6 Lecture 2 
is that of obtaining a directional receiving transducer with its beamwidth con- 
stant over this wide frequency range. It will probably be satisfactory for the 
transmitter to be omnidirectional, and forthe systemto rely only on the receiver 
for directionality; however, the method to be described can apply to a trans- 
mitter if necessary. Since target bearing is to be determined, directionality in 
the system is clearly essential; but it must obviously also be constant at all the 
frequencies concerned, since it is hardly possible to correct for the angular 
position of the target in the beam. 
There are several ways of achieving a constant beamwidth. The Birmingham 
method [11,12] is to have an array divided into sections, which are connected, 
via decoupling circuits, into a number of phase-shifting networks (or delay lines) 
as shown in Fig. 2.5. These have a phase shift which increases with frequency, 
so that at each end the effective directional pattern is deflected by an amount 
which increases with frequency. Successive delay lines have a successively 
larger range of phase shift. Then, after correcting the phase of the outputs from 
the various lines so that they are all in phase at all frequencies, the outputs are 
added together. Now, at the lowest frequency ofthe range, the phase shifts in the 
lines are small (ideally zero) and the directional patterns all very nearly coin- 
cident; their addition therefore produces only the ordinary (sin x)/x pattern, 
corresponding to the number of wavelengths in the length of the array at that 
frequency. As the frequency is increased, the individual patterns begin to sepa- 
rate, so that their addition leads to a wider beam in terms of wavelength, but it 
DECOUPLING 
IMPEDANCES TRANSDUCER IN 19 SECTIONS 
AND SIMILAR CONNEC TIONS 
>> FOR ALL SECTIONS OF TRANS— — — 
DUCER AND DELAY LINES 
de) | 
ff i oe 
Aad Pod oa 
tea Eth co | a eal [a a | pm [es em 
ae | 
ADDING UNIT 
WITH PHASE 
CORREC TORS 
OU T PUT 
Fig. 2.5. Wide-band constant-beamwidth array: block schematic for frequency band of 9 to 1. 
