Wireless communications device including side-by-side passive loop antennas and related methods

A wireless communications device may include a housing, and wireless communications circuitry carried by the housing. The wireless communications device may also include an antenna assembly carried by the housing and coupled to the wireless communications circuitry. The antenna assembly may include a substrate and a plurality of passive loop antennas carried by the substrate and arranged in side-by-side relation. Each of the plurality of spaced apart passive loop antennas may include a passive loop conductor and a tuning element coupled thereto. The antenna assembly may also include an active loop antenna carried by the substrate and arranged to be at least partially coextensive with each of the plurality of passive loop antennas. The active loop antenna may include an active loop conductor and a pair of feedpoints defined therein.

FIELD OF THE INVENTION

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

BACKGROUND OF THE INVENTION

Antennas 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 wireless communications device, which is nearly ubiquitous. A relatively small size, increased efficiency, and a relatively broad radiation pattern are generally desired characteristics of an antenna for a portable radio or wireless device. Additionally, as the functionality of a wireless device continues to increase, so too does the demand for a smaller wireless device which is easier and more convenient for users to carry. One challenge this poses for wireless device manufacturers is designing antennas that provide desired operating characteristics within the relatively limited amount of space available for antennas. For example, it may be desirable for an antenna to communicate over multiple frequency bands and at lower frequencies.

Newer designs and manufacturing techniques have driven electronic components to relatively small dimensions and reduced the size of many wireless communication devices and systems. Unfortunately, antennas, and in particular, broadband antennas, have not been reduced in size at a comparable level and often are one of the larger components used in a smaller communications device.

Indeed, antenna size may be based upon operating frequency or frequencies. For example, an antenna may become increasingly larger as the operating frequency decreases. Reducing the wavelength may reduce the size of the antenna, but a longer wavelengths may be desired for enhanced propagation. At high frequencies (HF), 3 to 30 MHz for example, used for long-range communications, efficient antennas, for example, transmitting antennas, may become too large to be portable, and wire antennas may be required at fixed stations. Thus, it may become increasingly important in these wireless 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 over the desired frequency bands.

The instantaneous 3 dB gain bandwidth, also known as half power fixed tuned radiation bandwidth, of electrically small antennas is thought to be limited under the Chu-Harrington limit (“Physical Limitations Of Omni-Directional Antennas, L. J. Chu, Journal of Applied Physics, Vol. 19, pp 1163-1175, December 1948). One form of Chu's Limit provides that the maximum possible 3 dB gain antenna bandwidth limited to 1600(πr/λ)3percent, where r is the radius of the smallest sphere that can enclose the antenna, and λ is the free space wavelength. This may be for single mode antennas matched into circuits. Unfortunately, such an antenna fitting inside a radius=λ/20 spherical envelope may not have more than 6.1% of this bandwidth. Further, practical antennas seldom approach the Chu's limit bandwidth. An example is a relatively small helix antenna enclosed by r=λ/20 sphere size operated at 1.2% bandwidth, e.g. ⅕ of Chu's Limit. Small antennas having increased bandwidth for size may thus be desired.

Canonical antennas include dipole and the loop antennas, in line and circle shapes. They translate and rotate electric currents to realize the divergence and curl functions, for example. Various coils may form hybrids of the dipole and the loop. Antennas may be linear, planar, or volumetric in form, e.g. they may be nearly 1, 2 or 3 dimensional. Optimal envelopes for antenna sizing may be Euclidian geometries such as a line, a circle, and a sphere, which may provide increased optimization of a relatively short distance between two points, increased area for circumference, and increased volume for decreased surface area respectively. It may be desirable to know the antennas that provide the greatest radiation bandwidth in these sizes. A broadband electrically large (r>λ/2π) antenna, for example, the spiral antenna, may provide a high pass response with theoretically unlimited bandwidth above a lower cutoff. At electrically small size, however, (r<λ/2π), the spiral may provide only a quadratic, bandpass type response with greatly limited bandwidth.

Planar antennas may be increasingly valuable for their ease of manufacture and product integration. The elementary planar dipole may be formed by radial electric currents flowing on a metal disc (“Theory Of The Circular Diffraction Antenna,” A. A. Pistolkors, Proceedings of the Institute Of Radio Engineers, January 1948, pp 56-60). Circular and linear notches for feeding may be desired. A circle of wire may give the same radiation pattern, and it may be preferred for ease of driving. Elements to extend the bandwidth of wire loop antennas may be desired. Radio wave expansion occurs at the speed of light. If the speed of light were reduced, antenna size would also be reduced.

U.S. Patent Application Publication No. 2009/0212774 to Bosshard et al. discloses an antenna arrangement for a magnetic resonance apparatus. In particular, the antenna arrangement includes at least four individually operable antenna conductor loops arranged in a matrix (i.e. rows and columns) configuration. Two antenna conductor loops adjacent in a row or column are inductively decoupled from one another, while two antenna loops diagonally adjacent to one another are capacitively decoupled from one another.

U.S. Patent Application Publication No. 2009/0009414 to Reykowsi discloses an antenna array. The antenna array includes multiple individual antennas arranged next to one another. The individual antennas are arranged within a radio-frequency closed conductor loop with capacitors inserted in each conductor loop.

U.S. Patent Application Publication No. 2010/0121180 to Biber et al. discloses a head coil to a magnetic resonance device. A number of antenna elements are carried by a supporting body. The supporting body has an end section that is shaped as a spherical cap. A butterfly antenna is mounted at the end of the section, and is annularly surrounded by at least one group antenna that overlaps the butterfly antenna. However, none of these approaches are focused on providing an antenna with multi-band frequency operation, while being small in size, and having desired gain for area.

SUMMARY OF THE INVENTION

In view of the foregoing background, it is therefore an object of the present invention to provide a relatively small size multi-band antenna.

This and other objects, features, and advantages in accordance with the present invention are provided by a wireless communications device that includes a housing and wireless communications circuitry carried by the housing. The wireless communications device also includes an antenna assembly carried by the housing and coupled to the wireless communications circuitry, for example.

The antenna assembly includes a substrate, and a plurality of passive loop antennas carried by the substrate and arranged in side-by-side relation. Each of the plurality of passive loop antennas includes a passive loop conductor and a tuning element coupled thereto, for example.

The antenna assembly also includes an active loop antenna carried by the substrate and arranged to be at least partially coextensive with each of the plurality of passive loop antennas. The active loop antenna includes an active loop conductor and a pair of feedpoints defined therein, for example. Accordingly, the antenna assembly has a relatively reduced size, while maintaining performance, for example, by providing multi-band frequency operation, and providing increased gain with respect to area.

Each of the plurality of passive loop antennas may have a respective straight side adjacent each neighboring passive antenna. Each of the plurality of passive loop antennas may have a polygonal shape, for example. The polygonal shape may be one of a square shape, a hexagonal shape, and a triangular shape. Each of the plurality of passive loop antennas may have a same size and shape.

The active loop antenna may have a circular shape, for example. The plurality of passive loop antennas may define a center point. The active loop antenna may be concentric with the center point, for example.

Each of the tuning elements may include a capacitor, for example. The plurality of passive loop antennas may be positioned on a first side of the substrate and the active loop antenna is positioned on a second side of the substrate, for example. Each of the passive loop conductors and the active loop conductor comprises an insulated wire.

A method aspect is directed to a method of making an antenna assembly to be carried by a housing and to be coupled to wireless communications circuitry. The method includes positioning a plurality of passive loop antennas to be carried by a substrate in side-by-side relation. Each of the plurality of passive loop antennas includes a passive loop conductor and a tuning element coupled thereto, for example. The method also includes positioning an active loop antenna to be carried by the substrate and to be at least partially coextensive with each of the plurality of passive loop antennas. The active loop antenna includes an active loop conductor and a pair of feedpoints defined therein, for example.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring initially toFIG. 1, a wireless communications device10includes a housing11and wireless communications circuitry12carried by the housing. The wireless communications circuitry12may be cellular communications circuitry or radiolocation tag circuitry, for example, and be configured to communicate voice and/or data. The wireless circuitry12may be configured to communicate over a plurality of frequency bands, for example, cellular, WiFi, and global positioning system (GPS) bands. Of course, the wireless communications circuitry12may be configured to communicate over other frequency bands. Other circuitry, for example, a controller13may be carried by the housing11and coupled to wireless communications circuitry12. Additionally, the wireless communications device10may include an input device (not shown), for example, input keys and/or a microphone, and an output device (not shown), for example, a display and/or speaker, coupled to the controller13and/or wireless communications circuitry12.

The wireless communications device10also includes an antenna assembly20carried by the housing11and coupled to the wireless communications circuitry12. The antenna assembly20illustratively includes a substrate21. The substrate21may be a printed circuit board substrate, for example, and may carry other components, as will be appreciated by those skilled in the art. The antenna assembly20also includes three same-sized hexagonal shaped passive loop antennas22a-22ccarried by the substrate21. The passive loop antennas22a-22care arranged in a side-by-side relation. In the illustrated embodiment, each of the three passive loop antennas22a-22chas a respective straight side adjacent each neighboring passive antenna. In a preferred embodiment, for example, the passive loop antennas22a-22ceach have a circumference of 0.5 wavelengths or less at the operating frequency, e.g. the passive radiating loop antennas are naturally resonant or electrically small relative to the wavelength.

As will be appreciated by those skilled in the art, each of the hexagonal passive loop antennas22a-22cmay be considered as an individual antenna element such that the combined electrical characteristics act like a loop antenna array. The hexagonal shape of the passive loop antennas22a-22ccreates a honeycomb lattice which advantageously provides an increased efficiency usage of space. The hexagonal tiling of space filling polyedra may be particularly advantageous in a portable wireless communications device where the housing21is relatively limited in size. The hexagonal shape of the passive loop antennas develop an increased radiation resistance at a reduced conductor loss for an increased efficiency gain and reduced overall size.

Each of the passive loop antennas22a-22cincludes a passive loop conductor27a-27cand a tuning element28coupled thereto. As will be appreciated by those skilled in the art, the tuning element28determines the frequency band of a particular passive loop antenna22, and not the size thereof. Instead, the size of each passive loop antenna22is related to the gain of the antenna assembly20at the frequency band corresponding to the respective passive loop antenna.

Each passive loop antenna22also includes a dielectric insulation layer29surrounding the passive loop conductor27. In other words, each passive loop antenna22may be an insulated wire. The tuning element28is illustratively a capacitor and coupled inline with the passive loop conductor27. Of course, the tuning element28may be another type of component, for example, an inductor, and may not be coupled inline, for example, a ferrite bead may instead surround the passive loop conductor27and the dielectric insulation layer29. When the tuning element28is a capacitor, for example, the passive loop antennas22a-22cbecome electrically loaded so that they operate at a smaller physical size and/or lower frequency. Thus, the tuning element28, or capacitor, reduces the size.

As will be appreciated by those skilled in the art, the active loop antenna23cooperates with the passive loop antennas22a-22cby inductive coupling such that the passive loop antennas act as three independent tunable antennas. Independent tuning of each of the passive loop antennas22a-22cis accomplished by selecting or changing the value of each of the tuning elements28, in particular, the capacitance.

The antenna assembly20also includes an active loop antenna23carried by the substrate21. The active loop antenna23illustratively has a circular shape and is partially coextensive with each of the plurality of passive loop antennas22a-22c. In other words, the areas of the active loop antenna23and passive loop antennas22a-22cmay overlap without touching one another. The active loop antenna includes an active loop conductor25and a pair of feedpoints26a,26bdefined therein. The active loop antenna23may also include an insulation layer36surrounding the active loop conductor25. In other words, the active loop antenna23may also be an insulated wire. The respective insulation layers advantageously provide dielectric spacing between the passive loop antennas22a-22cand the active loop antenna23so that they do not short circuit.

Illustratively, the side-by-side relation of the passive loop antennas22a-22cdefines a center point24, and the active loop antenna23is illustratively concentric with the center point. Of course, the active loop antenna23may not be concentric with the center point24in other embodiments. As will be appreciated by those skilled in the art, adjustment of an amount of offset may affect an amount of power coupled to each of the passive loop antennas22a-220.

A feed conductor31or cable may couple the antenna assembly20to the wireless communications circuitry12via the feedpoints26a,26b. The feed conductor31may be coaxial cable, for example, and may include a center conductor32coupled to one of the feedpoints26a,26band an outer conductor34coupled to the other of the feedpoints, and separated from the inner conductor by a dielectric layer33. Other types of cables or conductors may be used, such as, for example, a twisted pair of insulated wire. In some instances, the feed cable31may itself become an antenna. Advantageously, the active loop antenna23may provide a balun to reduce the effect of the feed cable31inadvertently becoming an antenna. This is because the passive loop antennas22a-22cdo not have a direct current (DC) connection to the feed cable31(i.e. there is no conductive contact, but rather inductive coupling). The active loop antenna23may also function as balun or “isolation transformer” to reduce common mode currents on coaxial feedlines, for example.

Referring now toFIG. 2, a graph50is shown of the measured frequency response, or voltage standing wave ratio, of a multiple band prototype antenna assembly similar to the antenna assembly20as illustrated inFIG. 1. The prototype antenna assembly included three hexagonal passive loop antennas and a circular active loop antenna. A first capacitor had a value of 30 picofarads, a second capacitor was 10 picofarads, and a third capacitor was 20 picofarads. Thus, each passive loop antenna loop had a different value tuning capacitor. The graph50illustratively includes three bands,51a,51b,51cat about 86 MHz, 106 MHz, and 144 MHz respectively, that were independently realized based upon the values of the respective capacitors. A summary of the multiple band prototype is as follows:

Individual electrically small antennas, for example, may have a quadratic frequency response. Thus, such antennas may cover a single frequency band that may be relatively narrow. The antenna assembly20, however, may be tuned so that each of the three frequency bands may be combined to form single enlarged or broad frequency band with respect to each frequency band individually. More particularly, the resonance of each hexagonal shaped passive loop antenna22a-22cmay be adjusted according to the Chebyschev polynomial to provide an increased bandwidth to a specified ripple. For example, each of the passive loop antennas may be stagger tuned to the zeroes of the nth order Chebyshev polynomial. For example, two passive loop antennas can provide a 4thorder Chebyschev response with 2 ripple peaks and about 4 times the bandwidth of a single passive loop antenna.

More particularly, for example, an antenna assembly having a single hexagonal shaped passive loop antenna has a quadratic response according to ax2+bx+c=0. For example, if the single hexagonal shaped passive loop antenna has a diameter of 0.12λ, the 6:1 voltage standing wave ratio (VSWR) bandwidth is about 1.52%. An antenna assembly according to the present invention, having, for example, two hexagonal shaped passive loop antennas has a Chebyshev polynomial response according to:
Σ=Tn(x)tn=(1−tx)/(1−2tx+t2)
Where:Tn=Chebyschev polynomial of degree nx=angular frequency=2πf

Thus, if each hexagonal shaped passive loop antenna also has a diameter of 0.12λ, the bandwidth is about 4×1.52% or 6.1%. The ripple frequency of the Chebyschev polynomial generally increases with the order n so when ripple amplitude is held constant, a diminishing return occurs with increasing order n. An infinite number of passive loop antennas, for example, may provide up to 3π more instantaneous bandwidth than a single radiating loop antenna, as will be appreciated by those skilled in the art. Testing has shown that two passive loop antennas provide four times the bandwidth of a single passive loop antenna. Thus, the embodiments advantageously provide a loop antenna array with versatile tunings for reduced size and increased instantaneous bandwidth. The embodiments advantageously provide the versatile tunings through radiating structures rather than external lumped element networks of passive components, for example, without a ladder network of inductors and/or capacitors. Referring now to the graphs61,62,63,64,65inFIGS. 3a-3d, and4, the radiation pattern of the antenna assembly20is generally toroidal. The graph61illustrates the plane of the antenna assembly20in a Cartesian coordinate system. As will be appreciated by those skilled in the art, the plane of the antenna assembly20lies in the XY plane. The graph62illustrates that the XY plane radiation pattern cut of the antenna assembly20is circular and omnidirectional.

Similarly, the graphs63,64, respectively illustrate that the shape of the radiation pattern cuts in the YZ and ZX planes are that of a two petal rose having the function cos2θ. The radiation pattern is a Fourier transform of the current distribution around the loop which is uniform at smaller loop sizes. The antenna assembly20radiation pattern shape is similar to a canonical ½ wave wire dipole oriented along the graph61Z axis, although the ½ wave dipole will be vertically polarized and the antenna assembly20will be horizontally polarized. Horizontal polarization may be particularly advantageous to aid in long range propagation by tropospheric refraction, for example. Moreover, the antenna assembly20has radiation pattern nulls broadside the antenna plane, and the radiation pattern lobe is in the antenna plane. The half power beamwidth of the antenna assembly20in the YZ and ZX pattern cuts is about 82 degrees. The directivity is 1.5. When mismatch loss is zero, for example, the realized gain and radiating pattern, as will be appreciated by those skilled in the art, may be calculated according to:
Realized Gain=10 log10(ηDcos2θ)
Where:η=the radiation efficiency of the antenna assembly20D=the antenna directivity=1.5 for the antenna assembly20Θ=the elevation angle measured from normal to the plane of the antenna assembly20. (θ=0° normal to the antenna plane and θ=90° in the antenna assembly plane)

In practice, with relatively low loss tuning capacitors, the radiation efficiency η is mostly a function of the passive loop antenna22a-22cradiation resistance Rrrelative the passive loop antennas conductor loss resistance Rlso the radiation efficiency may be calculated as:
Radiation Efficiency η=RR/(Rr+Rl)
and the realized gain as:
Realized Gain=1.76−10 log10(Rr/(Rr+Rl) dBil

The graph65inFIG. 4illustrates the typical relationship (calculated) between size, realized gain, and frequency for a single hexagonal passive loop antenna. The graph65inFIG. 4also illustrates the typical realized gain provided by an embodiment of the antenna assembly. The antenna assembly corresponding to the graph65is a single passive loop antenna similar to the antenna assembly20inFIG. 1, and is copper and greater than 3 RF skin depths thick. The antenna assembly is tuned and matched, by using radiation pattern peak gain, for example, and the polarization is co-polarized. The tuning element is a capacitor having quality factor Q=1000, and the passive loop antenna trace width is about 0.15 inches at the passive loop antenna outer diameter. Illustratively, the lines66,67,68, and69correspond to +1.5, 0.0, −10.0, and −20.0 dBil realized gain, respectively. As will be appreciated by those skilled in the art, the embodiments advantageously allowing tradeoffs between antenna size and realized gain and provide increased efficiency with respect to size.

In a test of a prototype antenna assembly similar to the antenna assembly20ofFIG. 1, the antenna assembly was used for radiolocation purposes using Global Positioning System (GPS) satellites. The antenna assembly provided relatively high GPS satellite constellation availability so many satellites could be received at once. A performance summary for the prototype antenna assembly GPS reception is a follows:

The GPS prototype had the operative advantage of reduced deep cross sense circular polarization fades. Right hand circularly polarized microstrip patch antennas tend to become left hand circularly polarized when inverted, which can produce deep fades in GPS reception. Thus, when wireless communications circuitry includes a GPS radiolocation tag, for example, with an antenna assembly, the antenna assembly provided increased reliability reception than a microstrip patch antenna having circular polarization and higher gain, for example. In GPS radiolocation devices, the antenna is generally un-aimed and unoriented. Indeed, in the present embodiment, when the circumference of the passive loop antenna approaches ½ wavelength, the radiation pattern becomes nearly spherical and isotropic.

Referring now additionally toFIG. 5, the circuit equivalent model of the antenna assembly20may be regarded as a transformer with multiple secondary windings, so that a power divider is realized, for example. The signal generator S corresponds to the wireless communications circuitry12. As will be appreciated by those skilled in the art, the active loop antenna23corresponds to a primary winding L, while the three hexagonal passive loop antennas22a-22ccorrespond to respective secondaries k1, k2, k3. Power may be equally divided three-ways, by the active loop antenna23being concentric with the center point24defined by the three hexagonal passive loop antennas22a-22c. Adjustment of the amount of coextension of the three hexagonal passive loop antennas22a-22cover the active loop antenna23is equivalent to adjustment of the “turns ratio” of conventional transformers having multiple turn windings.

In the illustrated corresponding circuit schematic diagram, the equivalent tuning elements are the capacitors C1, C2, C3. The illustrated resistors Rr1, Rr2, Rr3, correspond to the radiation resistance. In other words, this is the resistance provided by the conductor itself, for example, a copper conductor. R11, R12, R13correspond to conductor resistance loss from joule effect heating. As will be appreciated by those skilled in the art, if the antenna assembly20is too small, R1increases, and performance may decrease to a potentially unacceptable level. R1is usually the predominant determinant of the antenna efficiency. In fact, tuning capacitor equivalent series resistance (ESR) losses often may be neglected. The radiation efficiency η of an individual passive loop antenna can be therefore be approximately by:
η=Rr1/(Rl1+Rr1)
and the realized gain approximated by:
G=10 log10{1.5[Rr1/(Rl1+Rr1)]} dBil.

As background, the loss resistance of metal conductors is generally a fundamental limitation to efficiency and gain of room temperature electrically small antennas. When electrically small, the directivity of an individual passive loop antenna is 1.76 dB. This value of directivity does not significantly increase or decrease with the number or passive loop antennas. In typical practice, the active loop antenna may be adjusted to provide 50 ohms of resistance, and the metal conductor loss of the active loop may be neglected.

The passive loop antennas typically do not significantly couple to one another when their loop structures do not overlap, e.g. the mutual coupling is less than about −15 dB in those circumstances. Overlapping of the passive loop antennas may alter the mutual coupling as desired. The degree of mutual coupling adjusts the spacing between the Chebyschev responses. Thus, the features of the present embodiments allow for control of driving resistance (active loop diameter), reactance (tuning capacitor), frequency (tuning element value), element mutual coupling (spacing between passive loop antennas, size (tuning element provides loading), gain (passive loop antenna diameter), and bandwidth (the number of passive loop antennas22adjust the frequency response ripple).

Referring now toFIG. 6, another embodiment of an antenna assembly20′ illustratively includes four passive loop antennas22a′-22d′ each having a square shape and carried by a first side37′ of the substrate21′. The four passive loop antennas22a′-22d′ are illustratively arranged in side-by-side relation and define a center point24′ corresponding to a corner of each of the square passive loop antennas. The active loop antenna23′, which is carried on a second side38′ of the substrate21′, or opposite side from the passive loop antennas22′, is partially coextensive with each of the four square shaped passive loop antennas22a′-22d′. Each of the four square passive loop antennas22a′-22d′ includes a respective tuning member28a′-28d′, or capacitor coupled to respective passive loop conductors27a′-27d′. As will be appreciated by those skilled in the art, each of the four passive loop antennas22a′-22d′ corresponds to a frequency band that is determined by respective capacitors28a′-28d′.

Referring now toFIG. 7, yet another embodiment of the antenna assembly20″ illustratively includes eight passive loop antennas22a″-22h″ each having a triangular or pie shape. The eight passive loop antennas22a″-22h″ are illustratively arranged in side-by-side relation and define a center point24″ corresponding to a point of each of the triangular passive loop antennas. The active loop antenna23″ is partially coextensive with each of the eight triangular shaped passive loop antennas22a″-22h″. Each of the eight triangular passive loop antennas22a″-22″ includes a respective tuning member28a′-28d′, or capacitor, coupled to respective passive loop conductors27a″-27h″. As will be appreciated by those skilled in the art, each of the eight passive loop antennas27a″-27h″ corresponds to a frequency band that is determined by respective capacitors28a″-28h″.

While each passive loop antenna22described herein is illustratively a same size shape, the passive loop antennas may have any polygonal shape. Additionally, in some embodiments, each of the passive loop antennas22may not be the same size.

A method aspect is directed to a method of making an antenna assembly20to be carried by a housing11and to be coupled to wireless communications circuitry12. The method includes positioning a plurality of passive loop antennas22to be carried by a substrate21in side-by-side relation. Each of the passive loop antennas22include a passive loop conductor27and a tuning element28coupled thereto. The method also includes positioning an active loop antenna23to be carried by the substrate21and to be at least partially coextensive with each of the passive loop antennas22. The active loop antenna23includes an active loop conductor25and a pair of feedpoints26a,26b, defined therein.

Referring now to the graph100inFIG. 8, the gain response of a double tuned/4thorder Chebyschev embodiment of the antenna assembly is illustrated. Illustratively, there is a rippled passband106with two gain peaks, but the two peaks of passband are considered as being a single continuous passband, e.g. so a single band antenna with ripple is formed. Ripple in the passband106may be particularly beneficial to provide increased bandwidth, for example. The antenna assembly corresponding to the graph100includes two (2) passive loop antennas are adjacent each other with one (1) active loop antenna overlapping each passive loop antenna. To realize the double tuned 4thorder Chebyschev polynomial response, the radiating loop antennas are preferentially of equal size, and they use similar or identical value tuning element capacitors. Thus, the individual resonant frequencies of the passive loop antennas are the same by themselves. However, when the passive loop antennas are brought relatively close to each other, mutual coupling may cause the two gain peaks106,108in the frequency response to form. The quadratic responses of two individual passive loop antennas thus combine to become a double tuned 4thorder Chebyschev response.

The ripple amplitude104and the bandwidth106may be adjusted by adjusting the spacing of the passive loop antennas with respect to each other. When the two passive loop antennas are further apart, the spacing between gain peaks102is reduced and so the bandwidth106?? is reduced, and the ripple level amplitude104is reduced.

When the spacing between the two passive loop antennas are closer, the spacing102between the gain peaks108,110is increased (the responses spread apart), so the bandwidth106is increased, and the ripple amplitude104is increased. The two passive loop antennas may even overlap each other (but not touch each other) to create relatively very large bandwidths. As can be appreciated, the double tuned 4thorder Chebyschev embodiment advantageously provides a wide and continuous range of tradeoff between ripple level104and bandwidth106.

In the double tuned 4thorder Chebyschev embodiment using two passive loop antennas, the diameter of the active loop antenna adjusts the circuit resistance that the antenna provides to the wireless communications circuitry. A larger diameter active loop increases the resistance provided to the transmitter, and a smaller diameter active loop reduces the resistance provided to the transmitter. 50 ohms resistance has been readily achievable in practice when the diameter of the active loop was about 0.2 to 0.5 the diameter of a passive loop antennas. The size of the active loop antenna may be adjusted to obtain active and 1 to 1 VSWR. Alternatively, the active loop antenna may be increased in size to provide an overactive trade for increased bandwidth with increased VSWR at the two gain peaks108,110.

The active loop antenna advantageously provides a resistance compensation over a given frequency. In other words, as the passive loop antennas become smaller, their radiation resistance drops, but the coupling factor of the active loop antenna increases as the passive loop antennas become smaller. Thus, the desired resistance seen by the electronics circuitry may be constant over a relatively broad bandwidth. The compensation behavior is thought to be due to the transition in the passive loop antennas' current distribution from sinusoidal to uniform with reduced passive loop antenna circumference. Loop antennas have stronger magnetic near fields when electrically small so they become better transformer secondaries. The passive loop antenna is a far field antenna for radiation, and also a near field antenna.

Highest gain results when the electrical conductor forming the passive loop antennas have a width near 0.15 that of the loop outer diameter. Thus, if a passive loop antenna has an outside diameter of 1.0 inch, and each passive loop antenna is wire, the highest realized gain typically occur when the wire diameter is 0.15 inches. If the passive loop antenna is 1 inch in diameter and formed as a printed wiring board (PWB) trace, the width of that trace should be also about 0.15 inches for increased radiation efficiency. Of course other conductor widths can be used if desired.

The conductor loss resistance is increased when the trace width is too small as there is too little metal to conduct efficiently. Yet, when the trace width is too large, proximity effect increases the conductor loss resistance. When conductor proximity effect occurs, the current hugs the inside edge of the loop conductor and not all the metal is put used for radiating. The loop conductor on the opposite side of the loop causes the proximity effect. The hole in the loop should generally be sized appropriately. The optimal loop conductor trace width for the passive loop antennas was verified by experiment.

The graph110ofFIG. 9illustrates the measured quality factor (Q)111of a PWB embodiment single passive loop antenna versus loop conductor trace width. Q is an indication of antenna gain so when the Q is highest the realized antenna gain is highest. The outer loop diameter was 1.0 inch and it was operated at 146.52 MHz so the outer loop diameter was λ/84. Thus, critical active and resonance at 146.52 MHz was considered and adjusted. The thickness of the PWB copper traces was greater than 3 skin depths thick. When the loop antenna hole was 90 percent of the outer diameter, a 22 picofarad capacitor was connected across a gap in the loop to cause set the resonance at 146.52 MHz. When the passive loop antenna internal hole size was zero, the antenna was effectively a notched metal disc. It used a 290 picofarad chip capacitor across the notch at the disc rim, and the resonance was again at 146.52 MHz. As illustrated from the graph110inFIG. 9the best measured Q111was 225, and this occurred when the diameter of the inner hole was 70 percent that of the loop outer diameter. The loop outer diameter was 1.0 inches, and the loop inner diameter equaled 0.7 inches at highest Q and realized gain. The trace width for the best realized gain was therefore (1.0−0.7)/2=0.15 the loop outer diameter.

The active loop antenna23typically does not radiate appreciably or have significant ohmic losses. As background, the active loop antenna23also provides a balun of the isolation transformer type.

Testing has shown that losses in G10 and FR4 type epoxy glass printed circuit board embodiments of the antenna assembly20have been negligible at UHF, e.g. at frequencies between 300 MHz and 3000 MHz. Thus, most commercial circuit materials are generally suitable for the substrate21. The antenna assembly20accomplishes this operative advantage by having stronger radial magnetic near fields rather than radial electric near fields which minimizes PWB dielectric losses. Additionally, the antenna assembly20tuning and loading is accomplished by component capacitors rather than the PWB dielectric. For example, chip capacitors are relatively inexpensive and low loss, and the NPO variety has relatively flat temperature coefficients. Stable capacitance over temperature means that the antenna assembly20can have relatively stable frequency of operation over temperature. This can be an advantage of the antenna assembly20over typical microstrip patch antennas, for example.

As background, microstrip patch antennas may require costly, low loss controlled permittivity materials as the antenna “patch” forms a printed circuit transmission line concentrating electric near fields in the PWB dialectic. The capacitance of microstrip patch antenna PWB materials is generally not as stable over temperature as are NPO chip capacitors. Thus antenna20may have stable tuning along and may be planar and relatively easy to construct at a relatively low expense.

The present embodiments advantageously provide multi-band operation and/or to provide relatively broad single band bandwidth with a Chebyschev passband response. However, embodiments of the antenna assembly also provide broad tunable bandwidth. Variable tuning over a wide range is accomplished by varying the reactance of a tuning element28, for example. Thus, the tuning element28may be a variable capacitor, for example. The tunable bandwidth can be over a 7 to 1 frequency range with a relatively low voltage standing wave ratio (VSWR). In an HF prototype, a VSWR under 2 to 1 was realized across a continuous 3 to 22 MHz tuning range using a vacuum variable capacitor having a range of 10 to 1000 picofarads, and the passive loop antenna22was formed from a hexagon of copper water pipe having a circumference of 18 feet. The change in the antenna operating frequency is the square root of the reactance change in tuning element28, such that, for example, to double the operating frequency the tuning element the capacitor value is reduced to ½2=¼ of original value. The tuning element28may be a varactor diode for electronic tuning, for example. The desired value of the tuning element28may be calculated from the common resonance formula ½π√LC once the inductance of the passive loop antenna22is known. The inductance of the passive loop antenna22can be measured or calculated using the formula:
Lin micro-henries=0.01595[2.303 Log10(8D/d−2)]
Where:D=the mean diameter of the passive loop antennad=the diameter of the wire conductor

Increasing the capacitance of the tuning element28lowers the operating frequency of the antenna assembly20, and decreasing the capacitance raises the frequency. In most circumstances it is preferential to use a capacitor as the tuning element28for reduced losses, although an inductor may be used if desired. An example and application for the antenna assembly20is for television and FM broadcast reception with extended range. Typical broadcasts in these frequency bands include horizontal polarization components, and the antenna assembly20advantageously responds to horizontal polarization components when oriented in the horizontal plane. Horizontal polarization is known to propagate over the horizon by tropospheric refraction. Thus, the antenna assembly20may provide greater range than a vertical ½ wave dipole. The antenna assembly20is omni-directional when horizontally polarized, aiming may not be desired. A passive loop antenna22a-22ccan render +1.0 dBil realized gain at 100 MHz when it is 19 inches in diameter, and thus may be used indoors.

Although there are many differences between loop antennas and dipole antennas, electrically small dipole antennas and loop antennas are typically loaded to smaller size with capacitors and inductors respectively. In the current art, and at room temperature, there are better insulators than conductors, so the efficiency and Q of capacitors is usually much better than inductors. Indeed, the quality factor of capacitors is typically 10 to 100 times better than inductors. Thus, loop antennas similar to the present embodiments of the antenna assembly may be preferred over dipole antennas as they may accomplish size reduction, loading, and tuning using relatively low loss and relatively inexpensive capacitors. Loop antennas also provide an inductor and a transformer winding with limited or reduced additional components. Thus, the present embodiments provide a compound design in which the antenna inductor, matching transformer, and balun are integrated into the antenna structure.