D. G. Tucker 37 
is arranged that the beamwidth in terms of physical angles remains very nearly 
constant; about + 10% variation over a9to1 frequency ratio was achieved. Figure 
2.6 shows the state of affairs at the maximum frequency. Above this frequency, 
dips occur and the pattern begins to break up. 
An experimental array covering the frequency range 9 to 81 kcps has been 
built and tested in water. The theoretical results have been matched very closely. 
It is, of course, difficult to operate the transducers at good sensitivity over 
this wide frequency range, but wearetryingto develop a satisfactory capacitance 
transducer. 
2.3. SONAR SYSTEMS 
2.3.1. Within-Pulse Scanning Systems Using Multielement Arrays 
The idea of electronic sector-scanning sonar systems in which the receiving 
beam is repeatedly swung across the search sector, insonified by a wide-beam 
transmitter, at least once during each period of time equal to the duration of the 
transmitted pulse, is by now probably quite familiar. By this method, a virtually 
simultaneous examination is made in all directions withinthe search sector. For 
a sector equal to n times the 3-db beamwidth of the receiving array, the array 
has to be divided into n sections. Alternative arrangements for swinging the 
beam have been published; the system developed at Birmingham [13,14] uses 
phase-shift networks to link the sections of the receiving arrays, with frequency 
changers interposed in the leads from each array section. The local oscillation 
supplied to the frequency changers is swept through a range of frequencies with 
a repetition rate equal to the desired scanning rate; and since the phase-shift 
networks have a phase shift varying with frequency, the beam direction is swept 
over a sector in sympathy with the sweep of frequency. 
—— COMPONENT PATTERN 
o-8L sss RESULTANT PATTERN 
RELATIVE RESPONSE 
ait 
Fig. 2.6. Wide-band constant-beamwidth array: component and resultant directional 
patterns at upper frequency limit. 
