either a circulator or an orthomode transducer (OMT) can play this role. So, which is better?

Circulators are non-reciprocal three-port devices. Their operation is based on the field-rotating properties of certain ferrite materials. By convention, circulators provide low attenuation for signals going from Port 1 to Port 2, from Port 2 to Port 3, or from Port 3 to Port 1. Conversely, they provide high attenuation (isolation) for signals going from Port 2 to Port 1, from Port 3 to Port 2, or from

Port 1 to Port 3. 

The basic configuration of a circulator is a ferrite bead positioned at the center of a three-port junction. A typical single-junction circulator provides about 20 dB of isolation between well-matched ports. In order to increase port isolation, cascaded two- and three-junction circulators are possible. A typical three-junction circulator provides about 35 dB of isolation. However, port isolation can be dramatically reduced if any of the ports becomes poorly matched.

In a typical communication or radar transceiver, Port 1 accepts the Transmit (TX) signal, Port 3 provides the Received (RX) signal, and Port 2 connects to a single antenna. Isolation between Ports 1 and 3 is essential to prevent the TX signal from spilling over into the RX channel, which could significantly degrade the receiver’s sensitivity. Unfortunately, channel isolation is limited by how well the other system components are matched to the isolator. This can be a significant problem when using circulators as duplexers. Other potential problems include the limited power-handling capability of circulators, and signal distortion caused by the inherently nonlinear characteristics of ferrite materials.

In contrast, the isolation provided by an OMT is not sensitive to port matching or the signal power. The OMT is a linear, reciprocal device that uses signal polarization to realize port isolation. The Common (antenna) port has a square waveguide opening that carries both vertically and horizontally polarized signals to and from a dual-polarized antenna. The OMT also has two rectangular waveguide ports, each of which carries either the vertically or horizontally polarized signal. 

In a common duplexing scenario, the Vertical port carries the transmit signal to the antenna while the Horizontal port provides the cross-polarized signal received by the antenna. Isolation between the Vertical and Horizontal ports is typically 40 dB regardless of port matching. This enables transmitted and received signals with low cross-polarization, providing high isolation and low interference between the duplexed channels. Due to the inherent mode-separating structure of the OMT, port matching does not affect port isolation appreciably. Therefore, if one can use vertically and horizontally polarized waveforms for the TX and RX signals separately, OMTs are usually preferred as signal duplexers.