circulator

A circulator is a 3 port device that allows signals to flow in a particular direction (clockwise if I'm not mistaken) from one port to the next. The wavelength form of a circulator consists of three arms at 120° increments joined to a central boss, at the center of which is placed a slab of ferrite which is magnetically biased by a permanent magnet. The ferrite effectively rotates the standing wave pattern so the coupling can only occur in one direction. The internal dimensions of the arms and boss determine the approximate frequency of operation.

Consider Port A to be connected to a transmitter, Port B to be connected to the antenna, and Port C to be connected to a dummy load. Power from port A will be delivered to port B. If port B accepts all of the power supplied by port A, there is no signal delivered to port C. If some or all of the power is reflected from port B (due to bad feedline or antenna) this power will be delivered to port C, and consumed as heat. None of the original power from port A will ever be delivered back to port A unless the match of port B, and C are bad. The continuous power handling of the dummy load should equal the transmitter power for 100% protection of mismatch. In practice, this load is usually smaller due to losses in transmission line.

A circulator, used as an isolator in this instance, is primarily used to protect your own transmitter, however a circulator can help prevent intermodulation products from occurring in the output section of your own transmitter, thus making the site cleaner. A circulator cannot clean up a dirty transmitter, and should not be used to do so. There is no substitute for a clean transmitter. If you start off clean, it is much easier to keep the whole site clean, not to mention making it easier to make your repeater duplex.

An isolator is the above device that has a 50 ohm load attached to the 3rd port (one being input and the other being output) to absorb any reflected energy. A circulator is the above device that does not have a 50 ohm load attached to the 3rd port, however a circulator can be configured as an isolator with the addition of an external load. A circulator allows the energy to be routed to another device such as a receiver as it does in microwave applications.

The isolator is most commonly use in the pursuit of IM reduction. Beware though, a isolator or a circulator will generate harmonics. There is no way around it, it's the nature of the beast. Always follow a circulator or an isolator with a bandpass cavity. The harmonics can be embarrassing - (2 x 450 {uhf} = 900). Something nailing your 900 Trunking RX?

One additional note from a experienced ham:

Years ago I heard about a 52.525 MHz remote base that had a circulator and no pass cavity on the 6m tx. At the same site was a 420.200 MHz RX that was one end of a point-to-point link. So? you ask... Well, the 8th harmonic of 52.525 is dead on 420.200. And 5 kHz dev at 52 MHz is 40 kHz at 420 MHz. It was trashing 420.175, 420.200 and 420.025... Oops.