Loop antenna including impedance tuning gap and associated methods

<u>Field of the Invention</u>

0001The present invention relates to the field of communications, and, more particularly, to antennas and related methods.

<u>Background of the Invention</u>

0002Antennas may be used for a variety of purposes, such as communications or navigation, and portable radio devices may include broadcast receivers, pagers, or radio location devices ("ID tags"). The cellular telephone is an example of a portable communications device, which is nearly ubiquitous. Antennas for portable radios or wireless devices should be small, efficient, and have a broad radiation pattern.

0003Orientation of a portable device may be a concern. It may be impractical to orient a radio location tag, or point a cell phone, and satellites may tumble unintentionally. When antennas having radiation pattern nulls become misoriented, unacceptable fading is a common problem. Communications need to be reliable, and increased transmitter power may be required. Thus, a nondirectional antenna having a full-coverage radiation pattern may be desirable to avoid fading.

0004An example of a nondirectional antenna, which does not have radiation pattern nulls, is the isotropic antenna, which has a spherical radiation pattern for equal radiation in all directions. Isotropic antennas may provide a constant signal level for all antenna orientations, for operation without fading when the antenna cannot be aimed or pointed. The directivity of an isotropic antenna is 0.0 dB and if 100 percent efficient, the isotropic antenna gain is 0 dBi. Omnidirectional antennas may have circular antenna patterns in a single plane, such as for the horizon, and an isotropic antenna may provide omnidirectional patterns in all planes.

0005Antennas are transducers between electric currents and radio waves, and they may have a variety of shapes. Euclidian geometric shapes, such as those known through the ages, can be favorable for antennas. They can provide the greatest area for the perimeter (circles) or the shortest length between points (lines), etc. Thus, the two canonical antenna shapes may be the line and circle, corresponding to the dipole and loop type respectively.

0006The thin-wire half wave dipole is an example of a line shaped antenna. It may have a cos² θ radiation pattern (two petal rose in plane) with two pattern nulls, a gain of 2.1 dBi, and a 3 dB gain bandwidth of 13%. Dipole antennas may be very common in the art, yet circle shaped antennas may have advantages for gain, polarization, and otherwise.

0007The full wave loop antenna is an example of a circle shaped antenna. It may have a circumference of 1 wavelength, a two petal rose radiation pattern (lobes broadside to the loop plane), and a gain of 3.6 dBi. <patcit id="pcit0001" dnum="US20080136720A" dnum-type="L"><text>U.S. Patent Application Publication No. 2008/0136720 to Parsche et al.</text></patcit>, assigned to the present assignee, and entitled "Multiple Polarization Loop Antenna and Associated Methods" discloses a full wave loop antenna with multiple feedpoints. Multiple polarizations may be provided from the single loop, including linear, circular, and dual polarizations.

0008A rectangular loop antenna was described by Heinrich Hertz in 1886. In his classic work, sparks were produced by radio, and the antenna was a 0.8 X 1.2 meter wire rectangle ("<nplcit id="ncit0001" npl-type="b"><text>Electric Waves", Heinrich Hertz, Macmillan 1893</text></nplcit>). Sparks were rendered at a gap in the antenna conductor, so the gap provided a detector and receiver. As the frequency neared 40 MHz, the loop was a half wavelength in perimeter, resonant (or "antiresonant"), and with a high impedance at the gap. While the high impedance was beneficial for high voltage sparks, high impedances may not be preferential for modern electronics since solid state devices operate at low voltages. For modern needs, a half wave circular loop antenna of a low driving impedance, for example, 50-Ohms may be desirable.

0009Newer designs and manufacturing techniques have driven electronic components to small dimensions and miniaturized many communication devices and systems. Unfortunately, antennas have not been reduced in size at a comparative level and often are one of the larger components used in a smaller communications device. Antennas become increasingly larger as the frequency decreases. At high frequencies (HF), 3 to 30 MHz for example, used for long-range communications, efficient antennas become too large to be portable, and wire antennas may be required at fixed stations. It becomes increasingly important in these communication applications to reduce not only the antenna size, but also to design and manufacture a reduced size antenna having the greatest gain for the smallest area.

0010<patcit id="pcit0002" dnum="US6252561B"><text>U.S. Patent No. 6,252,561 to Wu, et al.</text></patcit> is directed to a wireless LAN antenna with a dielectric substrate having a first surface and a second surface. The first surface of the dielectric substrate has a rectangular loop. A rectangular grounding copper foil is adhered within the rectangular loop. A signal feeding copper foil is further included. One end of the signal feeding copper foil is connected to the rectangular loop and the grounding copper foil, while another end of the signal feeding copper foil runs across another end of the rectangular loop. Moreover, a layer of copper foil is plated to the back side of the printed circuit board. This back surface copper foil covers one half of the loop on the front surface. Adjustment of the transversal dimensions of the grounding copper foil will impedance-match the antenna to the feeding structure of the antenna.

0011Also, <patcit id="pcit0003" dnum="US6590541B"><text>U.S. Patent No. 6,590,541 to Schultze</text></patcit> is directed to a half-loop antenna having an antenna half-loop positioned on top of a ground plane, the antenna half-loop forming an area whose outer edge forms a convex closed curve. The conductor half-loop has the form of an ellipse tapering to a point at its ends, and at the feed-in point of the conductor half-loop an inductance can be inserted, formed as a spring.

0012<patcit id="pcit0004" dnum="US4185289A"><text>U.S. Patent No. 4,185,289 to DeSantis et al.</text></patcit> discloses a spherical body dipole including an annular slot feed. Complimentary radiation patterns provide near isotropic coverage. Yet, a smaller, planar radiating structure may be needed for portable personal communications, and a wire structure may be required for HF applications.

0013United States Patent Application <patcit id="pcit0005" dnum="US10478234B"><text>10/478,234</text></patcit> discloses a loop antenna with at least one gap flanking the (input) port for regulating impedance. <nplcit id="ncit0002" npl-type="s"><text>The Note on Circular Loop Antennas with Non-Uniform Current Distribution in the Journal of Applied Physics by G. Glinski</text></nplcit> discloses a loop antenna with a symmetrical radiation pattern. The IEE article <i>Electronic beam tilting using a single reactively loaded circular wire loop antenna</i> by H. Scott and V.F. Fusco discloses inserting a capacitive reactive load in a loop antenna and that varying the capacitance of the inserted reactive load varies input impedance together with the antenna beam to provide a beam tilt mechanism for the antenna. The IEEE article <i>Determination of Reactance Loading for Circularly Polarized Circular Loop Antennas With a Uniform Traveling-Wave Current Distribution</i> by Li, et. al., discloses inserting a capacitance gap at 45 degrees from a feed point of a loop antenna, and notes that this results in asymmetric radiation patterns for the modified loop antenna.

0014United States Patent <patcit id="pcit0006" dnum="US7057574B"><text>7,057,574 (Shamir, et al.</text></patcit>) refers to a method for designing a small antenna matched to an input impedance, and small antennas designed according to that method. The antenna is matched in an extremely narrow bandwidth by a capacitance obtained preferably by a printed gap. To design a small antenna matched to a required input impedance and operating at a desired frequency, the Shamir discloses to choose an impedance matching point related to a singular point; and canceling the imaginary part of the input impedance, thereby obtaining a design of an antenna matched to the required impedance and operating at a desired frequency.

0015To find the singular point in a specific example, the singular point is found when the loop length is around 0.5 of the wavelength (C/.lamda.=0.5). The singular point for a loop will be found at a length that is a fraction, smaller than one, and more typically between about 0.2 and 0.7 of the wavelength. At the singular region, there are two points where the real part of the impedance is 200 ohms--one at around 0.4 wavelengths, and one at around 0.55 wavelengths. The method includes to choose the 0.4 wavelength point.

0016Prior approaches to forming isotropic antennas include optical approaches and or waveguides. <patcit id="pcit0007" dnum="US5859615A"><text>U.S. Patent No. 5,859,615 to Toland et al.</text></patcit> is directed to an omnidirectional isotropic antenna using a tubular waveguide and an ellipsoid lens. <patcit id="pcit0008" dnum="US7298343B"><text>U.S. Patent No. 7,298,343 to Forster et al.</text></patcit> is directed to an RFID tag that includes an antenna structure that is a hybrid loop-slot antenna.

0017Thus, there is a need for an easily manufactured, reduced size and cost, planar, isotropic loop antenna.

<u>Summary of the Invention</u>

0018In view of the foregoing background, it is an object of the present invention to provide an easily manufactured, reduced size and cost, loop antenna.

0019This and other objects, features, and advantages in accordance with the present invention as defined by the independent claim. are provided by a loop antenna that may include first and second electrical conductors arranged to define a circular shape with first and second spaced apart gaps therein. The loop antenna may further include opposing portions of the first and second electrical conductors at the first gap defining a signal feedpoint, for example. Opposing portions of the first and second electrical conductors at the second gap may also advantageously define an impedance tuning feature. The second gap may be circumferentially spaced from the first gap less than ninety degrees, for example. The second gap may be greater than the first gap to provide a predetermined impedance and an isotropic radiation pattern at a predetermined operating frequency for the loop antenna. Accordingly, the loop antenna provides an easily manufactured, reduced size, and reduced cost isotropic loop antenna.

0020Additionally, the second gap may be circumferentially spaced from the first gap by an angle in a range of 40 to 70 degrees. The second gap may also have an angular width in a range of 5 to 15 degrees, for example. Still further, the first gap may have an angular width in a range of 0.001 to 10 degrees.

0021The loop antenna may further include a dielectric substrate mounting the first and second electrical conductors thereon, for example.

0022The circular shape may have a circumference in a range of 0.3 to 0.6 times a wavelength of the predetermined operating frequency of the loop antenna. Additionally, the signal feedpoint may define a 50-Ohm signal feedpoint, for example.

0023In some embodiments, a portion of the first electrical conductor may include an outer conductor of a coaxial transmission line. The second electrical conductor may include an inner conductor of the coaxial transmission line extending outwardly beyond an end of the outer conductor. At least one dielectric body may be positioned at the second gap to define a frequency tuning feature.

0024Another aspect is directed to a method of making the loop antenna. The method may include arranging first and second electrical conductors to define a circular shape with first and second spaced-apart gaps therein so that opposing portions of the first and second electrical conductors at the first gap define a signal feedpoint. The method may also include arranging first and second electrical conductors so that opposing portions of the first and second electrical conductors at the second gap define an impedance tuning feature. The second gap may be circumferentially spaced from the first gap less than ninety degrees and located to provide a predetermined impedance and an isotropic radiation pattern at a predetermined operating frequency for the loop antenna.

<u>Brief Description of the Drawings</u>

0025<ul id="ul0001" list-style="none" compact="compact"><li><figref idref="f0001">FIG. 1</figref> is a top plan view of a loop antenna in accordance with the present invention.</li><li><figref idref="f0002">FIG. 2A</figref> is a perspective view of the loop antenna of <figref idref="f0001">FIG. 1</figref> in a radiation pattern coordinate system.</li><li><figref idref="f0003">FIG. 2B</figref> is an XY plane cut radiation pattern graph for the loop antenna as shown in <figref idref="f0001">FIG. 1</figref>.</li><li><figref idref="f0004">FIG. 2C</figref> is a YZ plane cut radiation pattern graph for the loop antenna as shown in <figref idref="f0001">FIG. 1</figref>.</li><li><figref idref="f0005">FIG. 2D</figref> is a ZX plane cut radiation pattern graph for the loop antenna as shown in <figref idref="f0001">FIG. 1</figref>.</li><li><figref idref="f0006">FIG. 3</figref> is a voltage standing wave ratio response graph of the loop antenna as shown in <figref idref="f0001">FIG. 1</figref>.</li><li><figref idref="f0007">FIG. 4</figref> is a graph of the driving point resistance for the loop antenna as shown in <figref idref="f0001">FIG. 1</figref>, as a function of gap position.</li><li><figref idref="f0008">FIG. 5</figref> is a graph of the current distribution along the loop conductors for the loop antenna as shown in <figref idref="f0001">FIG. 1</figref>.</li><li><figref idref="f0009">FIG. 6</figref> is a top plan view of an another embodiment of the loop antenna in accordance the present invention.</li><li><figref idref="f0010">FIG. 7</figref> is a schematic block diagram of a communications device including the loop antenna as shown in <figref idref="f0001">FIG. 1</figref>.</li></ul>

<u>Detailed Description of the Preferred Embodiments</u>

0026The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout, and prime notation is used to indicate similar elements in an alternative embodiment.

0027Referring initially to <figref idref="f0001">FIG. 1</figref>, a loop antenna <b>10</b> includes first and second electrical conductors <b>11, 12</b> arranged to define a circular shape with first and second spaced apart gaps <b>13, 14</b> therein. The circular shape is configured so that the circumference is equal to a range of 0.3 to 0.6, and more preferably 0.5 times a wavelength of an operating frequency of the loop antenna <b>10.</b> In other words, the circumference of the loop antenna <b>10</b> will vary according to a desired operating frequency.

0028The first and second electrical conductors <b>11, 12</b> are preferably copper traces with tin lead plating. The first and second conductors <b>11, 12</b> may be, for example, metal wires, metal tubing, a printed-wiring board trace, metal strips, conductive ink on paper, or other conductors, as will be appreciated by those skilled in the art. Moreover, the first and second conductors <b>11, 12</b> may be about 0.254 cm (0.1 inches) wide, for example. Other widths may be contemplated by those skilled in the art, so long as the width is less than the total outer circumference diameter of the loop antenna <b>10</b> divided by five.

0029Opposing portions of the first and second electrical conductors <b>11, 12</b> at the first gap <b>13</b> define a signal feedpoint <b>15.</b> The signal feedpoint <b>15</b> may include a pair of terminals or a port, for example. The signal feedpoint <b>15</b> may be a 50-Ohm signal feedpoint, for example, however, the signal feedpoint can be configured for other resistances or even complex impedances. The signal feedpoint <b>15</b> may also receive a coaxial cable (not shown) that can be soldered across the first gap <b>13.</b> Additionally, the first gap <b>13</b> has an angular width, as noted by angle α in <figref idref="f0001">FIG. <b>1</b></figref><b>,</b> in a range of 0.001 to 10 degrees, and, for example, about 5 degrees between opposing portions of the first and second electrical conductors <b>11, 12.</b> As will be appreciated, by those skilled in the art, alternative angular gap widths may be implemented.

0030Opposing portions of the first and second electrical conductors <b>11, 12</b> at the second gap <b>14</b> define an impedance tuning feature. The second gap <b>14</b> illustratively has an angular width, noted by angle β, in a range of 5 to 15 degrees, and, for example, about 10 degrees between opposing portions of the first and second electrical conductors <b>11, 12.</b> As will be appreciated by those skilled in the art, alternative angular gap widths may be implemented. The center of the second gap <b>14</b> is circumferentially spaced from the center of the first gap <b>13</b> by an angle γ less than ninety degrees, and the second gap <b>14</b> is greater than the first gap <b>13</b> to provide a predetermined impedance and an isotropic radiating pattern at the predetermined operating frequency for the loop antenna. For example, the operating frequency may be UHF, in other words, in a range of 300 MHz to 3 GHz. In this case, as the preferred circumference C is 0.5λ_air, and the preferred diameter <b>d</b> is 0.5λ_air/π = 0.16 λ_air, the outside diameter <b>d</b> of antenna <b>10</b> at UHF may range from 16 to 1.6 cm (6.3 to 0.63 inches).

0031In a preferred embodiment, the center of the second gap <b>14</b> is circumferentially spaced from the center of the first gap by an angle γ in a range of 40-70 degrees from the first gap <b>13,</b> and, more preferably, the angle may be <b>50</b> degrees to provide a 50-Ohm impedance at the feedpoint <b>15.</b> As will be appreciated by those skilled in the art, the spacing between the second gap <b>14</b> and the first gap <b>13</b> may be varied to alter the impedance at the feedpoint <b>15.</b> For example, moving the second gap <b>14</b> closer to the feedpoint <b>15,</b> or in other words, decreasing the angle γ, raises the impedance seen at the feedpoint. Conversely, moving the second gap <b>14</b> further away from the feedpoint <b>15,</b> or increasing the angle γ, will reduce the impedance seen at the feedpoint.

0032Coarse adjustment of frequency of operation for the loop antenna <b>10</b> may be accomplished by linear scaling, e.g., reducing or enlarging the size of the entire structure as whole, as reducing the wavelength reduces the size of the antenna. Antenna size is of course the reciprocal of frequency (Size ∝ 1/Frequency) so loop antenna <b>10</b> is made smaller for a higher frequency. Fine frequency adjustment, e.g., frequency trimming after antenna fabrication, may be accomplished by adjusting the width of the second gap <b>14,</b> by ablation or otherwise. The width of the second gap <b>14</b> is denoted by angle β. As will be appreciated by those skilled in the art, antenna driving point impedance (z) is complex and expressed as z = r +jx, where r is the resistance and x is the reactance and j is the complex operator √-1. Loop antenna <b>10</b> is preferentially operated at resonance such that no reactance (jx = 0) exists at first gap <b>13.</b> Thus, adjustment of frequency, e.g., "tuning", is the reduction of driving point reactance to zero.

0033Antenna driving point resistance is independently adjustable from reactance, and may be accomplished by moving the position of the second gap <b>14</b> with respect to the first gap <b>13;</b> the geometry of this is denoted by angle γ. Moving second gap <b>14</b> closer to the first gap <b>13</b> raises the resistance obtained and moving the second gap <b>14</b> away from the first gap <b>13</b> lowers the resistance obtained.

0034Referring now briefly to <figref idref="f0007">FIG. <b>4</b></figref><b>,</b> the plot <b>30</b> shows the resistance obtained for the loop antenna <b>10</b> when it is at resonance, as a function of the angular position of the center of the second gap <b>14.</b> Mathematically, the resistance obtained varies approximately as: <maths id="math0001"><math display="block"><mi mathvariant="normal">R</mi><mo mathvariant="normal">=</mo><mn mathvariant="normal">12</mn><mo mathvariant="normal">+</mo><mfenced open="|" close="|"><mn mathvariant="normal">30</mn><mspace width="1em" /><mi>cot</mi><mspace width="1em" /><mfenced><mi mathvariant="normal">γ</mi><mo mathvariant="normal">/</mo><mn mathvariant="normal">2</mn></mfenced></mfenced></math><img file="EP2178166B1_D0001.tif" /></maths> Where: <ul id="ul0002" list-style="none" compact="compact"><li>R = Resistance at resonance at first gap <b>13</b> in Ohms</li><li>γ = Angle between center of the first gap <b>13</b> and the center of the second gap <b>14,</b> in degrees or radians.</li></ul> As will be appreciated by those skilled in the art, without the inclusion of the second gap <b>14,</b> e.g., if the second gap <b>14</b> were shorted, the resistance at the first gap <b>13</b> or driving point could approach infinity in theory and thousands of Ohms might occur in practice. Note that the value of the reactance at the first gap <b>13,</b> which is preferentially zero for resonance, is not appreciably affected by the angular position of the second gap <b>14.</b> Thus, separate independent controls of reactance and resistance at the first gap <b>13,</b> by adjustment of the second gap <b>14</b> width and the second gap <b>14</b> location respectively are provided.

0035Exact resonance in thin wire embodiments (i.e. <b>a</b> width smaller than diameter <b>d</b> divided by 20) has been observed with an antenna circumference C of 0.505 to 0.510 wavelengths, corresponding to an antenna outer diameter <b>d</b> of 0.161 to 0.162 wavelengths in air. Fat wire or wide trace embodiments of the loop antenna <b>10</b> (i.e. a width greater than the diameter <b>d</b> divided by 20) resonate at a smaller circumference <b>C,</b> for example, 0.45 wavelengths or less in some instances.

0036An optional variable capacitor <b>19</b> may be configured across the second gap <b>14</b> to provide a post-manufacture frequency adjustment, e.g., tuning. A simple formula to calculate the exact capacitance for a tuning shift may not be possible due to the stray capacitance of the second gap <b>14</b> geometry, but in general, the frequency shift is according to the circuit resonance formula F=1/2π√(LC). For example, the frequency shift is the square root of the capacitance change (ΔF=√(ΔC)). Electrically variable capacitors, such as varactor diodes are also suitable for electronic tuning, as are other tuners, as will be appreciated by those skilled in the art.

0037Radiation efficiency of the loop antenna <b>10</b> will now be considered. When copper is used for the first and second electrical conductors <b>11, 12,</b> resistive losses may be negligible and radiation efficiency may be increased. This is because the loop antenna <b>10</b> may have a radiation resistance (R_r) in the range of 8 to <b>14</b> Ohms, which is sufficient to overcome most conductor loss. A specific example for radiation efficiency is operation at 1000 MHz, for example, for PWB implementation, narrow copper traces 0.025 antenna diameters wide, and traces 0.00178 cm (0.0007 inches) thick. The loop antenna <b>10</b> diameter d is then 0.5λ_air/π = 0.16 λ_air = 4.826 cm (1.9 inches) 1.9 inches, the copper traces 0.05 (4.826) = 0.24 cm wide, and the radio frequency loss resistance (R₁) of the copper traces may be calculated to be 0.25 Ohms total. Radiation efficiency (η) is then approximately [R_r / (R_r + R₁) ] X 100% = [10 / (10 + 0.25)] X 100 % = 98 %. As will be appreciated by those skilled in the art, radiation resistance (R_r) is an artifice for analysis which indicates the transducer resistance at a current maxima in the antenna, and for electrically small loops with a uniform amplitude current distribution it is calculated by the well known formula R_r = 31,200 {[(πa²)/λ²]²}, which is about 10 Ohms for a uniform current loop antenna the size of the loop antenna <b>10.</b>

0038The loop antenna <b>10,</b> however, has slightly more radiation resistance as the current amplitude distribution is sinusoidal or nearly so. R_r has been measured at 12 to 14 Ohms in some prototypes. Note that the driving resistance provided at the first gap <b>13</b> is generally not the same as the radiation resistance, and the driving resistance may be adjusted to 50 Ohms or as otherwise desired by the location of the second gap <b>14.</b>

0039The loop antenna <b>10</b> further illustratively includes a dielectric substrate <b>17</b> mounting the first and second electrical conductors <b>11, 12</b> thereon. The dielectric substrate may be made of IsoClad® 933, a nonwoven fiberglass reinforced polytetrafluoroethylene (PTFE) composite material having a dielectric constant of about 2.33 and being available from Arlon Microwave Materials of Cucamonga, CA. Other materials may also be used, as antenna tuning is little effected by the substrate dielectric constant, unlike microstrip patch antennas, for example. The first and second electrical conductors <b>11, 12</b> are illustratively positioned on a topside of the dielectric substrate <b>17.</b> A bottom-side of the dielectric substrate <b>17</b> is preferably left bare; that is, no electrical conductors are mounted thereon.

0040The loop antenna <b>10</b> advantageously radiates in all directions forming a substantially spherical radiation pattern. As illustrated in <figref idref="f0002 f0003 f0004 f0005">FIGS. 2a-2d</figref>, for example, the principal plane radiation patterns are isotropic to about within +/- 1.5dB. The patterns illustrated in <figref idref="f0002 f0003 f0004 f0005">FIGS. 2a-2d</figref> are for total fields and were obtained by a method of moments calculation in the NEC4.1 Numerical Electromagnetic Code by Lawrence Livermore National Laboratory. Gain is defined in IEEE Standard 145-1993 and in units of dBi (decibels with respect to an isotropic antenna). As will be appreciated by those skilled in the art, 0.0 dBd (decibels with respect to a half wave dipole) equals 2.1 dBi. The isotropic pattern of the loop antenna <b>10</b> may reduce communication fades associated with orientation, for example, with tumbling satellites or misoriented pagers. If a circularly polarized antenna is used to link to the loop antenna <b>10,</b> the loop antenna may be randomly oriented, and the aiming fades may be about 6 dB or below. This is because the polarization loss factor between linear and circular polarization is 3 dB and the deepest radiation pattern null in the loop antenna <b>10</b> is about 3 dB down from pattern peak. The loop antenna <b>10</b> is linearly polarized or mostly so in all directions.

0041Referring now to <figref idref="f0006">FIG. <b>3</b></figref><b>,</b> the loop antenna <b>10</b> advantageously provides a reduced voltage standing wave ratio (VSWR) <b>31,</b> and about 1.2:1 In other words, the maximum standing wave amplitude is 1.2 times greater than the minimum standing wave value of 1:1 in a 50 Ohm system. The VSWR of 1.2:1 is indicative of lower losses and a reduced reflected power radiated by the loop antenna <b>10,</b> as will be appreciated by those skilled in the art. The outer circumference of the loop antenna <b>10</b> is measured at about 0.45 to 0.50 times the wavelength at the frequency of minimum VSWR, which is the first or fundamental resonance in the loop antenna <b>10.</b> The exact circumference depends on the width of first and second electrical conductors <b>11, 12.</b>

0042A performance summary for the loop antenna <b>10</b> is shown below in Table 1. <tables id="tabl0001" num="0001"><table frame="all"><title>Table 1</title><tgroup cols="3"><colspec colnum="1" colname="col1" colwidth="64mm" /><colspec colnum="2" colname="col2" colwidth="82mm" /><colspec colnum="3" colname="col3" colwidth="20mm" /><thead><row><entry namest="col1" nameend="col3" align="center" valign="top"><b>Table 1: Example And Prototype</b></entry></row><row><entry align="center" valign="top"><b>Parameter</b></entry><entry align="center" valign="top"><b>Value</b></entry><entry align="center" valign="top"><b>Method</b></entry></row></thead><tbody><row><entry align="center">Implementation</entry><entry align="center">Printed Wiring Board</entry><entry align="center">-</entry></row><row><entry align="center">PWB Material</entry><entry align="center">Teflon, ∈_r= 2.33 Farads/Meter</entry><entry align="center">-</entry></row><row><entry align="center">Conductors</entry><entry align="center">Copper, Greater Than 5 Skin</entry><entry align="center">Measured</entry></row><row><entry align="center" /><entry align="center">Depths (σ) Thick</entry><entry align="center" /></row><row><entry align="center">Frequency</entry><entry align="center">1868.4 MHz</entry><entry align="center">Specified</entry></row><row><entry align="center">Outer Diameter</entry><entry align="center">2.286 cm (0.90 Inches) (0.14λ_air)</entry><entry align="center">Measured</entry></row><row><entry align="center">Outer Circumference</entry><entry align="center">7.19 cm (2.83 Inches) (0.46λ_air)</entry><entry align="center">Measured</entry></row><row><entry align="center">Trace Width</entry><entry align="center">0.114 cm (0.045 Inches) (0.007λ_air)</entry><entry align="center">Measured</entry></row><row><entry align="center">First Gap <b>13</b></entry><entry align="center">2.5 &lt; δ &lt; -2.5 Degrees (Gap Width α = 5 Degrees)</entry><entry align="center">Measured</entry></row><row><entry align="center">Second Gap <b>14</b></entry><entry align="center">55 &lt; δ &lt; 65 Degrees (Gap Width β = 10 Degrees, Gap Spacing γ = 60 degrees)</entry><entry align="center">Measured</entry></row><row><entry align="center">Variable Capacitance <b>19</b></entry><entry align="center">0.0 pf (No Capacitor Used)</entry><entry align="center">-</entry></row><row><entry align="center">System Impedance</entry><entry align="center">50 Ohms Nominal</entry><entry align="center">Specified</entry></row><row><entry align="center">Complex Driving Point Impedance</entry><entry align="center">52 - 4.6j Ohms</entry><entry align="center">Measured</entry></row><row><entry align="center">VSWR At Resonance</entry><entry align="center">1.2 to 1 (VSWR Is A Dimensionless Ratio)</entry><entry align="center">Measured</entry></row><row><entry align="center">Gain (At Peak)</entry><entry align="center">+0.9 dBil (Decibels With Respect To Isotropic, Linear Polarization)</entry><entry align="center">NEC4.1</entry></row><row><entry align="center">Instantaneous 3 dB Gain Bandwidth</entry><entry align="center">3.2 %</entry><entry align="center">Calculated</entry></row><row><entry align="center">Polarization</entry><entry align="center">Linear</entry><entry align="center">Specified</entry></row><row><entry align="center">Polarization</entry><entry align="center">Horizontal When Antenna Is</entry><entry align="center">Measured</entry></row><row><entry align="center">Orientation</entry><entry align="center">Operated In Horizontal Plane.</entry><entry align="center" /></row><row><entry align="center">Radiation Pattern Shape</entry><entry align="center">Nearly Isotropic (Spherical)</entry><entry align="center">NEC4.1</entry></row><row><entry align="center">Radiation Pattern, Deviation From Isotropic</entry><entry align="center">Less than + - 1.5 dB</entry><entry align="center">NEC4.1</entry></row><row><entry align="center">Radiation Efficiency</entry><entry align="center">98 %</entry><entry align="center">Calculated</entry></row><row><entry align="center">Current Distribution Along Loop Conductors</entry><entry align="center">Sinusoidal Amplitude, Constant Phase</entry><entry align="center">NEC4.1</entry></row><row><entry align="center">Virtual Ground Node <b>18</b></entry><entry align="center">No connection thereto (Located At δ = 260 Degrees)</entry><entry align="center">-</entry></row></tbody></tgroup></table></tables>

0043The instantaneous bandwidth, e.g., fixed tuned bandwidth, of the loop antenna <b>10</b> varies with the trace width of the first and second electrical conductors <b>11, 12.</b> For narrow traces, as described above, the 3 dB gain bandwidth is near 3.2 percent. For wide, fat loop conductors, as described above, the 3 dB gain bandwidth rises to about 10 percent. The tunable bandwidth can exceed the instantaneous bandwidth of the loop antenna <b>10</b> as the radiation pattern shape is stable over a bandwidth of about 20 to 30 percent. Multiple tuning extends instantaneous gain bandwidth, and it may be applied to the loop antenna <b>10</b> by external elements, such as a lumped element LC network interposed at the signal feedpoint <b>15.</b> The double tuning form of multiple tuning generally provides about a 2² bandwidth enhancement (400 percent).

0044As will be appreciated by those skilled in the art, small antennas may operate according to Chu's Limit for instantaneous gain bandwidth (<nplcit id="ncit0003" npl-type="s"><text>Physical Limitations of Omni-Directional Antennas", L.J. Chu, Journal of Applied Physics, Volume 19, pp 1163 - 1175 December 1948</text></nplcit>). The 3 dB gain single tuning form of Chu's Limit is BW_3dB ≤ 200(r/λ)³ for single tuning, and for a sphere, the diameter of the loop antenna <b>10</b> Chu's Limit can be calculated as BW_3dB ≤ <b>(100%)200(0.16λ/λ)³ ≤</b> 82%. As the loop antenna <b>10</b> may operate to about 10% 3 dB gain bandwidth, the loop antenna can operate near 10%/82% = 8.2% of Chu's Limit for single tuning and 3 dB gain, which is sufficient for many purposes, and the loop antenna <b>10</b> may be advantaged for being planar rather than spherical. Antennas according to Chu's Limit may of course, be unknown.

0045It is also appropriate to consider the loop antenna <b>10</b> current distribution, as radiated far fields and antenna aperture distribution are reciprocal Fourier transforms. <figref idref="f0008">FIG. 5</figref> illustrates the calculated current magnitude <b>33</b> for the loop antenna <b>10,</b> along first and second electrical conductors <b>11, 12,</b> for a 1-volt excitation at the first gap <b>13.</b> As will be appreciated, the shape of the current magnitude distribution is sinusoidal and is a standing wave, e.g.: <maths id="math0002"><math display="block"><mi mathvariant="normal">I</mi><mo>∝</mo><mfenced open="|" close="|"><mi>sin</mi><mspace width="1em" /><mfenced open="[" close="]"><mfenced><mi mathvariant="normal">δ</mi><mo>/</mo><mn>2</mn></mfenced><mo>+</mo><mi mathvariant="normal">γ</mi></mfenced></mfenced></math><img file="EP2178166B1_D0002.tif" /></maths> Where: <ul id="ul0003" list-style="none" compact="compact"><li>I = the loop current in amps</li><li>δ, γ as depicted in <figref idref="f0001">FIG. 1</figref>.</li></ul> Although not plotted, the phase of the current distribution around the loop antenna <b>10</b> was nearly a constant value everywhere around the loop antenna, e.g., uniform in phase. In an NEC4.1 analysis of the Table 1 prototype, the phase of the current was between 2.8 and 4.6 degrees at all points along the loop. The current amplitude is always zero across the second gap <b>14,</b> so repositioning the second gap <b>14</b> moves the standing wave maxima and minima around the loop conductor, and the first gap <b>13</b> may lie at a current maxima, current minima, or anywhere in between, as may benefit driving resistance needs.

0046Referring again to <figref idref="f0001">FIG. 1</figref>, a virtual ground node <b>18</b> for the loop antenna <b>10</b> is at the current maxima along the second electrical conductor <b>12,</b> which occurred near δ = 260 degrees for the loop antenna in the Table 1 example. The virtual ground node <b>18</b> is a point at which an electrical connection can be made to the loop antenna <b>10</b> with minimal electrical disturbance. For example, a metallic mast or metal handle (not shown) may be attached to the loop antenna <b>10</b> at the virtual ground node <b>18</b> without significant change to antenna radiation patterns or driving impedance. For outdoor use, an earth ground wire (not shown) may be connected at the virtual ground node <b>18</b> to drain static charge.

0047Referring now to <figref idref="f0009">FIG. 6</figref>, an additional embodiment of the loop antenna <b>10'</b> is described. The loop antenna <b>10'</b> includes an inset coaxial feed, which may be mechanically coupled or for operation without a balun. The loop antenna <b>10'</b> illustratively includes a coaxial transmission line <b>74'</b> having an inner conductor <b>70'</b> and outer conductor <b>72'.</b> The coaxial transmission line <b>74'</b> may include a dielectric fill (not shown) between the inner conductor <b>70'</b> and the outer conductor <b>72'.</b> The outer conductor <b>72'</b> is removed at the first gap <b>13',</b> and the inner conductor <b>70'</b> illustratively extends beyond the first gap 13' to define the second electrical conductor 12'. The first gap <b>13'</b> is measured by the radial distance separating the inner conductor <b>70'</b> and the outer conductor <b>72',</b> and is illustratively smaller than the second gap <b>14'.</b>

0048Additionally, as can be appreciated by those in the art, a coaxial connector (not shown) may be configured at the first gap <b>13',</b> and the second electrical conductor <b>12'</b> may be formed by a separate conductive structure. The virtual ground node <b>18'</b> conductively attaches the first electrical conductor <b>11'</b> to the outer conductor <b>72'</b> of coaxial transmission line <b>74'</b> at bend <b>32'.</b> Attachment may be by soldering or clamping, for example, or other form of attachment, as will be appreciated by those skilled in the art. The inner conductor <b>72'</b> does not make any conductive connection to the first electrical conductor <b>11'</b> at the bend <b>32'.</b> The bend <b>32'</b> in the coaxial transmission line <b>74'</b> may be in any direction, although it may be preferred that the coaxial transmission line exit at a right angle to loop. Between the bend <b>32'</b> and the first gap <b>13',</b> the loop antenna <b>10'</b> is formed from the outside of the outer conductor <b>72',</b> e.g., an "inset feed".

0049Additionally, when the bend <b>32'</b> occurs at the virtual ground node <b>18'</b> of the loop antenna <b>10',</b> common mode currents are diminished along the coaxial transmission line <b>74'</b> beyond the first and second electrical conductors <b>11', 12',</b> such that a balun function is provided by the inset feed geometry of the loop antenna. As will be appreciated by those skilled in the art, coaxial transmission lines <b>74'</b> are capable of carrying radio frequency (RF) currents on their outer surface, in addition to the internal RF currents associated with power transmission. This effect is advantageously used to provide a portion of the loop antenna 10', and on the portion of the coaxial transmission line <b>74'</b> external to the loop antenna. This effect is also avoided by joining the coaxial transmission line 74' at a current maxima or virtual ground point <b>18'</b> of a low RF impedance and electrical symmetry in the loop antenna <b>10'.</b> Thus, the coaxial transmission line <b>74'</b> is coupled to radiate internally to the loop antenna <b>10'</b> and to not radiate externally to the loop antenna.

0050Illustratively, two optional dielectric bodies <b>20'a, 20'b</b> are adjacent each side of the second gap <b>14'</b> to provide fine frequency adjustment post manufacture, e.g., tuning. The dielectric bodies <b>20'</b> may have different dielectric constants. Suitable materials for the dielectric bodies <b>20'</b> can include styrene (C₈H₈), alumina (Al₂O₃), or barium titanate (BaTiO₃), or other dielectric material as will be appreciated by those skilled in the art. No dielectric bodies <b>20'</b> may be used if no tuning effect is needed. Although cylindrical shapes may be preferred for the dielectric bodies <b>20',</b> other shapes may be used. In other embodiments, the dielectric bodies 20' may be coupled to at each side of the second gap <b>14',</b> and may be attached with adhesives, plastic clamps (not shown), or other forms of attachment.

0051Referring now to <figref idref="f0010">Fig. 7</figref>, another aspect is directed to a communications device <b>20</b> illustratively including a housing <b>21.</b> The loop antenna <b>10</b> is illustratively carried by the housing <b>21</b> and includes first and second electrical conductors <b>11, 12</b> arranged to define a circular shape with first and second spaced apart gaps <b>13, 14</b> therein. The loop antenna <b>10</b> further includes opposing portions of the first and second electrical conductors <b>11, 12</b> at the first gap <b>13</b> defining a signal feedpoint <b>15.</b>

0052Opposing portions of the first and second electrical conductors <b>11, 12</b> at the second gap <b>14</b> define an impedance tuning feature. The second gap <b>14</b> is circumferentially spaced from the first gap <b>13</b> less than ninety degrees. The second gap has a greater angular width than the first gap to provide a predetermined impedance and an isotropic radiating pattern at a predetermined operating frequency for the loop antenna as discussed above.

0053The communications device <b>20</b> also includes circuitry <b>22</b> carried by the housing <b>21</b> and cooperating with the loop antenna <b>10</b> to process a signal therethrough. Additionally, the communications device <b>20</b> also includes a feed line <b>23</b> coupling the loop antenna <b>10</b> to the circuitry <b>22.</b> Moreover, it should be understood that the loop antenna <b>10</b> may be embodied in various communications devices <b>20,</b> such as RFID tags, RFCD radios, GPS receivers, cellular telephones, pages, WLAN cards, or other mobile wireless communications devices.

0054Referring again to <figref idref="f0001">FIG. 1</figref>, another aspect is directed to a method of making the loop antenna <b>10.</b> The method includes arranging first and second electrical conductors <b>11, 12</b> to define a circular shape with first and second spaced apart gaps <b>13, 14</b> therein so that opposing portions of the first and second electrical conductors at the first gap <b>13</b> define a signal feedpoint <b>15.</b> The first and second electrical conductors <b>11, 12</b> are also arranged so that their opposing portions at the second gap <b>14</b> define an impedance tuning feature. The method further includes arranging the first and second electrical conductors <b>11, 12</b> so that the second gap <b>14</b> is circumferentially spaced from the first gap <b>13</b> less than ninety degrees and forming the second gap to be greater than the first gap to provide a predetermined impedance and an isotropic radiating pattern at a predetermined operating frequency for the loop antenna <b>10.</b>

0055As can be appreciated, isotropic antennas provide omnidirectional radiation patterns in all planes. Thus, the loop antenna <b>10</b> is also an omnidirectional antenna at any orientation. When mounted in the horizontal plane, the loop antenna <b>10</b> is well suited for FM broadcast reception with horizontal polarization, and is significantly smaller in size than the ½ wave dipole or dipole turnstile. At United States FM broadcast frequencies (88 - 108 MHz), the diameter of the loop antenna <b>10</b> is about <b>48.26</b> cm (19 inches), while a half wave dipole is 152.4 cm (60 inches) long.

0056The loop antenna <b>10</b> is also useful for HF (high frequency) service as the radiation pattern includes NVIS (near vertical incidence) coverage, and it may be a wire structure supported on poles. The poles need only form loop conductors <b>11, 12</b> in a polygonal shape, which approximates the circular embodiment illustrated in <figref idref="f0001">FIG. 1</figref>. Of course the loop antenna <b>10</b> may operate on other frequencies.

0057Thus, the loop antenna <b>10</b> provides a substantially isotropic radiation pattern with high radiation efficiency and sufficient gain for many purposes. It operates at a reduced size relative wavelength, is planar for inexpensive manufacture, and it may avoid the need for antenna aiming. Accordingly, the loop antenna <b>10</b> is particularly advantageous for portable, unoriented devices, such as personal communications or radio location devices, such as tracking tags. Of course, the loop antenna <b>10</b> may be used in other devices, as will be appreciated by those skilled in the art.