Multifrequency magnetic dipole antenna and methods for re-using the volume of an antenna

<u style="single">BACKGROUND OF THE INVENTION</u>

1. CROSS REFERENCE TO RELATED APPLICATIONS

This application relates to co-pending application Serial No. <patcit id="pcit0001" dnum="US09901134B"><text>09/901,134</text></patcit>, entitled "Multimode Grounded Multifinger Patch Antenna" by Gregory Poilasne et al., owned by the assignee of this application and incorporated herein by reference.

This application also relates to co-pending application Serial No. <patcit id="pcit0002" dnum="US09781779B"><text>09/781,779</text></patcit>, entitled "Spiral Sheet Antenna Structure and Method" by Eli Yablonovitch et al., owned by the assignee of this application and incorporated herein by reference.

2. FIELD OF THE INVENTION

The present invention relates generally to the field of wireless communications, and particularly to the design of an antenna.

3. BACKGROUND

Small antennas are required for portable wireless communications. With classical antenna structures, a certain physical volume is required to produce a resonant antenna structure at a particular radio frequency and with a particular bandwidth. A fairly large volume is required if a large bandwidth is desired. Accordingly, the present invention addresses the needs of small compact antenna with wide bandwidth.

<u style="single">SUMMARY OF THE INVENTION</u>

The present invention provides a multiresonant antenna structure in which the various resonant modes share at least portions of the structure volume. The frequencies of the resonant modes are placed close enough to achieve the desired overall bandwidth. Various embodiments are disclosed. The basic antenna element comprises a ground plane; a first conductor extending longitudinally parallel to the ground plane having a first end electrically connected to the ground plane and a second end; a second conductor extending longitudinally parallel to the ground plane having a first end electrically connected to the ground plane and a second end spaced apart from the second end of the first conductor; and an antenna feed coupled to the first conductor. Additional elements are coupled to the basic element, such as by stacking, nesting or juxtaposition in an array. In this way, individual antenna structures share common elements and volumes, thereby increasing the ratio of relative bandwidth to volume.

<u style="single">BRIEF DESCRIPTION OF THE DRAWINGS</u>

<ul id="ul0001" list-style="none"><li><figref idref="f0001">Figure 1</figref> conceptually illustrates the antenna designs of the present invention.</li><li><figref idref="f0001">Figure 2</figref> illustrates the increased overall bandwidth achieved with a multiresonant antenna design.</li><li><figref idref="f0002">Figure 3</figref> is an equivalent circuit for a radiating structure.</li><li><figref idref="f0002">Figure 4</figref> is an equivalent circuit for a multiresonant antenna structure.</li><li><figref idref="f0002">Figure 5</figref> is a perspective view of a basic radiating structure.</li><li><figref idref="f0003">Figure 6</figref> is a perspective view of an alternative basic radiating structure.</li><li><figref idref="f0003">Figure 7</figref> is a top plan view of one embodiment of a multiresonant antenna structure.</li><li><figref idref="f0004">Figure 8</figref> is a perspective view of the antenna structure of <figref idref="f0003">Figure 7</figref>.</li><li><figref idref="f0005">Figure 9a</figref> is a perspective view of another embodiment of a multiresonant antenna structure.</li><li><figref idref="f0005">Figure 9b</figref> is a perspective view of a further embodiment of a multiresonant antenna structure.</li><li><figref idref="f0005">Figure 10</figref> is a perspective view of still another embodiment of a multiresonant antenna structure.</li><li><figref idref="f0006">Figure 11</figref> is a perspective view of yet another embodiment of a multiresonant antenna structure.</li><li><figref idref="f0007">Figure 12</figref> is a perspective view of another embodiment of a multiresonant antenna structure.</li><li><figref idref="f0007">Figure 13</figref> is a perspective view of another embodiment of a multiresonant antenna structure.</li><li><figref idref="f0008">Figure 14</figref> is a perspective view of another embodiment of a multiresonant antenna structure.</li><li><figref idref="f0008">Figures 15a-b</figref> are top plan and side views, respectively, of another embodiment of a multiresonant antenna structure.</li><li><figref idref="f0009">Figure 16</figref> diagrammatically illustrates a multiresonant antenna structure with parasitic elements.</li><li><figref idref="f0010">Figure 17</figref> is a Smith chart illustrating a non-optimized multiresonant antenna.</li><li><figref idref="f0010">Figure 18</figref> is a Smith chart illustrating an optimized multiresonant antenna.</li><li><figref idref="f0009">Figure 19</figref> is a side view of one of the elements of the antenna structure of <figref idref="f0009">Figure 16</figref>.</li><li><figref idref="f0009">Figure 20</figref> illustrates optimization of the coupling of the elements of the antenna structure of <figref idref="f0009">Figure 16</figref>.</li><li><figref idref="f0011">Figure 21</figref> illustrates optimization of the feed point of a driven element of the antenna structure of <figref idref="f0009">Figure 16</figref>.</li><li><figref idref="f0011">Figure 22</figref> illustrates an antenna structure with a two-dimensional array of radiating elements.</li><li><figref idref="f0011">Figures 23a-23d</figref> illustrate alternative antenna structures with two-dimensional arrays of radiating elements.</li><li><figref idref="f0012">Figure 24</figref> illustrates a physical embodiment of a radiating element for the antenna structures of <figref idref="f0011">Figures 22-23</figref>.</li><li><figref idref="f0012">Figures 25a and 25b</figref> illustrate alternative physical embodiments of radiating elements for the antenna structures of <figref idref="f0011">Figures 22-23</figref>.</li><li><figref idref="f0012">Figure 26</figref> illustrates a parasitic antenna element having a spiral configuration.</li></ul>

<u style="single">DETAILED DESCRIPTION OF THE INVENTION</u>

In the following description, for purposes of explanation and not limitation, specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be apparent to one skilled in the art that the present invention may be practiced in other embodiments that depart from these specific details. In other instances, detailed descriptions of well-known methods and devices are omitted so as to not obscure the description of the present invention with unnecessary detail.

The volume to bandwidth ratio is one of the most important constraints in modem antenna design. One approach to increasing this ratio is to re-use the volume for different orthogonal modes. Some designs, such as the Grounded Multifinger Patch disclosed in patent application Serial No. <patcit id="pcit0003" dnum="US09901134B"><text>09/901,134</text></patcit>, already use this approach, even though the designs do not optimize the volume to bandwidth ratio. In the previously mentioned patent application, two modes are generated using the same physical structure, although the modes do not use exactly the same volume. The current repartition of the two modes is different, but both modes nevertheless use a common portion of the available volume. This concept of utilizing the physical volume of the antenna for a plurality of antenna modes is illustrated generally in <figref idref="f0001">Figure 1</figref>. V is the physical volume of the antenna, which has two radiating modes. The physical volume associated with the first mode is designated V1, whereas that associated with the second mode is designated V2. It can be seen that a portion of the physical volume, designated V12, is common to both of the modes.

We will express the concept of volume reuse and its frequency dependence with what we refer to as a "K law". The common general K law is defined by the following: <maths id="math0001" num=""><math display="block"><mi mathvariant="normal">Δf</mi><mo mathvariant="normal">/</mo><mi mathvariant="normal">f</mi><mo mathvariant="normal">=</mo><mi mathvariant="normal">K</mi><mo mathvariant="normal">•</mo><mi mathvariant="normal">V</mi><mo mathvariant="normal">/</mo><msup><mi mathvariant="normal">λ</mi><mn mathvariant="normal">3</mn></msup></math><img file="EP1959518A2_D0001.tif" /></maths>

Δf/f is the normalized frequency bandwidth. λ is the wavelength. The term V represents the volume that will enclose the antenna. This volume so far has been a metric and no discussion has been made on the real definition of this volume and the relation to the K factor.

In order to have a better understanding of the K law, different K factors are defined:

K_modal is defined by the mode volume V_i and the corresponding mode bandwidth: <maths id="math0002" num=""><math display="block"><msub><mi mathvariant="normal">Δf</mi><mi mathvariant="normal">i</mi></msub><mo mathvariant="normal">/</mo><msub><mi mathvariant="normal">f</mi><mi mathvariant="normal">i</mi></msub><mo mathvariant="normal">=</mo><msub><mi mathvariant="normal">K</mi><mi>modal</mi></msub><mo mathvariant="normal">•</mo><msub><mi mathvariant="normal">V</mi><mi mathvariant="normal">i</mi></msub><mo mathvariant="normal">/</mo><msubsup><mi mathvariant="normal">λ</mi><mi>i</mi><mn>3</mn></msubsup></math><img file="EP1959518A2_D0002.tif" /></maths> where i is the mode index.

K_modal is thus a constant related to the volume occupied by one electromagnetic mode.

K_effective is defined by the union of the mode volumes V₁ ∪ V₂ U ..V_i and the cumulative bandwidth. It can be thought of as a cumulative K; <maths id="math0003" num=""><math display="block"><mstyle displaystyle="false"><munder><mo>∑</mo><mi mathvariant="normal">i</mi></munder><mo>⁢</mo><msub><mi mathvariant="normal">Δf</mi><mi mathvariant="normal">i</mi></msub><mo mathvariant="normal">/</mo><msub><mi mathvariant="normal">f</mi><mi mathvariant="normal">i</mi></msub><mo mathvariant="normal">=</mo><msub><mi mathvariant="normal">K</mi><mi>effective</mi></msub><mo mathvariant="normal">•</mo></mstyle><mfenced separators=""><msub><mi mathvariant="normal">V</mi><mn>1</mn></msub><mo>∪</mo><msub><mi mathvariant="normal">V</mi><mn>2</mn></msub><mo>∪</mo><mn>..</mn><msub><mi mathvariant="normal">V</mi><mi mathvariant="normal">i</mi></msub><mo>)</mo><mo mathvariant="normal">/</mo><msubsup><mi mathvariant="normal">λ</mi><mi mathvariant="normal">C</mi><mn>3</mn></msubsup></mfenced></math><img file="EP1959518A2_D0003.tif" /></maths> where λ_c is the wavelength of the central frequency.

K_effective is a constant related to the minimum volume occupied by the different excited modes taking into account the fact that the modes share a part of the volume. The different frequencies f_i must be very close in order to have nearly overlapping bandwidths.

K_physical or K_observed is defined by the structural volume V of the antenna and the overall antenna bandwidth: <maths id="math0004" num=""><math display="block"><mi mathvariant="normal">Δf</mi><mo mathvariant="normal">/</mo><mi mathvariant="normal">f</mi><mo mathvariant="normal">=</mo><msub><mi mathvariant="normal">K</mi><mi>physical</mi></msub><mo mathvariant="normal">•</mo><mi mathvariant="normal">V</mi><mo mathvariant="normal">/</mo><msup><mi mathvariant="normal">λ</mi><mn mathvariant="normal">3</mn></msup></math><img file="EP1959518A2_D0004.tif" /></maths>

K_physical or K_observed is the most important K factor since it takes into account the real physical parameters and the usable bandwidth. K_physical is also referred to as K_observed since it is the only K factor that can be calculated experimentally. In order to have the modes confined within the physical volume of the antenna, K_physical must be lower than K_effective. However these K factors are often nearly equal. The best and ideal case is obtained when K_physical is approximately equal to K_effective and is also approximately equal to the smallest K_modal. It should be noted that confining the modes inside the antenna is important in order to have a well-isolated antenna.

One of the conclusions from the above calculations is that it is important to have the modes share as much volume as possible in order to have the different modes enclosed in the smallest volume possible.

For a plurality of radiating modes i, <figref idref="f0001">Figure 2</figref> shows the observed return loss of a multiresonant structure. Different successive resonances occur at the frequencies f₁,f₂, f_i,...f_n. These peaks correspond to the different electromagnetic modes excited inside the structure. <figref idref="f0001">Figure 2</figref> illustrates the relationship between the physical or observed K and the bandwidth over f₁ to f_n.

For a particular radiating mode with a resonant frequency at f₁, we can consider the equivalent simplified circuit L₁C₁ shown in <figref idref="f0002">Figure 3</figref>. By neglecting the resistance in the equivalent circuit, the bandwidth of the antenna is simply a function of the radiation resistance. The circuit of <figref idref="f0002">Figure 3</figref> can be repeated to produce an equivalent circuit for a plurality of resonant frequencies.

<figref idref="f0002">Figure 4</figref> illustrates a multiresonant antenna represented by a plurality of LC circuits. At the frequency f₁ only the circuit L₁C₁ is resonating. Physically, one part of the antenna structure resonates at each frequency within the covered spectrum. Again, neglecting real resistance of the structure, the bandwidth of each mode is a function of the radiation resistance.

As discussed above, in order to optimize the K factor, the antenna volume must be reused for the different resonant modes. One example of a multimode antenna utilizes a capacitively loaded microstrip type of antenna as the basic radiating structure. Modifications of this basic structure will be subsequently described. In all of the described examples, the elements of the multimode antenna structures have closely spaced resonant frequencies.

<figref idref="f0002">Figure 5</figref> illustrates a single-mode capacitively loaded microstrip antenna. If we assume that the structure in <figref idref="f0002">Figure 5</figref> can be modeled as a L₁C₁ circuit, then C₁ corresponds to a fringing capacitance across gap g. Inductance L₁ is mainly contributed by the loop designated by the numeral 2. Another configuration of a capacitively loaded microstrip antenna is illustrated in <figref idref="f0003">Figure 6</figref>. The capacitance in this case is a facing capacitance at the overlap designated by the numeral 3.

A top plan view of a tri-mode antenna structure is shown in <figref idref="f0003">Figure 7</figref>. This structure comprises three sections corresponding to three different frequencies. The feed is placed in area 7, which is similar to the feed arrangement used for the antennas of <figref idref="f0002">Figure 5</figref> and <figref idref="f0003">Figure 6</figref>. This structure has three sets of fingers, 4/5, 8/9, and 10/11, configured similarly to the antenna of <figref idref="f0002">Figure 5</figref>. The different inductances are defined by the lengths of fingers 4, 5, 8, 9, 10 and 11. The different capacitances are defined by the gaps 6, 12 and 14.

<figref idref="f0004">Figure 8</figref> is a perspective view of the antenna structure shown in <figref idref="f0003">Figure 7</figref>. In this configuration, there is a separate capacitance and inductance for each of the frequencies. The different L_i and C_i are set in order to have closely spaced frequencies f_i. The slots S₁ and S₂ isolate the different parts of the antenna and therefore separate the frequencies of the antenna. This case shows that it is possible to partially reuse the volume of the antenna structure since the area 7 associated with the feed is common to all of the modes. However, some portions of the volume are dedicated to only one of the frequencies.

Another solution for the reuse of the structure volume is depicted in <figref idref="f0005">Figures 9a and 9b. Figure 9a</figref> is a variation of the basic structure shown in <figref idref="f0002">Figure 5</figref>, whereas <figref idref="f0005">Figure 9b</figref> is a variation of the basic structure shown in <figref idref="f0003">figure 6</figref>. In each case, slits 15 are placed near the sides of the antenna, along its length. The slits create a resonant structure at one frequency, but are electromagnetically transparent at a second characteristic frequency of the structure. The spacing of the resonant frequencies of the structure is mainly controlled by the dimensions 16, 17, 18 and 19. In both <figref idref="f0005">figures 9a and 9b</figref>, two different antennas can be visualized - one by removing the material in the slits 15, which resonates at a first frequency, and the other by filling in the slits, which resonates at a second frequency. These two antennas in one clearly share the same volume.

An embodiment of a multifrequency antenna structure composed of overlapping structures is shown in <figref idref="f0005">Figure 10</figref>. A plate 20 connected to another plate 21 is placed over a structure S like that shown in <figref idref="f0003">Figure 6</figref>. The underlying structure S defines a capacitance C₁ and an inductance L₁ and is resonant at a frequency f₁. The plate 20 is placed at a distance 23 from one edge. The plate 21 is placed at a distance 22 from the underlying structure, which defines a second capacitance C₂. A second frequency f₂ is characterized by the inductance L₂ of loop 24 and the capacitance C₂ associated with gap 22 (the size of which is exaggerated in the figure). By optimizing C₁, C₂, L₁ and L₂ it is possible to achieve a set of two close frequencies that will indeed increase the K factor while reusing the same volume. In this case the volume V₁ is included within the volume V₂. It should be noted that f₂ is not necessarily lower than f₁.

<figref idref="f0006">Figure 11</figref> illustrates an extension of the structure shown <figref idref="f0005">Figure 10</figref> in which several plates 20-21, 29-30, 31 and 32 have been superposed on an underlying structure S to create a plurality of loops 25, 26, 27, 28. Each of these loops is associated with a different resonant frequency. This concept can be extended to an arbitrary number of stacked loops.

<figref idref="f0007">Figure 12</figref> illustrates an antenna having a first structure 34 of the type shown in <figref idref="f0002">Figure 5</figref> included within a second such structure 33. The feeding point could be coupled to the end of either plate 35 or plate 36 or along any of the open edges. Here, the volume of one antenna is completely included in the volume of the other.

<figref idref="f0007">Figure 13</figref> illustrates another embodiment in which a plurality of structures share common parts and volumes. In this case, the loops associated with the characteristic inductances of the structures are numbered 37 and 38. This concept can be extended to more than two frequencies. The dimensions of the structures may be adjusted to achieve the desired capacitance values as previously described. It should be noted that the selected dimensions may give rise to parasitic frequencies and that these may be used in adjusting the overall antenna characteristics.

Another approach to making a multiresonant antenna is illustrated in <figref idref="f0008">Figure 14</figref>. Here, multiple antennas are combined in such a way that the coupling is low. The basic antenna element is the same as shown in <figref idref="f0003">Figure 6</figref>: A set of such elements Fp1, Fp2, ...Fpi are stacked upon one another. One part of each Fpi is also a part of Fpi+1 and Fpi-1. The common parts will help to define the related capacitances C_i. The entire structure may have a common feeding point at Fp1 or separate feeding points may be located at Fp2...Fpi.

It is interesting to note that the width of the antenna structure does not have a critical influence on either the resonant frequency or the bandwidth. There is an optimum width for which the bandwidth of the basic element is at a maximum. Beyond this, the bandwidth does not increase as the width is increased.

The limited effect of the antenna width on bandwidth allows consideration of the structure shown in <figref idref="f0008">Figures 15a-b</figref>, which nests the individual antenna elements in both the vertical and horizontal directions. This allows more freedom in organizing the capacitive and inductive loading. This arrangement provides for the total inclusion of the inner antenna elements within the overall antenna volume, each element sharing a common ground. At different frequencies, only one element is resonating.

<figref idref="f0009">Figure 16</figref> illustrates an antenna structure comprising an array of elements, each of the general type shown in <figref idref="f0003">Figure 6</figref>, having a driven element 40 and adjacent parasitic elements 41-43. Impedance matching of this structure is illustrated by the Smith chart shown in <figref idref="f0010">Figure 17</figref>. The large outer loop 50 corresponds to the main driven element 40, whereas the smaller loops 51-53 correspond to the parasitic elements. This is a representation of a non-optimized structure. Various adjustments can be made to the antenna elements to influence the positions of the loops on the Smith chart. The smaller loops may be gathered in the same area in order to obtain a constant impedance within the overall frequency range.

In the case of a typical 50 ohm connection, an optimized structure will have all of the loops gathered approximately in the center of the Smith chart as shown in <figref idref="f0010">Figure 18</figref>. In order to gather the loops in the center of the Smith chart (or wherever it is desired to place them), the dimensions of the individual antenna elements are adjusted, keeping in mind that each loop corresponds to one element.

<figref idref="f0009">Figure 19</figref> illustrates a single element, such as 41, of the antenna structure shown in <figref idref="f0009">Figure 16</figref>. By reducing the dimension 1, the corresponding loop rotates clockwise on the Smith chart. By adjusting the length of the parasitic elements, all of the different loops can be gathered. Then, if necessary, the group of loops can be rotated back in the counter-clockwise direction on the Smith chart by reducing the length of the main driven element.

In order to optimize the bandwidth of the antenna structure, the main loop must have a large enough diameter. With reference to <figref idref="f0009">Figure 20</figref>, the diameter of the main loop is controlled by the amount of coupling between each element and its neighbor, which is determined by the distance dl between the adjacent elements. The amount of coupling is also controlled by the width of the elements. The narrower the elements are, the closer the elements can be in order to keep the same loop diameter. The ultimate size reduction is obtained when each element comprises a single wire. Furthermore, the elements can also be placed closer together by making the gap 45 smaller.

Finally, the main loop may be centered on the Smith chart by adjusting the location of the antenna feed on the main driven element. Referring to <figref idref="f0011">Figure 21</figref>, impedance matching of the antenna structure is optimized by adjusting the dimension If. By increasing If, the diameter of the main loop is increased. In this way, the small loops can be centered at the desired location on the Smith chart.

<figref idref="f0011">Figure 22</figref> illustrates a polarized multi-resonant antenna structure in which polarization diversity is achieved through the use of two interleaved arrays of antenna elements. In the case illustrated, the two arrays are arranged orthogonally to provide orthogonal polarization. The two arrays may be interconnected in various ways or they may be totally separated. It is easiest to have the arrays make contact where they cross, otherwise the manufacturing is more difficult. However it is not necessary that the arrays contact one another, and, in some cases, isolating the array elements from each other can be used for adjusting the impedance matching characteristics of the antenna. In any case, it is always possible to match the antenna by adjusting the various dimensions of the array elements as discussed earlier.

The use of one- or two-dimensional arrays of antenna elements allows the antenna structure to be co-located on a circuit board with other electronic components. The individual array elements can be placed between components mounted on the board. The electronic behavior of the components may be slightly affected by the presence of the radiating elements, but this can be determined through EMC studies and appropriate corrective measures, such as shielding of sensitive components, may be implemented. However, the electronic components will generally not perturb the electromagnetic field and will therefore not change the characteristics of the antenna.

The two-dimensional array shown in <figref idref="f0011">Figure 22</figref> can be extrapolated to other array designs as illustrated in <figref idref="f0011">Figures 23a-d</figref>. The elements of the array can be arranged in various configurations to achieve spatial and/or polarization diversity. Other configurations in addition to those shown in <figref idref="f0011">Figures 23a-d</figref> are possible. In each case, the elements of the array may be interconnected in various ways or may be electrically isolated from one another. In addition, the individual elements may or may not be shorted to ground. All of these design parameters, including those previously discussed, permit the design of an antenna structure having the desired electromagnetic characteristics.

The design of an antenna structure must, of course, take into account manufacturing considerations, the objective being to achieve an antenna with both high efficiency and a low manufacturing cost. In achieving this objective, the problem of loss maybe a big issue. The electric field inside the capacitive part of the antenna is very high. Therefore, no material should be in between the two metallic layers.

A first solution, as illustrated in <figref idref="f0012">Figure 24</figref>, utilizes an antenna element consisting of two wires 60, 61 connected to a ground. The distance between the two wires is very important for frequency tuning. Therefore, it is important to have a spacer that maintains the two wires at a fixed distance. In order to minimize the loss contributed by the presence of the spacer, the spacer should not intrude into the space between the wires. <figref idref="f0012">Figure 24</figref> shows a simple solution configured like a conventional surface mounted resistor. The wires are secured within a plastic hollow cylinder 62 and the protruding wires are then soldered to the ground.

A second solution, as illustrated in <figref idref="f0012">Figures 25a-b</figref>, utilizes an antenna element constructed as a printed circuit. Each element is printed on a very thin, low-loss dielectric substrate in order to achieve good efficiency. The printed circuit element is then placed vertically on the ground. <figref idref="f0012">Figure 25a</figref> shows a simple two-arm element. <figref idref="f0012">Figure 25b</figref> shows a similar two-arm element with the ground printed on the substrate.

The parasitic elements of the antenna array need not be limited to the basic two-wire design shown in <figref idref="f0002">Figures 5</figref> and <figref idref="f0003">6</figref> and in the later described structures based on these elements. Referring to <figref idref="f0012">Figure 26</figref>, the parasitic elements may instead have a spiral configuration. The resonant frequency of the spiral element will be a function of the number of turns. It should be noted that when such a spiral element is coupled to a driven element having the configuration shown in <figref idref="f0002">Figure 5</figref> or <figref idref="f0003">Figure 6</figref>, the capacitive coupling is reduced since the driven element acts as a dipole, whereas the spiral element acts as a quadrupole.

It will be recognized that the above-described invention may be embodied in other specific forms without departing from the spirit or essential characteristics of the disclosure. Thus, it is understood that the invention is not to be limited by the foregoing illustrative details, but rather is to be defined by the appended claims.