Isolators — Radio's Good Cop, Bad Cop

By Alfred T. Yerger II

Most of the columns you read here on RF Globalnet deal with the latest breakthroughs in the RF world. However, in this quarter’s “Bird’s Eye View,” I would like to take a closer look at something old, the ferrite isolator.

First, the correct name for the device is a circulator. Isolator is the name of one of the more common applications, especially in the land mobile radio (LMR) industry. Circulators come in several different forms. However, the form that is most common in land mobile field work is the three-port circulator.

A three-port circulator operates on the principle of the cancellation of two waves travelling through two different paths in a strong magnetic field. The actual construction consists of a transmission line sounding a ferrite disk sandwiched between two magnets. This creates a three-port device where RF energy entering any port is rotated, or circulated, to the next port with very little energy present at the third port. This concept is used in microwave and radar applications to create a duplexer, where the transmitter signal passes through the circulator to the antenna, and the received signals can flow from the antenna to the receiver with very little transmit energy present at the receiver input. This is especially useful in radar, where something like a relay or electronic transmit receive switch would not be fast enough to allow the receiver to hear the echoes.