Electronically Modulated Tacan Antenna

Abstract

A TACAN antenna array comprising a central radiating element, providing carrier frequency f.sub.c radiation omnidirectionally in the azimuth plane, which is surrounded by two or more concentric arrays of 15Hz and 135Hz modulation radiating elements. The first concentric array is preferably comprised of 4 elements used to provide a f.sub.c .+-. 15Hz modulation and is comprised of two orthogonally disposed antiphased subarrays which are excited with the suppressed carrier sideband energy of a 15Hz modulated carrier frequency f.sub.c with the two orthogonally arranged subarrays being excited in phase quadrature with one another. The second concentric array is preferably comprised of 36 radiating elements divided into two subarrays of 18 elements each, which are excited with 135Hz modulated suppressed carrier f.sub.c sideband energy. The elements of the two 18 element subarrays are alternately dispersed in a circular pattern. The two subarrays respectively are fed in phase quadrature with adjacent elements of each subarray being fed in antiphase relationship so that as the circumference is traversed a +l, +j, -l, -j phase relationship exists in consecutive sets of four elements around the 36 radiating elements.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electronically controlled antenna system and more particularly to one which is capable of providing a rotating radiation pattern particularly adapted to TACAN.

2. Description of the Prior Art

TACAN is a tactical air navigation system which provides bearing and range information on direct reading instruments located in an aircraft. The heart of the system lies in the ground beacon antenna which generates a certain amplitude modulation in space in all azimuth directions to provide bearing information. Essentially, this is a shaped beam in the horizontal plane which rotates in this plane. Course bearing information is generated by a rotating limacon-shaped pattern which results in a 15Hz amplitude modulation. Fine bearing information is generated by a nine-lobed radiation pattern which is superimposed on the limacon pattern. To provide vertical coverage, the antenna is shaped similar to a cosecant-squared pattern.

In order to better understand the design of an array for TACAN, an existing mechanically scanned antenna design and its operation will be briefly described. The antenna consists of a stationary central vertically polarized element or array to provide an omnidirectional pattern in the azimuth plane. This is the only active element in the antenna. All other elements are parasitically excited. A rotating subassembly is placed around this stationary central element or array to distort the omnidirectional pattern into a scalloped pattern which is shaped to produce two amplitude modulations when viewed from a distance.

This subassembly consists of two coaxial plastic cylinders or radomes which are rigidly held to each other and rotated at a controlled speed of 15Hz. In turn, these coaxial cylinders are concentric with the inner central feed array which is stationary. Two biconical horns which are stacked vertically are fastened between the inner and outer rotating plastic cylinders and thus rotate with them. These two biconical horns are fed by the stationary array. The purpose of the horns is to shape the antenna pattern in the vertical plane. In other words, these horns determine the shape of the primary radiation pattern in the vertical plane and so determine the vertical coverage.

The heart of the amplitude modulating mechanism lies in the inclusion of small parasitic radiators which are fastened to the two rotating cylinders. Without these parasitic radiators, the antenna pattern is essentially omnidirectional in the azimuth plane. A single parasitic reflecting element in the form of fine wires is cemented on the inner cylinder. By placing these wires in the electric field radiating from the central stationary array, the omnidirectional pattern is distorted into a more directional one. The shape of this distorted pattern is that of a limacon. Next nine parasitic elements uniformly spaced at 40.degree. intervals are cemented on the outer rotating cylinder. Thus placed in the electric field of the dipoles, the azimuth plane pattern is distorted by the reradiated energy from these plastic wires. With the absence of parasitic wires on the inner cylinder, the pattern resulting from the nine parasitics is that of a nine lobed one.

As noted above, these two dielectric cylinders with their parasitic elements are rotated at the controlled speed of 15Hz. Because of the radiation pattern distortion, the radiated energy goes through periodic variations in amplitude when viewed from anywhere in space. The amplitude variation produced by a single parasitic element on the inner cylinder can be represented as a sine wave. Obviously, the frequency of this modulation is 15Hz. The amplitude variation produced by the nine parasitic elements on the outer cylinder also produces a sine wave. Because the pattern is nine lobed, the frequency of this modulation is 9 .times. 15 or 135Hz. With the simultaneous modulation by both sets of parasitics, the nine lobed pattern is superimposed on the limacon pattern. The resultant wave radiated by the antenna thus contains two modulating frequencies 15Hz and, its ninth harmonic, 135Hz.

These mechanically rotated systems, although simple and reliable, are bulky, heavy and consume relatively large amounts of primary power. Because of this, great interest has recently arisen for an electronically scanned TACAN antenna. Three electronically scanned TACAN antenna systems are shown and described in the following patents:

U.S. Pat. No. 3,474,446, L. N. Shestag et al.;

U.S. Pat. No. 3,474,447, L. Melancon; and

U.S. Pat. No. 3,560,978, L. Himmel et al.

SUMMARY

Briefly, the subject invention comprises, inter alia, an electronically scanned TACAN antenna comprised of a first array configured as a stationary central element or array which is excited from a TACAN transmitter with a carrier frequency signal f.sub.c. A second and third fixed array of n elements and 4n elements, respectively, are concentrically positioned relative to the first array. The second array is preferably comprised of four elements configured as two orthogonally disposed antiphased subarrays with the subarrays being excited by a f.sub.c .+-. 15Hz suppressed carrier sideband signal in phase quadrature with one another. The third array is excited with a f.sub.c .+-. 135Hz suppressed carrier signal and preferably is comprised of 36 radiating elements. The 36 elements are excited in groups of 18 elements in phase quadrature respectively with adjacent elements of each group being antiphased with one another. The two groups of elements are interspersed to provide repeating sets of four elements, each having relative phases of +1, +j, -1, and -j, respectively.

BRIEF DESCRIPTION OF THE DRAWING

The DRAWING is an electrical schematic diagram of the preferred embodiment of the subject invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the DRAWING, reference numeral 10 designates a central radiating element excited by a carrier frequency signal f.sub.c coupled thereto from a TACAN transmitter 12. The central element 10 is comprised of a selected number of vertical elements forming an array suitable to provide a desired vertical plane coverage pattern which is omnidirectional in azimuth. An impedance matching device 14 is shown coupled between the output terminal 16 and the junction 18 for providing a suitable impedance match between the central element 10 and the TACAN transmitter 12 for obtaining maximum power transfer.

A second array of at least four active radiating elements, designated by reference numerals 20.sub.1, 20.sub.2, 20.sub.3 and 20.sub.4, respectively, are disposed around the central element 18 with the elements 20.sub.1 and 20.sub.3 being located on a common axis equidistantly on opposite sides of the central element 10 and with the elements 20.sub.2 and 20.sub.4 being located on an orthogonal axis relative to the first axis and also being disposed equidistantly on either side of the central element 10. The elements thus located around the central element 10 are excited by means of a suppressed carrier sideband signal having a frequency of f.sub.c .+-. 15Hz appearing at output terminal 22. The four elements 20.sub.1 . . . 20.sub.4 are excited as two subarrays which each comprise two elements which are fed in antiphase relationship relative to one another, i.e., 180.degree. out of phase with one another. Moreover, the two subarrays are respectively excited in phase quadrature with one another so that for example the elements 20.sub.1, 20.sub.2, 20.sub.3, and 20.sub.4 have a respective phase of +1, +j, -1, and -j which ocrrespond to 0.degree. , 90.degree. , 180.degree. and 270.degree. of relative phase shift.

More particularly, the f.sub.c .+-. 15Hz signal is applied from output terminal 22 to a signal buss 24. Radiating element 20.sub.1 is coupled to signal buss 24 through the impedance matching devices 26 and 28 and is thus said to have a zero or +1 phase shift. Its opposing radiating element 20.sub.3 is excited from signal buss 24 through the impedance matching elements 28 and 30 as well as a phase inverter 32 which applies 180.degree. phase shift to the signal. Thus elements 20.sub.1 and 20.sub.3 are said to have a +1 and -1 phase, that is a mutual antiphase relationship. The radiating elements 20.sub.2 and 20.sub.4 which are orthogonally disposed with respect to the first pair of elements 20.sub.1 and 20.sub.3 are excited from signal buss 24 after passing through a 90.degree. or quadrature phase shifter 34. Moreover, the radiating element 20.sub.2 is coupled to the output of the 90.degree. phase shifter 34 by means of the impedance matching devices 36 and 38 and thus is said to have a +j or 90.degree. phase shift relative to the phase of the signal appearing on the circuit buss 24 and the radiating element 20.sub.1. With respect to the fourth radiating element 20.sub.4, it is coupled to the output of the 90.degree. phase shifter 34 through the impedance matching devices 36 and 40 as well as a phase inverter 42, thus making its radiated signal 180.degree. out of phase with respect to the signal radiated from element 20.sub.2 and thus is said to have a -j phase.

The amplitude of the energy in the sidebands radiated from the elements 20.sub.1 . . . 20.sub.4 is maintained below the amplitude of the energy in the carrier radiated from the central element 10 which results in an amplitude modulated signal, where the phase of the modulation varies linearly as the direction varies from 0.degree. to 360.degree. in azimuth which is one of the characteristics in the TACAN system.

Concentric with the second array including the radiating elements 20.sub.1 . . . 20.sub.4 is a third radiating array comprised of 4n active radiating elements and more particularly comprised of 36 radiating elements 46.sub.1, 46.sub.2 . . . 46.sub.35 and 46.sub.36 arranged in a circular configuration around the central array 10. These 36 radiating elements are excited by means of a suppressed carrier sideband signal having a frequency f.sub.c .+-.135Hz which appears at output terminal 48 of the TACAN transmitter 12. Moreover, the 36 radiating elements 46.sub.1 . . . 46.sub.36 are divided into two groups of 18 elements which are interlaced with one another, i.e., alternating. The first or even-numbered group of radiating elements are comprised of elements 46.sub.2, 46.sub.4, 46.sub.6 . . . 46.sub.34 and 46.sub.36 while the second or odd numbered group of elements are comprised of elements 46.sub.1, 46.sub.3, 46.sub.5, 46.sub.7 . . . 46.sub.35. The even numbered elements are powered from a signal feed line 52 coupled to the signal buss 50 by means of an impedance matching device 54 with each succeeding even numbered element being excited in antiphase relationship with the preceding even numbered element. Taking element 46.sub.36 as an illustrative example, it is coupled to the signal feed line 52 by means of an impedance matching element 56 and exhibits a +1 phase or 0.degree. phase shift with respect to the signal appearing on circuit buss 50. The even numbered elements 46.sub. 2 and 46.sub.34 on either side of the +1 phase element 46.sub.36 are coupled to the signal feed 52 by means of impedance matching elements 58 and 60 and phase inverters 62 and 64, respectively, and thus exhibit a -1 phase or 180.degree. phase shift relative to element 46.sub.36.

The odd numbered elements 46.sub.1, 46.sub.3 . . . 46.sub.35 are powered from a signal feeder line 66 which has its f.sub.c .+-.135Hz sideband signal applied thereto in phase quadrature or 90.degree. out of phase with the signal appearing on signal feed line 52. This is provided by a quadrature phase shifter 68 and impedance matching element 70 coupled between the signal buss 50 and the feeder line 66. Radiating element 46.sub.1 is coupled to the feeder line 66 through the impedance matching element 72 and thus exhibits a +j phase or 90.degree. phase shift between the signal applied to its immediately adjacent elements 46.sub.36 and 46.sub.2 which are phased +1 and -1 respectively. Odd numbered elements 46.sub.3 and 46.sub.35 succeeding and preceding odd numbered element 46.sub.1 are coupled to the signal feed through the impedance matching devices 74, 76 and phase inverters 78 and 80, respectively and thus provide a -j phase or 180.degree. phase shift with respect to the signal applied to the element 46.sub.1.

The sequence of phasing of the f.sub.c .+-. 135Hz sideband energy as the circular array including the elements 46.sub.1 . . . 46.sub.36 is traversed, is +1, +j, -1, and -j which is repetitive in that order in each set of four elements and thus the radiated signal from the 36 element array will provide in conjunction with the carrier radiated from the central element 10 a 135Hz modulation radiated signal in which the phase of the modulation goes through a 360.degree. linear phase variation for each 40.degree. of azimuth direction angle which is the other characteristic of a TACAN antenna system.

The choice of exact radius on which to locate the elements of the second and third concentric arrays with respect to the central element comprises a design detail; however, the vertical plane pattern of the four element array comprising radiating elements 20.sub.1 . . . 20.sub.4 is approximated by J.sub.1 [(.pi.d/.lambda.) sin.theta.] while the 36 element array including the radiating elements 46.sub.1 . . . 46.sub.36 is approximated by J.sub.9 [(.pi.d/.lambda.) sin.theta.] where .lambda. is the wavelength of the carrier frequency and .theta. is the angle measured from the zenith and would be equal to 90.degree. on the horizon. Normally the radius of the elements of the array would be chosen so that the Bessel function is a maximum on the horizon although for synthesizing specific vertical coverage patterns, other values might be used as well as a combination of additional four element and additional 36 element arrays to provide a desired coverage pattern; however, these additional elements can be arranged both in the vertical plane and the azimuth plane on different radii.

What has been shown and described, therefore, is a TACAN antenna system in which the modulation or scanning is introduced onto the radiated signal electronically and except for this fact, provides a radiated signal which is identical in all respects to that realized from a mechanically rotating TACAN antenna.

Having thus described what is at present considered to be the preferred embodiment of the subject invention,

Claims

I claim as my invention:

1. An electronically modulated TACAN antenna system for providing a rotating radiation pattern, comprising in combination:

a TACAN transmitter generating and providing at least three output signals having frequencies of f.sub.c, f.sub.c .+-. 15Hz, and f.sub.c .+-. 135Hz, respectively, where f.sub.c is the carrier frequency;

central radiation means coupled to said f.sub.c output signal and providing an omnidirectional radiation pattern in the azimuth plane thereby;

an array of at least four elements containg two pairs of mutually orthogonal active radiation elements centrally located with respect to said central radiation means;

circuit means coupling said array of at least four radiation elements to said f.sub.c .+-. 15Hz output signal and including circuit means feeding said f.sub.c .+-. 15Hz output signal to said at least four radiation elements with respective f.sub.c .+-. 15Hz output signals phased +1, +j, -1, and -j, consecutively where +1 corresponds to 0.degree. phase shift, +j corresponds to 90.degree. phase shift, -1 corresponds to 180.degree. phase shift and -j corresponds to 270.degree. phase shift whereby said +1 phase and -1 phase excited radiation elements are located on a common axis on opposite sides of said central radiation means and said +j phase and -j phase excited radiation elements are located on a common orthogonal axis on opposite sides of said central radiation means;

another array of 4 n radiation elements, where n>1, circularly arranged concentrically with respect to said central radiation means; and

circuit means coupling said circular array of 4n radiation elements to said f.sub.c .+-. 135Hz output signal and including circuit means feeding said f.sub.c .+-. 135Hz output signal to repetitive sets of four radiation elements each with respective f.sub.c .+-. 135Hz output signals phased +1, +j, -1, and -j, consecutively around the entire array.

2. An antenna system according to claim 1 wherein n is equal to 9 whereby the phase of f.sub.c .+-. 135Hz signal goes through a 360.degree. linear phase variation for each 40.degree. of direction angle in azimuth.

3. The apparatus as defined in claim 1 wherein said output signals of f.sub.c .+-. 15Hz and f.sub.c .+-. 135Hz comprise suppressed carrier sideband signals.

4. The invention as defined by claim 1 wherein said feeding means for said array of at least four radiation elements includes:

a f.sub.c .+-. 15Hz signal buss;

signal coupling means coupled from said signal buss to a first radiation element of said four radiation elements and providing the +1 phase radiation;

signal coupling means including a phase inverter coupled between said signal buss and a second radiation element, said second element being located on said common axis with said first radiation means and providing the -1 phase radiation;

quadrature phase shifter means coupled to said signal buss;

signal coupling means coupled from the output of said quadrature phase shifter to a third radiation element, said third radiation element being located on said orthogonal axis and providing the +j phase radiation; and

signal coupling means including a phase inverter coupled from the output of said quadrature phase shifter to a fourth radiation means, said fourth radiation means being located on said orthogonal axis opposite said third radiation element and providing the -j radiation.

5. The invention as defined by claim 4 and wherein said feeding means for each set of 4n radiation elements of said circular array includes:

a f.sub.c .+-. 135Hz signal buss;

signal coupling means coupling said last recited circuit buss to the first radiation element of each set of elements and thereby providing the +1 phase radiation;

signal coupling means including a phase inverter coupling said last recited circuit buss to the third radiation element of each set of elements and thereby providing the -1 phase radiation;

a quadrature phase shifter coupled to said last recited circuit buss;

signal coupling means coupling the output of said quadrature phase shifter to the second radiation element of each set of elements and thereby providing the +j phase radiation; and

signal coupling means including a phase inverter coupling the output of said quadrature phase shifter to the fourth radiation element of each set of elements and thereby providing the -j phase radiation.

6. The apparatus as defined in claim 5 wherein said 4n radiation elements comprise 36 elements.

7. The apparatus as defined in claim 6 wherein said f.sub.c .+-. 15Hz and f.sub.c .+-. 135Hz output signals each comprise suppressed carrier sideband signals.