Patent Publication Number: US-11024981-B2

Title: Multi-band endfire antennas and arrays

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 62/657,093 filed on Apr. 13, 2018, the entirety of which is incorporated by reference herein. 
    
    
     TECHNICAL FIELD 
     Embodiments of the invention relate to an antenna assembly that radiates in two or more frequency bands, in a direction parallel to a ground plane. 
     BACKGROUND 
     Wireless devices use antennas to transmit and receive wireless signals. Modern wireless devices, such as those operating in the 5G (fifth generation) mobile communication networks, use multi-band antennas capable of signaling (transmitting and/or receiving) at multiple frequency bands in the millimeter frequency spectrum (e.g., 24.0-300 GHz). Operation at these frequencies may encounter significant challenges. For example, millimeter wave communications typically do not navigate around or through obstacles effectively. Thus, millimeter wave signals may be substantially attenuated during signal propagations. In addition, many wireless devices, such as smartphone and smart watches, have a limited form factor which constrains the size of the antennas. 
     SUMMARY 
     In one embodiment, there is provided an antenna assembly comprising a first antenna element coupled to RF circuitry via a first feeder, and a second antenna element coupled to the RF circuitry via a second feeder. The first feeder and the second feeder have different shapes. The first antenna element and the second antenna element radiate in different frequency bands and in a direction parallel to a ground plane. The ground plane is disposed on at least one layer in a substrate that includes a plurality of layers parallel to one another. The first antenna element is disposed on first one or more of the layers and the second antenna element is disposed on second one or more of the layers, which are different from the first one or more of the layers. 
     In another embodiment, there is provided an antenna assembly comprising a first antenna subarray including a plurality of first antenna elements, each first antenna element coupled to RF circuitry via a first feeder; and a second antenna subarray including a plurality of second antenna elements, each second antenna element coupled to the RF circuitry via a second feeder. The first feeder and the second feeder have different shapes. The first antenna element and the second antenna element radiate in different frequency bands and in a direction parallel to a ground plane. The ground plane is disposed on at least one layer in a substrate that includes a plurality of layers parallel to one another. Each of the first antenna elements and the second antenna elements is disposed on one or more of the layers, and each first antenna element and a corresponding one of the second antenna elements are stacked in a perpendicular direction with respect to the ground plane. 
     Advantages of the embodiments will be explained in detail in the following descriptions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings in which like references indicate similar elements. It should be noted that different references to “an” or “one” embodiment in this disclosure are not necessarily to the same embodiment, and such references mean at least one. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to effect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described. 
         FIG. 1  illustrates an antenna assembly including two antenna elements according to one embodiment. 
         FIG. 2  illustrates a side view of the antenna assembly of  FIG. 1  according to one embodiment. 
         FIG. 3  illustrates a perspective view of the antenna assembly of  FIG. 1  according to one embodiment. 
         FIG. 4  illustrates a perspective view of an antenna assembly including two antenna subarrays according to one embodiment. 
         FIG. 5  illustrates a perspective view of a multi-band antenna assembly including three antenna elements according to one embodiment. 
         FIG. 6  illustrates a top view of the two antenna elements of  FIG. 1  according to one embodiment. 
         FIG. 7  illustrates a top view of two antenna elements with an offset in a first direction according to one embodiment. 
         FIG. 8A  illustrates a top view of two antenna elements with an offset in a second direction according to one embodiment. 
         FIG. 8B  illustrates a top view of two antenna elements with another offset in a second direction according to one embodiment. 
         FIG. 9  illustrates a top view of two interleaved antenna subarrays according to one embodiment. 
         FIG. 10  illustrates a top view of two antenna subarrays with different numbers of antenna elements according to one embodiment. 
         FIG. 11  is a schematic diagram of two antenna elements, each coupled to a three-terminal switch according to one embodiment. 
         FIG. 12  is a schematic diagram of two antenna elements using a filter to enhance signal isolation according to one embodiment. 
         FIG. 13  illustrates a wireless device according to one embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known circuits, structures and techniques have not been shown in detail in order not to obscure the understanding of this description. It will be appreciated, however, by one skilled in the art, that the invention may be practiced without such specific details. Those of ordinary skill in the art, with the included descriptions, will be able to implement appropriate functionality without undue experimentation. 
     Embodiments of an end-fire antenna assembly are described herein. The antenna assembly may be a multi-band antenna that includes multiple antenna elements electromagnetically resonate in multiple different frequencies. In one embodiment, the antenna assembly includes at least a low-band antenna element and a high-band antenna element. The two antenna elements electromagnetically resonate in two different frequencies (e.g., a low-frequency band and a high-frequency band, respectively). Both antenna elements lie on planes that are parallel to a ground plane. Both antenna elements radiate electromagnetic waves (e.g., wireless signals) propagating in the direction parallel to the ground plate. The low-band antenna element and the high-band antenna element may be coupled to different transceivers; e.g., a low-band transceiver and a high-band transceiver, respectively. Alternatively, the low-band antenna element and the high-band antenna element may be coupled to a transceiver with front-end circuitry supporting two or more frequency bands. The different antenna elements may be disposed on different layers of a multi-layer substrate with a small spacing (e.g., up to a half wavelength of their highest resonant frequency) between the antenna elements. In one embodiment, the antenna assembly includes two or more antenna subarrays. A first antenna subarray includes a plurality of the first antenna elements and a second antenna subarray includes a plurality of the second antenna elements. Each antenna element and each antenna subarray described herein radiate wireless signals in the end-fire direction; i.e., in the direction parallel to the ground plane; more specifically, directions on the X-Y plane as defined in the following description. 
     For ease of description, the plane on which the ground plane lies is referred to as the X-Y plane, and the thickness of the ground plane is aligned with the Z direction. In one embodiment, the thickness of the substrate (i.e., the Z direction) is much smaller than its length (the X direction) and width (the Y direction). In some conventional systems, antenna elements are disposed across the X-Y plane and radiate wireless signals in the broad-side direction; i.e., the Z direction. There is more room on the X-Y plane of the substrate to spread out broad-side antenna elements than in the thickness dimension (e.g., the Y-Z plane) of the substrate. Embodiments of the antenna assembly described herein arrange end-fire antenna elements on the limited Y-Z plane of the substrate while maximizing cross-band signal isolation and antenna gain. To reduce the footprint of the antenna assembly on the Y-Z plane of the substrate, antenna elements of different frequency bands are stacked along the Z direction. However, stacking these antenna elements in the limited space along the Z direction may cause potential signal isolation problems and reduced antenna gain. Embodiments of the antenna assembly use different types of antenna elements (e.g., the low-band antenna element is a dipole antenna and the high-band antenna element is a loop-shaped antenna) and different shapes of antenna feeders to increase the signal isolation and antenna gain for the antenna elements of different frequency bands stacked in the Z direction. 
     Thus, the antenna assembly described herein has a compact size suitable for portable wireless devices having a limited form factor. The antenna assembly may be used for millimeter wave communications, such as 5G mobile communications. 
     In the following description, the term “parallel” is used herein to mean that two lines, layers or planes are parallel or slightly deviated from being parallel. The slight deviation may come from the antenna manufacturing process and is within an allowable tolerance range. Thus, the terms “parallel” and “substantially parallel” are interchangeable in this disclosure to mean that two or more lines, layers, and/or planes are parallel within an allowable tolerance range. Furthermore, the terms “parallel” and “substantially parallel” are also interchangeable in this disclosure to mean that a line of direction and a plane/layer are parallel within an allowable tolerance range. 
       FIG. 1  illustrates an antenna assembly  100  including two antenna elements according to an embodiment. The antenna assembly  100  includes a first antenna element  110  coupled to a first radio frequency (RF) circuit  115  via a first feeder  116 . The antenna assembly  100  further includes a second antenna element  120  coupled to a second RF circuit  125  via a second feeder  126 . The first feeder  116  and the second feeder  126  have different geometric shapes. The first RF circuit  115  and the second RF circuit  125  are collectively referred to as the RF circuitry. In one embodiment, the first RF circuit  115  is a first transceiver and the second RF circuit  125  is a second transceiver. Alternatively, the first RF circuit  115  and the second RF circuit  125  are different front-end circuits for different frequency bands in one transceiver. 
     The first RF circuit  115  and the second RF circuit  125  may be disposed on a circuit block  140 . The circuit block  140  may be disposed on a surface of a substrate  150 ; e.g., the surface facing the (−Z) direction as shown. At least one ground plane  130  is in the substrate  150 . The first RF circuit  115  and the second RF circuit  125  are further coupled to a processing circuit  160  for processing incoming and outgoing wireless signals. In one embodiment, the antenna assembly  100  and the ground plane  130  are disposed in the substrate  150 , e.g., a multi-layer substrate (which is outlined in dashed lines). The substrate  150  also includes a circuit routing  145  infrastructure which is composed of conducting materials for routing electrical signals between circuit components. The circuit block  140  and the components thereon are assembled on the substrate  150 . 
     The first antenna element  110  and the second antenna element  120  resonate in different frequencies or frequency bands. In one embodiment, the first antenna element  110  radiates RF signals in a first frequency band and the second antenna element  120  radiates RF signals in a second frequency band, where the first frequency band is lower than the second frequency band. Thus, the first antenna element  110  may also be referred to as a low-band antenna element and the second antenna element  120  may also be referred to as a high-band antenna element. In one embodiment, the first antenna element  110  may have a resonance frequency at 28 GHz, and the second antenna element may have a resonance frequency at 39 GHz. In alternative embodiments, the antenna elements  110  and  120  may have other different resonance frequencies. In one embodiment, the widths (in the Y direction) of the first antenna element  100  and the second antenna element  120  may be a half wavelength of their respective resonance frequencies. 
     Moreover, the first antenna element  110  and the second antenna element  120  radiate RF signals in a direction parallel to the ground plane  130 . Thus, the antenna assembly  100  is also referred to an end-fire antenna assembly, and the antenna elements  110  and  120  may be referred to as end-fire antenna elements. For ease of description, the plane on which the ground plane  130  lies is referred to as the X-Y plane. To orient the antenna elements  110  and  120  to radiate in the end-fire direction, the first antenna element  110  is disposed on a first plane and the second antenna element  120  is disposed on a second plane above the first plane, where the first plane and the second plane are parallel to the ground plane  130 . In the description herein, the direction “above” or “top” is the direction that perpendicularly points toward the (+Z) direction. 
     Furthermore, the antenna elements  110  and  120  may be disposed side-by-side with the circuit routing  145 , the ground plane  130 , and the circuit block  140 . A metal wall  170  may be disposed at the interface, dividing the side of the antenna elements  110  and  120  and the side of the circuit block  140 , the circuit routing  145 , and the ground plane  130 . The metal wall  170  may serve as the reflector of the antenna elements  110  and  120  to improve the antenna gain. The metal wall  170  can be formed by a plurality of vias in the substrate  150 . The placement and the orientation of the antenna elements  110  and  120  are shown more clearly with the side view in  FIG. 2 . 
       FIG. 2  illustrates a side view of the antenna elements  110  and  120  according to one embodiment. The antenna elements  110  and  120  are disposed in the multi-layer substrate  150 . The layers of the substrate  150  are parallel with each other and parallel with the X-Y plane. Each layer is shown in  FIG. 2  as a horizontal line across the substrate  150 . Each layer contains current-conducting materials. Each of the antenna elements  110  and  120  may be disposed on one, two, or more layers of the substrate  150 . In this embodiment, the first antenna element  110  is disposed on a first layer  210  and a second layer  211 , and the second antenna element  120  is disposed on a third layer  220  and a fourth layer  221 , with the third layer  220  and the fourth layer  221  above or on top of the first layer  210  and second layer  211  in the Z direction. One or more vias may be formed between the layers to allow current-conducting wires to pass through from one layer to another. The area of the substrate  150  between the first antenna element  110  and the second antenna element  120  consists of dielectric materials. In an embodiment where there are three or more antenna elements resonating at different frequencies, the three or more antenna elements may each be disposed on one or more different layers and stacked in the Z direction. The top surfaces of the antenna elements  110  and  120  may be parallel or substantially parallel with the X-Y plane. 
       FIG. 2  also shows that the first antenna element  110  and the second antenna element  120  have separate feeding ports ( 216 ,  217 ) and ( 226 ,  227 ) (marked in dashed ellipses), respectively, for connecting to the respective feeders  116  and  126 . Either or both of feeding ports ( 216 ,  217 ) and feeding points ( 226 ,  227 ) may be single-ended. For example, feeding ports ( 216 ,  217 ) may be single-ended (e.g., feeding port  216  connects to RF signals from the first RF circuit  115  and feeding port  217  connects to ground), and/or feeding ports ( 226 ,  227 ) may be single-ended (e.g., feeding port  226  connects to RF signals from the second RF circuit  125  and feeding port  227  connects to ground). Alternatively, either or both of feeding ports ( 216 ,  217 ) and feeding points ( 226 ,  227 ) may be a differential pair. For example, feeding ports  216  and  217  connect to RF+ and RF− signal from the first RF circuit  115 , respectively, and/or feeding ports  226  and  227  connect to RF+ and RF− signals from the second RF circuit  125 , respectively. The feeding ports ( 216 ,  217 ) and ( 226 ,  227 ) may be the same type of port or different types of ports. 
     The ground plane  130  spans on the X-Y plane. The ground plane  130  is disposed side-by-side with the antenna elements  110  and  120 . More specifically, the Z-direction projections of the antenna elements  110  and  120  fall in an area next to, and non-overlapping with, the ground plane  130 . Each of the ground plane  130 , the circuit routing  145  and the metal wall  170  is disposed in one or more of the layers in the multi-layer substrate  150 . In the embodiment of  FIG. 2 , the ground plane is disposed on the layer  230 . In one alternative embodiment, one of the layers on which the antenna element ( 110  or  120 ) is disposed may be the layer  230 , the same layer on which the ground plane is disposed. 
     Although this disclosure describes various embodiments in which the second antenna element  120  is stacked on top of (or substantially on top of) the first antenna element  110 , in alternative embodiments the first antenna element  110  may be stacked on top of (or substantially on top of) the second antenna element  120 . 
       FIG. 3  illustrates a perspective view of the antenna assembly  100  according to one embodiment. This perspective view more clearly shows that the first feeder  116  and the second feeder  126  have different shapes. More specifically, the second feeder  126  includes two line elements that form a fork with angle A 1  toward the second antenna element  120 . Angle A 1  may be any angle between, non-inclusively, 0 and 180 degrees. In contrast, the two line elements of the first feeder  116  are parallel to each other; that is, 180 degrees between the two line elements. A designer may choose angle A 1  such that angle A 1  is different from the angle between the two line elements of the first feeder  116 . The difference between the two angles improves isolation between the two antenna elements  110  and  120 , thereby improving the antenna gain. 
     In one embodiment, the first antenna element  110  and the second antenna element  120  may be different types of antennas and have different antenna shapes. For example, the first antenna element  110  may be a dipole antenna, and the second antenna element  120  may be a folded dipole antenna, a loop antenna, or another loop-based antenna. In an alternative embodiment, the first antenna element  110  may be a folded dipole antenna, a loop antenna, or another loop-based antenna, and the second antenna element  120  may be a dipole antenna. 
       FIG. 3  also shows a spacing S between the first antenna element  110  and the second antenna element  120 . S denotes the vertical distance, in the Z direction, between the two antenna elements  110  and  120 . S is smaller than the height (i.e., thickness) of the substrate  150 . A typical substrate, such as the substrate  150 , has a thickness (in the Z direction) much smaller than its length (in the X direction) and width (in the Y direction). 
     The spacing S may be determined at the antenna design time based on the frequency range(s) and the corresponding wavelengths for which the antenna elements provide. In one embodiment, S may be a non-zero value less than or equal to λ d /2, wherein λ d  is the highest resonant frequency of the first antenna element  110  and the second antenna element  120 . 
       FIG. 4  illustrates a perspective view of an antenna assembly including two antenna subarrays (a 1 -a 3  and b 1 -b 3 ) according to one embodiment. The first antenna subarray includes antenna elements a 1 -a 3 , each of which may be the first antenna element  110  in  FIGS. 1-3 . The second antenna subarray includes antenna elements b 1 -b 3 , each of which may be the second antenna element  120  in  FIGS. 1-3 . Each antenna element (a 1 , a 2 , a 3 , b 1 , b 2 , or b 3 ) may have the same orientation and shape, and operates in the same frequency or frequency band as the corresponding antenna element  110  or  120 . In alternative embodiments, the number of antenna elements in each subarray may be any plural number different from three. 
     In this embodiment, the antenna elements (a 1 -a 3  or b 1 -b 3 ) in each subarray form an equidistant linear array that spans in the width (Y) direction. Stacking the second antenna subarray (b 1 -b 3 ) on top of the first antenna subarray (a 1 -a 3 ) significantly reduces the footprint of the antenna assembly as compared to spreading all antenna elements (a 1 -a 3  and b 1 -b 3 ) on the same plane along the width (Y) direction. In an alternative embodiment, the first antenna subarray (a 1 -a 3 ) may be stacked on top of the second antenna subarray (b 1 -b 3 ). 
     Furthermore, all of the antenna elements a 1 -a 3  in the first antenna subarray have a first polarization, and all of the antenna elements b 1 -b 3  in the second antenna subarray have a second polarization. The first polarization may be the same as, or different from, the second polarization. 
       FIG. 5  illustrates a perspective view of an antenna assembly  500  including three antenna elements  110 ,  120  and  510  according to one embodiment. The three antenna elements  110 ,  120  and  510  radiate in three different frequencies or frequency bands; e.g., the antenna element  110  operates in a low-frequency band, the antenna element  120  operates in a middle-frequency band and the antenna element  510  operates in a high-frequency band, where the terms “low,” “middle” and “high” denote frequency ranges relative to one another in the frequency spectrum. The three antenna elements  110 ,  120  and  510  radiate in an end-fire direction; i.e., the direction parallel to the ground plane  130 . The immediately adjacent two antenna elements (e.g., ( 110  and  120 ), and ( 120  and  510 )) in the Z direction are coupled to feeders of different shapes. For example, the feeder  116  of the antenna elements  110  includes two parallel line elements. The feeder  126  of the antenna element  120  includes two line elements that fork toward the antenna element  120  at an angle between 0-180 degrees, non-inclusively. The feeder  516  may have the same shape as the feeder  116 . In an alternative embodiment, the feeder  516  may have a shape different from the feeder  116  and the feeder  126 . 
     Furthermore, the immediately adjacent two antenna elements (e.g., ( 110  and  120 ), and ( 120  and  510 )) in the Z direction are different types of antennas. As mentioned before, the first antenna element  120  may be a dipole antenna, and the second antenna element  110  may be a folded dipole antenna, a loop antenna, or another loop-based antenna. The third antenna element  510  may also be a dipole antenna. An alternative embodiment of an antenna assembly may include more than three antenna elements, each radiating in a different frequency band in the end-fire direction. In such an antenna assembly, any two immediately adjacent antenna elements (where the adjacency is in the Z direction) are different types of antennas and are coupled to different shapes of feeders. Antenna elements that are not immediately adjacent in the Z direction may be the same type of antenna and coupled to feeders of the same shape. 
     Although not shown in the figures herein, an antenna assembly may include three or more antenna subarrays, where the antenna elements in each subarray form an equidistant linear array that spans in the width (Y) direction, and the antenna elements of different subarrays are disposed on different parallel planes or in different parallel layers. For example, the antenna assembly  500  in  FIG. 5  may be replicated a number of times and placed along the Y direction of the substrate. The corresponding antenna elements in the three subarrays may be stacked on top of one another as shown in the three-antenna-element stack in  FIG. 5 . A similar construction is applicable to an antenna assembly having more than three antenna subarrays. 
       FIG. 6  illustrates a top view of the two antenna elements  110  and  120  of  FIG. 1  according to one embodiment. This top view shows that the two antenna elements  110  and  120  are aligned in the middle of their respective width (Y) dimensions. That is, the respective middle lines (shown as a dotted line) of their respective width (Y) dimensions are on top of (i.e., aligned with) each other.  FIG. 7  illustrates a top view of the two antenna elements  110  and  120  with an offset in the Y (or −Y) direction according to one embodiment. That is, the middle lines (shown as two dotted lines) of their respective width (Y) dimensions are not on top of each other.  FIG. 8A  illustrates a top view of the two antenna elements  110  and  120  with an offset in the X (or −X) direction according to one embodiment. That is, endpoints ( 810 ,  820 ) of their respective feeders ( 116 ,  126 ) are not on top of each other. In some designs, the offset may be used to increase isolation between the antenna elements.  FIG. 8B  illustrates a top view of the two antenna elements  110  and  120  with another offset in the X (or −X) direction according to one embodiment. The feeder  816  or  826  may be lengthened or shortened in the X (or −X) direction by an offset compared to its counterpart  116  or  126 , such that the endpoints ( 810 ,  820 ) are aligned when viewed from the top. 
     In one embodiment, an antenna assembly that includes two or more subarrays may use the arrangements shown in  FIGS. 6-8 . For example, the antenna subarrays (a 1 -a 3  and b 1 -b 3 ) may be arranged to be aligned with respect to their respective width (Y) dimensions for each corresponding pair of first and second antenna elements  110 ,  120  as in  FIG. 6 . Alternatively, the antenna subarrays (a 1 -a 3  and b 1 -b 3 ) may be arranged to have an offset in the Y direction or in the X direction respect to their respective width (Y) dimensions for each corresponding pair of first and second antenna elements  110 ,  120  as in  FIG. 7  or  FIG. 8 . 
       FIG. 9  illustrates a top view of an antenna assembly including two interleaved antenna subarrays (a 1 -a 3  and b 1 -b 3 ) according to one embodiment. The antenna elements a 1 -a 3  and b 1 -b 3  may be the same antenna elements as in  FIG. 4 . The first antenna subarray (a 1 -a 3 ) interleaves with the second antenna subarray (b 1 -b 3 ) such that, in a direction parallel to the ground plane (e.g., the Y direction), any two antenna elements that are immediately adjacent to each other include one of the first antenna elements and one of the second antenna elements. That is, the antenna elements of the same antenna subarray are not all disposed on the same plane. For example, the antenna elements a 1 , b 2  and a 3  are on the same plane, and the antenna elements b 1 , a 2  and b 3  are on the same plane. That is, the two antenna subarrays are interleaved. The interleaving is applicable to an antenna assembly having more than two subarrays, and each subarray may include any numbers of antenna elements. To enhance signal isolation and improve the antenna gain, the interleaved antenna assembly maintains that different types of antennas and different shapes of feeders are used for any immediately adjacent two antenna elements in the Z direction. 
       FIG. 10  illustrates a top view of two antenna subarrays (a 1 -a 3  and b 1 -b 4 ) with different numbers of antenna elements according to one embodiment. The antenna elements in each subarray form an equidistant linear array that spans in the width (Y) direction. Each of the antenna elements a 1 -a 3  may be the first antenna element  110  in  FIGS. 1-3 . Each of the second antenna subarray (b 1 -b 4 ) may be the second antenna element  120  in  FIGS. 1-3 . The antenna elements of the same subarray are disposed on the same plane parallel to the ground plane  130 , and the antenna elements of different subarrays are disposed on different parallel planes. In this embodiment, the second antenna subarray (b 1 -b 4 ) includes more antenna elements than the first antenna subarray. In an alternative embodiment, the first antenna subarray may include more antenna elements than the second antenna subarray. In an alternative embodiment, each antenna subarray may include any number of the same antenna elements. 
       FIG. 11  is a schematic diagram of the antenna elements  110  and  120 , each coupled to a three-terminal switch ( 1140   a  or  1140   b ) according to one embodiment. The three terminals of the switch  1140   a ,  1140   b  are coupled to a power amplifier (PA)  1110  to amplify outgoing signals in the transmission path, a low-noise amplifier (LNA)  1120  to amplify incoming signals in the receiving path, and a load  1130 . The load  1130  is optimized with respect to at least the impedance to minimize the interference caused by an antenna element not in active operation (i.e., not transmitting or receiving signals). For example, the switch  1140   a  connects the antenna element  110  to the PA  1110  when the antenna element  110  is transmitting a signal; at the same time the switch  1140   b  connects the antenna element  120 , which is not transmitting or receiving, to the load  1130 . Thus, when a low-band antenna element is in a transmit or receive state, the high-band antenna element is connected to the load  1130 , and vice versa. Connecting the non-active antenna element to an optimized load (e.g., open or short) can reduce interference and improve the antenna gain of the active antenna element. In one embodiment, the three-terminal switches  1140   a ,  1140   b  and the optimized load  1130  may be used in addition to the aforementioned different antenna types and different feeder shapes in connection with the first antenna element  110  and the antenna element  120 . 
       FIG. 12  is a schematic diagram of two antenna elements using a filter (e.g., a low pass filter) to enhance signal isolation according to one embodiment. In this embodiment, a low-band antenna element  1210  operates in a frequency band which is the same or substantially the same as the aforementioned first antenna element  110 . Each of the antenna elements ( 1210  and  120 ) is coupled to a PA or an LNA via a two-terminal switch  1240   a  or  1240   b . In one embodiment, the low-band antenna element  1210  is integrated with a low-pass filter F 1 . In an alternative embodiment, the low-band antenna element  1210  is coupled to a low-pass filter F 2  on the signal path to/from the RF circuitry. The pass-band of the low-pass filters F 1  or F 2  does not overlap with the second frequency band at which the second antenna element  120  operates. Adding a low-pass filter, such as F 1  or F 2 , can improve the isolation between the two antenna elements  1210  and  120  at the expense of an insignificant amount of insertion loss. The high-band antenna element, such as the antenna element  120 , may use an antenna type and a feeder shape different from those of the antenna element  1210 , as mentioned before in connection with the first antenna element  110  and the antenna element  120 . 
       FIG. 13  illustrates an example of a wireless device  1300  according to one embodiment. The wireless device  1300  may include any of the aforementioned antenna assemblies or their variations for transmitting and/or receiving wireless signals. The wireless device  1300  includes processing circuitry  1310 , which may further include one or more of: arithmetic and logic units (ALUs), control circuitry, cache memory, and/or other processing circuitry. Non-limiting examples of the wireless device  1300  include smartphones, smartwatches, tablets, laptops, Internet-of-things (IoT) devices, navigation devices, multimedia devices, and other computing and/or communication devices having wireless communication capabilities. 
     The wireless device  1300  further includes memory and storage circuitry  1320  coupled to the processing circuitry  1310 . The memory and storage circuitry  1320  may include memory devices such as dynamic random access memory (DRAM), static RAM (SRAM), flash memory and other volatile or non-volatile memory devices. The memory and storage circuitry  1320  may further include storage devices, for example, any type of solid-state, magnetic and/or optical storage device. 
     The wireless device  1300  also includes input/output (I/O) circuitry  1330  which may further include user interface devices  1340 , such as one or more of: a display, a speaker, a microphone, a camera, touch sensors, buttons, a keyboard and/or a keypad, etc. The I/O circuitry  1330  further include wireless communication circuitry  1331  for communicating wirelessly with external systems. The wireless communication circuitry  1331  may include radio-frequency (RF) transceiver circuitry  1332  for handling various RF communication bands used in one or more of: WiFi, Bluetooth, cellular, Global Positioning System (GPS), millimeter wave, any short-range and/or long-range networks. In one embodiment, the wireless communication circuitry  1331  includes an antenna assembly  1333  coupled to the RF transceiver circuitry  1332 . The antenna assembly  1333  may include the aforementioned antenna elements, antenna subarrays and/or their variations; e.g., the end-fire antenna elements and the end-fire antenna subarrays shown and/or described with reference to  FIGS. 1-12 . 
     In one embodiment, the RF transceiver circuitry  1332  is disposed on a ground plane (not shown) which is parallel to the X-Y plane. The antenna assembly  1333  radiates in two or more frequency bands in the end-fire direction; i.e., in a direction parallel to the X-Y plane. In one embodiment, the antenna assembly  1333  may additionally include broad-side antenna elements and/or antenna subarrays radiating in two or more frequency bands in a direction perpendicular to the X-Y plane. 
     While the invention has been described in terms of several embodiments, those skilled in the art will recognize that the invention is not limited to the embodiments described, and can be practiced with modification and alteration within the spirit and scope of the appended claims. The description is thus to be regarded as illustrative instead of limiting.