Network Approach For Reducing The Number Of Phase Shifters In A Limited Scan Phased Array

Abstract

An arrangement for scanning of an array by means of controllable phase shifters. A network approach is used to substantially reduce the number of phase shifters required for a limited-scan array. Each phase shifter output is distributed over more than one antenna element of the array. The concept is applicable to both one-dimensional and two-dimensional scanning. The grating lobes of the array factor are allowed to exist in real space but are excluded from the scan region by selecting the subarray spacing. The network is designed to suppress other grating lobes in real space.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to scanning antennas as commonly used in the radar arts, and more particularly, to the so-called phased array types.

2. Description of the Prior Art

Phased arrays find major application where it is desirable to scan a beam electronically and, therefore, in an inertialess manner, rather than relying on partially or fully rotatable, mechanical arrangements. The advantages of inertialess scanning are well understood. Such scanning arrangements have been extensively described in the literature. In the textbook entitled "Radar Handbook" by Merrill Skolnik, a McGraw-Hill book (1970), Chapter 11 is devoted entirely to array antennas and the inertialess scanning of the beams formed by those antennas. FIG. 3 of that chapter described a phased array equivalent to that shown as prior art in FIG. 1 of the drawings of the present application. That figure shows the classical form of phased array operated in accordance with programmed variation of phase shifters. Each of the N phase shifters controls a corresponding one of the N elements of the array. A power distribution network 7 would normally provide distribution of excitation energy in the transmitting mode, and would also collect the signals through the N phase shifters and connect them to terminal 8 in the receiving mode.

It will be seen that the number of phase shifters required is equal to the antenna length divided by the elements spacing. The element spacings, in turn, are determined by the requirement to avoid grating lobes in real space, as will be discussed in more detail hereinafter. The maximum element spacing is, therefore, limited to:

S/.lambda. = 1/1 + .vertline. sin .theta..sub.o max .vertline. Equation I

where,

S = element spacing,

.theta..sub.o max = maximum scan angle, and

.lambda. = wavelength of operation.

The major problem encountered in applying this approach to scanning over a limited angular range is that the number of phase shifters required is usually excessive. Moreover, if the maximum scan angle required is reduced from 90.degree., for example, to only 10.degree., the number of phase shifters required is reduced by only 40 percent.

The manner in which the disadvantages of the known prior art phased-array scanning arrangements are overcome will be understood as this description proceeds.

The prior art disadvantages include difficulty in managing grating lobe distribution in practical arrays, as well as the equipment multiplicity referred to above.

SUMMARY OF THE INVENTION

It may be said to be the general objective of the present invention to provide a new approach for substantially reducing the number of active elements required for a limited-scan phased array. The concept is applicable to both one-dimensional and two-dimensional scanning as will be more fully described.

In accordance with the general concept of the present invention, the N radiating elements are fed from a subarray interconnecting network having a smaller number M of network ports, as compared to the number N of antenna elements in the array. Each of the said M subarray network ports is fed from a distribution network, much as in the prior art, through a discrete phase shifter. Accordingly, there are M phase shifters for the N antenna elements, M being a substantially smaller number than N. Spacing of subarray inputs is chosen so as to locate the grating lobe area (in terms of angular coordinates) where the normalized pattern voltage ratio is minimized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a prior art form of phased-array scanning configuration.

FIG. 2 is a grating lobe and desired lobe location diagram for the FIG. 1 arrangement.

FIG. 3 is a basic block diagram illustrating the subarray interconnecting network of a typical implementation of the present invention with a reduced number of phase shifters.

FIG. 4 is a grating lobe and desirable lobe location diagram for FIG. 3.

FIG. 5 is a detail showing typical interconnections within the subarray feed network of FIG. 3.

FIG. 6 is a normalized pattern diagram of a typical subarray.

FIG. 7 is a typical subarray network feed arrangement for an area array with limited-scan volume in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

As indicated in connection with the description of the prior art, FIG. 1 represents the well known array geometry of a conventional phased array in which each of N antenna elements has a corresponding phase shifter. The antenna elements are typically 1, 2 and 3 and the corresponding phase shifters are typically 4, 5 and 6, respectively. The distribution network 7 is a power divider or combiner receiving power from an input terminal 8 in the transmitting mode, or providing a collection point for energy combined in 7 in the receiving mode.

FIG. 2 illustrates the lobe limitations imposed by the well-known phenomenon of grating lobes in real space. The parameters and symbols depicted in FIG. 2 are defined as follows:

.theta. = observation angle

.theta..sub.o = scan angle of desired lobe

.theta..sub.g = angle of grating lobe

.lambda. = wavelength

S = element spacing

U.sub.o = sin .theta..sub.o

U.sub.g = sin .theta..sub.g

U.sub.max = sin .theta..sub.o max

It will be noted that a desired scan region defined by .+-.U.sub.max is shown. The nominal or central location of the useful lobe is shown at U.sub.o and an undesirable grating lobe U.sub.g is depicted at a spacing of .lambda./S from U.sub.o. From this illustration, the interrelationship of element spacing, wavelength and the angular extent of the desired scan region are depicted.

Referring now to FIG. 3, an idealized illustration of the concepts involved in the present invention is presented in block form. Here the N antenna elements, typically 9, 10 and 11 are comparable to elements 1, 2 and 3 of FIG. 1. A subarray interconnecting network 15 is provided, however, and the M phase shifters, typically 12, 13 and 14, are fewer in total number than the N antenna elements. An input-output terminal 17 is comparable to terminal 8 of FIG. 1 and the distribution network 16 compares with 7 of FIG. 1.

The nature of the subarray interconnecting network 15 will be described hereinafter. Each subarray input from each of the M phase shifters feeds several of the N antenna elements. The spacing between subarray phase centers is S and is greater than the element spacing. Employing superposition with respect to the subarray inputs, the resultant array pattern is equal to:

.rho.(u) = F (u) A (u),

where,

F (u) = (Subarray factor) .sub.o (u),

.rho..sub.o (u) = element pattern,

A (u) = array factor, ##SPC1##

U = sin .theta. observation parameter,

= sin .theta..sub.o scan parameter of desired lobe.

The grating lobes of the array factor occur at:

sin .theta..sub.g = sin .theta..sub.o .+-. (.lambda./S) n Equation III

where,

n = 1, 2, ----.

It will be realized that Equation II provides a mathematical definition of the arrangement of FIG. 3, and Equation III is illustrated in the grating lobe limitation diagram of FIG. 4. This diagram of FIG. 4 bears the same relationship to FIG. 3, as did FIG. 2 to FIG. 1.

The grating lobes of the array factor are allowed to exist in real space, but are excluded from the scan region by selecting the subarray spacing such that:

S/.lambda. .ltoreq. 1/2 .vertline. sin .theta..sub.o max .vertline. Equation IV

accordingly, these extra, or grating lobes, in real space still exist as a problem; however, they are inhibited by the ideal subarray pattern illustrated in FIG. 4. The factor reduction in the number of phase shifters in this idealized array compared to the conventional limited-scan array is equal to

G = S/S = 1 + .vertline. sin .theta..sub.o max .vertline./2.vertline.sin .theta..sub.o max .vertline. .gtoreq. 1 Equation V

hereinafter, a practical technique for approximating the idealized subarray pattern to effect a significant reduction in the number of phase shifters required will be presented. It should be recognized that if the subarray size is restricted dimensionally to S, then it is not possible to synthesize an approximation to the ideal subarray pattern. A solution to this problem is available, however, through the expedient of having each subarray input, with its associated phase shifter, feed overlapping subgroups of elements by means of interconnecting circuits. Thus, the identification of the system of the present invention as a "network approach" will be seen to be apropos. The increased size of the subarray allows the ideal subarray pattern to be approximated, thereby inhibiting the undesirable grating lobes.

FIG. 5 simply illustrates the design approach for a linear array with .+-.10.degree. (typical) scan requirement.

FIG. 6 illustrates a subarray pattern (for example, for the configuration of FIG. 5) which inhibits the grating lobes by 23 decibels. By "locating" the grating lobes in the identified grating lobe range on FIG. 6, by means of selection of the spacing of subarray inputs, in such a configuration as FIG. 5, the effect is to greatly reduce the grating lobe amplitude.

It should be recognized that, in the general case, each input (as shown in FIG. 5) could feed a much larger number of elements with interconnecting circuits to best approximate an ideal subarray pattern. In the FIG. 5 case, each input point (typically 24 or 26) contributes power to each of three antenna elements. Thus, elements 18, 19 and 20 are excited from the power divider at 24 and elements 20, 21 and 22 are excited by power from the power divider 26. In that example, element 20 receives energy from both 24 and 26. The components 23, 25 and 27 are typical couplers for the corresponding antenna elements, and in that way they act to add power along the branches from the power dividers feeding the corresponding antenna elements. It will be noted that each of the phase shifters, typically 28 and 29, feeds a corresponding power divider 24 and 26, etc., respectively, in this case. Looking typically at the power divider 26, it will be noted that there is one output S.sub.o to element 21, and two S1 branch outputs to couplers 25 and 27. The relative amplitude of the S.sub.o branch is 0.6635 and that of each of the S1 branches is 0.3348 .sqroot.2. Each of elements 20 and 22 is, therefore, excited by an amplitude equal to S1/.sqroot.2.

The antenna element spacing in FIG. 5 is 0.7.lambda..sub.o, this and the foregoing power division parameters combining to produce the effect described graphically in FIG. 6, considering the effect of greater subarray center-to-center spacing.

Thus, it may be said that the general design procedure involves maximizing the subarray pattern in the scan region, and minimizing the subarray pattern in the grating-lobe regions in real space. The grating-lobe region is defined as:

grating lobe region = <U.sub.g > = <v> .+-. .lambda.n/S

where,

n = 1, 2, ---- and

<v> = scan region.

The maximum numerical reduction in the number of phase shifters is, of course, limited by the factor G previously defined in Equation V. The realized reduction will depend on the degree of suppression of the grating lobes required, and the maximum complexity allowable for the interconnecting network. It should also be emphasized that the scan region can be shifted from broadside. In that case, the reduction factor is even larger. In that instance,

G = 1 + .vertline. sin .theta..sub.o max .vertline./sin .theta..sub.o max - sin .theta..sub.o min

where,

<v> = sin .theta..sub.o max - sin .theta..sub.o min = scan region.

Such a shift is accomplished by appropriate phase shifter programming.

To illustrate the utility of the novel approach of the present invention, consider the case of a scan requirement taken to be .+-.10.degree.. In that case, the maximum reduction in the number of phase shifters is 3.35 over that required in the conventional linear array of FIG. 1. The relatively simple interconnecting network illustrated in FIG. 5 results in a two-to-one reduction in the number of phase shifters, as compared to the aforementioned conventional phased array. It may be said that FIG. 5 is a linear array of overlapping subarrays, elements 20, 21 and 22 comprising the antenna elements of a typical subarray therein. Looking at each subarray as an entity, it will be realized that the effect of greater element spacing is achieved from subarray center to subarray center, insofar as grating lobe generation is concerned.

The network approach herein described for reducing the number of phase shifters in a limited-scan array can readily be extended to an area array which scans in both planes (horizontal and vertical, for example). The subarray in this case comprises an area rather than a single linear dimension. Accordingly, the general design procedure is to maximize the subarray pattern in the scan volume, and minimize the subarray pattern in the grating lobe region of real space. A subarray feed network for an area array with a triangular arrangement of elements is illustrated in FIG. 7. This configuration is well suited for scanning a maximum of 10.degree. off broadside in any scan plane, while providing 23 decibels of suppression of the undesirable grating lobes.

Referring now to FIG. 7 specifically, seven main elements 30, 31, 32, 33, 34, 35 and 36 are illustrated. These main elements are comparable to the elements such as 19 and 21 in FIG. 5. THe coupled elements illustrated include 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47 and 48. That is, each of these elements is comparable to elements such as 20 and 22 in FIG. 5, in that they are excited by a pair of S1 branches taken from couplers at the element locations and located similarly in respect to the coupled and main elements, as depicted in FIG. 5. In FIG. 7, S.sub.o = 1.00 voltage ratio and S1 = 0.576 .times. .sqroot.2 voltage ratio. Signals are added at coupled elements through the use of a hybrid at each coupled element location, these hybrids being similar to the couplers 23, 25 and 27, illustrated on FIG. 5. A significant difference, as compared to FIG. 5, is that in the case of each main element, for example, element 30, there are six branches from the power divider feeding the S.sub.o signal to element 30. These branch signals are represented by the S1 signals provided to the coupled elements at 37, 38, 39, 40, 41 and 42 via the hybrid couplers also coupling in signal from the corresponding power dividers feeding elements 32, 33, 34, 35, 36 and 31, respectively. Thus, the configuration of FIG. 7 is an area form of that depicted at FIG. 5.

To summarize, this disclosure will be seen to have presented a new technique and structure for limited-scan phased array construction, substantially reducing the number of active devices required in either a linear or area type array.

Claims

1. A scanning phased array antenna system having a predetermined number N of antenna elements, and a distribution network having a common input terminal and a predetermined number of distribution ports M, where M is a smaller number than N, comprising:

M phase shifters each connected at its input discretely from a corresponding one of said M distribution ports;

a subarray interconnecting network having N output ports and M input ports, each of said N output ports being connected discretely to a corresponding one of said antenna elements, and each of said M input ports being connected discretely to the output of a corresponding one of said phase shifters;

divider means within said subarray interconnecting network for dividing the power input from each of said phase shifters into a primary branch and a plurality of secondary branches, said primary branch being connected to a selected one of said antenna elements and said secondary branches feeding an equal number of antenna elements adjacent on each side of said selected element;

and means associated with each antenna element fed from one of said secondary branches for combining feeds from said branches providing feed

2. A scanning phased array antenna system comprising:

an array of antenna elements comprising a first group of main elements and a second group of spaced coupled intermingled elements comprising a subarray;

means including a plurality of discretely controllable phase shifters for providing excitation to corresponding ones of said main elements;

a plurality of power dividers connected, one to the output of each of said phase shifters, said dividers each having a main output and a plurality of branch outputs each providing power output at a predetermined fraction of the power supplied by the corresponding phase shifter, each of said main outputs being connected to an antenna element of said first group of main elements;

and a plurality of coupler means one of which is discretely connected to each element of said subarray, said couplers also being connected to mix power from at least one of said branch outputs from at least each of said power dividers corresponding to the adjacent main elements on each side of

3. Apparatus according to claim 2 in which said array is a linear array.

4. Apparatus according to claim 2 in which said array comprises a linear array of plural subarrays, said main elements are first alternate elements of said array, and said subarrays comprise at least one element on each

5. Apparatus according to claim 4 in which said subarray comprises one

6. Apparatus according to claim 2 in which said array is an area type array in which said main elements are distributed over a surface in a regular pattern, said subarray comprises an element spaced on a line between each of said main elements, said power divider branch outputs are six in number corresponding to six subarray elements comprising the nearest antenna elements surrounding each of said main elements, and each of said coupler means being connected to a branch output from the power dividers corresponding to a pair of adjacent, but oppositely disposed, main

7. Apparatus according to claim 4 in which said divider is arranged to apportion the power in said branch outputs with respect to said main elements to produce a predetermined subarray pattern, the phase centers between said subarrays being greater than the element spacing of said array.