Note: Descriptions are shown in the official language in which they were submitted.
CA 02775410 2012-04-26
ANTENNA ASSEMBLY UTILIZING METAL-DIELECTRIC STRUCTURES
Background
[00011 The present disclosure relates generally to antennas for portable,
handheld
communication devices, and more particularly to designing an antenna for
operation
at specific radio frequencies.
[00021 Different types of wireless mobile communication devices, such as
personal
digital assistants, cellular telephones, and wireless two-way email
communication
equipment, cellular smart-phones, wirelessly enabled notebook computers, are
available. Many of these devices are intended to be easily carried on the
person of a
user, often compact enough to fit in a shirt or coat pocket.
[00031 As the use of wireless communication equipment continues to increase
dramatically, a need exists for increased system capacity. One technique for
improving
the capacity is to provide uncorrelated propagation paths using Multiple
Input, Multiple
Output (MIMO) systems. A MIMO system employs a number of separate independent
signal paths, for example by means of several transmitting and receiving
antennas.
[00041 MIMO systems, employing multiple antennas at both the transmitter and
receiver offer increased capacity and enhanced performance for communication
systems without the need for increased transmission power or bandwidth. The
limited
space in the enclosure of the mobile communication device, however presents
several
challenges when designing such multiple antennas assemblies. An antenna should
be
compact to occupy minimal space and its location is critical to minimize
performance
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CA 02775410 2012-04-26
degradation due to electromagnetic interference. Bandwidth is another
consideration
that the antenna designers face in multiple antenna systems.
[0005] The size of the antenna is dictated by the radio frequency or band of
frequencies at which the antenna is intended to resonate and operate
Typically, the
physical length of the antenna is a fraction of the wavelength of the
operating
frequency, for example one-fourth or one-half the wavelength of the radio
frequency
signal, thus enabling the antenna to resonate at the respective operating
frequency.
The required physical size for the antenna, to resonate at a certain
frequency, is
known as the resonant length. For example, an antenna which requires a length
equal
to quarter of the wavelength of the resonance frequency is known to have a
resonant
length of a quarter of a wavelength. This size requirement limits how small
the
antenna can be constructed and thus the amount of space in the housing of the
mobile
communication device that is occupied by the antenna.
[0006] Nevertheless, it is desirable to further reduce the size of the antenna
so it
can be fit in the small space designated for the antenna in the communication
device, especially when the communication device has multiple antennas.
Brief Description of the Drawings
[0007] FIGURE 1 is a schematic block diagram of a mobile, wireless
communication device that incorporates the present antenna assembly;
[0008] FIGURE 2 is pictorial view of a printed circuit board on which a first
version
of a multiple antenna assembly is formed;
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CA 02775410 2012-04-26
10009] FIGURE 3 is an enlarged view a portion of one side of a printed circuit
board
in Figure 2;
[00101 FIGURE 4 is an enlarged view of a portion of the opposite side of a
printed
circuit board showing an alternative arrangement of metal-dielectric
structures;
[00111 FIGURE 5 is a detailed view of one metal-dielectric structure in Figure
3;
[00121 FIGURE 6 depicts one of the metal-dielectric structures in Figure 4;
[00131 FIGURE 7 illustrates a first alternative embodiment of a metal-
dielectric
structure;
[0014] FIGURE 8 illustrates a second alternative embodiment of a metal-
dielectric
structure;
[00151 FIGURE 9 is an enlarged partial view of one side of a printed circuit
board
with slot type antennas;
[00161 FIGURE 10 is an enlarged view of a portion of the opposite side of a
printed circuit board showing an alternative arrangement of metal-dielectric
structures
for a slot type antenna; and
[00171 FIGURE 11 is a cross sectional view through a printed circuit board
that
has yet another type of metal-dielectric structures.
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CA 02775410 2012-04-26
Detailed Description
[0018] The present antenna array for communication devices provides a
mechanism
for altering the effective electrical size of an antenna so that the antenna
can have a
smaller physical size and still be tuned to a desired radio frequency. The
exemplary
antenna assembly has two identical radiating elements, which in the
illustrated
embodiments, comprise slot (gap) antennas or inverted-F antennas. It should be
understood, however, that other types of radiating elements can be tuned using
the
techniques and structures described herein. Also, the antenna assembly can
have a
single radiating element or more than two radiating elements.
[0019] The embodiments of the antenna array described herein have a printed
circuit
board (PCB) with a first major surface with an electrically conductive layer
thereon to
form a ground plane At least one antenna is disposed on that first major
surface. For
example, a pair slot antennas are formed by two straight, open-ended slots at
two
opposing edges of that conductive layer. The slots are located along one edge
of the
PCB opposing each other. The dimensions of the slots, their shape and their
location
with respect to the any edge of the PCB can be adjusted to optimize the
resonance
frequency, bandwidth, impedance matching, directivity, and other antenna
performance
parameters. Each antenna in this configuration operates with a relatively wide
bandwidth. Furthermore the slots may be tuned to operate at different
frequencies
using microelectromechanical systems (MEMS), for example by opening or closing
conductive bridges across a slot. The opposite side of the PCB is available
for mounting
other components of the communication device.
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[00201 One or more metal-dielectric structures are formed either in the
conductive
layer on the first major surface of the PCB or on the opposite second major
surface.
Each metal-dielectric structure resonates at a frequency in the bandwidth of
radio
frequency signals to be transmitted or received by the antenna. These metal-
dielectric
structures are placed around and underneath the antenna on the ground plane at
locations
where a high current density exists. Thus the structures are strategically
placed only at
locations where they are effective for tuning the antennas. The placement of
one or
more metal-dielectric structures at such locations adjacent the antenna
enables the
antenna to have a smaller physical size than it is required for the antenna to
resonate at
its resonant frequency. In particular, these structures can allow the antenna
to be
physically smaller than its resonant length at a particular frequency, and
still efficiently
transmit or receive radio signals at that frequency.
[00211 When the antenna can be tuned to different operating frequencies, a
mechanism for corresponding tuning the metal-dielectric structures also is
provided.
[00221 Examples of specific implementations of the present antenna assembly
now
will be provided. For simplicity and clarity of illustration, reference
numerals may be
repeated among the figures to indicate corresponding or analogous elements. In
addition, numerous specific details are set forth in order to provide a
thorough
understanding of the embodiments described herein. The embodiments described
herein may be practiced without these specific details. In other instances,
well-known
methods, procedures and components have not been described in detail so as not
to
obscure the embodiments described herein. Also, the description is not to be
considered as limited to the scope of the embodiments described herein.
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CA 02775410 2012-04-26
[0023] Referring initially to Figure 1, a mobile, wireless communication
device
10, such as a cellular telephone, illustratively includes a housing 20 that
may be a
static housing or a flip or sliding housing as used in many cellular
telephones.
Nevertheless, other housing configurations also may be used. A battery 23 is
carried
within the housing 20 for supplying power to the internal components.
[0024] The housing 20 contains a main printed circuit board (PCB) 22 on which
the
primary circuitry 24 for the wireless communication device 10 is mounted. That
primary
circuitry 24, typically includes a microprocessor, one or more memory devices,
along
with a display and a keyboard that provide a user interface for controlling
the
communication device.
[0025] An audio input transducer, such as a microphone 25, and an audio output
transducer, such as a speaker 26, function as an audio interface to the user
and are
connected to the primary circuitry 24.
[0026] Communication functions are performed through a radio frequency
transceiver 28 which includes a wireless signal receiver and a wireless signal
transmitter that are connected to a MIMO antenna assembly 21. The antenna
assembly
21 may be carried within the upper portion of the housing 20 and will be
described in
greater detail herein.
[0027] The mobile wireless, device 10 also may comprise one or more auxiliary
input/output (1/0) devices 27, such as for example, a WLAN (e.g., Bluetooth"',
IEEE.
802.11) antenna and circuits for WLAN communication capabilities, and/or a
satellite
positioning system (e.g., GPS, Galileo, etc.) receiver and antenna to provide
position
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CA 02775410 2012-04-26
locating capabilities, as will be appreciated by those skilled in the art.
Other examples
of auxiliary I/O devices 27 include a second audio output transducer (e.g., a
speaker
for speakerphone operation), and a camera lens for providing digital camera
capabilities, an electrical device connector (e.g., USB, headphone, secure
digital (SD)
or memory card, etc.).
[00281 Figure 2 illustrates an exemplary a first antenna assembly 30 that can
be
used as the MIMO antenna assembly 21. The first antenna assembly 30 is formed
on a
printed circuit board 32 that has a non-conductive substrate 31 of a
dielectric material
with a first major surface on which an electrically conductive layer 34 is
applied to
form a ground plane 35. The substrate 31 and likewise the conductive layer 34
have a
first edge 36 and second and third edges 37 and 38 that are orthogonal to the
first edge.
First and second antennas 41 and 42 are located along the first edge 36 and
extend
inwardly from the opposite second and third edges 37 and 38.
[00291 Each antenna 41 and 42 is an inverted-F type formed by a radiating
element 44 that is parallel to and spaced from the conductive layer 34. A
shorting
element 46 is connected between the inner end of the radiating element 44 and
the
conductive layer 34. A signal feed pin 48 extends from a central area of the
radiating element 44 through an aperture in the printed circuit board 32 and
is
connected to the radio frequency transceiver 28. The first and second antennas
41
and 42 oppose each other across a width of the ground plane 35 and may have
substantially identical shapes.
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CA 02775410 2012-04-26
[0030] Although the present apparatus is being described in the context of an
assembly of two antennas, it should be appreciated that the assembly can have
a single
antenna or a greater number of antennas.
[0031] With additional reference to Figure 3, a separate set of four identical
metal-dielectric structures 51, 52, 53 and 54 are located on the ground plane
35
adjacent the signal feed pin 48 of each of the first and second antennas 41
and 42. In
the exemplary illustrated arrangement the four identical metal-dielectric
structures
51-54 are located around the feed pin 48 at least partially underneath the
associated
radiating element 44.
[0032] Each metal-dielectric structure 51-54 is placed at a location on the
ground
plane 35 that has a high current density as determined from the emission
pattern of the
two antennas 41 and 42. Those locations in the ground plane are places having
the
maximum current density level or a current density that is at least some
percentage of
the maximum current density level, such as at least eighty percent. Note that
locating the
metal-dielectric structures 51-54 based on this criterion does not necessarily
form a
periodic array, i.e., the spacing between adjacent pairs of the metal-
dielectric structures
is not identical. It should be understood that the number and location of
these metal-
dielectric structures 51-54 in the drawings is for illustrative purposes and
may not
denote the actual number and locations for a given antenna assembly design.
[0033] As shown in detail in Figure 5, the metal-dielectric structures 51-54
in the
embodiment of Figure 2 comprise a frequency selective surface formed by two
concentric rings 55 and 56 formed by annular slots which extend entirely
through the
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CA 02775410 2012-04-26
conductive layer 34 that defines the ground plane 35. Each ring 55 and 56 is
not
continuous, but has a gap 57 or 58 in the respective slot which gap is created
by a
portion of the conductive layer 34. The gap 57 in the slot of the inner ring
55 is
oriented 180 from the gap 58 in the slot of the outer ring 56. In other
words, the gap is
on a side of one ring that is opposite to a side of the other ring on the
other gap is
located.
[0034] The metal-dielectric structure 51-54 can be modeled as an inductor-
capacitor
network that forms tuned circuit which provides a frequency selective surface.
The
metal-dielectric structures are designed to have a specific frequency stop
band that
reflects radio frequency signals or prohibits the transmission of signals at
that frequency
band. The maximum dimensions of each structure may be about one-tenth of the
free
space wavelength of the operating frequency of the antenna. If each of the
first and
second antennas 41 and 42 function at a single frequency, i.e. not be
dynamically
tunable, then the metal-dielectric structures can have a fixed stop band that
includes the
radio frequencies of the signals to be transmitted and received by the
adjacent antenna
41 or 42.
[0035] The placement of one or more metal-dielectric resonant structures at
such
locations adjacent the antenna enables the antenna to have a physical size
that is not its
resonant length at the operating frequency of the signal applied by the radio
frequency
transceiver 28. In some embodiments, these structures enable the antenna to be
physically shorter than the resonant length and still efficiently transmit or
receive the
radio frequency signal. The metal-dielectric structures, however, alter the
resonant
frequency of the antenna so that the antenna has an effective electrical
length which is
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CA 02775410 2012-04-26
longer than the physical length and thus is tuned to the wavelength of the RF
signal
from the radio frequency transceiver 28. In other words, although the physical
size of
the antenna that is much smaller than its resonant length, interaction with
the metal-
dielectric structures 51-54 causes the antenna to function as through its
physical size is
equal to its resonant length at the operating frequency.
[00361 If the first and second antennas 41 and 42 are intended to transmit and
receive signal at different radio frequencies, then the metal-dielectric
structures can
be dynamically tunable so that the structures still alter the resonant
frequency of the
adjacent antenna. One way of accomplishing that dynamic tuning or
configuration
of an antenna is to place one or more switches 59 at selected locations across
one of
both of the slots of the metal-dielectric structure. Each switch 59, for
example, may
be a microelectromechanical system (MEMS) that is controlled by a signal from
the
tuning control 29. When closed, the respective switch 59 provides an
electrical path
between the across the slot thereby altering the electrical length of the ring
55 or 56.
Such alteration changes the resonant frequency of the metal-dielectric
structure and
thus also the frequency to which the associated antenna is tuned.
[00371 Figure 4 illustrates an alternative placement of the metal-dielectric
structures
for the antennas 41 and 42 in Figure 2. Instead of placing the sets of metal-
dielectric
structures 51-54 on the ground plane near the antennas, a set of metal-
dielectric
structures 61, 62, 63 and 64 is located on the opposite second major surface
40 of the
printed circuit board 32. Thus the metal-dielectric structures 61-64 are
formed on a non-
conductive surface of the substrate 31 underneath the first and second
antennas 41 and
42. As with the placement of the structures 51 and 54, each of these metal-
dielectric
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CA 02775410 2012-04-26
structures 61-64 is located at a position where the current density in the
substrate 31, as
determined from the antenna emission pattern, is greater than a given
threshold level.
[00381 As shown in detail in Figure 6, each metal-dielectric structure 61-64
is formed
by a frequency selective surface structure having a pair of concentric rings
83 and 84 of
metal that is deposited on that second major surface 40. The inner ring 83 has
a gap 85
that is diametrically opposite to the gap 86 in the outer metal ring 84.
several switches
87 are placed between the two rings 83 and 84 of the metal-dielectric
structure at
selected radial locations. Each switch 87 may be a microelectromechanical
system
(MEMS), for example, that is controlled by a signal from the tuning control
29. When
closed, a respective switch 87 provides an electrical path between the inner
and outer
rings 83 and 84. A tuning circuit 89 can be connected across the gap of one of
the two
rings instead of using the switches 87.
[00391 Although the metal-dielectric structures 51-54 and 61-64 in Figures 2-4
are
implemented utilizing circular ring resonators, other types of resonant cells
may be
employed. For example as shown in Figure 7, an alternative metal-dielectric
structure
90 has inner and outer rectilinear, e.g. square, rings 94 and 92. If these
rings are on the
second major surface of the substrate, that is opposite from the ground plane
conductive
layer, the rings are formed by metal strips, whereas the rings are slots when
located on
the ground plane conductive layer. Each rectilinear ring 92 and 94 has a gap
96 and 98,
respectively, with the gap on one ring being on the opposite side from the gap
on the
other ring. Another type of metal-dielectric structure is formed by a single
slotted ring
similar to outer ring 56 in Figure 5, outer ring 84 in Figure 6, or ring 94 in
Figure 7.
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[00401 Figure 8 denotes another configuration of a metal-dielectric structure
100 that
can be used as a resonant tuning cell. This structure 100 is an
electromagnetic band gap
device that has a square ring 102 that is continuous and does not have a gap.
Within the
square ring 102 is an interior element 104 having a shape of a Jerusalem
cross.
Specifically the interior element has four T-shaped members 105, 106, 107 and
108,
each having a cross section extending parallel to and spaced from one side of
the square
ring 102. Each T-shaped member 105-108 has a tie section that extends from the
respective cross section to the center of the square ring 102 at which point
all the
T-shaped members are electrically connected. Switches can be connected at
various
locations between the T-shaped members 105, 106, 107 and 108 and the square
ring
102 to dynamically tune the resonate frequency of the metal-dielectric
structure 100.
[00411 Figure 9 depicts a second antenna assembly 110 in which the first and
second
antennas 120 and 121 have radiating elements formed by slots 122 and 123,
respectively, in a ground plane 117. The physical length of each slot 122 and
123 is not
equal to the resonant length of the antennas 122 and 123, which the resonant
length is
one-fourth the wavelength of the radio frequency signal that is applied to the
antennas
by the radio frequency transceiver 28 operating in a transmitting mode. For
example,
the physical length of each slots 122 and 123 may be least than one-fourth
that
wavelength. In this embodiment, a printed circuit board 11 I that has a non-
conductive
substrate 112 with three adjacent edges 113, 114 and 115. A conductive layer
116
forms the ground plane 117 on a first major surface of the substrate 112. The
first and
second open-ended slots 122 and 123 extend through the conductive layer 116
beginning at the opposite edges 114 and 115. The slots have interior closed
ends that
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CA 02775410 2012-04-26
are spaced apart by a portion of the conductive layer 116. Each antenna 120 or
121 has
a separate signal port 124 or 125 to which a radio frequency signal from the
radio
frequency transceiver 28 is applied to excite the respective antenna.
[0042] A plurality, in this instance four, metal-dielectric structures 126,
127, 128 and
129 are located around each antenna slot 122 and 123. Each of these metal-
dielectric
structures 126-129 is formed by a pair of concentric rings and has the same
formation
as the metal-dielectric structure shown in Figure 5.
100431 Without the metal-dielectric structures 126-129, the physical length of
each
antenna slot 122 and 123 typically would be one-quarter of the wavelength of
the radio
frequency signal for which the antenna is desired to operate. The metal-
dielectric
structures, however enable the length of each antenna slot 122 and 123 to be
substantially less than one-quarter of the wavelength, e.g. 60% of one-quarter
of the
wavelength.
[00441 Alternatively, instead of placing the metal-dielectric structures on
the ground
plane 117, sets of metal-dielectric structures 131, 132 and 133 are formed on
the
opposite second major surface 118 of the printed circuit board 111 as
illustrated in
Figure 10. These metal-dielectric structures 131-133 may be located directly
beneath
the slots 122 and 123 of the first and second antennas 120 and 121. In this
instance,
each metal-dielectric structure 131-133 is formed by a pair of concentric
rings of metal
with the same configuration as shown in Figure 6. Nevertheless, the metal-
dielectric
structures in Figures 7 and 8 may be used instead. As noted previously single
slotted
ring metal-dielectric structures also can be used.
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CA 02775410 2012-04-26
[0045] The metal-dielectric structures 131-133, however, do not have the
switches
between the concentric rings and employ a different tuning mechanism. The
metal-
dielectric structures 131-133 are formed on a layer 134 of a liquid crystal
polymer
that is deposited upon the opposite major surface 118 of the printed circuit
board
substrate 112. In this embodiment, the concentric rings form the metal portion
of
each metal-dielectric structure 131-133 with the substrate 112 and the liquid
crystal
polymer layer 134 forming the dielectric component of the structure. Liquid
crystal
polymers have a dielectric characteristic that changes in response to
variation of a DC
voltage applied thereto. Therefore, when the radio frequency transceiver 28
applies a
signal with a different radio frequency to the first or second antenna 120 or
121, a
control signal is sent to the tuning control 29 which responds by which
applying a DC
voltage that biases the liquid crystal polymer layer 134 with respect to the
ground
plane 117. This biasing alters the dielectric characteristic of the metal-
dielectric
structures 131-133 and their stop band frequencies, thereby changing the
electrical
size and the resonant frequency of the first and second antennas 120 and 121.
As
illustrated a single liquid crystal polymer layer 134 extends beneath the
metal-dielectric structures 131-133 for both antennas. Alternatively, a
separate liquid
crystal polymer layer can be placed under the set of metal-dielectric
structures for
each antenna or a separate liquid crystal polymer layer can be formed under
each
individual metal-dielectric structure.
[00461 In both embodiments depicted in Figures 9 and 10, the metal-dielectric
structures 126-129 and 131-133 enable the adjacent antenna slot 122 or 123 to
have a
physical length that is not one-fourth the wavelength of the radio frequency
signals
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CA 02775410 2012-04-26
applied by the radio frequency transceiver 28. In some instances, those
structures
enable the antenna to be physically shorter than one-fourth that wavelength
and still
efficiently transmit or receive the radio frequency signal. The metal-
dielectric
structures, however, alter the electrical length and thus the resonant
frequency of the
antenna so that the antenna has an effective electrical length that is longer
than the
physical length. Thus the antenna is tuned to the wavelength of the RF signal
from
the radio frequency transceiver 28.
[0047] Figure 11 illustrates another embodiment of an antenna assembly 150
that incorporates a further type of metal-dielectric structures 152. This
antenna
assembly 150 includes first and second inverted F type antennas 154 and 156
mounted on a printed circuit board 160. The printed circuit board 160
comprises a
substrate 162 of dielectric material with a first major surface that has a
layer 164 of
electrically conductive material thereon, thereby forming a ground plane.
[0048] The first and second antennas 154 and 156 are disposed on the same
surface of the substrate 162 as the electrically conductive layer 164. Each
antenna
has a first leg 153 parallel to and spaced from the conductive layer 164. A
second
leg 155, that forms a shorting pin, is connected between the conductive layer
and the
first leg 153. Each antenna 154 and 156 has a third leg 157, forming a feed
connection, to which a radio frequency signal is applied by the transceiver 28
to
excite the respective antenna. The length of the antenna 154 or 156 is the
combined
lengths of the radiating element 153 summed with length (or height) of the
first leg
155.
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[0049] One or more metal-dielectric tuning structures 152 are provided that
enable the length of the first and second antennas 154 and 156 to be less than
one-
fourth the wavelength of the radio frequency signals transmitted or received
by the
antenna, which is the resonant length of the antenna. Each of these metal-
dielectric
tuning structures 152 is a "mushroom" type electromagnetic band gap device
comprising a patch style metal pattern 168 formed on the opposite surface 166
of the
printed circuit board from the antennas 154 and 156. The metal pattern
alternatively may be one of the resonant cells previously described herein,
however
in this instance the metal pattern 168 is connected to a via 170.
[0050] The metal-dielectric structure 152 is dynamically tuned to alter the
electrical
length and the resonant frequency of the associated antenna 154 or 156. That
dynamically tuning is accomplished by the tuning control 29 operating a switch
171,
such as a MEMS, for example, that selectively connects the via 170 to the
electrically
conductive layer 164.
[0051] It should be appreciated that more than one such metal-dielectric
structures
152 can be employed in this antenna assembly, depending upon the locations of
high
current density regions around and underneath the two antennas 154 and 156.
[0052] The foregoing description was primarily directed to a certain
embodiments of
the antenna. Although some attention was given to various alternatives, it is
anticipated
that one skilled in the art will likely realize additional alternatives that
are now apparent
from the disclosure of these embodiments. Accordingly, the scope of the
coverage
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CA 02775410 2012-04-26
should be determined from the following claims and not limited by the above
disclosure.
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