1300051 Radar; aerials W L MAXSON CORP 12 Sept 1956 27951/56 Headings H4A and H4D In a pulse radar system for scanning a 200 nautical mile range the transmitter frequency is continuously varied over a certain frequency range, e.g. 2925-2700 Mc, whereby the directivity of the antenna is shifted and the beam is swept in agimuth, e.g. through 45 degrees. The beam may be of 1 degree width and swept at the rate of 1 degree per 500 Ásec., a pulse being transmitted at each degree stage. Since in an interval of 500 Ásec. the maximum range from which an echo can be received is 40 miles, the 200 mile range is divided into five zones each extending over 40 miles, viz. 0-40, 40-80, &c. miles. Thus each zone returns an echo having a characteristic frequency, each zone differing from the next by 5 Mc, the amount by which the frequency changes in 500 Ásec., the different frequencies, being selectively recieved by separate channels. The arrangement permits a higher pulse repetition frequency. In Fig. 2 the loci of the receiver frequencies for the 2nd and 5th zones are shown. The antenna system 10, Fig. 5, employs four linear antenna arrays 24 ... 27 each as shown in Fig. 4 and comprising radialing elements 16 spaced ¢ wavelength apart at a frequency corresponding to a bearing of 0 degrees with respect to the direction broadside to the array. The elements are helically fed by waveguides 15 so that an R.F. source shifting 225 Mc moves the beam through 90 degees, the arrays being fed in succession through an antenna switch 35 to continually scan the 360 degrees. The switch is connected to a pulsed oscillator 36 through a wave guide 41 and directional coupler 42, the oscillator having a variable frequency (2À925-2À7 KMc.) and being pulsed by a generator 37. The oscillator frequency is swept at a linear rate by a voltage shaper 38 fed with a sawtooth voltage from an azimuth sweep generator 39 synchronized by a mechanical connection 40 with the antenna switch 35. The frequency of oscillator 36 is tracked with the frequency of local oscillator 47 by an A.F.C. circuit, including a mixer 46, I.F. amplifier 48 and a frequency discriminator 49. The discriminator produces an output corresponding to the frequency error which is fed to a shaper 50 to control oscillator 47. The frequency of oscillator 47 is swept over the range 3À225-3 KMc in synchronism with the oscillator 36 by the shaper 50 fed by the generator 39. The output of oscillator 47 is supplied to a mixer 55 where it is combined with the signal received over connections 41, 56 the output being supplied to an amplifier 58 in turn supplying a mixer 59 connected to a local oscillator 60. The output of mixer 59 is fed through channels 61 ... 65 having 5 Mc separations so that echo signals from the 5th zone pass through the 40 Mc channel 61 whereas the 60 Mc signal from the 1st zone passes through channel 65. To provide a greater bandwidth oscillator 60 is varied through a range of Œ2À5 Mc every 500 Ásec. by a range sweep generator 87 over lead 66. The channels are fed through detectors 71 ... 75 to display means 81 ... 85 connected to display units giving a type "B" presentation (Fig. 6, not shown), there being four such units arranged in quadrature. The range sweep voltage is supplied to the displays from generator 87 and the azimuth sweep voltage from generator 39 and lead 90. A switch 91 operating in synchronism with the antenna switch 35 switches the input to the appropriate display units in turn. In a system permitting azimuth and range tracking, Fig. 7, a tracking resonant cavity 110 is connected by a line 109 to an antenna system 100 which may be the same as antenna 10 of Fig. 5. The cavity is manually tunable by elements 111, 112 to select the bearing of the selected target. A quadrant sensing device 121 is connected at 120 to the antenna switching means and to a quadrant identification device 123 whereby the tuning of the cavity is shifted through a transmission device 124 when the target changes quadrants. The cavity 110 is maintained tuned to the selected target by a servo loop comprising a phase sensing detector 125 connected to a coincidence gate 128 receiving a D.C. pulse, corresponding to the range of the selected target, over a connection 129, the gate output being supplied to a servo system 130 having a mechanical connection 131 to the transmission device 124. The azimuth information is supplied by a connection 113 to an indicating or computing apparatus. The cavity output is also supplied over connection 135 to a mixer 136 where it is combined with the oscillations of the local oscillator 47, the mixer output being supplied to amplifier 137 and thence to a range tuned circuit 138. Circuit 138 is initially manually tuned to the range of the selected target by means 139 and after- wards maintained tuned by a servo system 142 supplied with a signal from a phase sensitive detector 141 receiving the combined outputs of circuit 138 and amplifier 137. The output of circuit 138 is also supplied to a detector 150 for supplying the D.C. pulses to the gate 128. In a modified arrangement (Fig. 8 not shown) permitting tracking in elevation without searching in elevation, an additional antenna array is provided displaced from the first array in a vertical direction by a distance of one-wavelength, the phase difference of the energy received from the target by the two antennµ giving the elevation in the range 0 to 60 degrees. The azimuth information obtained from the Fig. 7 system is accurate to one beamwidth and to obtain a much higher order of precision the system of Figs. 9, 9A is employed in which a pair of cavities 190, 191 are connected to the antenna 100<SP>11</SP> and tuned to frequencies different by one beamwidth, the response curves being denoted by A, B respectively and the intermediate frequency by f c . The cavities are tuned in unison by a manual control 193 and their outputs connected to microwave hybrid devices 195, 196, the device 195 having an output corresponding to the sum of its inputs and the device 196 an output corresponding to the difference of its inputs. The outputs are supplied to mixers 197, 198 and heterodyned by local oscillator 199, the sum and difference signals from the mixers being fed to I.F. amplifier 200, 201, the output of the amplifier 200 having the form denoted by curve C. The amplifier outputs are connected to a phase sensitive detector 205 having an output denoted by curve D, the detector producing a positive D.C. voltage for incoming signal frequencies on one side of f c and a negative voltage for frequencies on the other side. The detector output is fed to an azimuth servo system 206 operating a mechanical coupling device 207 to control the tuning drive 208. A moving target indication system, Fig. 11, in which pairs of pulses having a 100 Ásec. spacing are transmitted to permit cancellation of signals due to stationary targets comprises an antenna 300 energized by an amplifier 301 pulsed by a generator 302 producing pairs of spaced pulses 303 304 every 600 Ásec. The first pulse of each pair is applied to a voltage stepper 305 producing a step voltage 306 which is supplied to a variable frequency local oscillator 307 having its output heterodyned in a mixer 308 with oscillations from a local oscillator 309. Mixer 308 is connected to the amplifier 301 through filter 310 whereby the frequency supplied to the amplifier is varied and it provides pairs of pulses of the same frequency. The echo signals are supplied to a mixer 316 and combined with the output of oscillator 307 to provide an I.F. signal which is fed to an amplifier 317. This output is fed to a mixer 318 where it is heterodyned by local oscillator 319 and the signal fed to amplifiers 321 ... 325 turned at 5 Mc intervals. These amplifiers are connected to 100 Ásec. delay circuits 331 ... 335 via limiters 326 ... 330, the delayed and undelayed outputs of the limiters being fed to cancellation circuits provided by phase sensing detectors 336 ... 340. The outputs of the detectors are fed to display means as in Fig. 5, there being no output for stationary targets. The sweep generators of the display units are supplied with the second pulses 304 of each pair over the line 341 since the sweeps must be delayed by 100 Ásec.; also, since two pulses spaced 100 Ásec. are transmitted in each azimuth angle an interval of 600 Ásec. must be allocated to each range zone.