period in this condition, the particles will form a detuning wall within the cavity and eventually prevent
the tube from functioning. A second cause of failure is the absorption of gas within the enclosure by the
metal electrodes. This results in a gradual reduction of pressure within the tube to a point where gap
breakdown becomes very difficult. The final result is that extremely strong signals (from the transmitter)
are coupled to the receiver. Because both types of failures develop gradually, the tr tube periodically must
be checked carefully to determine performance level.
Q27. What type of switches are used as duplexers?
Q28. What tube in a duplexer has the primary function of disconnecting the receiver?
Q29. How may the tr tube ionization speed be increased?
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, lets 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.