Radar System Engineering

SEC. 10.10] LINE-TYPE PULSERS 377
In large radar equipments, it is often desirable to separate pulser and
r-f units by considerable dktances. The pulse transmission cable necessary
in such cases has been standardized at an impedance level of 50
ohms. Many pulse networks designed for high-power radar have an
impedanceof 500hmsin order toavoid the use ofa matching pulsetransformer
between the network and the cable. As a result, many of the
more desirable switching devices have been designed to give maximum
power output when used with a 50-ohm network.
TABLE10.6.—PULSE-FOEWNG hTETWOItKCH.*RACTEFUSTICS
N’etwork (Fig. 10.38)
Pulse length, ~ec
PRF, 13pS
Power
h’o. 1
0.8
2,2
840
420
25 kw
25 kw
No. 2
0 25
05
2.6
52
1600
800
4oil
200
250 kw
250 kw
250 kw
250 kw
No. 3
1
1000
250 kw
No. 4
1
800
3 3fw-
An important design consideration in pulse network applications is
the average power to be handled. A pulse network designed to have
adequate life at one pulse rate would overheat and perhaps be ruined by
operation at a higher repetition frequency. Since overheating is a function
of both applied voltage and repetition rate, little flexibility remains
in a line-type pulser designed to achieve maximum economy of weight,
space, and power. Provision can be made for shortening pulse length
by bringing out taps on the pulse line. However, this is rather difficult
at high power, because of the problem of designing suitable line switches.
The Sw”tch.—The possible advantages of the line-type pulser greatly
stimulated the design of low-impedance spark switches and thyratrons,
since it became possible to secure flat-topped pulses without the necessity
of opening the switch. Rotary spark gaps, ‘‘ trigatrons,” series gaps,
hydrogen thyratrons, and mercury thyratrons have all been used as network
switching devices. Each has its field of application. Each switch
is limited in one or more of the following respects: (1) poor precision of
firing, (2) low maximum pulse rate, (3) short life on long pulses, (4) narrow
operating range of voltage, (5) occasional erratic firing, (6) inefficient
cathode, and (7) unnecessary complication. The ideal switch has not
yet been designed.
Figure 10.39 shows schematically the simple “series gap” s\vitch.
The Western Electric’ 1B22 is a good example of this class of switch; it
consists of a cathode cylinder of aluminum surrounding an anode rod of