Updated July 5,
to go to our main page on radar
here to go to our page on circulators
here to go to our discussion of diplexers
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.
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
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
- 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
This discussion was provided
by Chris, with some edits by us... and anyone else that wants to
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