Patent GB-1425141-A (GB)
Title: Antenna system for radiating doppler coded pattern using sequential modal excitation
IPC: G01S1/38
Filed: 1974-02-11
Abstract: 1425141 Radio-navigation; aerials HAZELTINE CORP 11 Feb 1974 [3 April 1973] 06114/74 Headings H4D and H4A An aerial system comprises an array of parallel aerial element columns arranged with their centres along a circular arc lying in a selected plane, said columns comprising one or more elements, and means for coupling wave energy signals thereto. The system also comprises means for supplying a sequence of pulse type signals to each of the columns simultaneously during a period of time, the phase difference between pairs of corresponding signals in the sequences supplied to any two adjacent columns being equal to a different integral multiple of the angular separation of said columns for each of said pairs, and the phase of the signals applied to one of said columns remaining substantially constant. The system radiates with a desired radiation pattern into a region of apace during the time period, and the frequency of the radiated energy varies with a component of angular direction from said system measured in said plane. As described, eight aerial elements 10a to 10h, Fig. 1, such as dipoles, slotted waveguides or spirals, are equally spaced to form a complete circle. The spacing between adjacent elements is, typically, about a half-wavelength. The elements are connected to respective output parts of a Butler matrix 11 by transmission lines 12 of equal phase lengths. The eight inputs 13 of the Butler matrix are connected by transmission lines 14 to respective outlets of a sequential switching device 15, which receives input from an oscillator 16. Each of the input ports of the Butler matrix corresponds to a different "mode" of the circular array. In each mode the phase difference between the signals coupled to adjacent elements of the array is an integral multiple of #/4, and ranges from - #(-4 mode) to + ¥# (+3 mode). The modes are shown in Fig. 2, and when the input ports 13, Fig. 1, are sequentially connected to the oscillator 16 by the switch 15 the elements 10 have signals supplied to them with a sequentially changing phase, except for the element 10a. This sequentially changing phase during the time period required for the switch 15 to complete one cycle of operation causes an apparent frequency shift of the radiated signal, when viewed by an observer located in the far field of the array. The total phase change, and hence the frequency shift observed, is dependent on the angle of the observer from the array, there being no phase change for an angle of 0 degrees and a 7# phase change for an angle of 180 degrees. At 180 degrees there is ambiguity because of the large phase change with each pulse, but in other regions positive or negative frequency shifts occur which are linearly dependent on angle. The lengths of the transmission lines 14 are adjusted to compensate for differing near field phase velocities of the various modes, so as to give the required far field phases indicated in Fig. 2. The switch and oscillator shown in Fig. 1 may be replaced by a tapped delay line connected to a pulse generator. The aerial elements may be spaced along an arc constituting only a portion of, instead of, a complete circle. Frequency conversion means may be included in the transmission lines 12, if desired.