Device and method of reducing mutual coupling of two antennas by adding capacitors on ground

Radio frequency antennas sharing a ground plane are largely decoupled using one or more lumped capacitive elements set into holes within the ground plane. The holes, which are precisely placed, can extend to a side of the ground plane. A stub extends from a fringe of the hole either straight or bending in an L shape, and a capacitor connects between an end of the stub and another side of the hole. Capacitive elements can also be supported on raised solder pads above a ground plane or off to one side of the ground plane. Methods for manufacturing the decoupling apparatus are described.

CROSS-REFERENCES TO RELATED APPLICATIONS

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BACKGROUND

1. Field of the Invention

The present application generally relates to means for reducing coupling between antennas. Specifically, the application is related to the placement of lumped capacitive elements upon structures inset within, protruding above, or protruding from a ground plane shared by different antennas in order to reduce mutual coupling between the antennas.

2. Description of the Related Art

Multiple-input and multiple-output, or MIMO, technology has been widely used in today's wireless communication systems, from base stations to Wi-Fi modules and various mobile terminals such as smart phones and tablets. It has become an essential component of industry standards, not only in IEEE 802.11n and LTE 4G, but also in 5G wireless systems. By using multiple antennas, a MIMO system sends and receives more than one data signal stream simultaneously over the same radio channel by utilizing uncorrelated channel paths in a multipath environment.

One of the issues to accommodate the high demands of a high number of antennas on a smart phone is how to reduce the mutual coupling among tightly packed antennas that are attached to a compact circuit board, which is full of densely populated surface mounted electronic components. A common design scenario is that space for antennas is very limited. There are mainly three basic types of antennas commonly seen on wireless terminals: inverted-F antennas (IFAs), monopole antennas, and loop antennas. Mutual coupling is inevitable among antennas regardless of antenna type. It has been well understood that mutual coupling will significantly degrade the data throughput in a MIMO system. The impact of mutual coupling can be attributed by three factors: (1) it lowers the total efficiency of the antenna array as the coupled antenna dissipates the coupled energy; (2) it increases the correlation of different channels and deteriorates the MIMO performance; and (3) it decreases the signal-to-noise (S/N) ratio of each communication channel. Research also reveals that a coupled signal also degrades the linearity of the power amplifier in the victim channels.

There is a need in the art for reducing mutual coupling between antennas in smart phones and other wireless terminals.

BRIEF SUMMARY

Generally, one or more decoupling capacitors are inlaid within, set above, or extended from the perimeter of a ground plane that is shared by antennas. The decoupling capacitors are placed at “acupoints” in the ground plane. Each capacitor acts as a coherent current source that generates an interference signal that is of the same magnitude but opposite phase as the coupled signal at the coupled antenna port, resulting in mutual coupling cancellation. The x, y positions of the acupoints are within 0.2λ of a feeding port or a shorting end of the offending antenna.

Each capacitor extends between a stub, a small protrusion of conductive material, from the ground plane, back to the ground plane. No feature of the stub, or aperture in which it is set, has a dimension longer than 0.1λ, making the stub electrically small or shallow.

Within these parameters the capacitance of the lumped capacitor can be optimized using electromagnetic (EM) simulation software.

Some embodiments of the present invention are related to an antenna decoupling apparatus for antennas that share a ground plane. The apparatus includes a first antenna having an operative wavelength λ, a second antenna, a ground plane connecting the first antenna and the second antenna, the ground plane having an aperture located within 0.2λ of a feeding port or a shorting end of the first antenna, the aperture having no continuous edge longer than 0.1λ, a stub extending from a first edge of the aperture, and a discrete capacitor connecting the stub to a second edge of the aperture.

The aperture can be a reentrant opening extending from a periphery of the ground plane. The ground plane can have a second aperture located within 0.2λ of a feeding port or a shorting end of the second antenna, the second aperture having no continuous edge longer than 0.1λ. The apparatus can further include a second stub extending from a first edge of the second aperture, and a second discrete capacitor connecting the second stub to a second edge of the second aperture. The second aperture can be a reentrant opening extending from a periphery of the ground plane.

Either antenna can be an inverted F antenna (IFA), and the aperture can be located within 0.2λ of a shorting end of the IFA. The stub can be L-shaped and the second edge of the aperture inward, away from a nearest edge of the ground plane.

Either antenna can be a bent monopole antenna, and the aperture can be located within 0.2λ of a feeding port of the bent monopole antenna. The stub can be rectangular and extend parallel with a nearest edge of the ground plane.

Either antenna can be a loop antenna, and the aperture can be located within 0.2λ of a feeding port of the loop antenna. The stub can be L-shaped and the second edge of the aperture be inward, away from a nearest edge of the ground plane.

The first and/or second antenna can be an inverted F antenna (IFA), monopole antenna, or loop antenna. The first antenna or the second antenna can comprise a metal frame of a mobile electronic device. The discrete capacitor can be a variable capacitor. The discrete capacitor can be a surface mount device (SMD) capacitor. Another discrete capacitor can be in parallel with the first capacitor. The apparatus can include a printed circuit board (PCB) dielectric supporting the ground plane, first antenna, and second antenna and filling the aperture. The discrete capacitor can be directly connected to the ground plane at the second edge of the aperture. The discrete capacitor can be connected at an end of the stub.

The first and second antennas can share operating frequency bands or be in two adjacent frequency bands. The first and/or second antenna can operate in a long-term evolution (LTE) band frequency between 2.11 GHz and 2.17 GHz and thus has an operative wavelength λ between 142 mm and 138 mm, or an industrial, scientific, and medical (ISM) frequency between 2.400 GHz and 2.4835 GHz and thus has an operative wavelength λ between 125 mm and 121 mm, or a global positioning system (GPS) L1 frequency at 1.57542 GHz (L1) and L2 frequency at 1.22760 GHz and thus has an operative wavelength λ of 190 mm or 244 mm.

Some embodiments are related to an antenna decoupling apparatus for antennas that share a ground plane. The apparatus can include a first antenna having an operative wavelength λ, a second antenna, a ground plane connecting the first antenna and the second antenna, a protrusion extending no more than 0.1λ from the ground plane and being located within 0.2λ of a feeding port or a shorting end of the first antenna, and a discrete capacitor connecting the protrusion to the ground plane within 0.2λ of the feeding port or the shorting end of the first antenna.

There can be a second protrusion extending no more than 0.1λ from the ground plane and being located within 0.2λ of a feeding port or a shorting end of the second antenna, and a second discrete capacitor connecting the second protrusion to the ground plane within 0.2λ of the feeding port or the shorting end of the second antenna. The protrusion can include a soldering pad raised above the ground plane. The protrusion can include a stub extending laterally in a same plane as the ground plane.

Some embodiments are related to an antenna decoupling apparatus for antennas that share a ground plane. The apparatus can include a first antenna having an operative wavelength λ1, a second antenna having an operative wavelength λ2, a ground plane connecting the first antenna and the second antenna, a first discrete capacitor located within 0.2 λ1 of a feeding port or a shorting end of the first antenna and having both terminals electrically shorted with the ground plane, and a second discrete capacitor located within 0.2 λ2 of a feeding port or a shorting end of the second antenna and having both terminals electrically shorted with the ground plane.

The operative wavelength λ1 can be between 600 mm and 60 mm, or corresponding frequencies of 0.5 GHz and 5 GHz, and a capacitance of the first capacitor can be between 0.56 pF and 10 pF.

Some embodiments are related to a method for reducing coupling between a first antenna and a second antenna that share a ground plane. The method includes forming an aperture in the ground plane within 0.2λ of a feeding port or a shorting end of the first antenna, the aperture having no continuous edge longer than 0.1λ, fashioning a stub extending from a first edge of the aperture, and soldering a discrete capacitor to the stub and connecting the discrete capacitor to a second edge of the aperture.

The aperture can be a reentrant opening extending from a periphery of the ground plane. The method can include forming a second aperture in the ground plane within 0.2λ of a feeding port or a shorting end of the second antenna, the second aperture having no continuous edge longer than 0.1λ, fashioning a second stub extending from a first edge of the second aperture, and soldering a second discrete capacitor to the second stub and connecting the second stub to a second edge of the second aperture of the ground plane. The second aperture can be a reentrant opening extending from a periphery of the ground plane. The method can further include modeling the dimensions of the first and second antennas, ground plane, aperture, and stub using electromagnetic (EM) simulation software, and selecting a capacitance of the discrete capacitor based on the modeling. The method can include providing a printed circuit board (PCB) dielectric, and milling or etching metal on the PCB for the forming and fashioning.

DETAILED DESCRIPTION

In general, what is described is a self-curing decoupling scheme for two or more antennas that share a ground plane. One or more capacitors that are inlaid within, above, or alongside the ground plane creates an incremental current on top of that of original coupled antennas. The interference signal generated by the incremental current is of the same magnitude but opposite phase as that of the coupled signal so that the coupled signal is canceled out at the victim port. A numerical electromagnetic (EM) simulation of the configuration can be used to select the capacitances of the capacitors and fine tune the configuration.

Technical advantages of this self-curing decoupling scheme are many. For example, there does not need to be a direct physical connection or obstruction between coupled antennas. It requires negligible real-estate. It is versatile for virtually all of the commonly used antennas on wireless terminals. The scheme tends to improve the matching conditions of the decoupled antennas without needing any extra impedance matching circuit. It is highly flexible in implementation as the electrically small decoupling capacitors are detached from the antennas. And the design method can be potentially extended to multiple-input, multiple output (MIMO) antenna arrays with more than two antenna elements. These and other advantages can be found in some embodiments described herein.

FIG. 1is an isometric view of a system100of two antennas104and124on a smart phone ground plane102with inlaid decoupling apparatus103.

On one end of ground plane102, loop antenna104extends from feeding port106and meanders around to shorting point108, which is electrically connected to ground plane102. Loop antenna has a nominal frequency or frequency range, for which a nominal wavelength λ can easily be computed from frequency f by the equation λ=c/f, where c is the speed of light. The speed of light in a vacuum is 299,792,458 meters per second.

In some embodiments, the antennas can be part of the metal frame of a smart phone or other mobile electronic device.

Near feeding port106, ground plane102has L-shaped stub110, capacitor111, and reentrant aperture112. They are all within 0.2λ of feeding port106. Being close to the feeding port exposes the stub and capacitor to stronger currents from the ground plane than if they were further away from the feeding port. Using the stronger currents, the stub and capacitor perturb the current distribution on the ground plane and equivalently create a coherent current source. The coherent current source generates a signal at the victim port with the same magnitude but opposite phase as that of the coupled signal to mitigate the mutual coupling. Stubs and/or capacitors within 0.01λ, 0.5λ, 0.10λ, 0.15λ, and 0.20λ of a feeding port or ground port are suitable. Stubs and/or capacitors within 0.25λ, 0.30λ, 0.35λ, 0.40λ, 0.45λ, and 0.50λ of a feeding port or ground port of an antenna may be suitable.

A “reentrant” opening or aperture includes a slot, channel, or other opening that extends inward from an edge of an item, or as otherwise known in the art.

Aperture112includes edges114,116, and118. The combined total length of edges114,116, and118, along with the edges of stub110, which form a continuous edge, is less than 0.1λ. This is electrically very small, such that the introduction of the aperture to the ground plane does not change the antenna characteristics. The magnitude and phase of S-parameters of the coupled antennas are not changed noticeably. This would likely be different for apertures on the order of ¼ λ and greater. Thus, apertures with no continuous edge longer than 0.01λ, 0.05λ, 0.10λ, are suitable. Apertures with no continuous edge longer than 0.15λ, and 0.20λ may be suitable.

Stub110extends from edge118of the aperture and turns inward toward the center of the ground plane. The right angle turn forms an L shape. The portion of stub110that extends from edge118is not even, or aligned, with the outer edge of ground plane102. Instead, it is slightly inset from the perimeter. This inset distance can be zero to align them; however, making it non-zero, as shown here, gives one freedom to tune the decoupling effect.

Discrete capacitor111, a lumped capacitive element, connects from stub110to edge116. It connects from an end of stub110but may connect from another portion of the stub.

In some embodiments, multiple discrete capacitors in parallel may be used instead of a single discrete capacitor. This may give additional design room for achieving a desired capacitance.

On the opposite end of ground plane102, another loop antenna124extends from feeding port126and meanders around to shorting point128, which is electrically connected to ground plane102. This second loop antenna has a nominal frequency or frequency range, for which there is a nominal wavelength λ2.

Aperture132includes edges134,136, and138. The combined total length of edges134,136, and138, along with the edges of stub130, which form a continuous edge, is less than 0.1 λ2. In other embodiments, apertures with no continuous edge longer than 0.01 λ2, 0.05 λ2, 0.15 λ2, and 0.20 λ2 may be suitable.

Stub130extends from edge138of the aperture and turns inward toward the center of the ground plane. The right angle turns create an L shape. Like the other stub, the portion of stub130that extends from edge138is not even with the outer edge of ground plane102.

Discrete capacitor131, a lumped capacitive element, connects from stub130to edge136. It connects from an end of stub130but may connect from another portion of the stub.

Inlaid capacitor111is positioned at particular x, y coordinate, an “acupoint,” on ground plane102. In theory, the position of the acupoint, along with the features of the connecting stub, uses the electrical energy near feeding port106for antenna104and slightly delays it to form another signal on the ground plane. This signal is akin to a current source. This signal ends up at the victim port, feeding port126of antenna with the same magnitude but opposite phase as that of the coupled signal between antennas104and124, resulting in mutual coupling cancellation.

In some embodiments, the discrete capacitor can be a variable capacitor for tuning the decoupling apparatus. It can be a surface mount device (SMD), and/or the capacitor can include two or more capacitors in parallel with a first capacitor.

The antennas can share operating frequency bands, be in adjacent frequency bands, or a mixture of the two. As common in some mobile devices, the antennas can operate in a long-term evolution (LTE) band frequency between 2.11 GHz and 2.17 GHz and thus has an operative wavelength λ between 142 mm and 138 mm, operate in an industrial, scientific, and medical (ISM) frequency between 2.400 GHz and 2.4835 GHz and thus has an operative wavelength λ between 125 mm and 121 mm, and/or operate in a global positioning system (GPS) L1 frequency at 1.57542 GHz (L1) and L2 frequency at 1.22760 GHz and thus has an operative wavelength λ of 190 mm or 244 mm

FIGS. 2A-2Cillustrate loop antennas with decoupling apparatus inlaid within a ground plane. A loop antenna is a commonly-used antenna form in mobile terminals. It is well known that loop antennas are less vulnerable to the body proximity effect because the current induced on the ground plane is weak.

InFIG. 2A, system200has two identical loop antennas204and224placed antisymmetrically on two opposite edges of a printed circuit board (PCB). Reentrant openings212and232are made in ground plane202. Antennas204and224are supported by ground plane202and dielectric240that extends laterally from edges of the ground plane.

FIG. 2Bis a close up view of the antenna204with reentrant opening212to one side. A leg of antenna204connects to feeding port206, and an opposite leg connects directly to ground plane202through shorting point208.

FIG. 2Cis a further close up of the right side of antenna204and feeding port206with aperture212nearby. Stub210projects from the side of the aperture nearest feeding port206and then turns downward toward the center of the ground plane. Capacitor211connects an end of stub210to an edge of the aperture.

The dimensions of the exemplary loop antenna as well as a reentrant opening for inlaying a decoupling capacitor are marked with reference identifiers l3, w3, and lT−lZand wL−wP. Specific values are shown in Table 1.

With the specific numerical dimensions in the table, two decoupling capacitors with a value of 1.1 pF have been experimentally shown to improve isolation between the loop antennas from about 14 to 26 dB at 2.14 GHz and the matching condition is almost untouched.

As summarized in Table 2, the measured total efficiencies at 2.14 GHz is improved from 50 to 54% after decoupling. The average throughputs for the LTE module with the coupled and decoupled loop antennas are measured under the UMi and UMa environments, respectively. In the UMi channel environment, when throughput drops 10% from the maximum value of 14.386 Mbps, about 0.6 dB or 13% of power saving, can be achieved after decoupling. Similarly, under the UMa channel environment, about 1.2 dB power saving can be achieved when the throughput drops from the maximum value 14.386 Mbps to 13 Mbps. Similar to the case of IFAs and monopole antennas, the shape of radiation patterns does not change significantly after decoupling.

TABLE 1Dimensional values for different antenna cases. Units are in millimeters (mm).Inverted-Fl1lAlBlClDlElFlGlHlIlJlKAntennas100196.54.55201292.512.52.3lLw1wAwBwCwDwEwF1.5653.132101.21Monopolel2lMlNlOlPlQlRlSw2wGwHwIAntennas12019628111.813703.1210wJwK1.21.7Loopl3lTlUlVlWlXlYlZw3wLwMwNAntennas12051.77.2232.513.72.1753.127wOwP1.71.5

FIGS. 3A-3Cillustrate inverted F antennas (IFAs) with decoupling apparatus inlaid within a ground plane.

InFIG. 3A, system300has two IFAs304and324mounted on the two opposite edges of a phone sized PCB board. Electrically shallow openings312and332are cut from the edge of ground plane302and a short stub is stretched from a fringe of each opening. Antennas304and324are supported by ground plane302and dielectric340that extends laterally from edges of the ground plane. The dimension of the openings should be electrically very small to ensure the influence of the opening and the stub on original attributes of the antennas is negligible.

FIG. 3Bis a close up view of the antenna304with feeding port306and shorting end308on the shorting arm. Reentrant opening312is near shorting end308.

FIG. 3Cis a further close up of the shorting arm of antenna304with aperture312nearby. Stub310projects from the side of the aperture nearest shorting end308and then turns downward toward the center of the ground plane. Lumped capacitor element311is soldered between an end of stub310and an edge of the aperture to the ground plane.

In the embodiment, the two IFAs are in long-term evolution (LTE) band 1 (2.11-2.17 GHz). Dimensions of the exemplary IFA and reentrant opening are marked with reference identifiers l1, w1, lA−lL, and wA−wF. Specific values are shown in Table 1. Measured performance improvements are shown in Table 2.

With the specific numerical dimensions in Table 1, a set of optimal solutions for capacitive loads can to be sought. This searching process can be carried out graphically, such as with a contour plot for the mutual coupling S21at 2.14 GHz with respect to capacitor311and the capacitor near the other antenna. It can be found that for a given allowable mutual coupling level, say −20 dB, there is a solution range for the two capacitors, which are not necessary equal even for two symmetric antennas.

The definition of the objective function can be flexible. For example, the highest mutual coupling in the band from 2.13 to 2.15 GHz can be collected. For the same mutual coupling level, the solution range for decoupling in a frequency band is narrower than that for decoupling at a single frequency point, say 2.14 GHz.

When distance lH(i.e., the distance between the shorting leg and aperture) is changed from 2.5 to 6 mm, the solution range for the two capacitors can be narrower and the achievable minimum mutual coupling level is higher. It can be concluded that the position of the opening plays an important role in decoupling using this decoupling method.

One should find an optimal position for a wider solution region, which also leads to a wider decoupling frequency band. It is understandable that when the length becomes smaller, the capacitor values become larger. In some simulations, the real part of the loads for the capacitors is assumed to be 0.148Ω, although the value is not sensitive to the solution region.

FIGS. 4A-4Cillustrate bent monopole antennas with decoupling apparatus inlaid within a ground plane. Monopole antennas are widely used in mobile terminals due to their low profile, compact size, and convenience of layout.

InFIG. 4A, system400has two monopole antennas404and424mounted on the top edge of a PCB board, symmetrically. Electrically shallow openings412and432are cut from the edge of ground plane402, and a short, straight stub extends from one edge to an opposite edge of the opening. Antennas404and424are supported by ground plane402and dielectric440that extends laterally from edges of the ground plane.

FIG. 4Bis a close up view of the left antenna404with feeding port406. Reentrant opening412is near feeding port406.

FIG. 4Cis a further close up of the feeding port arm of antenna404with aperture412nearby. Stub410projects from the side of the aperture nearest feeding port404and extends straight out. Lumped capacitor element411is soldered between an end of stub410and an edge of the aperture to the ground plane.

The dimensions of the exemplary monopole antenna as well as a reentrant opening for inlaying a decoupling capacitor are marked with reference identifiers l2, w2, lM−lSand wG−wK. Specific values are shown in Table 1.

With the specific numerical dimensions in the table, and assuming real parts of the impedances of the capacitors are 0.106Ω, two decoupling capacitors with a value of 3 pF have been experimentally shown to improve isolation between the loop antennas from about 7 to 30 dB at 2.14 GHz, and the matching condition is, interestingly, improved.

As summarized in Table 2, the measured total efficiencies at 2.14 GHz is improved from 57 to 73% after decoupling. The average throughputs for the LTE module with the coupled and decoupled loop antennas are measured under the UMi and UMa environments, respectively. In the UMi channel environment, when throughput drops 10% from the maximum value of 33.356 Mbps, about 0.4 dB or 9% of power saving, can be achieved after decoupling. Similarly, under the UMa channel environment, about 0.9 dB power saving can be achieved when the throughput drops from the maximum value 14.386 Mbps to 13 Mbps. Similar to the case of IFAs, the shape of the radiation patterns does not change significantly after decoupling. Gain improvement is noticeable.

FIG. 5illustrates soldering pad decoupling apparatus raised above a ground plane. In system500, antennas504and524are connected through ground plane502at feeding ports506and526.

Instead of capacitors inlaid within the ground plane, stubs544protrude above ground plane502by distance542. Stubs544, which are essentially soldering pads raised above the ground plane, are connected by capacitor511. The stubs extend no more than 0.1λ from the ground plane, and they are located within 0.2λ of feeding port506of antenna504.

Similarly, another set of stubs are joined by capacitor531within 0.2 λ2 of feeding port506of antenna524.

No feature of the protrusions is greater than 0.2λ in order to remain electrically shallow. Properly located, stub protrusions544and capacitors511generate a signal from the currents near feeding port506of the same magnitude yet opposite phase of that of a coupling current between antennas504and524. The generated signal largely cancels the coupling current at victim port526of antenna524.

FIG. 6illustrates decoupling apparatus protruding laterally in the same plane as a ground plane. In system600, antennas604and624are connected through ground plane602at feeding ports606and626.

Protrusions644extend laterally, in the same plane as the ground plane, from an edge of ground plane602. The protrusions extend no more than 0.1λ from the ground plane, and they are located within 0.2λ of the feeding port606of antenna604.

Similarly, another set of stubs are joined by capacitor631within 0.2 λ2 of feeding port626of antenna624. No feature of the protrusions is greater than 0.2 λ2.

FIGS. 7-10illustrate different embodiments of apertures and stubs near the shorting end of an IFA antenna. These aperture and stub configurations are applicable to other antenna types, such as loop and monopole antennas. These configurations are a sample of different design configurations that can be used in different embodiments.

FIG. 7illustrates stub710that is aligned and even with a side of a ground plane. That is, an outer edge of the stub is just the outer edge of the ground plane. The aperture, and not the stub, is L-shaped. Capacitor711extends from the end of stub710to an opposite side of the aperture.

FIG. 8illustrates straight stub810with capacitor811off to one side. That is, capacitor811is not connected at the end of stub810, but rather the side of the stub. This shows that a capacitor does not need to be at the end of the stub. But the capacitor should have one end electrically connected with the stub and the other end electrically connected with the ground plane.

FIG. 9illustrates L-shaped stub910in an interior hole of a ground plane. That is, the hole or aperture is not reentrant. Capacitor911extends from an end of stub910to a side of the hole.

FIG. 10illustrates straight stub1010in an interior hole of a ground plane. Like in the previous figure, the hole is not reentrant. Capacitor1011extends from an end of stub1010to a side or fringe of the hole.

For the interior hole configurations inFIGS. 9-10, it is found that the decoupling effect still exists but performance is not very good as compared to the cases in which the cut is made from the edge of the ground plane. That is, reentrant apertures appear to have better decoupling performance.

FIG. 11illustrates aperture1114, or slot, in a ground plane with a long continuous edge of the prior art. Although the slot is serpentine with several 90 degree bends, its edge is continuous. As shown in the figure, the “continuous edge” of the aperture extends uninterrupted from point A to point B. This long continuous edge, or discontinuity between the metal of the ground plane and the air or dielectric in the slot, could be resonant with antenna frequencies if it is on the order of ¼ λ or more.

For embodiments of the present application, it has been found that avoiding long slots with continuous edges, and keeping edges and features less than 0.1λ, is less likely to negatively affect antenna performance.

FIG. 12is a flowchart of process1200in accordance with an embodiment. In operation1201, an aperture in the ground plane is formed within 0.2λ of a feeding port or a shorting end of a first antenna, the aperture having no continuous edge longer than 0.1λ. In operation1202, a stub extending from a first edge of the aperture is fashioned. In operation1203, a discrete capacitor is soldered to the stub, and the capacitor is connected to a second edge of the aperture. In operation1204, a second aperture is formed in the ground plane within 0.2 λ2 of a feeding port or a shorting end of the second antenna, the second aperture having no continuous edge longer than 0.1 λ2. In operation1205, a second stub extending from a first edge of the second aperture is fashioned. In operation1206, a second discrete capacitor is soldered to the second stub, and the second capacitor is connected to a second edge of the second aperture of the ground plane. One can mill or etch metal on a PCB for the forming and fashioning.

Although specific embodiments of the invention have been described, various modifications, alterations, alternative constructions, and equivalents are also encompassed within the scope of the invention. Embodiments of the present invention are not restricted to operation within certain specific environments, but are free to operate within a plurality of environments. Additionally, although method embodiments of the present invention have been described using a particular series of and steps, it should be apparent to those skilled in the art that the scope of the present invention is not limited to the described series of transactions and steps.

Further, while embodiments of the present invention have been described using a particular combination of hardware, it should be recognized that other combinations of hardware are also within the scope of the present invention.