Patent Publication Number: US-8525745-B2

Title: Fast, digital frequency tuning, winglet dipole antenna system

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to aircraft antennas. 
     2. Description of the Related Art 
     To enhance their operational capabilities, it is often desirable for modern aircraft to carry a variety of antenna systems that operate over wide frequency bands. Most of these systems require the presence of a ground plane so that they must generally be carried on an aircraft&#39;s fuselage. This restriction has placed serious limitations on aircraft performance. 
     BRIEF SUMMARY OF THE INVENTION 
     Compact, high-gain, fast-tuning, self-contained, dipole antenna systems are provided that are especially useful for operation over multiple frequency bands and for mounting in a wide variety of locations on aircraft because they are configured to operate in the absence of a ground plane. The drawings and the following description provide an enabling disclosure and the appended claims particularly point out and distinctly claim disclosed subject matter and equivalents thereof. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an exemplary airplane and also illustrates winglet antenna embodiments that are carried on the airplane&#39;s wingtips; 
         FIG. 2A  is an enlarged side view of the winglet antenna within the ellipse  2 A in  FIG. 1 ; 
         FIGS. 2B and 2C  are respectively bottom and rear views of the winglet antenna of  FIG. 2A ; 
         FIG. 3A  is a view of an antenna system embodiment within a radome of the winglet antenna of  FIG. 2A ; 
         FIG. 3B  is a frequency chart that shows exemplary operational frequency bands of the antenna system of  FIG. 3A ; 
         FIG. 4  is a block diagram of the antenna system of  FIG. 3A ; 
         FIGS. 5A and 5B  are diagrams of measured return loss in a prototype of the antenna system of  FIG. 3A ; and 
         FIGS. 6A and 6B  are measured gain patterns in a prototype of the antenna system of  FIG. 3A . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 1-6B  illustrate antenna system embodiments and the performance of these embodiments. The antenna system embodiments are compact and are especially suited for operation in the absence of a ground plane so that they may be mounted on various portions of an aircraft&#39;s structure. For example, they may be configured as winglets and situated far out on wingtips to thus free the remainder of an aircraft&#39;s structure for other operational systems. In addition, the antenna system embodiments can be rapidly switched between channels within multiple frequency bands and provide superior performance of antenna parameters, e.g., return loss and gain. 
     In particular,  FIG. 1  illustrates winglet antenna embodiments  20  that are mounted to the wingtips  21  of an airplane  22 . Because the antennas  20  are carried far out on the wingtips, the fuselage  24  of the airplane is completely available for mounting of other auxiliary aircraft structures. For example, a monopole antenna  26  is carried on the upper portion of the metallic fuselage which then forms a ground plane for this monopole antenna. Radiation reflection off of this ground plane causes the monopole antenna to respond substantially as if it were a dipole antenna. Because only the monopole portion extends away from the fuselage, the height of fuselage-mounted monopole antennas is considerably reduced and this reduction is highly desirable in the wind stream environment provided by airplanes in flight. 
     However, a considerable portion of the aircraft&#39;s wings  25  may be formed of electromagnetically-transparent materials (e.g., fiberglass) and the winglet antennas must therefore operate in the absence of a ground plane. To adapt to this absence, the antennas  20  have been configured as dipole antennas which effectively operate without the presence of a ground plane. They may thus be carried on the airplane&#39;s wingtips  21  and this important feature advantageously frees up the fuselage for the mounting of other antennas and other systems. 
       FIG. 2A  is an enlarged view of the winglet antenna system  20  of  FIG. 1 .  FIGS. 2B and 2C  are respectively bottom and rear views of the antenna system of  FIG. 2A .  FIG. 2A  particularly shows an antenna system  40  mounted within the radome portion  31  of a housing  30 . The housing forms a radome portion  31 , a stub portion  32  that extends away from radome, and a rim  33  that extends away from the stub portion. The radome and the stub are aerodynamically-shaped to generally conform with the outer shapes of the airplane ( 22  in  FIG. 1 ). In particular, the radome is arranged to extend above and below the stub which is shaped to match the shape of the wingtip ( 21  in  FIG. 1 ). 
     When the stub is mounted to the wingtip, the rim  33  slips inside the wingtip. The rim surrounds connection structures (e.g., a multi-pin logic signal connector  34  and a TNC RF connector  35 ) that functionally connect the antenna system  20  to electronic systems within the airplane ( 20  in  FIG. 1 ). As shown, a navigation light  36  may be carried on the outer surface of the radome  31 . 
     The radome  31  is cut away in  FIG. 2A  to reveal an antenna system embodiment  40  that is protectively carried within the radome  31  which is preferably formed with electromagnetically-transparent materials so that it does not interfere with the antenna system&#39;s operation. As shown in  FIG. 2A , the system  40  includes a planar, top-loaded antenna  42  and a planar elliptical antenna  50 . Interfacing with the antennas are electronics. The antennas and the electronics are shown in detail in  FIGS. 3A and 4 . 
     A somewhat-enlarged view of the antenna system  40  of  FIG. 2A  is shown in  FIG. 3A . Because the block diagram of  FIG. 4  includes many of the elements of  FIG. 3A , the following description applies to both of these figures. In the embodiment shown, the antenna system  40  includes a planar, top-loaded dipole antenna  42  and a planar elliptical dipole antenna  50  that is arranged substantially coplanar with the antenna  42 . The antenna  42  comprises first and second opposed portions that are each formed with a top-loaded blade  43 . A leading edge  44  of the blade is preferably swept back as it rises from a base edge  45  to conform to the aerodynamically-shaped radome ( 31  in  FIG. 2A ). In the embodiments of  FIGS. 3A and 4 , the blade  43  has a substantially triangular shape and an elongate top load  46  protrudes rearward from the outer edge of the blade  43 . The top-loaded blade is formed by a metallic coating that is carried on an electromagnetically-transparent carrier  47  (formed, for example, by fiberglass). 
       FIGS. 3A and 4  show that an elliptical dipole antenna  50  is positioned between the top loads  46  of the top-loaded dipole antenna  42 . This antenna is also formed by a metallic coating on the carrier  47  so that it is substantially coplanar with the antenna  42 . The antenna  50  comprises a first set  51  of nested elliptical rings  52  and a second set  53  of nested elliptical rings that are arranged in a dipole relationship with the first set. As shown in this embodiment, centers of the elliptical rings in each of the first and second sets are progressively spaced towards the other of the first and second sets. Accordingly, the inner ends of the elliptical rings  52  can be easily joined together to facilitate electrical connection to the antenna. Because the elliptical dipole antenna  50  can be sized to lie between the top loads  46 , the size of the antenna system  20  is small enough to facilitate its use on aircraft wingtips. 
       FIG. 3A  also shows a first string  55  of inductors  56  that is coupled to the inner edge of one of the blades  43  (via a circuit board pad  57 ) and a second string  58  of similar inductors that is coupled to the inner edge of the other of the blades (the strings are referred to in  FIG. 4  as frequency agility tuning chains). These inductors are realized with conductive plating that is preferably carried on a printed-circuit board  59  that is mounted on the carrier  47 . 
     The top-loaded dipole antenna  42  is configured to operate over a first frequency band. Although the antenna system  40  of  FIG. 3A  can be configured to conform to various first frequency bands, the frequency chart of  FIG. 3B  illustrates an exemplary first frequency band of 30-88 MHz. In this frequency band, the blade  43  has been found to present a small resistance in series with a capacitive reactance which decreases from a rather large initial value at the lower edge of the frequency band to a smaller final value at the upper edge of the frequency band. 
     As shown in  FIG. 3A , the spiral inductors  56  have different numbers of windings so that their inductance progressively varies from a low value to a high value. In an exemplary binary progression, if the smallest inductor has an inductance L, then the second inductor has an inductance  2 L, the third an inductance  4 L and so on. Pairs  60  of opposed fast switching diodes (e.g., PIN diodes formed by positive and negative type regions separated by an intrinsic region) are arranged in parallel with each of the inductors. Signals applied to ports  61  of each diode pair can selectively reverse and forward bias those diodes so that the associated inductor is either included in the string of inductors or is excluded (in the latter case, the inductor is bypassed by a shorted path through the diodes). As indicated by an arrow  62 , these signals are preferably applied to the ports  61  through the isolation of a low-pass filter such as the series inductor and shunt capacitor  63 . 
     With the pairs  60  of switching diodes, inductors  57  of each of the strings  55  and  58  can be selected to form a combined inductance that will, when presented to the associated top-loaded blade  43 , substantially cancel that blade&#39;s capacitance at the selected frequency within the first frequency band. Accordingly, only the small resistance introduced above remains and that is fed into the balun  64  (balun is an abbreviation of “balanced impedance to unbalanced”) which is arranged to convert the balanced impedance of the frequency agility tuning chains  55  and  58  to an unbalanced single-ended impedance that is referenced to ground. The output of the balun  64  goes into an impedance matching circuit  65  to convert the remaining small resistance to approximate a 50 ohm resistance. A diplexer  66  is coupled to the impedance matching circuit  65  and the output of the diplexer is fed to an antenna output port  68 . 
     To this point, the description has assumed the antenna system  40  is operating in the exemplary first frequency band of 30-88 MHz that is shown in  FIG. 3B . It is noted that switching diodes  72  (and associated ports for application of switching signals) are arranged to couple the circuit pads  57  to a second balun  73 . In the first frequency band these diodes would be biased off. When the antenna system is operated in the exemplary second frequency band of 108-174 MHz (shown in  FIG. 3B ), these diodes are biased on so that signals from the top-loaded dipole antenna  42  are guided to a second balun  73 . The single-ended output of the second balun is coupled to a second impedance matching circuit  74  and the output of this circuit can be switched via an appropriate one of switching diodes  75  to the diplexer  66 . 
     The second impedance matching circuit is configured to convert, for channels within the second frequency band, the impedance of the top-loaded dipole antenna  42  to substantially 50 ohms Output signals from the second impedance matching circuit  74  are then applied to the diplexer  66  through a respective one of a pair of switching diodes  75 . It is noted that the capacitance of the top-loaded dipole antenna  42  is substantially lower in the second frequency band so that the impedance of the antenna can be substantially converted to 50 ohms in this band without the need for an intervening tuning coil chain such as the chains  55  and  58  that were used in association with the first frequency band. The impedance of these chains in the second frequency band is sufficiently high enough so that signals in this band are diverted through the switching diodes  75  when they are biased on. 
       FIGS. 3A and 4  show that a third balun  76  couples the elliptical dipole antenna  50  to a third impedance matching circuit  77 . Signals in the exemplary third frequency band of 225-600 MHz (shown in  FIG. 3B ) are sent by the elliptical dipole antenna  50  through the third balun  76  and the third impedance matching circuit  77 . Signals from the output of the third impedance matching circuit can then be switched via an appropriate one of switching diodes  75  to the diplexer  66 . Exemplary performance of the elliptical dipole antenna and the third balun and third impedance matching circuit is shown in  FIG. 5B . 
     Embodiments of the impedance matching circuits  65 ,  74  and  77  may be formed with inductors and capacitors that are arranged in ways well known in the impedance-matching art to convert input impedances, in each of the three frequency bands of  FIG. 3B , to ones approximating 50 ohms for application of signals to the diplexer  66 . 
     Although the planar inductors  56  of the strings  55  and  58  of  FIG. 3A  are uniquely suited for fabrication processes of the printed circuit board  59 , other inductor forms may be used in other antenna system embodiments. For example, a replacement arrow  86  in  FIG. 3A  indicates that the planar inductors  56  can be replaced by inductors  87  that are formed with wire that has been formed into coils. The inductors  87  are carried on the printed circuit board  59  but rise above the board. As shown, the coils of neighboring inductors  87  are preferably arranged orthogonally to thereby reduce inductive coupling between them. 
       FIG. 4  illustrates an exemplary application of the antenna system  40  in which it interfaces with a transceiver  80  to form a radio  70 . The transceiver also supplies  28  VDC to a DC-DC converter  81  and sends a frequency word to a processor  82  which is configured to then set up the antenna system  40  for processing of signals having the frequency denoted by the frequency word. After the processor receives the frequency word, it accesses a digital lookup table to determine the appropriate code to switch the pairs  60  of switching diodes, the switching diodes  72  and the switching diodes  75 . A logic controller  84  is shown in  FIG. 4  to facilitate digital control of the circuits described above. This control may support operational modes such as frequency hopping to thereby realize a secure communication system. The controller preferably interfaces with the multi-pin connector  35  of  FIG. 2C . 
     For operation in the first frequency band (30-88 MHz) of  FIG. 3B , the processor turns off the switching diodes  72  and  75  and switches on appropriate ones of the pairs  60  of diodes in  FIG. 3A  to thereby leave appropriate inductors of the sets  55  and  58  in the signal paths. For each operational channel the selected inductors of the strings  55  and  58  provide inductances that substantially match the capacitance of the top-loaded antenna  42  at that channel. The first balun  64  then converts the balanced configuration of the tuning coil chains to the unbalanced configuration (i.e., circuit above a ground plane) of the impedance matching circuit  65 . 
     The processes of the tuning coil chains  55  and  58 , the balun  64 , and the matching circuit  65  provide a well-matched transmission line so that energy losses between the top-loaded dipole antenna  42  and the diplexer  66  are minimized. In an important feature, the balun  64  follows the tuning coil chains  55  and  58  rather than preceding them so that it can effectively process resistive impedances rather than complex impedances. It has been found that interchanging these elements substantially degrades antenna tuning performance and gain. 
     For operation in the second frequency band (108-174 MHz) of  FIG. 3B , the processor  82  turns on the switching diodes  72  and an appropriate one of the switching diodes  75  to connect the balun  73  and the impedance matching circuit  74  to the diplexer  66 . And for operation in the third frequency band (225-600 MHz) of  FIG. 3B , the processor  82  turns on an appropriate one of the switching diodes  75  to connect the balun  76  and the impedance matching circuit  77  to the diplexer  66 . 
     In  FIG. 3A , the first, second and third impedance matching circuits  65 ,  74  and  77 , the second and third baluns  73  and  76 , and the diplexer  66  have been shown as functional blocks. In an antenna system embodiment, the elements of these circuits may be carried on different sides of the printed circuit board  59  and properly arranged (e.g., with short circuit lines) for operation at the high frequencies of the signal bands of  FIG. 3B . 
     The measured performance of the antenna system  40  is indicated by the graph  90  of  FIG. 5A  that shows a plot  91  of return loss which is the ratio of the power reflected back to the top-loaded dipole antenna  42  to the power inserted into the tuning coil chains  55  and  58  from the antenna. The greater the return loss, the greater the power that is successfully delivered at the diplexer  66  (equivalently, the greater the power successfully provided to the top-loaded dipole antenna  42  by the transceiver  80  of  FIG. 4 . 
     The plot  91  of  FIG. 5A  indicates return losses at the output of diplexer  66  of seven discrete frequencies in the first frequency band of 30-88 MHz (specifically 30, 40, 50, 60, 70, 80 and 88 MHz) when the radio  70  of  FIG. 4  commands these frequencies one by one. The return loss is shown in the plot  91  to exceed 9 dB at all of the seven frequencies so that the radiated power at these frequencies is more than 85% of the power applied to the antenna. The narrowness of the responses in  FIG. 5A  is an indication that the top-loaded dipole antenna ( 42  in  FIG. 3A ) provides high selectivity in reception mode. When reception is commanded at each of the seven frequencies, it is thus assured that nearby noise and interference will be substantially rejected. The signal-to-noise ratio performance of the radio  70  of  FIG. 4  is thus seen to be excellent. 
     As shown by the plot  92  of graph  91  of  FIG. 5B , it has also been found that the elliptical dipole antenna  50  of  FIG. 3A , with its sets  51  and  53  of elliptical rings  52 , is especially suited for reception and transmission of energy over wide frequency bands. In this particular embodiment, the ratio, for each ring, of the vertical to horizontal dimensions is substantially 1.4:1. The selection of ten rings was made to optimize return loss across the 225-600 MHz band. As shown by the peaks of  FIG. 5B , the return loss generally exceeds 5 dB. It is noted that the matching circuit  77  of  FIGS. 3A and 4  improves this to approximately 9 dB. In general, it has been found that the more elliptical rings  51  that are nested in each of the dipoles  52  and  53 , the greater the number of channels at which effective power transmission is realized. 
     Antenna gain relates the power intensity of an antenna in a given direction to the intensity that would be produced by a hypothetical ideal antenna that radiates equally in all directions (i.e., isotropically) and that has no losses. The plot  97  of the graph  96  of  FIG. 5A  was measured on an antenna prototype at 39 MHz and the plot  99  of the graph  98  was measured at 75 MHz. Dots on the plot  97  indicate gains of −3 dB and −4 dB and dots on the plot  99  indicate gains of +0.5 dB and +1 dB. 
     For the antenna prototype that exhibited the measured return loss of  FIGS. 5A and 5B  and the gain of  FIGS. 6A and 6B , the physical size of the winglet antenna  20  is shown in  FIG. 2A  in terms of wavelength λ at 30 MHz. As shown, the overall dimensions are a height and a width of λ/20. As also shown, the parallel portions at the top and bottom of the antenna have a width of λ/44. The height of the winglet antenna  20  must be sufficiently restricted to insure clearance from the ground when the airplane  22  of  FIG. 1  is landing and taking off and when it is parked. The height of the antenna must also be sufficiently limited to insure the airplane&#39;s aerodynamic performance is not degraded. In general, the selected height is then a compromise between these physical restraints and antenna gain. 
     Antenna structure, operation and performance has been generally described above in terms of received signals. Because antenna performance is reciprocal, however, the descriptions are also applicable to transmitted signals. 
     The embodiments of the invention described herein are exemplary and numerous modifications, variations and rearrangements can be readily envisioned to achieve substantially equivalent results, all of which are intended to be embraced within the spirit and scope of the appended claims.