ATR Tube

The atr tube is usually a simpler device than a tr tube. An atr tube might use a pure inert gas, such as argon, because recovery time generally is not a vital factor. Furthermore, a priming agent, such as keep-alive voltage, is not needed. The absence of either a chemically active gas or a keep-alive voltage results in atr tubes having longer useful lives than tr tubes.

Tr and atr tubes may contain radioactive material. Handle with care to avoid breakage and possible contamination.

There are two basic tr-atr duplexer configurations. They are the parallel-connected and the series-connected duplexer systems. The following paragraphs describe the operation of both systems.

Parallel Connected Duplexer Operation

First, let's consider a PARALLEL-CONNECTED DUPLEXER system, as shown in figure 2-14. The tr spark gap shown in figure 2-14 is located in the receiver coupling line one-quarter wavelength from the T-junction. A half-wavelength, closed-end section of transmission line, called a STUB, is shunted across the main transmission line. An atr spark gap is located in this line one-quarter wavelength from the main transmission line and one-quarter wavelength from the closed end of the stub. As shown in the figure, antenna impedance, line impedance, and transmitter output impedance, when transmitting, are all equal. The action of the circuit during transmission is shown in figure 2-15.

Figure 2-14. - Parallel-connected duplexer showing distance and impedance.

Figure 2-15. - Parallel-connected duplexer during transmission.

During the transmitting pulse, an arc appears across both spark gaps 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 and atr 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, as shown in figure 2-16, the amplitude of the received echo is not sufficient to cause an arc across either spark gap. 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 T-junction, 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.

Figure 2-16. - Parallel-connected duplexer during reception.

Series-Connected Duplexer Operation

In the SERIES-CONNECTED DUPLEXER SYSTEM, shown in figure 2-17, the tr spark gap is located one-half wavelength from the receiver T-junction. The atr spark gap is located one-half wavelength from the transmission line and three-quarters wavelength from the receiver T-junction. During transmission, the tr and atr gaps fire in the series-connected duplexer system, as shown in figure 2-18. This causes a short-circuit to be reflected at the series connection to the main transmission line one-half wavelength away. The transmitted pulse "sees" a low impedance path in the direction of the antenna and does not go into the atr stub or the receiver.

Figure 2-17. - Series-connected duplexer showing distance and impedance.

Figure 2-18. - Series-connected duplexer during transmission.

During reception, neither spark gap is fired, as shown in figure 2-19. The atr acts as a half-wave stub terminated in an open. This open is reflected as a short-circuit at the T-junction three-quarters of a wavelength away. Consequently, the received signal sees a low impedance path to the receiver, and none of the received signal is lost in the transmitting circuit.

Figure 2-19. - Series-connected duplexer during reception.

DUPLEXER TYPES

Duplexers are constructed in many forms, such as RESONANT-CAVITY COAXIAL SYSTEMS, WAVEGUIDE SYSTEMS, and HYBRID RINGS. Since waveguide and hybrid-ring duplexers are most common in radar systems, those will be discussed in this section.