Duplexers, Diplexers and Circulators
By Don Steinbach, AE6PM
Duplexers, Diplexers and Circulators are passive devices that are connected between atransmitter, a receiver and an antenna to perform various functions like radio-frequency
signal combining, separating, or isolation. They are typically designed for use at vhf
frequencies and higher.
Duplexers
The duplexer is a passive frequency-selective three-port device designed specifically toallow simultaneous transmission and reception on adjacent frequencies on the sameantenna. They have separate transmitter and receiver ports (connectors) and a common(shared) antenna port. Obviously, the transmitter and receiver cannot operate on the same
frequency, but they can be very close to the same frequency. This is accomplished byusing highly frequency-selective band-pass and band-reject filters in the two independenttransmit and receive channels. Duplexers are carefully tuned to the specific transmit andreceive frequencies being used. The primary duplexer attribute for each channel is low
insertion loss through the channel and high isolation between the channels. The duplexer
protects the receiver from overload and desensitization while the transmitter is operating
at full power, or from other strong signals in the immediate area. It also protects thetransmitter from “receiving” strong local signals that might result in spurious signals
being produced in the transmitter final amplifier. Typical duplexer specifications might
be insertion loss of less than 1 dB and isolation greater than 75 dB.
Duplexers are based on very high-Q tuned circuits that often take the form of cavity
resonators. Some cable lengths need to be exact quarter- or half-wavelengths, theresonators themselves are sometimes silver plated internally, and mechanical adjustingrods might be made from Invar for dimensional stability over temperature changes. Aduplexer ‘set’ typically consists of 4 or 5 resonators such as shown in Figure 1.
Typically, two of the resonators (also called “cavities”) would be in the receiver path andpass the receive frequency and notch the transmit frequency. The other two resonators
would be in the transmit path and pass the transmit frequency and notch the receivefrequency.
Diplexers
The diplexer is similar in function to the duplexer in that it combines two (or more) ports
into a single port. Like the duplexer, the diplexer is frequency selective, but typicallybroadband. A typical application for a diplexer is to allow two transceivers on two
different bands to share a common transmission line. For example, a 2-meter transceiver
and a 70-cm transceiver might each be connected a single diplexer which is connected toa single coax. On the other end of the coax might be a dual-band antenna, or anotherdiplexer that separates the signals for connection to two separate antennas.
Remember the splitter we used to separate the VHF and UHF TV signals from the single
300 ohm twinlead from the antenna so we could get the signal to the separate VHF andUHF tuner inputs at the TV set? That’s functionally equivalent to a diplexer consistingof a low-pass filter for the VHF channels (54-216 MHz) and a high-pass filter for theUHF channels (470-890 MHz).
The diplexer concept can be expanded to more ports, such as three inputs for a triplexer.
Gary Gordon, K6KV, described a triplexer to connect 10- 15- and 20-meter field day
stations to a single triband beam. See QST magazine for June 2010.
Circulators
The circulator is a 3-port ferromagnetic-based device that allows rf energy to to becoupled only from port A to port B, from port B to port C, and from port C to port A.
The isolation between the ports is such that almost no rf energy can flow in the reverse
direction. If port A is connected to a transmitter, and port B is connected to an antenna,
the forward power will pass from port A to port B and any reflected power back from theantenna will pass to port C. An isolator is a special case of a circulator with the third port
(port C) terminated by a dummy load equal to the system characteristic impedance inwhich case any power from port C will be dissipated as heat.
In another application, a transmitter could be connected to port A, an antenna to port B
and a receiver to port C, similar to a duplexer connection. Energy from the transmitter(the transmitted signal) would flow to the antenna, and energy from the antenna (thereceived signal) would flow to the receiver. Unfortunately, any reflected energy from thetransmitted signal due to mismatch at the antenna would also flow back to the receiverconnection at port C possibly damaging or desensitizing the receiver. Circulators are
somewhat frequency dependant and also require exact impedance matching to be fullyeffective. The reverse isolation between ports is typically 20 to 25 dB.

Fig.1. A 4-resonator duplexer similar to the one used on the W6UU 70-cm repeater. Aunit like this costs about $1,800, has an insertion loss of 1.0 dB or less and isolation of 90
dB. A set for 2-meters is much larger than this. A resonator for 6-meters approaches thesize of a trash can. The panel in this photo is 19” wide.