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Duplexers are often confused with diplexers. Read Microwaves101 and you will learn the difference! A duplexer is the network that permits a transmitter and receiver to use the same antenna, at or very near the same frequency. This is used in radar, where the returned signal is going to be very close to the transmitted frequency, such is the case in a T/R module. In a diplexer, the signals have to be offset in frequency by an appreciable percentage so the filters can do their job sorting them out. Diplexers are used in communications, not radar. You dig?
Before we get too far, let's start by saying our previous statements on Microwaves101 that a duplexer and a circulator are one and the same were overly simplistic, and we stand corrected, thanks to Tony D!
There are two references on duplexers and receiver protectors that are particularly useful. The first is by Merrill Skolnik, "Introduction to Radar Systems" (see our page on recommended microwave books). The second reference is an article written by a great guy named Dick Bilotta, entitled "Receiver Protectors: A technology Update", Microwave Journal, August 1997.
Attention Ham radio enthusiasts... your definition of "duplexer" might be different from ours (and be more like a diplexer). Hams are all just a little strange, why would anyone have a hobby involving antennas?
What is a duplexer? It is a three-port network that allows the transmitter and receiver in a radar or communications system to use the same antenna. The duplexer can be as simple as a circulator in low-power applications, or it may be a radioactive gas-discharge T/R tube for megawatt radars. Important properties of a duplexer are:
- Low loss between transmitter and antenna in transmit (less than 1 dB is desirable)
- High isolation from transmitter to receive in transmit (as much as 80 dB for megawatt systems)
- Low loss between antenna and receiver in receive (less than 1 dB is desirable)
- Fast switching between the transmit and receive state, sometimes "automatically switched by the transmit signal, sometimes by command signal.
Receiver protector circuits
Sometimes the duplexer by itself cannot provide enough isolation to the receiver during transmit, and other components are added in from of the receiver. One component that helps in this regard is the PIN diode limiter.
T/R tubes
This discussion was provided by Chris, with some edits by us... and anyone else that wants to contribute!
T/R tubes simply act as shunt diodes, in that they saturate when spanked with high power (like transmitter leakage), yet allow low-power signals (like received signals) to pass unattenuated. The addition of a radioactive gas in a TR tube facilitates tailoring so that the tube sparks at a preset power level. This device is most often realized in waveguide, with "windows" on each end to seal in all that nasty gas. They have the advantage of being able to quickly ionize (faster than the radar's PRF). Yet they serve a different purpose from ionizing keyers (also very fast switches), such as thyratrons (which operate in a modulator and switch high voltage which later becomes transmitted RF).