Electrically Scanned Microstrip Antenna

Abstract

A microstrip antenna including a dielectric layer loaded with a ferrite material disposed between a ground plane and a generally planar or single layer arrangement of electrical conductors constituing both r.f. radiators and r.f. feedlines. Either or both of the r.f. radiators and feedlines include special d.c. circuits for passing d.c. electrical currents. When the d.c. electrical currents are passed through the r.f. radiators, the permeability of the ferrite loaded dielectric is altered thus scanning the resonant frequency of a radiator in accordance with the applied d.c. current or voltage. Furthermore, when the d.c. currents are passed through the r.f. feedline, or portions thereof, the magnetic fields set up in the ferrite loaded dielectric causes controlled phase shifts to occur in r.f. energy passing there along thus effecting controlled phase shifts and hence beam scanning of an array of such radiators as a function of the d.c. current or voltage.

Description

This application is related to my co-pending U.S. application Ser. No. 352,005 filed concurrently herewith. It is also related to commonly assigned United States Pat. No. 3,713,162 and to the commonly assigned co-pending application Ser. No. 99,481 filed Dec. 18, 1970.

This invention relates generally to antenna structures utilizing a ferrite loaded dielectric layer disposed between a ground plane and another layer of conductors comprising the antenna elements and/or feedlines. In particular, it relates to an antenna structure of this kind wherein the resonant frequency of an r.f. radiator and/or the beam direction of an array of such radiators is controlled by passing d.c. electrical currents through the radiator and/or feedlines respectively.

The antenna structure to be described below is a form of microstrip antenna wherein the actual r.f. feedlines and/or r.f. radiators are preferably formed on one face of a dielectric sheet using conventional photo-resist/etching techniques as are used in forming electrical circuit boards.

As will be appreciated by those in the art, it is often desirable to increase the bandwidth or range of possible operating frequencies for any given antenna structure whether that structure is utilized alone or in an array of similar structures. As will be explained in more detail below, when the dielectric substrate of a microstrip antenna is loaded with a ferrite material the resonant frequency for the microstrip radiator may be altered by passing a direct current through the radiator thus changing the permeability of the ferrite loaded dielectric in the vicinity of the radiator. Such changes in the relative permeability of the dielectric will change the effective electrical length or other dimension of the microstrip radiator thus altering the operating frequency as will become apparent. Accordingly, the antenna structure of this invention permits controlled increases in the effective bandwidth for any given antenna dimensions.

An electronically scanned antenna array is usually a costly and complex apparatus. However, as will be explained in more detail below, the antenna structure of this invention may be formed through economical printed circuit board techniques to provide a microstrip antenna array having a radiation beam direction that may be selectively or controllably scanned as a function of a variable voltage or current. One variable voltage will permit scanning in one coordinate direction while two variable voltages will permit two dimensional scanning along two coordinate directions as will become apparent. Accordingly, this antenna structure provides an extremely simple, reliable, and cheap scannable array. Furthermore, it is extremely simple to operate the scanned array of this invention since all that is required is a variable d.c. voltage or current. Since techniques are readily available for providing variable voltages/currents having complex predetermined wave shapes, it should readily be apparent that the antenna array of this invention may be easily controlled to follow complex scanning patterns.

Furthermore, since the antenna structure of this invention is extremely thin compared to conventional antenna structures, it is readily adaptable for use in streamlined vehicles such as airplanes and rockets where design considerations necessitate the minimum possible protuberance either inside or outside of the vehicular skin. As will be appreciated, since the antenna is really in the nature of a thin printed circuit board, the whole structure will be flexible if the dielectric material is properly chosen thus making it easy to conform the entire antenna structure with the skin of such a vehicle or to any other desired shape.

These and many other objects and advantages of this invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, of which:

FIG. 1 is a plan view of an exemplary microstrip antenna array according to this invention which permits beam scanning in one dimension by controlling a d.c. current along portions of the r.f. feedline therein;

FIG. 2 is a cross sectional view of the exemplary antenna array shown in FIG. 1;

FIG. 3 is a sequence of schematic diagrams illustrating the variation of beam direction with a d.c. current for the exemplary antenna structure of FIG. 1;

FIG. 4 is an exemplary plan view of one microstrip radiator including a d.c. circuit for altering its resonant frequency;

FIG. 5 illustrates an exemplary modification of the FIG. 1 embodiment using switchable diodes to further selectively alter the beam steering characteristics of the array; and

FIG. 6 is a schematic illustration of an exemplary two dimensional microstrip antenna array including two separate d.c. current circuits in the r.f. feedlines for steering the beam direction of the two dimensional array along two coordinate axes.

The printed circuit board 10 shown in FIG. 1 is shown in cross section at FIG. 2. Broadly stated, it comprises the usual printed circuit board construction wherein a conductive layer 12 is selectively etched away from a dielectric layer 14 to result in a generally planar or single layer arrangement of electrical conductors on top of the dielectric substrate 14. In the case of this invention, the planar arrangement of conductors 12 comprises r.f. feedlines as well as r.f. radiator sections and superposed d.c. electrical circuits as will be explained. The dielectric substrate 14 has been loaded with conventional microwave quality ferrite materials as indicated in FIG. 2 and is disposed between the planar arrangement of electrical conductors 12 and a ground plane surface of electrically conducting material 16. As will be appreciated by those in the art, the ground plane 16 may, in fact, comprise a layer of conductive material adhered to the backside of the dielectric layer 14 thus being co-extensive with the dielectric substrate 14. On the other hand, the ground plane 16 might also comprise part of a conducting vehicular surface such as an airplane or missile skin, etc., as will be appreciated.

As indicated in FIG. 2, the ferrite loaded dielectric 14 may comprise a conventional dielectric material in which conventional microwave quality ferrite powder has been dispersed by conventional techniques. Alternatively, it is possible to load the dielectric substrate with a sheet of ferrite material as will be apparent to those in the art. In the preferred embodiment, the planar or single layer arrangement of electrical conductors 12 is formed by conventional photo resist-chemical etching processes commonly used in the manufacture of printed circuit boards.

As seen in FIG. 1, a linear array of microstrip radiators N.sub.1, N.sub.2, N.sub.3 and N.sub.4 is provided. Each of these radiators is fed from an r.f. feedline emanating from an overall corporate feedline structure wherein the original r.f. input signal at 18 is first divided at point 20 into two equal power signals as should be apparent. In the absence of any d.c. electrical current (as will be described in more detail below), these divided half power signals will also be of equal phase relative to one another as they travel along the corporate structure r.f. feedline.

These half power signals are again divided at points 22 and 24 into quarter power signals of equal power and equal relative phases which are then fed directly to the microstrip radiators as shown in FIG. 1. Accordingly, as will be appreciated, in this case of zero d.c. current, all elements of the linear array will be fed with equal power and equal relative phase signals so that the resultant radiation will be a high gain beam pattern directed normally to the plane of the array and as is schematically illustrated in the top line of FIG. 3.

Ferrite materials have been used in the past to cause relative phase shifts in r.f. signals as shown for instance in United States Pat. No. 3,553,733 to Buck and in United States Pat. No. 3,377,592 to Robieux et al. However, these prior structures have involved bulky waveguides and/or external electromagnet assemblies that have made them relatively complex and costly.

It has now been discovered that the antenna structure of this invention utilizing a ferrite loaded dielectric may be conveniently modified to produce the necessary phase shifts between the array radiators thus achieving beam steering capability for the array. For example, as shown in FIG. 1, a variable d.c. electrical current source 26 is connected to a d.c. circuit within the generally planar arrangement of electrical conductors 12 for achieving the necessary relative phase shifts. Of course, as those in the art will appreciate, the current source 26 could be just as well be replaced with a voltage source; however, since the phase shifting and/or other effects to be described herein are believed to be proportional to the d.c. current, the exemplary embodiment has been explained using a variable d.c. current source for explanatory purposes.

In essence, the exemplary embodiment in FIG. 1 provides for a d.c. electrical current to flow along the isolated segments 28, 30 and 32 of the r.f. feedline structure. It has been discovered, that when d.c. currents are passed through these microstrip feedlines above the ferrite loaded dielectric, a controlled phase shift may be introduced into r.f. signals also passing therealong.

Accordingly, as can be appreciated from FIG. 1, the r.f. signals routed to the microstrip radiator N.sub.1 will not experience any further additional relative phase shifts. However, those r.f. signals routed to microstrip radiator N.sub.2 will experience an additional phase shift proportional to the current being passed along isolated segment 28 and to the length of segment 28.

Likewise, the r.f. signal being passed to radiator N.sub.3 will experience a similar added relative phase shift (since the d.c. circuit is a series circuit exactly the same d.c. current must be flowing in segment 30 as in segment 28) but since the length of segment 30 is twice the length of segment 28, these signals will have experienced twice as much relative phase shifting as those which are supplied to radiator N.sub.2. Similarly, the signals supplied to radiator N.sub.4 undergo a still further phase shift along segment 32 of the d.c. circuit which is equal in length to one-half of segment 30 and to the full length of segment 28. Accordingly, the signals reaching radiator N.sub.4 will be shifted three times as much in relative phase as those signals which are supplied the radiator N.sub.2.

The segments 28, 30 and 32 of the r.f. feedline in FIG. 1 are isolated from the other portions of the r.f. feedline with respect to d.c. currents by d.c. blocking-r.f. passing means 34 which are somewhat analogous to the d.c. blocking capacitors used in low frequency electronics circuits. Here, to insure maximum passage of r.f. currents, the "plates" of these coupling means should be approximately one-fourth of a wave length long (taking into account the dielectric and magnetic parameters of the ferrite loaded dielectric) and spaced, preferably, no more than two to three thousandths of an inch apart.

These isolated segments of the r.f. feedline are interconnected by d.c. passing-r.f. blocking means 36, 38 and 40. These d.c. passing-r.f. blocking segments of the d.c. circuit are somewhat analagous to r.f. chokes commonly used in electrical circuits. To minimize the interference with r.f. currents in the r.f. feedlines, the d.c. circuits 36, 38 and 40 should include any necessary open circuited line segments 42 dimensioned and spaced to reflect an r.f. open circuit condition at the actual points of connection to the r.f. feedline for the anticipated operating frequency.

Thus, as should now be appreciated, a complete d.c. circuit has been described within the planar or single layer arrangement of electrical conductors 12. This circuit comprises the d.c. passing-r.f. blocking portions 36, 38 and 40 together with the isolated r.f. feedline segments 28, 30 and 32. Of course, the d.c. circuit is returned to ground as at 44 to complete the electrical circuit.

Referring now to FIG. 3, the situation as it would exist with no current flowing in the d.c. circuit from current source 26 is shown at the top line of FIG. 3 wherein all four of the microstrip radiators are receiving equal power and equally phased excitations to result in a beam direction normal to the plane of the linear array. However, when the current source 26 is activated to produce some current I.sub.1 relative phase shifts will be introduced in the r.f. signals traversing isolated segments 28, 30 and 32 which phase shifts will be proportional to the magnitude of the current I.sub.1 and to the length of r.f. feedline conducting such d.c. currents along which the various r.f. signals are propagating. Accordingly, for some value I.sub.1 a situation can be expected as shown in line 2 of FIG. 3 where the relative phase angles between the excitation or driving signals to the four microstrip radiators differ by 10.degree. to cause the beam direction to be deviated as shown in the second line of FIG. 3.

For a further increase in the d.c. current to a second higher value I.sub.2, a situation will be reached as depicted in line 3 of FIG. 3 where the array elements are excited by signals 30.degree. out of phase with respect to their nearest neighbors to even further deviate the beam direction as is also indicated in FIG. 3. Accordingly, as should now be apparent, the beam of the linear array may be swept along the dimension of the array (i.e., rotated with respect to the fixed array) by merely sweeping the current or voltage source 26.

Besides sweeping the beam direction of such an array, it has also been discovered that it is possible to change the actual resonant frequency of the microstrip radiators with this antenna structure as is schematically depicted in FIG. 4. Here, one of the microstrip radiators 50 has been connected into a d.c. circuit via the d.c. passing-r.f. blocking segments 52 and 54 with a d.c. current or voltage source 56. Thus, as the d.c. current or voltage source 56 is varied, the ferrite material is caused to take on different values of magnetic permeability which will, in turn, change the effective electrical length (approximately one-half wavelength at resonance) of the radiator 50 according to the well-known electrical formula .lambda../2.sqroot..mu..sub.r .epsilon..sub.r which, of course, will change the effective resonant frequency of the microstrip radiator 50 as should now be apparent. Accordingly, in spite of the fact that microstrip antennas are relative narrow bandwidth radiators, the resonant frequency may be changed by this technique to effectively increase the potential operating bandwidth of the microstrip radiators.

As should now be apparent, the scanning of the resonant frequency of the radiators as depicted for one radiator in FIG. 4 may be utilized with a single microstrip radiator as shown in FIG. 4 or for one or more of the microstrip radiators of an array such as, for example, the array of FIG. 1.

Since d.c. electrical currents are being utilized within the r.f. feedline of FIG. 1, it is also possible to selectively alter the beam sweeping or scanning operation of the array as a function of the voltage by using switchable diodes such as shown in FIG. 5. The diodes may be Zener type diodes which are selectively actuated by various d.c. voltage levels and/or they may be controllable diodes that are under the control of a mini-computer or some other conventional control means to alter either the r.f. and/or the d.c. elecrical circuits to obtain further selected changes in the relative phase shifts to be attained between the various radiators in the array as should now be appreciated by those in the art.

A two dimensional array of microstrip radiators is depicted in FIG. 6 together with an arrangement for scanning the pencil beam of radiation produced by such a two dimensional array in two coordinate directions. For instance, as shown in FIG. 6, four linear arrays 100, 102, 104 and 106 may be placed side by side to provide the two dimensional array. Each of the linear arrays includes a corporate structured r.f. feedline as shown in FIG. 1 together with appropriate d.c. circuits for scanning the beam in the X direction (that is between the bottom and top of FIG. 6) in response to variations in the I.sub.x current from current source 108 as should now be apparent from the previous discussion.

In addition, each of the r.f. inputs 110, 112, 114 and 116 are carried upward to another corporate structured r.f. feedline which includes a d.c. electrical circuit by which the relative phases of the r.f. signals being input to each of the linear arrays may be selectively shifted for causing the beam to scan in the Y direction between left and right in FIG. 6). That is, selective phase changes proportional to the current I.sub.y from current source 118 are introduced for r.f. signals traversing the isolated r.f. feedline portions 120, 122 and 124, which isolated segments are interconnected by d.c. passing-r.f. blocking circuits 126, 128 and 130 as shown in FIG. 6.

The operation of the two dimensional array is exactly analagous to the one dimensional array previously discussed and it should now be apparent that a pencil beam of radiation produced by the two dimensional array may be selectively directed to any desired direction along the X and Y coordinates by selectively choosing the appropriate current magnitudes for current I.sub.x and I.sub.y from current sources 108 and 118 respectively.

Although only a few specific embodiments of this invention have been described in detail above, those in the art will readily appreciate that there are many possible modifications to the exemplary embodiments without departing from the spirit and teaching of this invention. Accordingly, this invention is intended to encompass all such modifications and/or variations.

Claims

1. An antenna structure comprising:

an electrically conducting ground surface,

a single layer arrangement of electrical conductors including at least one r.f. radiator and r.f. feedline connected thereto,

a dielectric layer loaded with a ferrite material disposed between said ground surface and said single layer arrangement, and

said single layer arrangement including d.c. circuit means for passing d.c. electrical current through at least a predetermined portion of said

2. An antenna structure as in claim 1 wherein said d.c. circuit means is connected to said r.f. radiator whereby the resonant frequency of the radiator can be controlled by controlling the permeability of said ferrite

3. An antenna structure as in claim 2 wherein said d.c. circuit means comprises r.f. blocking-d.c. passing means for connecting said radiator

4. An antenna structure as in claim 1 wherein said d.c. circuit means is connected to said r.f. feedline whereby controlled phase shifts in r.f. energy travelling therealong can be achieved by controlling the d.c.

5. An antenna structure as in claim 4 wherein said d.c. circuit means comprises:

d.c. blocking-r.f. passing means disposed in said r.f. feedline for isolating predetermined portions of the r.f. feedline with respect to d.c. electrical currents, and

r.f. blocking-d.c. passing means disposed for interconnecting said isolated

6. An antenna structure as in claim 5 wherein said d.c. blocking-r.f. passing means comprises:

two closely spaced but physically separated parallel electrical conductors

7. An antenna structure as in claim 5 wherein said r.f. blocking-d.c. passing means comprises:

electrical conductors including open circuited stubs disposed to reflect an r.f. open circuit condition at an anticipated r.f. operating frequency

8. An antenna structure as in claim 1 wherein:

said at least one r.f. radiator comprises a plurality of r.f. radiators disposed to form a phased antenna array providing a beam pattern of radiation along a predetermined direction,

said r.f. feedline comprises a corporate structure feedline for dividing an r.f. input energy between the r.f. radiators at predetermined relative phase angles in the absence of said d.c. electrical current, and

said d.c. circuit means includes d.c. electrical current paths along selectively predetermined portions of the r.f. feedline to control said relative phase angles as a function of said d.c. electrical current whereby the predetermined direction of said beam patterns of radiation is

9. An antenna structure as in claim 8 wherein said d.c. circuit means comprises:

d.c. blocking-r.f. passing means disposed in said r.f. feedline for isolating predetermined portions of the r.f. feedline with respect to d.c. electrical currents, and

r.f. blocking-d.c. passing means disposed for interconnecting said isolated

10. An antenna structure as in claim 8 wherein:

said plurality of r.f. radiators are disposed in a two dimensional phased array, and

said d.c. circuit means comprises:

a first d.c. circuit for passing a first d.c. electrical current to control the predetermined direction of the beam pattern in a corresponding first coordinate direction, and

a second d.c. circuit for passing a second d.c. electrical current to control the predetermined direction of the beam pattern in a corresponding

11. An antenna structure as in claim 8 further comprising controllable switch means connected to at least one of said r.f. feedline and said d.c. circuit means for providing further selectable changes in the relative

12. An antenna structure as in claim 11 wherein said controllable switch means comprises at least one diode.