Patent Publication Number: US-8115690-B2

Title: Coupled multiband antenna

Description:
TECHNICAL FIELD 
     The present application relates to antennas. More specifically, the application relates to a multiband antenna containing a coupled radiating element. 
     BACKGROUND 
     With the recent increase in portability of communication devices, it has been desirable to provide communications in different frequency bands. Such an arrangement permits communications in different locations around the world in which one or more of the different bands are used, provides a backup so that the same information can be provided at the different bands, or permits different types of information to be provided to the device at the different frequencies. 
     In many instances, for example due to space/design considerations, it is desirable to limit the number of separate antennas to a single combined structure that functions in the multiple bands. One particularly useful combination of bands includes very high frequency (VHF) band (about 136-174 MHz) and the global positioning satellite (GPS) band (about 1575 MHz, 10 times higher than the VHF band). This combination is particularly desirable for public safety providers (e.g., police, fire department, emergency medical responders, and military) who have used the VHF band maintained exclusively for public safety purposes. With the advent of GPS, it has become desirable to be able to determine locations of the public safety providers to better manage increasingly scarce resources, coordinate quicker response, and guide personnel safely through potentially dangerous situations. 
     It is especially challenging however to combine individual antennas with these bandwidths into a single structure. To be an effective radiator, antennas (also called radiating elements) have electrical lengths of λ/4. Thus, a VHF radiating element has a relatively long electrical length of λ/4 at the center of the VHF band, or about 50 cm, while the GPS radiating element of λ/4 is about 5 cm. 
     Unlike the VHF radiating element, the peak gain of the GPS radiating element is directed upward (away from feed point or the base of the radiating element) toward the GPS satellites. Unfortunately, the upward pointing antenna peak gain of GPS radiating elements of length λ/4 is relatively low in antenna structures combining VHF and GPS radiating elements. Simulations have shown that it would be desirable to extend the length of the GPS radiating element to 3λ/4 at the center of the GPS band to increase this gain and improve the upward radiation pattern. However, increasing this length to 3λ/4 detrimentally affects the performance in both bands when implemented in certain structures. Specifically, in these structures, the GPS radiating element consumes the majority of the current when attempting to excite the VHF radiating element, thereby suppressing the gain of the VHF radiating element. Further, in some of these certain structures, exciting the GPS radiating element instead excites the VHF radiating element, decreasing the gain of the GPS radiating element. 
     Accordingly, it is desirable to provide a combined antenna structure that has sufficient peak gain for multiple frequency bands while retaining a relatively small form factor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments will now be described by way of example with reference to the accompanying drawings, in which: 
         FIG. 1  is a side view of an embodiment of a combined antenna structure. 
         FIG. 2  is a top view of the combined antenna structure of  FIG. 1 . 
         FIG. 3  is a perspective view of an embodiment of a combined antenna structure. 
         FIG. 4  is a side view of the embodiment of  FIG. 2  showing the first radiating element. 
         FIG. 5  is a side view of the embodiment of  FIG. 2  showing the second radiating element. 
         FIGS. 6 and 7  are top views of embodiments of combined antenna structure of variations of  FIG. 2 . 
         FIG. 8  is a simulation of current distribution in VHF and GPS radiating elements when attempting to excite the VHF radiating element in an embodiment in which a single 3λ/4 GPS monopole wire is disposed within the VHF helix. 
         FIGS. 9A and 9B  are simulations of current distribution in VHF and GPS radiating elements when attempting to excite the GPS radiating element in embodiments in which a single 3λ/4 GPS monopole wire is disposed within and outside, respectively, the VHF helix. 
         FIGS. 10A and 10B  are simulations of current distribution in VHF and GPS radiating elements when attempting to excite the VHF radiating element in the embodiments of  FIGS. 1 and 3 . 
         FIGS. 11A and 11B  are simulations of current distribution in VHF and GPS radiating elements when exciting the GPS radiating element in the embodiments of  FIGS. 1 and 3 . 
         FIG. 12  is a simulation of the VHF gain in the embodiments of  FIGS. 1 and 3  and embodiments of  FIGS. 9A and 9B . 
         FIG. 13  is a simulation of the GPS gain in the embodiments of  FIGS. 1 and 3  and embodiments of  FIGS. 9A and 9B . 
         FIGS. 14A and 14B  are simulations of GPS radiation patterns at different angles of an embodiment. 
         FIG. 15  illustrates an embodiment of a portable communication device containing the antenna structure. 
     
    
    
     DETAILED DESCRIPTION 
     Free space antenna structures are presented in which multiple radiating elements are disposed proximate to each other. At least one of the radiating elements is split into a monopole and a dipole that are electrically, but not physically, coupled to each other. The radiating element having the longer wavelength may be compressed into a helical structure (helix) to reduce the physical length of the radiating element without reducing the electrical length. One or more sections of the shorter wavelength radiating element may be disposed outside this helix. The monopole, which is shorter than the dipole, drives the dipole at the fundamental resonant frequency. The radiating element having the longer wavelength does not drive either the monopole or the dipole. 
       FIG. 1  illustrates a side view of one embodiment of a free space combined antenna structure. The free space antenna structure is formed from individual conductive wires and assembled rather than being fabricated, for example, by deposition on a multilayer substrate. The antenna structure  100  contains first and second radiating elements  110 ,  120 . The first and second radiating elements  110 ,  120  are connected to other circuitry and electronics (not shown) at a base  104  of the antenna structure  100 . 
     The first radiating element  110  is, for example, a VHF antenna whose fundamental resonance is at VHF band frequencies. The VHF radiating element  110  is coiled into a helical spiral to compress the length of the VHF radiating element  110 . The uncoiled length of the VHF radiating element  110  is λ longer /4 (about 50 cm) while the length of the helix is much less (e.g., 16 or 18 cm). As used herein, the wavelength, λ, is the fundamental resonant frequency of the radiating element. This allows the VHF radiating element  110  to be accommodated within a much shorter physical length than the electrical length, allowing the VHF radiating element  110  to be implemented in portable electronics in which design considerations require a much shorter antenna. Although a helix is shown, other structures that compress the length of the radiating element (e.g., an element that extends back and forth multiple times laterally along the length of the structure) may be used instead or in addition to the helical element. Such structures may be used as long as desired electrical and physical antenna characteristics such as gain, radiation pattern, and form factor are able to be maintained. 
     The second radiating element  120  is, for example, a GPS antenna whose fundamental resonance is at GPS band frequencies. The second radiating element  120  contains two sections: a first section  122  (also called a stub) coupled to the base  104  of the antenna structure and a second section  124 . The second section  124  is floating, i.e., it is proximate enough to the first section  122  to be electrically coupled to and driven by the first section  122 , but does not physically contact the first section  122  (or the VHF radiating element  110 ). The first section  122  drives the second section  124  at the fundamental resonant frequency. The fundamental resonant frequencies of the first and second radiating elements  110 ,  120  are unrelated to each other (i.e., not harmonics). The first section  122  is, as shown in  FIG. 1 , a monopole wire whose length is λ shorter /4, or about 5 cm. As this length is much less than that of the VHF radiating element  110 , the first section  122  is able to be disposed within the helix of the VHF radiating element  110  without extending from the VHF radiating element  110 . The first section  122  shares the same feed as the first radiating element  110 . 
     The second section  124 , shown in  FIG. 1 , is a dipole wire whose length of the second section  124  is λ shorter /2, or about 10 cm. The second section  124  overlaps the first section  122  sufficiently to electrically couple to the first section  122  but does not physically contact the first section  122 . This is to say that although the second section  124  does not contact the first section  122 , the monopole wire  122  inside the helix serves to excite the dipole wire  124 . As shown, the monopole and dipole overlap each other laterally, i.e., along the direction of extension of the wires from the end of the monopole connected to the base  104  to the end of the dipole most distal from the base  104 . As above, although the monopole and dipole are illustrated as straight wires, other shapes may be used as long as desired electrical and physical antenna characteristics such as gain, radiation pattern, and form factor are able to be maintained. 
     The second section  124 , as can be seen, is external to the helix. Thus, the total electrical length of the second radiating element  120  is 3λ shorter /4 of the center GPS frequency, only λ shorter /4 of which is disposed within the helix. Although it is shown as floating in  FIG. 1 , the second section  124  is retained in the antenna structure  100  through any manner (e.g., retained between non-conductive inner and outer sleeves) as long as it does not electrically contact the first section  122  or the VHF radiating element  110 . For example, non-conductive shrink tubing may be used to retain the second section  124  in the desired location. 
     A top view of the embodiment shown in  FIG. 1  is illustrated in  FIG. 2 . As shown, the first section  122  of the second radiating element  120  is disposed within the helix forming the first radiating element  110  and the second section  124  of the second radiating element  120  is disposed outside of the helix. The second section  124  is separated from the first radiating element  110  by a non-conductive sheath  130 . The sheath  130  extends along substantially the entire length of the first radiating element  110 , although it may be shortened to extend only to cover the portion of the first radiating element  110  that overlaps with the second section  124  of the second radiating element  120 . The first section  122  of the second radiating element  120  is disposed proximate to the coils of the helix where the second section  124  is disposed to sufficiently couple to the second section  124 . A non-conductive cover  140  is disposed around the entire antenna structure  100  and retains the second section  124 . An additional non-conductive cover (not shown) may be disposed around the first section  122  between the first section  122  and the first radiating element  110 . 
     Another embodiment of a combined free space antenna structure is illustrated in the perspective view of  FIG. 3 . The combined antenna structure  300 , like the combined antenna structure  100  of  FIG. 1 , contains a first radiating element  310  and first and section sections  322 ,  324  forming a second radiating element  320 . The first radiating element  310  is, as in the above example, a λ longer /4 VHF antenna that provides resonance in VHF band frequencies and is coiled into a helical spiral. The first and second sections  322 ,  324 , as in the example above, are non-physically contacting, electrically coupled monopole and dipole wires (respectively) that overlap and form a total electrical length of 3λ shorter /4. The first section  322  drives the parasitic second section  324 . The first radiating element  310  and first section  322  of the second radiating element  320  are supplied with current at the base  304  of the antenna structure  300  by the same feed  306  (shown in  FIGS. 4 and 5 ). The overlapping portions of the first and second sections  322 ,  324  may be disposed radially adjacent to each other and may have a fitted sleeve therebetween. Similar to the embodiment of  FIG. 1 , the total physical length of the first and section sections  322 ,  324  is about ⅔ that of the first radiating element  310  (although this can differ, depending on the diameter and distance between adjacent coils of the helix). However, in the embodiment of  FIG. 3 , the first and section sections  322 ,  324  both lie outside the helix of the first radiating element  310 . 
     As shown in the side views of  FIGS. 4 and 5 , the base  304  has a connection portion  308  that may be inserted into a portable electronic communication device, such as a push-to-talk (PTT) device used by public safety personnel. The connection portion  308  is shown as having threads for a screw-type connector, however other types of connectors, such as snap-fit connectors may be used for easy connection to the body of the portable communication device. The first radiating element  310  is shown in  FIG. 4  as being connected to the base  304  of the antenna structure  300  by the feed  306 . Similarly, the second radiating element  320  is shown in  FIG. 5  as being connected to the base  304  of the antenna structure  300  at a portion of the feed point  306  more closely to the connection portion  308  than the first radiating element  310 . 
     Top views of variations of the embodiment shown in  FIGS. 3 and 4  are illustrated in  FIGS. 5 and 6 . As shown in both variations, both the first and second sections  322 ,  324  of the second radiating element  320  are disposed outside of the helix of the first radiating element  310 . The second radiating element  320  is separated from the first radiating element  310  by a non-conductive sheath  330  that extends along substantially the entire length of the first radiating element  310 . As shown in  FIG. 6 , the first and second sections  322 ,  324  are disposed radially adjacent and may be separated by a non-conductive shield  332  that extends at least around the overlapping portions of the first and second sections  322 ,  324 . The shield  332  is disposed such that the first and second sections  322 ,  324  are completely protected from physical contact with each other. As shown in  FIG. 7 , the first and second sections  322 ,  324  are disposed circumferentially adjacent with the non-conductive protection  332  extending at least around the overlapping portions of the first and second sections  322 ,  324 . The sheath  330  and protection  332  prevent accidental contact between the various portions of the antenna structure  300  if the antenna structure  300  is bent or otherwise damaged. A non-conductive cover  340  is disposed around the entire antenna structure  300  and retains the second section  324 . 
     In other unshown embodiments, the relative positions of the first and second sections  322 ,  324  may be reversed from that of  FIG. 6  such that the second section  324  is radially closer to the first radiating element  310  than the first section  322 . In other embodiments, the protection  332  may extend along either only the overlapping portions of the first and second section  322 ,  324  or over an extensive amount of the first and/or second section  322 ,  324 . In other embodiments, not shown, the protection  332  may extend entirely around the first or second section  322 ,  324  further protecting the closer of the two from the first radiating element  310  and from each other, or may be eliminated entirely, e.g., if the first and second sections  322 ,  324  are sufficiently circumferentially separated from each other. 
     In each of the embodiments of  FIGS. 1-7 , the first radiating element  110 ,  310  is shown as having a non-uniform helical structure. As is apparent, the portion of each first radiating element  110 ,  310  more proximate to the base  104 ,  304  of the antenna structure  100 ,  300  has a diameter larger than the diameter of that distal from the base  104 ,  304  of the antenna structure  100 ,  300 . Such an arrangement may be desirable, for example, to satisfy a desired form factor of the antenna structure. In other embodiments, a helix having a constant diameter can be used. 
     Various simulations shown in  FIGS. 8-14  are provided using the Method of Moment (MoM). A simulation of the current distribution in a combined antenna structure when attempting to excite the VHF radiating element is shown in  FIG. 8 . In this structure, a 3λ shorter /4 GPS monopole wire extends through the helix. The monopole wire is a single wire, unlike the embodiments shown in  FIGS. 1-7 . While such an antenna may be easier to fabricate, the 3λ shorter /4 GPS monopole wire electrically couples to the VHF helix, draining current from the VHF radiating element. Thus, even though it is desired to excite the VHF radiating element, the majority of the current is being undesirably used by the GPS radiating element, leaving the VHF signal dominated by the GPS signal. Similar results were obtained for an embodiment in which the 3λ shorter /4 GPS monopole wire is disposed outside the helix. 
     Simulations of the current distribution in a combined antenna structure when attempting to excite the GPS radiating element are shown in  FIGS. 9A and 9B . In this structure, a 3λ shorter /4 single GPS monopole wire extends through the helix in  FIG. 9A  and outside the helix in  FIG. 9B . As can be seen in  FIG. 9A , the majority of the current is being undesirably used by the VHF radiating element, leaving the GPS signal dominated by the VHF signal. The GPS signal fares better when the 3λ/4 single GPS monopole wire extends outside the helix, as shown in  FIG. 9B . 
     Simulations of the current distribution in the combined antenna structures  100 ,  300  of  FIGS. 1 and 3  when attempting to excite the VHF radiating element are shown respectively in  FIGS. 10A and 10B . The coupling impedance between the GPS monopole and GPS dipole is relatively large in the lower frequency range (about 150 MHz), leading to minimal current being induced in the GPS dipole. This is confirmed as shown in the simulation, the majority of the current is now being used by the VHF radiating element. The feed point of the radiating elements is the lower left position (0.0) of the simulations. As each simulation illustrates, the VHF current dominates over the entire length of the VHF antenna, the overlapping current curves at the lower portions of the simulations being the GPS stub and coupled dipole. 
     Simulations of the current distribution in the combined antenna structures  100 ,  300  of  FIGS. 1 and 3  when attempting to excite the GPS radiating element are shown respectively in  FIGS. 11A and 11B . The coupling impedance between the GPS monopole and GPS dipole is relatively small in the upper, GPS, frequency range (about 1575 MHz), leading to minimal current being induced in the GPS dipole. This is confirmed as shown in the simulation, the majority of the current is being used by the GPS radiating element. The only locations at which the VHF radiating element consumes more current than the GPS radiating elements are at the end points of the dipole. 
     Comparison simulations of the gain of the different radiating elements at different frequencies for far field radiation patterns are shown in  FIGS. 12-13 . A comparison simulation of the gain of the VHF radiating element at VHF frequencies (VHF gain) vs. angular distribution is shown in  FIG. 12 . This simulation illustrates that the VHF gain in the embodiments of  FIGS. 1 and 3  is larger than that of embodiments of  FIGS. 9A and 9B  at all angles (note: θ is defined along the length of the radiating element). Similarly, a comparison simulation of the gain of the GPS radiating element at GPS frequencies (GPS gain) vs. angular distribution is shown in  FIG. 13 . This simulation illustrates that the GPS gains in all embodiments are comparable. Similar case for the  FIG. 13 , it is a far field radiation pattern, but in a polar plot. The  FIG. 13  shows a comparable GPS performance. 
     Simulated GPS radiation patterns (at about 1.575 GHz) of the antenna structure of  FIG. 3  are shown in  FIGS. 14A and 14B . The radiation pattern in an elevation plane through the center of the device is illustrated in both figures. Specifically,  FIG. 14A  shows the radiation pattern with the figure (in outline) facing into the page and a radio containing the antenna structure facing right (φ=0°), while  FIG. 14B  shows the radiation pattern with the figure (in outline) facing right and the radio containing the antenna structure facing out of the page (φ=90°). As can be observed, the peak is consistent around 60° from the azimuth. 
     One example of a portable communication device containing the antenna structure of  FIG. 1  or  3  is shown in  FIG. 15 . The communication device  1500  has a body  1510  to which the antenna structure  1530  is connected via, e.g., screwing in the antenna structure  1530 . The body  1510  contains internal communication components (such as a microprocessor, transmitter, receiver, and memory) and circuitry to enable the device  1500  to communicate wirelessly with other devices. The body  1510  also contains I/O devices such as a keyboard  1512  with alpha-numeric keys  1514 , a display  1516  that displays information about the device  1500 , a PTT button to transmit  1518 , a channel selector knob  1522  to select a particular frequency for transmission/reception, a microphone  1524 , and a speaker  1526 . The channel selector knob  1522  and/or keyboard  1512 , for example, may be used choose which of the first and second radiating elements in the antenna structure  1530  to use. 
     Although the above description has focused on VHF/GPS antenna structures due to their use in the public safety environment, similar designs may be used in various antenna structures in which the frequency band difference is large (e.g., UHF/VHF or UHF/GPS). The various wavelength ranges and centers are as follows: VHF (136-174 MHz) center at 150 MHz, UHF (380-520 MHz) center at 450 MHz, 800 MHz (764-870 MHz), GPS (1575 MHz). Thus, for example, in a combined VHF/UHF antenna, the center frequency of the UHF band is 3 times larger than the VHF band, and in a combined UHF/GPS antenna, the center frequency of the GPS band is 3.5 larger than the UHF band. Both of these center frequency differences are sufficient to permit a combined antenna structure to be produced. Such designs include a λ/4 monopole wire coupled to a λ/2 dipole to form a 3λ/4 radiating element and effectively decouple the lower-frequency radiating element from the higher-frequency radiating element. Thus, exciting the lower-frequency radiating element will excite the higher-frequency radiating element by a minimal amount. This can also be extended to tri-frequency (or larger) antenna structures. For example, multiband antenna structures such as UHF/800 MHz/GPS, VHF/800 MHz/GPS, VHF/UHF/GPS. Such antenna structures can be used in a variety of situations, for example, to provide a duplicate communication channel in case messages at one of the frequencies are unable to be transmitted/received. 
     It will be understood that the terms and expressions used herein have the ordinary meaning as is accorded to such terms and expressions with respect to their corresponding respective areas of inquiry and study except where specific meanings have otherwise been set forth herein. Relational terms such as first and second and the like may be used solely to distinguish one entity or action from another without necessarily requiring or implying any actual such relationship or order between such entities or actions. The terms “comprises,” “comprising,” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. An element proceeded by “a” or “an” does not, without further constraints, preclude the existence of additional identical elements in the process, method, article, or apparatus that comprises the element. 
     Those skilled in the art will recognize that a wide variety of modifications, alterations, and combinations can be made with respect to the above described embodiments without departing from the spirit and scope of the invention defined by the claims, and that such modifications, alterations, and combinations are to be viewed as being within the scope of the inventive concept. Thus, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of present invention. The benefits, advantages, solutions to problems, and any element(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential features or elements of any or all the claims. The invention is defined solely by any claims issuing from this application and all equivalents of those issued claims. 
     The Abstract of the Disclosure is provided to allow the reader to quickly ascertain the nature of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. In addition, in the foregoing Detailed Description, it can be seen that various features are grouped together in various embodiments for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting an intention that the claimed embodiments require more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive subject matter lies in less than all features of a single disclosed embodiment. Thus the following claims are hereby incorporated into the Detailed Description, with each claim standing on its own as a separately claimed subject matter.