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ELECTRONICS |
building 20 - January 1999  |
 |
| by means of a capacitance
joint. A sketch is shown in Fig. 9.
The sections bb can be considered as forming a transmission line of very low characteristic impedance by themselves. Because this line is open circuited at the antenna end and is a quarter wave long, it is effectively short circuited at the transmitter end. The transmitter energy therefore passes from the stationary to the moving part. The function of the skirt c is for broad banding and also allows the bearing to be placed at a zero-current point. This bearing carries the total thrust load of the super-structure. The inner conductor functions in essentially the same manner except that it is unnecessary to fold the line back upon itself in a skirt as in the case of the outer conductor. In the inner conductor, the 1/4-inch rod extending beyond the rotary joint itself performs the same function as the skirt in the outer conductor. Oilite bearings are used on the inner conductor. Moving down the transmission line toward the transmitter, the next interesting feature is the t-r or transmit-receive system. As shown in the block diagram (Fig. 4) the transmitter and receiver are connected to the same antenna. Some means must be provided to protect the input of the receiver from the high power present in the line when the transmitter is operating and to prevent the received signal from being dissipated in the transmitter instead of flowing into the receiver. This is accomplished by a combination called a receiver-disconnect switch and a transmitter-disconnect switch. The combination of these two components is called the t-r system. A functional diagram is shown in Fig. 10. The transmitter-disconnect switch consists of a quarter-wave section of line with a spark gap in the end. When the spark gap fires, this quarter-wave section is short circuited and therefore presents a very high impedance at its open end. When the spark gap is not firing, the quarter-wave section is open circuited and presents a short circuit at the transmission line. A quarter wavelength toward the antenna from the transmitter-disconnect switch is the receiver-disconnect switch. This consists of a high-Q cavity a quarter wave- |
length long, short-circuited
at one end with a spark gap between the center conductor and ground at
the open end. The transmission line and the receiver are both coupled into
this cavity by means of loops. When the spark in this cavity is firing,
the cavity is detuned and the receiver is decoupled from the transmission
line. When the spark gap is not firing the cavity is tuned and energy
passes freely from the transmission line to the receiver.
The operation is as follows: When the transmitter is operat-ing, a high voltage is present in the line. This fires the spark gap in the transmitter-disconnect switch, which then acts as a quarter-wave-length short-circuited stub and has no effect on the line. A high volt-age is also built up in the cavity of the receiver-disconnect switch, firing its spark gap as well. This effectively decouples the receiver from the transmission line. When a received signal comes down the transmission line from the antenna, the voltage is extremely small. This voltage is far too small to fire either the spark gap in the trans-mitter-disconnect switch or the spark gap in the receiver-disconnect switch. The transmitter disconnect switch will therefore be an open-circuited wavelength and will appear to be a short circuit at the transmission line end. A quarter wavelength toward the antenna, at the point where the receiver-disconnect switch ties in, this will be reflected as an open circuit. Therefore, no energy will flow toward the transmitter. On the other hand, the receiver-disconnect switch is now a high-Q tuned cavity. The received energy will therefore flow freely from the transmission line cavity into the receiver. The transmission line is connected to the transmitter through a network which provides a means of matching impedances. This network consists of two stubs of variable lengths spaced 3/5 of a wave-length apart. These are known as tuning stubs. Since a short-circuited stub of variable lengths acts as a pure variable reactance either capactive or inductive, depending on its length, such a stub when properly placed may be used to match a transmitter of arbitrary impedance to a transmission line. |
However to avoid the
mechanical difficulty of properly placing this stub on the transmission
line two stubs are used and spaced 3/5 of a wavelength apart. By
properly adjusting the length of these stubs, any impedance from infinity
to one half the impedance of the line can be matched to the line.
The next and concluding installment on the AN/TPS-3 will describe the transmitter, modulating system, receiving and indicating system. Responsibility for the construc-tion of the AN/TPS-3 was
directly assigned to J. W. Marchetti, who was assisted by William P. Gold-berg
as civilian engineer in charge. Many individuals of the Camp Evans Signal
Laboratory contrib-uted in the design of various fea-tures of this equipment.
The modu-lator, indicators, and transmitter were turned over to a group
headed by Dr. John E. Gorham. Within this group H. P. Pacini was re-sponsible
for the design of the indicators and I. Sager for the modulator. The transmitting
tube, which was designed by the first named author, required very little
additional development, since it had been in semi-production for some time
and required only minor changes to adapt it for use in a lightweight radar
set being styled for mass production. Later, when the set went into production,
Dr. Gorham handled the manufacturing problems incidental to the con-struction
of the transmitting tube. The physical arrangement of the set was due to
William J. Smith and he further played a large part in the overall coordination.
The ma-jor part of the mechanical design of this set was due to Arthur
H. Hood.
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