Duplexer

Duplexer

Whenever a single antenna is used for both transmitting and receiving, as in a radar system, an electronic switch must be used! Switching systems of this type are called duplexers. Switching the antenna between the transmit and receive modes presents one problem; ensuring that maximum use is made of the available energy is another. The simplest solution is to use a switch to transfer the antenna connection from the receiver to the transmitter during the transmitted pulse and back to the receiver during the echo pulse. No practical mechanical switches are available that can open and close in a few microseconds. Therefore, electronic switches must be used.

Branch- Duplexer

A half-wavelength, closed-end section of transmission line, called a „stub”, is shunted across the main transmission line. An atr tube is located in this line one-quarter wavelength from the main transmission line and one-quarter wavelength from the closed end of the stub.

During the transmitting pulse, an arc appears across both the tr tube (at the point D) and the atr tube (at the point C) and causes the tr and atr circuits to act as shorted (closed-end) quarter-wave stubs. The circuits then reflect an open circuit to the tr (at the point B) and atr (at the point A) circuit connections to the main transmission line. None of the transmitted energy can pass through these reflected opens into the atr stub or into the receiver. Therefore, all of the transmitted energy is directed to the antenna. During reception the amplitude of the received echo is not sufficient to cause an arc across either tube. Under this condition, the atr circuit now acts as a half-wave transmission line terminated in a short-circuit. This is reflected as an open circuit at the receiver Tjunction (at the point B), three-quarter wavelengths away. The received echo sees an open circuit in the direction of the transmitter. However, the receiver input impedance is matched to the transmission line impedance so that the entire received signal will go to the receiver with a minimum amount of loss.

Balanced Duplexer

Duplexer using λ /4- stubs have a relatively small bandwidth caused of the frequency depended length of the stubs. This is very impractical by existing electronic jamming on a particular frequency. Radar sets working on a wider frequency band, need a duplexer with a wider bandwidth

The „Balanced Duplexer”. This duplexer works in accordance with the following principle: A -3 dB-hybride divides the transmitters power in two parts; this part passed the slot of the hybride take a phase-shift of 90°; both parts of power cause an arc across both spark gaps these arcs short-circuit the waveguide and the power would be reflected; the power divides in the -3 dB-hybride once again; this part passed the slot of the hybride again take a phase-shift of 90°; among the parts in the direction of the transmitter occurs a phaseshift of 180° and these parts of power compensates among each other; both parts in the direction of the antenna have the same phase and accumulate to the full power. During reception the amplitude of the received echo is not sufficient to cause an arc across either spark gap. both parts of the received echo can pass the spark gaps. The echoes recur both hybrides and accumulate their parts in-phase. The loss of this duplexer is about 0.5 to 1.5 dB.

Ferrite Circulators

The Ferrite duplexer uses a magic T, two non-reciprocal ferrite phase shifters and a short slot hybrid junction to perform the duplexer operation as shown in fig. the phase shifter provides 45’phase shift either it advanced to the phase or it may retart the phase according to the energy flow. A TR may be used to protect the energy flow . The Ferrite duplexer has lot of advantages over gas discharge tubes . It has wide bandwidth and fast recovery time than the gas discharge tube. But it has size and weight and it requires magnetic field for operation.

Fully solid state duplexers Fully solid state duplexers based on PIN diodes are becoming an attractive alternative in the context of good isolation, fast recovery time and long life. A diode limiter passes low power signals with negligible attenuation but above a given threshold it attenuates the signal to a constant level. However, in high power environments, the devices have to be actively switched in order to achieve low loss and good isolation. This complicates the design and leads to increased risks of the catastrophic failure of the receiver if the duplexer drive circuits fail. Also high power handling capability comes at the price of reduced isolation and longer recovery times. More than one set of limiters is often used to achieve the required level of isolation. This module switches a short circuit with two PIN diodes in a line strip and switches therefore the RF-energies of the entrance into dependence of a control voltage either on the dummyloads or to the exit of this module (that means: to the receiver).