WO1996021255A1

WO1996021255A1 - Curtain antenna

Info

Publication number: WO1996021255A1
Application number: PCT/US1996/000224
Authority: WO
Inventors: Edward Bennett Joy; Robert King Feeney; David Richard Hertling; B. Joseph Zimmerman
Original assignee: Georgia Tech Research Corporation
Priority date: 1995-01-06
Filing date: 1996-01-03
Publication date: 1996-07-11
Also published as: AU4896396A

Abstract

A phased antenna array (1) comprises a plurality of antennas (10) with a quarter wavelength transformer (12) and a phase delay element (14) inserted between adjacent antennas. Preferably, the antennas (10) are dipoles and have their lengths selected so that their input impedances become entirely real in the array environment. The quarter wavelength transformers (12) are connected to the input of the dipoles (10) and are designed to achieve a desired division of current among the antennas (10). The phase delay elements (14) introduce phase delays to steer the main beam of the antenna array (1) to a desired beam angle. The array may further comprise more than one column of antennas with the current distribution and the phasing between the columns being adjustable with a transformer. The array is formed on both sides of a flexible substrate and separated from a ground plane by a quarter wavelength.

Description

CURTAIN ANTENNA

PIELD OF INVENTION

This invention generally relates to a phased antenna array and, more particularly, to a phased array of dipoles fabricated onto a flexible substrate.

BACKGROUND OF THE INVENTION

In general, an array of antennas is a plurality of radiating elements in an electrical and geometrical configuration. Often, the radiating elements are the same type of antenna, such as a slot, microstrip patch, or a dipole antenna. In many applications, the gain of an array of antennas is preferred over a single antenna element since the directivity of the single antenna element would be too low and the radiation pattern of the single antenna would be too wide.

The radiated field pattern generated by an array of antennas is influenced by a number of factors and can be easily adjusted by controlling these factors. Two of these factors are the excitation amplitude and the phase of the currents of the individual antennas. The field pattern is also dependent upon the geometrical configuration of the overall array, such as a linear arrangement of antennas or a circular configuration of antennas, and the relative displacement between the antennas in the array. The relative displacement between the antennas in the array also changes the mutual coupling between the individual antennas. Since the field pattern generated by an array of antennas is the vector addition of the field patterns of all the individual antennas, the field pattern can be adjusted by selecting an antenna with a different field pattern. Because of the various factors influencing the field pattern, especially the mutual coupling among elements. It has traditionally been difficult in the industry to predict the field pattern that would be generated by a specific array of antennas. As a result, a scale model of an antenna array would typically be constructed in order to measure the performance of the antenna array. Ideally, the scale model would be an exact replica in both the physical and electrical sense of the full-sized antenna array in order to produce highly accurate results.

Based on the results measured from the scale model, the antenna array would frequently be redesigned to more closely approach the desired field pattern. The redesigned antenna array might then be reevaluated based upon another scale model. After one or more iterations of designing and testing, a full-sized antenna array with approximately the desired field pattern would be built. The field pattern for the full-sized antenna array would then be measured and the full-sized antenna array might then be redesigned to more closely approximate the desired field pattern.

The aforementioned prior art method of manufacturing an antenna array suffered from a disadvantage in that the process was very laborious and costly. Oftentimes, the antenna array might not be built until more than a year after the desired field pattern and other specifications have been defined. Thus, there is a great need in the industry for a method of producing an antenna array which can quickly and inexpensively design and manufacture an antenna array that has the desired field pattern as well as other desired specifications.

A custom built antenna array would also be very costly due to the lengthy testing involved in the designing of the antenna array. Thus, there is also a need in the art for an antenna array that is relatively inexpensive to fabricate.

Another disadvantage with many antennas is that they must be placed in undesirable locations in order to most efficiently transmit or receive signals. For instance, a reflector antenna is commonly used to transmit and receive signals from a satellite. The reflector antenna must be physically rotated to point in the direction of a satellite and requires mechanical supports for the structure of the antenna. As a result, the reflector antenna is relatively aesthetically displeasing and often architecturally unacceptable.

Thus, there is a heretofore unaddressed need in the prior art for an antenna that is hidden from view and one that is not aesthetically displeasing. There is also a need in the industry for an antenna that is efficient in use of space and conformal to existing planar surfaces. SUMMARY OF THE INVENTION It is a general object of the invention to overcome the disadvantages of the prior art antennas, as noted above and as generally known in the art. It is an object of the present invention to produce a thin sized antenna array that can be easily hidden from view and that does not require additional mechanical supports.

It is another object of the present invention to quickly and inexpensively produce an antenna array without having to produce scale models of the antenna array.

The objects, advantages and novel features of the invention are set forth in the description which follows, and will become readily apparent to those skilled in the art. To achieve the foregoing and other objects, in accordance with the present invention, in a preferred embodiment thereof, a phased antenna array comprises at least two antennas with the first antenna having a first input impedance and the second antenna having a second input impedance. Both the first antenna and the second antenna are dimensioned so that the impedances of the first and second antennas have only real components and no reactive components, including mutual coupling effects.

The first and second antennas are separated by an impedance transformer connected to an input of the first antenna and dimensioned so that an input impedance of the impedance transformer is equal to a third impedance. The third impedance is selected to achieve a desired distribution of current between the first antenna and the second antenna.

Preferably, the first and second antennas are dipole antennas and the impedance transformer is a quarter wavelength transformer. A phase delay element, for example, may additionally be inserted between the impedance transformer and the second antenna to introduce a desired amount of phase delay between the two antennas. The phase delay element is preferably a transmission line having a characteristic impedance equal to the impedance of the impedance transformer.

A method of manufacturing an antenna array according to the invention comprises the steps of forming a first antenna with dimensions such that an input impedance of the first antenna is at a first impedance having only a real component and no reactive components. An impedance transformer is connected to the first antenna and is sized to have an input impedance equal to a second impedance. A second antenna is formed with dimensions and a location such that an input impedance to the second antenna equals a third impedance having only a real component and no reactive components. The characteristic impedance of the impedance transformer is selected to achieve a desired distribution of current between the first and second antennas.

In a further aspect of the invention, a method of designing and manufacturing an antenna array having a plurality of individual antennas with adjacent antennas separated by an impedance transformer, comprises the steps of inputting desired specifications of the antenna array to a computer. The dimensions of the individual antennas are selected by the computer, accounting for the mutual impedance among the individual antennas, so that input impedances to the individual antennas consist of only real components and no reactive components. The dimensions for the impedance transformers are then determined by the computer to achieve a desired distribution of current between the antennas in the array.

Based on the determined impedances of the individual antennas and the impedance transformers, the computer defines a pattern of conductive material on a dielectric substrate. The conductive pattern defines the individual antennas, the impedance transformers, and the transmission lines. The computer also produces command codes based upon the pattern of conductive material. The pattern of conductive material is then manufactured by known equipment based upon the command codes. The pattern of conductive material defines an antenna array having the desired specifications.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are incorporated in, and form a part of, the specification, illustrate preferred embodiments of the present invention and, together with the description, serve to illustrate and explain the principles of the invention. The drawings are not necessarily to scale, emphasis instead being placed on clearly illustrating the principles of the invention. In the drawings:

Fig. 1 is a block diagram of an antenna array according to a first embodiment of the invention; Fig. 2 is a block diagram of an antenna array according to a second embodiment of the invention;

Figs. 3(A) and 3(B) respectively show top and bottom views of an antenna array fabricated onto a flexible substrate by photolithography;

Fig. 4 depicts a flow chart for a process of designing and producing an antenna array; and

Fig. 5 is a cross-sectional view of an antenna array having a circular polarization.

DETAILED DESCRIPTION Reference will now be made in detail to the preferred embodiments of the invention, which are illustrated in the accompanying drawings. To simplify the description of the invention, the antenna arrays will be described as being in a transmitting mode of operation. It should be understood that the antenna arrays are not limited to only transmitting but, due to reciprocity in the antenna arrays, are equally capable of receiving.

With reference to Fig. 1, an antenna array 1, according to one aspect of the invention, comprises a plurality of antennas 10, to 10_n connected with a cascade feed and forming a column of antennas 10 along an x-axis. The antennas 10 in the array 1, for example, may be tapped off of a transmission line at regular intervals. An impedance transformer 12 is formed between each pair of antennas 10. Thus, a first impedance transformer 12, is connected between a first antenna 10, and a second antenna 10₂ and a second impedance transformer 12₂ is connected between the second antenna 10₂ and a previous antenna 10₃. The leading antenna 10_n in the array 1 is coupled to an input port 18 through an impedance transformer 12_n.

The dimensions of the first antenna 10, are selected so that the input impedance R, to the antenna 10, is entirely real. The process of varying the dimension of an antenna 10 to adjust the input impedance R of the antenna 10 is defined as "tuning" the antenna 10. The second antenna 10₂ is also tuned to have an entirely real input impedance of R.-_J and the other antennas 10 are similarly tuned to have entirely real input impedances R₃ to R,.. The characteristic impedances of the impedance transformers 12 are selected to achieve a desired distribution of current among the antennas 10 in the array 1. In general, to match the first antenna's input impedance R, to a transmission line having a characteristic impedance Rr and supplying current to said impedance transformer 12,, the characteristic impedance Z₀ of the impedance transformer 12, should be related to the first antenna's input impedance R, and the impedance R,. according to the following equation:

(EQ. 1) . The impedances of the impedance transformers 12 are therefore selected to achieve a desired distribution of current among the antennas 10 in the array.

To introduce an additional amount of phase delay between the antennas 10 in the array 1, a phase delay element 14 is formed between adjacent antennas 10. Each phase delay element 14 may comprise a transmission line having a characteristic impedance equal to the input impedance of an associated impedance transformer 12. When the phase difference between the antennas 10 is set at any multiple of 360°, the main beam of the antenna array 1 is directed along a z axis perpendicular to an x- y plane defined by the antenna array 1. In order to point the main beam of the antenna array 1 along the x axis, defined along the column of antennas 10, a phase delay other than a multiple of 360° is introduced between antennas 10 by the phase delay elements 14. The phase delay introduced by each phase delay element 14 is preferably adjusted by varying the length of the transmission line forming the phase delay element 14. The leading impedance transformer 12_n matches the input impedance of the entire antenna array 1 to the characteristic impedance of an input transmission line connected to the input port 18. Frequently, the characteristic impedance of the transmission line is either 50 ohms or 75 ohms and the impedance transformer 12_n consequently sets the input impedance of the antenna array 1 to either the 50 ohms or 75 ohms of the input transmission line. In this manner, the maximum amount of power is supplied to the antenna array l from the input transmission line.

In a preferred embodiment, the antenna array 1 comprises a plurality of dipole antennas. The dipole antenna's reactive component of its input impedance is a function of its length. Thus, the length of the dipole may be varied in order to tune the antennas 10 to have only a real component and no reactive components. This tuning of antennas includes the mutual coupling effects of all antennas in the array. While the preferred antenna array 1 uses dipole antennas, the antenna array 1 could alternatively or additionally be formed with slot antennas, patch antennas, spiral antennas, as well as other types of antennas. The impedance transformers 12 are preferably quarter wavelength transformers. A quarter wavelength transformer has an electrical length equal to a quarter wavelength of the operating frequency of the antenna array 1 and has a width that is varied in order to vary the impedance of the impedance transformer 12. As shown above in Equation 1, the impedance of the quarter wavelength transformer should be selected to achieve the desired current distribution. Rather than a quarter wavelength transformer, the antenna array l may instead comprise other elements for transforming one input impedance to a second input impedance, such as a multi- section Tschebyscheff transformer.

The design of the antenna array will now be discussed in more detail. The dimensions of the first antenna 10, are first selected so that the input impedance R, of the first antenna 10, is entirely real. As discussed above, the process of varying the dimensions of an antenna 10_; to adjust the input impedance Rj of the antenna lOj is defined as "tuning" antenna 10;. The second antenna 10₂ is also tuned to have an entirely real input impedance R₂ and the other antennas 10₃ to 10_n are similarly tuned to have entirely real impedances R₃ to H_ . The antennas must be simultaneously tuned as the input impedance of one antenna depends on the lengths of all the others.

The input impedance R_T1 of the first impedance transformer 12, is selected to give the proper current division between a current I, of the first antenna 10, and a current l₂ of the second antenna 10₂ according to the following relationship:

(EQ. 2) . The characteristic impedance Z₀ of the quarter wavelength transformer 12, is chosen to give the required input impedance π by the relationship: ^Zo ~ yJ^Rι^Rτι

(EQ. 3) . The characteristic impedance of the phase delay element 14, is set by varying a width of a transmission line forming the phase delay element 14 to equal the input impedance R_j, of the impedance transformer 12,. The length of the phase delay element 14, is adjusted to provide a phase delay for a desired main beam pointing direction. The input impedance of the phase delay element 14, is therefore defined independent of the length of the phase delay element 14,.

The characteristic impedance of the quarter wavelength transformer 12₂ is determined to achieve a desired ratio between the resistance R₃ of the third antenna 10₃ and the input resistance to the second antenna I0₂ which is equal to the impedance R_τ, of the impedance transformer 12, in parallel with the input impedance R₂ of the second antenna 10₂. With the desired ratio of impedances, a current from the input port 18 is divided such that the second antenna 10₂ has the current I₂ and the third antenna has a required current I₃. This iterative process is then repeated until all of the elements in the array have been defined.

The quarter wavelength transformers 12 introduce a phase delay of 90° between adjacent antennas 10 in the array 1. To introduce an additional amount of phase delay, the phase delay elements 14 are formed between the antennas 10 in the array and are designed to have the same value of resistance as the input impedance of an associated impedance transformer 12. The lengths of the phase delay elements 14 are then varied to adjust the value of the phase delay.

In another embodiment of the invention, as shown in Fig. 2, an antenna array 1' comprises more than one column of antennas 10 with each column formed along the x-axis. Each column of antennas 10 is similar to the column of antennas 10 shown in Fig. 1 in that each antenna 10 has an associated impedance transformer 12 and phase delay element 14. The leading antennas 10_nl, 10^, and 10„₃ respectively have impedance transformers 12_nt, 12, , and 12„₃ for converting the input impedances of the columns to a desired input impedance. The impedance transformers 12_nl, 12,^, and 12^ are designed to achieve the desired current division among the columns.

Since the array 1' has more than one column of antennas 10, an array transformer 16 is placed between the input port 18' and the leading phase delay elements 14_nl, 1 „₂, and 1 „₃. The array transformer 16 matches the combined input impedance of the columns of antennas 10 to the impedance of the input transmission line of the input port 18' . In the unusual case of when the combined impedances of the column of antennas 10 equals the impedance of the input transmission line, the array transformer 16 would not be necessary.

The phase delay elements 14_nl, 14..₂, and 14,_ may also be connected between the leading antennas 10_nl, 10^, and lO in the columns and the input port 18' . The phase delay elements 14_nl, 14^, and 14^ introduce phase delays between the columns of antennas 10. When a phase delay other than a multiple of 360° is introduced between the currents supplied to the columns of antennas 10, the radiated main beam of the antenna array 1' no longer points perpendicular to the antenna array 1' but is directed toward the y axis, which runs transverse to the columns. The phase delays introduced by the individual phase delay elements 14 can point the main beam of the antenna array 1' along either the x-axis or the y-axis. Consequently, the radiated main beam of the antenna array 1' can be directed to almost any angle by proper adjustment of the phase delay elements 14. The antenna arrays 1 and 1' are preferably manufactured on a flexible substrate using photolithography, which is a well known method used in manufacturing printed circuit boards. It should be understood that the antenna arrays l and 1' may be manufactured by other processes, such as by printing, silk-screen printing, vacuum deposition, flame spraying, xerography, or by simply applying strips of highly conductive material to the substrate.

When photolithography is used to manufacture the antenna arrays 1 and 1' , the flexible substrate comprises a low-loss thin layer of dielectric material initially coated on both sides with thin layers of a highly conductive material, such as copper, gold, silver, or aluminum. The conductive material, which is typically copper, is then selectively removed in order to define a desired antenna array 1. The low-loss thin layer of dielectric material is preferably glass-reinforced teflon but may comprise other materials, such as "Mylar," "Kevlar," "Kapton, " "Polyflon, " corrugated PVC, or paper coated expanded polystyrofoam (EPS) .

In general, the first step of photolithography is the application of a photoresist evenly over the surface of the metal-coated substrate. After the metal-coated substrate and the photoresist have been soft-baked to remove solvents in the photoresist, the photoresist is selectively exposed to ultraviolet (UV) light. When a "positive" photoresist is used to define the desired pattern of copper, the areas on the surface of the metal- coated substrate that are exposed to UV light are later removed. On the other hand, when a "negative" photoresist is used to define the desired pattern of copper, the areas exposed to UV light remain on the substrate. Preferably, a "positive" photoresist is used to define the pattern of the antenna array l on the substrate.

After defining the photoresist and exposing the photoresist to UV light, a developer removes the portions of the photoresist that have been softened by the UV light. Next, the portions of copper not protected by the photoresist are etched away with suitable solvents, thereby leaving the desired pattern of copper. As the last step, the photoresist is removed from the pattern of copper with suitable solvents, thereby leaving the desired pattern of copper on the flexible substrate.

The exposure pattern for the UV light is defined by a piece of photomechanical film placed over the metal- coated substrate. The photomechanical film prevents the UV light from exposing the desired pattern of copper but allows the UV light to expose all other areas of the photoresist. The photomechanical film is developed by directing a beam of light onto those areas of the photomechanical film that will later define the desired pattern of copper. When the film is developed, the areas on the film that have been exposed to the beam of light will prevent the UV light from exposing the photoresist. On the other hand, the areas on the film that were not exposed to the beam of light will allow the UV light to pass through and expose the photoresist.

One conventional technique for defining the pattern on the photomechanical film is with a mechanical x-y plotter, which physically moves a beam of light about the surface of the film. This technique, however, is rather slow and inaccurate. A more sophisticated and preferred method directs a laser beam onto the film in order to define the desired conductive pattern. With either method, the photomechanical film will pass the UV light through in the areas that were not exposed to the light beam and will block the UV light in the areas that were exposed to the light beam.

An industry standard code of commands, called a Gerber code, has been developed for describing geometrical patterns, such as the dipole elements and components of an antenna array according to the invention. The production of the Gerber code from a specified conductive pattern is within the capability of one of ordinary skill in the art and will thus not be described in detail. Furthermore, depending upon the equipment used to fabricate the antenna arrays of the invention, command codes other than the Gerber code may be generated. An antenna array 41 fabricated on a flexible substrate 46 by photolithography is depicted in Figs. 3(A) and 3(B), with Fig. 3(A) depicting the top of the substrate 46 and Fig. 3(B) depicting the bottom of the substrate 46. As shown in Figs. 3(A) and (B) , the antenna array 41 comprises a plurality of antennas 40, which are preferably dipole antennas, arranged in two columns along the x-axis. While the antenna array 41 is shown with only two columns, the antenna array 41 may be constructed with more than two columns. Each of the antennas 40 in the array 41 has one of its two arms fabricated on one side of the substrate 46 and has the other arm fabricated on the other side of the substrate 46. A phase shift of 180° can be introduced by simply switching the placement of the dipole arms on the substrate 46.

The antenna array 41 also has a plurality of impedance transformers 42, which are preferably quarter wavelength transformers. With reference to Fig. 3(A), the impedance transformers 42 all have a length of a quarter wavelength but have varying widths in order to produce varying impedances. With reference to Fig. 3(B), the impedance transformers 42 on the bottom side of the substrate 46 are mirror images of the top surface of the substrate 46.

The antenna array 41 comprises a plurality of phase delay elements 44 for a respective plurality of antennas 40. The phase delay elements 44 have their widths varied in order to adjust the characteristic impedance of the transmission line forming the phase delay element 44 to match the input impedance of an associated impedance transformer 42. In order to distribute current from an input signal in a desired distribution among all of the antennas 40, the widths of the phase delay elements 44 and impedance transformers 42 are typically greater closer to the input port 48 than for the phase delay elements 44 and impedance transformers 42 further from the input port 48. The phase delay elements 44 also have varying lengths in order to introduce phase delays between adjacent antennas 40.

An array transformer 47 is placed between the column of antennas 40 and the input port 48. The array transformer 47 matches the combined impedance of the column of antennas 40 to the impedance of the input transmission line, which is typically 50 or 75 ohms. The array transformer 47 preferably comprises a quarter wavelength transformer but may comprise another type of element, such as a Tschebyscheff transformer. The phase delay elements 44 and the transmission lines in general in the antenna array 41 are not limited to the exact forms disclosed. For instance, while the phase delay elements 44 have been roughly formed in the shape of a triangular waveform having curved corners, the phase delay elements 44 may be formed in other shapes, such as a generally sinusoidal waveform. Moreover, the design of the transmission lines will be apparent to those of ordinary skill in the art and discontinuities in the lines may be compensated to the extent permitted by the state of the art. It will apparent to those of ordinary skill in the art that the design of the transmission lines in the antenna array will contain a number of features not disclosed in detail, such as junctions which may take the form of a 90° bend, a beveled corner, a T-intersection, a cross-intersection, or a junction where the width of the transmission lines changes.

A ground plane is preferably separated from the antenna array 41 by a quarter wavelength layer of dielectric material in order to make the antenna unidirectional and to increase the field strength of the antenna radiating pattern of the array by 6 dB. The ground plane, for example, comprises a planar conductive sheet of material separated from one surface of the flexible substrate 46 by a one quarter wavelength layer of closed-cell expanded polystyrofoam having a dielectric constant of 1.033. The use of a flexible substrate simplifies the shipping of the antenna. Whereas an antenna array typically was constructed on a relatively rigid structure, an anntenna array according to the invention may be rolled up and transported in a tube.

The thin antenna array according to the invention also provides several advantages over prior art antennas. The antenna array according to the invention may be easily hidden from view by placing the antenna array between layers of a wall structure or on the surface of a wall structure. Because the main beam of the antenna array can be adjusted to almost any angle, the main beam of the antenna array can be designed to point in a desired direction, such as towards a satellite. The antenna arrays according to the invention have been found to operate efficiently over a wide range of frequencies at least from 500 MHz to 8 GHz and are therefore ideal replacements for mobile phone antennas and for satellite reflector antennas. A preferred process of designing and producing an antenna array is generally shown in Fig. 4. In this process, desired specifications of the antenna array, such as beam angle, operating frequency, input impedance, and gain, are first input at a step 100. Based on the desired specifications, the dimensions and locations of each antenna in the array are determined at a step 102 such that the input impedance to each antenna has only a real component and no reactive components. As generally known in the art, the reactive component of the input impedance of a dipole antenna is dependent upon its length and can therefore be reduced to zero by proper adjustment of its length. The placement and length of the antennas takes into account the effects of mutual coupling between the antennas, as is known in the art.

At step 104, the dimensions of the impedance transformers are determined after considering the input impedances of each antenna in the array. The impedance transformers are preferably quarter wavelength transformers which have their widths varied in order to achieve a desired distribution of current among the dipole antennas in the array.

If an additional amount of phase delay between adjacent antennas is desired, then at step 106 the dimensions of the phase delay elements are determined. The widths of the phase delay elements are determined based upon the impedances of the impedance transformers, which have been defined to achieve a desired distribution of current between the antennas. The lengths of the phase delay elements are determined based upon a desired amount of phase delay between the antennas in the array. The required amount of phase delay between adjacent antennas is a function of the desired beam angle and can be determined by one of ordinary skill in the art. In the preferred embodiment, the phase delay elements comprise transmission lines that have their lengths varied in order to adjust the phase delay and which have their widths varied in order to adjust the impedance of the transmission line.

At step 108, a pattern of conductive material is defined based upon the impedances determined for the antennas, the impedance transformers, and the phase delay elements. The definition of a conductive pattern for a transmission line, a quarter wavelength transformer, or for an antenna so that the element has a certain impedance is well within the capability of one of ordinary skill in the art and will not be described in further detail herein.

Once the pattern of conductive material has been defined, Gerber codes for the fabrication of the antenna array are then produced at a step 110. Preferably, the Gerber codes are further translated into codes for controlling a laser plotter. The generation of Gerber codes or other such codes based upon a desired conductive pattern is within the capability of one of ordinary skill in the art. Finally, at step 112, the antenna array is fabricated by photolithography using the command codes. While the invention preferably uses photolithography to manufacture the antenna array, the antenna array may be formed by other processes, such as silk-screen printing, flame spraying, vacuum deposition, printing, the use of metallic tape, or xerography.

With the process depicted in Fig. 4, an antenna array having certain desired specifications can be rapidly and accurately produced. For instance, after the desired specifications have been defined, an antenna array can be produced in just a day without requiring scale models or any redesigns of the antenna array. The foregoing description of the preferred embodiments of the invention has been presented for purposes of illustrating the features and principles thereof. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations are possible in light of the above teaching.

For example, as shown in a cross-sectional view in Fig. 5, an antenna array 51 according to the invention may be fabricated to have a circular polarization. The antenna array 51 comprises a first array of antennas 52, a second array of antennas 58, quarter wavelength layers of dielectric material 54 and 60, a half wavelength layer of dielectric material 56, a ground plane 62, and a uni¬ directional ground plane 64.

The antennas in the first antenna array 52 are placed along a first direction, such as the x-axis, and the uni-directional ground plane 64 forms a ground plane coincident with the direction of the antennas in the first array 52 along the x-axis. The antennas in the second array 58 are placed along a line perpendicular to the antennas in the first array 52 and are therefore placed along the y-axis. The ground plane 62 forms a ground plane in the entire x-y plane ^"but, alternatively, could be limited to a ground plane along just the y-axis. Due to the spacing of the two antenna arrays 52 and 62 and the perpendicular placement of the antennas within the arrays 52 and 62, the antenna array 51 generates fields that have circular polarization.

Also, an antenna array according to the invention may be constructed by photolithography and may additionally comprise hybrid components. For instance, the antenna array may employ capacitors to cancel out any reactive components in the input impedances to the individual antennas. The embodiments were chosen and described in order to explain the principles of the invention and their practical application; various other possible embodiments with various modifications as are suited to the particular use are also contemplated and fall within the scope of the present invention.

Claims

We claim: 1. .An antenna array, comprising: a first antenna and a second antenna; said first antenna having a characteristic input impedance equal to a first impedance and said second antenna having a characteristic input impedance equal to a second impedance; an impedance transformer connected to an input of said first antenna and dimensioned so that an input impedance of said impedance transformer is equal to a third impedance; wherein said first antenna and said second antenna are dimensioned so that said first impedance and said second impedance consist essentially of real components and said third impedance is selected to achieve a desired distribution of current between said first antenna and said second antenna.

2. The antenna array as set forth in claim 1, further comprising a phase delay element between said first antenna and said second antenna, said phase delay element introducing a desired amount of phase delay between said first antenna and said second antenna.

3. The antenna array as set forth in claim l, wherein said first antenna and said second antenna comprise dipole antennas.

4. The antenna array as set forth in claim 1, wherein said phase delay element has a characteristic impedance equal to said third impedance.

5. The antenna as set forth in claim 1, wherein said impedance transformer comprises a quarter wavelength transformer.

6. The antenna array as set forth in claim 1, further comprising: a third antenna being dimensioned so that its characteristic input impedance equals a fourth impedance, said fourth impedance consisting essentially of a real component; and a second impedance transformer connected to an input of said second antenna and dimensioned so that an input impedance of said second impedance transformer is equal to a fifth impedance; wherein said fifth impedance is selected to achieve a desired distribution of current between said third antenna and said second antenna.

7. The antenna array as set forth in claim 1, wherein said antenna array is fabricated on a flexible substrate.

8. The antenna array as set forth in claim 7, wherein said first and second antennas are formed on opposite sides of said flexible substrate.

9. The antenna array as set forth in claim 7, wherein said impedance transformer is formed on opposite sides of said flexible substrate.

10. The antenna array as set forth in claim 1, wherein said first antenna, said second antenna, and said impedance transformer are fabricated by photolithography onto a flexible substrate.

11. The antenna array as set forth in claim 1, wherein said antenna array comprises at least four antennas with said first antenna and said second antenna forming a first column of antennas and a third antenna and a fourth antenna forming a second column of antennas.

12. The antenna array as set forth in claim 11, further comprising a column current divider for dividing current from an input port between said first column of antennas and said second column of antennas.

13. The antenna array as set forth in claim 11, further comprising a column phase delay element for introducing a phase delay between a first input signal supplied to said first column of antennas and a second input signal supplied to said second column of antennas.

14. The antenna array as set forth in claim 11, further comprising an array transformer for matching an impedance of the first and second column of antennas with an input transmission line.

15. A method of manufacturing an antenna array having at least a first antenna and a second antenna, comprising the steps of: forming said first antenna with dimensions such that an input impedance to said first antenna equals a first impedance consisting essentially of a real component; connecting an impedance transformer to said first antenna and sizing said impedance transformer to have an input impedance equal to a second impedance; and forming said second antenna with dimensions such that an input impedance to said second antenna equals a third impedance consisting essentially of a real component; and selecting said second impedance to achieve a desired distribution of current between said first antenna and said second antenna.

16. The method of manufacturing said antenna array as set forth in claim 15, wherein said step of selecting said second impedance comprises the step of selecting said second impedance to equal said third impedance.

17. The method of manufacturing said antenna array as set forth in claim 15, wherein said steps of forming said first and second antennas comprise the steps of forming first and second dipole antennas with each dipole antenna having two arms, placing one arm of each dipole on one side of a dielectric, and placing the other arm of each dipole on the opposite side of said substrate.

18. The method of manufacturing said antenna array as set forth in claim 15, wherein said step of connecting said impedance transformer comprises the steps of placing a first conductive path on one side of a substrate and placing a second conductive path on the opposite side of said substrate.

19. The method of manufacturing said antenna array as set forth in claim 15, further comprising the step of separating a ground plane from said antenr.i array by a quarter wavelength layer of dielectric material.

20. The method of manufacturing said antenna array as set forth in claim 15, further comprising the step of forming a phase delay element between said first antenna and said second antenna to introduce a desired amount of phase delay between said first antenna and said second antenna.

21. The method of manufacturing said antenna array as set forth in claim 20, wherein said step of forming said phase delay element comprises the steps of placing a first portion of said phase delay element on one side of a substrate and placing a second portion of said phase delay element on the opposite side of said substrate.

22. A method of designing and manufacturing an antenna array having a plurality of individual antennas with adjacent antennas separated by an impedance transformer, comprising the steps of: inputting desired specifications on said antenna array to a computer; selecting dimensions of said individual antennas with said computer so that input impedances to said individual antennas consist essentially of real components and no reactive components; determining dimensions for said impedance transformers with said computer to achieve a desired distribution of current between said individual antennas; defining with said computer a pattern of conductive material on a dielectric substrate based on said impedances of said individual antennas and said impedance transformers; producing command codes with said computer based upon said pattern of conductive material; and based upon said command codes, producing said pattern of conductive material onto said dielectric substrate, said pattern of conductive material being said antenna array having said desired specifications.

23. The method of designing and producing an antenna array as set forth in claim 22, wherein said step of producing said command codes comprises the step of producing Gerber codes and said step of producing said pattern of conductive material comprises the step of forming said pattern of conductive material by photolithography.

24. The method of designing and producing an antenna array as set forth in claim 23, wherein said step of forming said pattern of conductive material by photolithography comprises the step of removing a metal from a flexible substrate in areas other than said pattern.

25. The method of designing and producing an antenna array as set forth in claim 22, wherein said step of inputting desired specifications comprises the step of inputting an operating frequency, a beam angle of said antenna array, an input impedance, and gain of said antenna array.

26. The method of designing and producing an antenna array as set forth in claim 22, further comprising the steps of: inserting a phase delay element between individual antennas in said array; determining phase delay values for said phase delay elements in order for said antenna array to have a desired beam angle; and defining said pattern of conductive material to have said phase delays elements.

27. The method of designing and producing an antenna array as set forth in claim 22, wherein said individual antennas are dipole antennas and said step of selecting dimensions for said individual antennas comprises the step of determining lengths of said dipole antennas.

28. The method of designing and producing an antenna array as set forth in claim 22, wherein said impedance transformers comprise quarter wavelength transformers and said step of determining dimensions for said impedance transformers comprises the step of determining widths for said quarter wavelength transformers.

29. The method of designing and producing an antenna array as set forth in claim 26, wherein said phase delay elements comprise transmission lines and said step of determining phase delay values for said phase delay elements comprises the step of determining a length for each of said phase delay elements.

30. The method of designing and producing an antenna array as set forth in claim 26, further comprising the step of varying a width of said phase delay elements to equal the impedance of an associated impedance transformer.

31. An antenna fabricated onto a dielectric substrate, comprising: a dielectric substrate having a top surface and a bottom surface; a first radiating structure formed on said top surface of said dielectric substrate and a second radiating structure formed on said bottom surface of said dielectric substrate; and a transmission line having one conductive path on said top surface of said dielectric substrate leading from one terminal of an input terminal to said first radiating structure and having a second conductive path on said bottom surface of said dielectric substrate leading from a second terminal of said input terminal to said second radiating structure; wherein said dielectric substrate comprises a flexible substrate.

32. The antenna as set forth in claim 31, wherein said antenna comprises a dipole antenna with said first radiating structure and said second radiating structure being arms of said dipole antenna.

33. The antenna as set forth in claim 31, further comprising a ground plane separated from said bottom surface of said dielectric substrate by a quarter wavelength layer of dielectric material.

34. The antenna as set forth in claim 31, further comprising: a second antenna having a third radiating structure formed on said top surface of said dielectric substrate and a fourth radiating structure formed on said bottom surface of said dielectric substrate; and a second transmission line having a first conductive path on said top surface of said dielectric substrate connecting said first radiating structure to said third radiating structure and a second conductive path on said bottom surface of said dielectric substrate connecting said second radiating structure to said fourth radiating structure.

35. The antenna as set forth in claim 34, wherein said first and second radiating structures are the upper and lower arms of a first dipole antenna and said third and fourth radiating structures are the lower and upper arms of a second dipole antenna.