Abstract:
There is disclosed a meanderline loaded antenna comprising a ground plane, two vertical elements orthogonally affixed thereto, and a horizontal element between the two vertical elements. A meanderline coupler interconnects the horizontal element, at each of its ends, to the vertical elements. The antenna further includes an additional radiating element extending from the horizontal element in approximately planer relationship therewith. Additionally, the antenna includes a tuning element extending from the horizontal element and forming an acute angle with the adjacent vertical element.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to antennas loaded by one or more meanderlines (also referred to as variable impedance transmission lines), and specifically to such an antenna providing high gain and frequency tunability through the use of wings affixed to the antenna structure. 
     It is generally known that antenna performance is dependent upon the antenna shape, the relationship between the antenna physical parameters (e.g., length for a linear antenna, diameter for a loop antenna) and the wavelength of the operating frequency. These relationships determine several antenna parameters, including input impedance, gain, and the radiation pattern shape. Generally, the minimum physical antenna dimension must be on the order of a quarter wavelength of the operating frequency, thereby allowing the antenna to be excited easily and to operate at or near its resonant frequency, which in turn limits the energy dissipated in resistive losses and maximizes the antenna gain. 
     The burgeoning growth of wireless communications devices and systems has created significant needs for physically smaller, less obtrusive, and more efficient antennas. As is known to those skilled in the art, there is an inherent paradox between the physical antenna size and the antenna gain, at least with respect to single-element antennas. Increased gain requires a physically larger antenna, while users continue to demand physically smaller antennas. As a further constraint, to simplify the system design and strive for minimum cost, equipment designers and system operators prefer to utilize antennas capable of efficient multi-frequency and wide bandwidth operation. Finally, it is known that the relationship between the antenna frequency and the antenna length (in wavelengths) determines the antenna gain. That is, the antenna gain is constant for all quarter wavelength antennas (i.e., at that frequency where the antenna length is a quarter of a wavelength). 
     One prior art technique that addresses certain of these antenna requirements is the so-called “Yagi-Uda” antenna, which has been successfully used for many years in applications such as the reception of television signals and in point-to-point communications. The Yagi-Uda antenna can be designed with high gain (or directivity) and a low voltage-standing-wave ratio (i.e., low losses) throughout a narrow band of contiguous frequencies. It is also possible to operate the Yagi-Uda antenna in more than one frequency band, provided that each band is relatively narrow and that the mean frequency of any one band is not a multiple of the mean frequency of another band. 
     Specifically, in the Yagi-Uda antenna, there is a single element driven from a source of electromagnetic radio frequency (RF) radiation. That driven element is typically a half-wave dipole antenna. In addition to the half-wave dipole element, the antenna has certain parasitic elements, including a reflector element on one side of the dipole and a plurality of director elements on the other side of the dipole. The director elements are usually disposed in spaced apart relationship in that portion of the antenna pointing in the transmitting direction or, in accordance with the antenna reciprocity theorem, in the receiving direction. The reflector element is disposed on the side of the dipole opposite from the array of director elements. Certain improvements in the Yagi-Udi antenna are set forth in U.S. Pat. No. 2,688,083 (disclosing a Yagi-Uda antenna configuration to achieve coverage of two relatively narrow non-contiguous frequency bands), and U.S. Pat. No. 5,061,944 (disclosing the use of a full or partial cylinder partly enveloping the dipole element). 
     U.S. Pat. No. 6,025,811 discloses an invention directed to a dipole array antenna having two dipole radiating elements. The first element is a driven dipole of a predetermined length and the second element is an unfed dipole of a different length, but closely spaced from the driven dipole and excited by near-field coupling. This antenna provides improved performance characteristics at higher microwave frequencies. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention discloses an antenna comprising one or more conductive elements, including a horizontal element and one or more vertical elements interconnected by meanderline couplers, and a ground plane. The meanderline has an effective electrical length that affects the electrical length and operating characteristics of the antenna. Further, the antenna conductive elements include one or more radiating wings conductively connected to the horizontal element and substantially parallel to the ground plane. The radiating wings increase the coupling between the ground plane and the horizontal element, improving the antenna gain. Further, the antenna can include one or more tuning wings forming an acute angle with one of the vertical elements to provide a frequency tuning capability for the antenna. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention can be more easily understood and the further advantages and uses thereof more readily apparent, when considered in view of the description of the preferred embodiments and the following figures in which: 
     FIG. 1 is a perspective view of a meanderline loaded antenna of the prior art; 
     FIG. 2 is a perspective view of a prior art meanderline conductor used as an element coupler in the meanderline loaded antenna of FIG. 1; 
     FIGS. 3A through 3B illustrate two embodiments for placement of the meanderline couplers relative to the antenna elements; 
     FIG. 4 shows another embodiment of a meanderline coupler; 
     FIG. 5 is an embodiment of the present invention illustrating the availability of a plurality of meanderline couplers; 
     FIGS. 6 through 9 illustrate exemplary operational modes for a meanderline loaded antenna; 
     FIGS. 10 through 14 illustrate embodiments of meanderline loaded antennas constructed according to the teachings of the present invention; and 
     FIG. 15 illustrates an antenna array constructed with the meanderline loaded antennas of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Before describing in detail the particular meanderline loaded antenna constructed according to the teachings of the present invention, it should be observed that the present invention resides primarily in a novel and non-obvious combination of apparatus related to meanderline loaded antennas and antenna technology in general. Accordingly, the hardware components described herein have been represented by conventional elements in the drawings and in the specification description, showing only those specific details that are pertinent to the present invention, so as not to obscure the disclosure with structural details that will be readily apparent to those skilled in the art having the benefit of the description herein. 
     FIGS. 1 and 2 depict a prior art meanderline loaded antenna (See U.S. Pat. No. 5,790,080) to which the teachings of the present invention can be advantageously applied to increase the antenna gain and provide the antenna with frequency tunability, while maintaining an optimum input impedance characteristics. An example of a meanderline loaded antenna  10 , also known as a variable impedance transmission line antenna, is shown in a perspective view in FIG.  1 . Generally speaking, the meanderline loaded antenna  10  includes two vertical conductors  12 , a horizontal conductor  14 , and a ground plane  16 . The vertical conductors  12  are physically separated from the horizontal conductor  14  by gaps  18 . But the vertical conductors  12  are electrically interconnected to the horizontal conductor  14  by two meanderline couplers, one for each of the two gaps  18 , to thereby form an antenna structure capable of radiating and receiving RF energy. The meanderline couplers electrically bridge the gaps  18  and are designed to adjust the electrical length of the meanderline loaded antenna  10 . In addition, in one embodiment of the meanderline coupler, segments of the meanderline can be switched in or out of the circuit quickly and with negligible loss, to change the effective length of the meanderline couplers and therefore the electrical length of the meanderline loaded antenna  10 . The antenna parameters can therefore be changed by changing the meanderline lengths. The active switching devices are located in high impedance sections of the meanderline, thereby minimizing the current through the switching devices and resulting in very low dissipation losses in the switch and thereby maintaining high antenna efficiency. The operational parameters of the meanderline loaded antenna  10  are substantially affected by the frequency of the input signal. According to the antenna reciprocity theorem, the antenna parameters are also substantially affected by the receiving signal frequency. Two of the various modes in which the antenna can operate are discussed herein below. 
     Although illustrated in FIG. 1 as having generally rectangular plates, it is known to those skilled in the art that the vertical conductors  12  and the horizontal conductor  14  can take on any of a variety of shapes. For instance, thin metallic conductors having a length significantly greater than a width, could be used as the vertical conductors  12  and the horizontal conductor  14 . Single or multiple lengths of heavy gauge wire or conductive material in a filamental shape could also be used. Finally, it is known that the vertical conductors  12  and the horizontal conductor  14  do not necessarily require parallel opposing sides. For example, a conductive plate having sinuous or wavy edges can be used for the vertical conductors  12  and the horizontal conductor  14 . 
     FIG. 2 shows a perspective view of a meanderline coupler  20  constructed for use in conjunction with the meanderline loaded antenna  10  of FIG.  1 . Two meanderline couplers  20  are required for use with the meanderline loaded antenna  10 . The meanderline coupler  20  is a slow wave meanderline in the form of a folded transmission line  22  mounted on a plate  24 . The transmission line  22  is constructed from microstrip line including alternating sections  26  and  27 . The sections  26  are mounted close to the plate  24 ; the sections  27  are spaced apart from the plate  24 . This variation in height of the alternating sections  26  and  27  from the plate  24  gives the sections  26  and  27  different impedance values with respect to the plate  24 . As shown in FIG. 2, each of the sections  27  is approximately the same distance above the plate  24 . However, those skilled in the art recognize that this is not a requirement for the meanderline coupler  20 . Instead, the various sections  27  can be located at differing distances above through the plat  24 . Making this modification will change the characteristics of the coupler  20  from the uniform distances embodiment. Further, the characteristics of the antenna with which the coupler  20  is utilized will also be changed. Also, the impedance presented by the meanderline coupler  20  can be changed by changing the material or thickness of microstrip substrate or by changing the width of the sections  26 ,  27  or  28 . In any case, the meanderline coupler  20  must present a controlled (but controllably variable if the embodiment so requires) impedance. 
     The sections  26 , which are located relatively close to the plate  24  to create a lower characteristic impedance, are electrically insulated from the plate  24  by any suitable dielectric positioned therebetween. The sections  27  are located a controlled distance from the plate  24 , wherein the distance determines the characteristic impedance of the section  27  in conjunction with the other physical characteristics of the folded transmission line  22 , as well as the frequency of the signal carried by the folded transmission line  22 . 
     The sections  26  and  27  are interconnected sections  28  mounted orthogonal to the plate  24 . In this embodiment, the entire folded transmission line  22  may be constructed from a single continuous folded microstrip line. 
     The meanderline coupler  20  includes terminating points  40  and  42  for interconnecting to the elements of the loop antenna  10 . Specifically, FIG. 3A illustrates two meanderline couplers  20 , one affixed to each of the vertical conductors  12  such that the vertical conductor  12  serves as the plate  24 , so as to form a meanderline loaded antenna  50 . One of the terminating points, for instance the terminating point  40 , is connected to the horizontal conductor  14  and the terminating point  42  is connected to the vertical conductor  12 . The second of the two meanderline couplers  20  illustrated in FIG. 3A is configured in a similar manner. FIG. 3B shows the meanderline couplers  20  affixed to the horizontal conductor  14 , such that the horizontal conductor  14  serves as the plate  24  of FIG.  2 . As in FIG. 3A, the terminating points  40  and  42  are connected to the vertical conductors  12  and the horizontal conductor  14  so as to interconnect the vertical conductors  12  and the horizontal conductor  14  across the gaps  18 . 
     FIG. 4 is a representational view of a second embodiment of the meanderline coupler  20 , including low impedance sections  31  and  32  and relatively higher impedance sections  33 ,  34 , and  35 . The low impedance sections  31  and  32  are located in a parallel spaced apart relationship to the higher impedance sections  33  and  34 . The sequential low impedance sections  31  and  32  and the higher impedance sections  33 ,  34 , and  35  are connected by substantially orthogonal sections  36  and by diagonal sections  37 . The FIG. 4 embodiment includes shorting switches  38  connected between the adjacent low and higher impedance sections  32 / 34  and  31 / 33 . The shorting switches  38  provide for electronically switchable control of the lengths of the meanderline coupler  20 . As discussed above, the length of the meanderline coupler  20  has a direct impact on the center frequency of the meanderline loaded antenna  50  to which the meanderline couplers  20  are attached, as shown in FIGS. 3A and 3B. As is well known in the art, there are several alternatives for implementing the shorting switches  38 , including mechanical switches or electronically controllable switches such as pin diodes. In the embodiment of FIG. 4, all of the low impedance sections  31  and  32  and the higher impedance sections  33 ,  34 , and  35  are of approximately equal length. 
     The operating mode of the meanderline loaded antenna  50  depends upon the operating frequency and the electrical length of the entire antenna, including the meanderline coupler  20 . Thus the meanderline loaded antenna  50 , like all antennas, has a specific electrical length, which will cause it to operate in a mode determined by the signal operating frequency. That is, different operating frequencies excite the antenna to operate in different modes and therefore produce different antenna radiation patterns. For example, the antenna may exhibit the characteristics of a monopole at a first frequency, but exhibit the characteristics of a loop antenna at a second frequency. Further, the length of the meanderline coupler  20  can be changed (as discussed above) to effect the antenna electrical length and in this way change the operational mode at a given frequency. Still further, a plurality of meanderline couplers  20  of differing lengths can be connected between the horizontal conductor  14  and the vertical conductors  12 . Depending upon the desired antenna operating mode, two matching meanderline couplers  20  can be selected to interconnect the horizontal conductor  14  and the vertical conductors  12 . Such an embodiment is illustrated in FIG. 5 including matching meanderline couplers  20 ,  20 A and  20 B. A controller (not shown in FIG. 5) is connected to the meanderline couplers  20 ,  20 A and  20 B for selecting the operative coupler. A well-known switching arrangement can activate the selected meanderline coupler to connect the horizontal conductor  14  and the vertical conductors  12 , dependent upon the desired antenna characteristics. 
     Turning to FIGS. 6 and 7, there is shown the current distribution (FIG. 6) and the antenna electric field radiation pattern (FIG. 7) for the meanderline loaded loop antenna  50  operating in a monopole or half wavelength mode and driven by a source  40 . That is, in this mode, at a frequency of between approximately 800 and 900 MHz and further given a specific length for the meanderline couplers  20 , the horizontal conductor  14  and the vertical conductors  12 , the horizontal conductor  14  has a current null near the center and current maxima at each edge. As a result, a substantial amount of radiation is emitted from the vertical conductors  12 , and little radiation is emitted from the horizontal conductor  14 . As a result, the field pattern has the familiar omnidirectional donut shape as shown in FIG.  7 . 
     Those skilled in the art will realize that a frequency of between 800 and 900 MHz is merely exemplary. The antenna characteristics will change when excited by other frequency signals and the various antenna components (the meanderline couplers  20 , the horizontal conductor  14  and the vertical conductors  12 ) can be modified to create an antenna having monopole-like characteristics at other frequencies. A meanderline loaded antenna such as that shown in FIGS. 3A and 3B will exhibit monopole-like characteristics at a first frequency and loop-like characteristics at second frequency, where there is a loose relationship to the two frequencies. Similar characteristics (i.e., monopole and loop characteristics) can be achieved at any other two loosely related frequencies by changing the antenna design. 
     A second exemplary operational mode for the meanderline loaded antenna  50  is illustrated in FIGS. 8 and 9. This mode is the so-called loop mode. Note in this mode the current maxima occurs approximately at the center of the horizontal conductor  14  (see FIG. 8) resulting in an electric field radiation pattern as illustrated in FIG.  9 . Note that the antenna characteristics displayed in FIGS. 8 and 9 are based on an antenna of the same electrical length (including the length of the meanderline couplers  20 ) as the antenna parameters depicted in FIGS. 6 and 7. Thus, at a frequency of approximately 800 to 900 MHz, the antenna displays the characteristics of FIGS. 6 and 7. For a signal frequency of approximately 1.5 GHz, the same antenna displays the characteristics of FIGS. 8 and 9. By changing the antenna design, monopole and loop characteristics can be attained at two other loosely related frequencies. 
     Although the meanderline loaded loop antenna  50  offers certain advantages as discussed above, including its small physical size, it does not provide sufficient gain in certain applications. Of course, it is known to form an array of single elements to increase antenna gain, but this disadvantageously increases the physical size of the antenna. Additional gain can also be realized by increasing the size of the ground plane  16 , but this too increases the physical size. Further, in certain applications, the meanderline loaded antenna  50  is required to have more than a single frequency of operation. Given this preference, it is known that matching the impedance of the antenna to the transmission line at more than one frequency can be problematic. 
     An antenna  52  constructed according to the teachings of the present invention is shown in FIG. 10, with the addition of a radiating wing  54  connected to the horizontal conductor  14 , as shown. As will be appreciated by those skilled in the art, in lieu of attaching a separately constructed radiating wing  54  to the horizontal conductor  14 , the radiating wing  54  can be created by simply extending the length of the horizontal conductor  14 . The radiating wing  54  significantly improves the gain of the antenna  52  when the antenna  52  is operating in the mode where the horizontal conductor  14  is the radiating element, i.e., the loop mode as discussed above. In one embodiment, the radiating wing is 0.8 inches in length, however, the length can be increased or decreased to optimize the gain in accord with the performance requirements and the operational frequency of the antenna  52 . For comparison purposes, in the embodiment where the radiating wing is 0.8 inches long, the horizontal conductor  14  is 0.7 inches in length. Thus, the total length of the horizontal radiating element is 1.5 inches. At higher operational frequencies, the radiating wing  52  length obviously can be made shorter to provide the same effective coupling and gain increase. The optimal length for the radiating wing  52  is also dependent upon the distance between the radiating wing  52  and the ground plane  16 , as the radiating wing provides additional coupling to the ground plane  16 . In one embodiment, the gain increased several dB with the addition of a 0.8 inch radiating wing  54 . 
     FIG. 11 illustrates another embodiment showing an antenna  56 , including the radiating wing  54  and a second radiating wing  58 . The radiating wing  58  functions in a manner similar to the radiating wing  54  by adding coupling to the ground plane  16  and thereby gain to the antenna  56  when operating in the loop mode. It should also be observed that adding the radiating wings  54  and/or  58  does not have a substantial effect on the impedance characteristics of the antenna  56  with respect to the feeding transmission line. 
     In yet another embodiment illustrated in FIG. 12, an antenna  60  constructed according to the teachings of the present invention is affixed to a curved ground plane  62 . In this embodiment, the radiating wings  54  and  58  are bent so as to approximately follow the curvature of the ground plane  62  and thereby increase the coupling between the ground plane  62  and the radiating wings  54  and  58 . Although the radiating wings  54  and  58  are shown as linear elements in FIG. 12, both can in fact be bent to more closely follow the shape of the curved ground plane  62 . Generally, sufficient coupling between the curved ground plane  62  and the radiating wings  54  and  58  is created when the radiating wings  54  and  58  are linear, however, in certain applications, additional gain may be required and thus bending the radiating wings  54  and  58  to the same curvature as the curved ground plane  62  may be beneficial. The bending of the radiating wings  54  and  58  can also be used to achieve a change in the characteristic impedance of the antenna  60 . For instance, assume an antenna  60  constructed according to the teachings of the present invention in free space has a characteristic impedance of 50 ohms. When the antenna is affixed to a ground plane, such as the ground plane  62 , the characteristic impedance will no longer be 50 ohms. However, according to the teachings of the present invention, the radiating wings  54  and  58  can be bent to provide a characteristic impedance of 50 ohms at the desired frequency. 
     Another embodiment of an antenna  63  constructed according to the teachings of the present invention is illustrated in FIG.  13 . The FIG. 13 embodiment includes a tuning wing  64  attached to the horizontal conductor  14  and forming an angle Ø with the adjacent vertical conductor  12 . In the monopole mode, there is significantly greater current flowing in the vertical conductors  12  than in the horizontal conductor  14 . Therefore, there is considerably greater coupling between the vertical conductor  12  and the tuning wing  64  so that the operational frequency in the monopole mode can be adjusted by changing the angle Ø. Tuning of both the monopole and loop mode frequencies is accomplished by adjusting the length of the radiating wings  54  and/or  58 . Then the monopole mode frequency can be further independently tuned by adjusting the angle Ø. 
     Note further that the tuning wing  64  of FIG. 13 has some effect on the antenna gain in the loop mode because there is some coupling between the tuning wing  64  and the ground plane  16 . However, the degree of coupling is minimal compared to the coupling provided by a horizontal radiating wing, such as the radiating wings  54  and  58  of FIG.  11 . Finally, although the tuning wing  64  is illustrated on the right-hand side of the antenna  62 , those skilled in the art will recognize that in another embodiment the tuning wing  64  could be located on the left-hand side with substantially identical effects. Further, two tuning wings, one on each side of the horizontal conductor  14 , can be employed as required to provide additional tuning capability. 
     Turning to FIG. 14, there is shown both the radiating wing  54  and the tuning wing  64 , which are two elements of an antenna  70 , constructed according to the teachings of the present invention. As discussed above, the radiating wing  54  substantially impacts the antenna gain in the loop mode, as well as the center frequency for that mode. The tuning wing  64  most directly effects the center frequency of the monopole mode, while having little effect on the gain in either mode. Thus, in operation, the length of the radiating wing  54  is established based on the operating frequency and gain characteristics desired. Then the tuning wing  64  is added and bent downwardly to establish the operating frequency in the monopole mode, without having significant effect on the loop mode operating frequency. 
     FIG. 15 depicts an exemplary embodiment wherein a plurality of meanderline loaded antennas  94  constructed according to the teachings of the present invention are used in an antenna array  90 . Depending upon the performance characteristics desired, the meanderline antenna  94  can comprise any of the embodiments illustrated in FIGS. 10,  11 ,  13 , and  14 . The meanderline antenna  94  are fixedly attached to a cylinder  92  that serves as a ground plane  16  and provides a signal path to the meanderline antennas  94 . Advantageously, the meanderline antennas  94  in an upper area are oriented so as to produce a horizontally polarized signal, while the meanderline antennas in the lower area are disposed to emit a vertically polarized signal. Although only two rows of the meanderline antennas  94  are illustrated in FIG. 15, those skilled in the art will recognize that additional parallel rows can be included in the antenna array  90  so as to provide additive gain. A gain of the antenna array  90  comprises both the element factor and the array factor, as is well known in the art. Although not illustrated in FIG. 15, the FIG. 12 embodiment can be applied to the antenna array  90 , where the cylinder  92  serves as the ground plane  62  of FIG.  12 . The tuning wing embodiment of FIG. 13 can also be used in the antenna array  90 . 
     While the invention has been described with reference to preferred embodiments, it will be understood by those skilled in the art that various changes may be made and equivalent elements may be substituted for elements thereof without departing from the scope of the present invention. In addition, modifications may be made to adapt a particular situation more material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.