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Hybrid Ring Duplexer

Figure 1: Simple rat-race coupler with four arms in waveguide technology

Figure 1: Simple rat-race coupler with four arms in waveguide technology

Hybrid Ring Duplexer

In radar technology, hybrid ring duplexers are transmit/receive (TR) switch based on a simple rat-race coupler. This design is suitable for switching very high pulse powers.

A rat-race coupler usually consists of a rectangular waveguide bent into a circle with a mean circumference of exactly 1.5 λ. (All data on wavelengths here refer to the phase velocity existing within the waveguide. They are slightly longer than the wavelength valid for this frequency in free space.) The connections are four E-type T-junctions with a distance of λ/4 each. Thus a section with a length of 3λ/4 remains.

When used as a duplexer, the antenna is connected to arm 1, the transmitter to arm 3, and the receiver to ports 2 and 4. The receiver connections have a TR-tube at a distance of λ/4, which is fired during transmission and creates a short circuit on these lines.

TR-Tube
TR-Tube
from
transmitter
to the antenna

Figure 2: Propagation of the E-field at the moment of transmission

TR-Tube
TR-Tube
from
transmitter
to the antenna

Figure 2: Propagation of the E-field at the moment of transmission

Transmitting pulse

When the transmitter works, its power (E-field) is divided at the T-junction at arm 3 into two equal parts which are 180° out of phase with each other. One part moves clockwise around the ring and the other counterclockwise. Both fields must have received an additional phase difference of 180° within the waveguide circuit at the location of a connection due to the line delay to compensate for the phase shift during feeding. Only then can the E-field propagate in the respective connection.

The field moving clockwise from arm 3 ionizes the gas discharge tube in arm 4 and the energy flow is blocked in front of the receiver. The line length of λ/4 transforms this short circuit to a very high impedance, which corresponds to an open circuit at arm 4. This high impedance prevents energy from entering the receiver tract, although the two fields at the input of arm 4 are out of phase. The field moving counterclockwise from arm 3 ionizes the gas discharge tube in arm 2, which reflects a short circuit to the ring junction. However, no energy is sent to the receiver because the fields arriving at arm 2 are in phase. The fields arriving clockwise and counterclockwise arrive at arm 1 180 degrees out of phase. With the phase shift during feeding, both partial fields are in phase and are forwarded to the antenna.

Figure 3: Response of a T-connection for in-phase and out-of-phase fields

Figure 3: Response of a T-connection for in-phase and out-of-phase fields

Receiving time

During the reception, the relatively weak field from the antenna enters arm 1 and splits at the T-junction into two 180° phase-shifted components, one of which propagates clockwise and the other counterclockwise on the ring line. At the opposite arm 4, both components have the same delay from ¾λ, so they still have their phase difference from the feed. The connection 2 is exactly ½λ away from arm 4, both components are there with only opposite polarity. Neither field is sufficient to ignite the gas discharge tubes in arms 2 and 4. Since the fields arrive at these arms out of phase due to the line delay, the energy is transmitted to the receiver. (The division into two lines to the receiver with an antiphase signal (differential feed) is advantageous for the subsequent downward mixing.)

The E-fields arriving at arm 3 from left and right have the same phase angle due to the different line delay. They are therefore coupled into arm 3 in the opposite phase and thus cancel each other out. Within the ring line, they superimpose each other and form a standing wave.


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