EP0454694B1 - Reactance buffered loop antenna - Google Patents

Reactance buffered loop antenna Download PDF

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Publication number
EP0454694B1
EP0454694B1 EP90901381A EP90901381A EP0454694B1 EP 0454694 B1 EP0454694 B1 EP 0454694B1 EP 90901381 A EP90901381 A EP 90901381A EP 90901381 A EP90901381 A EP 90901381A EP 0454694 B1 EP0454694 B1 EP 0454694B1
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EP
European Patent Office
Prior art keywords
wristband
conductor
reactance
antenna
loop antenna
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired - Lifetime
Application number
EP90901381A
Other languages
German (de)
English (en)
French (fr)
Other versions
EP0454694A4 (en
EP0454694A1 (en
Inventor
William Tan
Robert Kurcbart
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Motorola Solutions Inc
Original Assignee
Motorola Inc
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Filing date
Publication date
Application filed by Motorola Inc filed Critical Motorola Inc
Publication of EP0454694A1 publication Critical patent/EP0454694A1/en
Publication of EP0454694A4 publication Critical patent/EP0454694A4/en
Application granted granted Critical
Publication of EP0454694B1 publication Critical patent/EP0454694B1/en
Anticipated expiration legal-status Critical
Expired - Lifetime legal-status Critical Current

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Classifications

    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01Q—ANTENNAS, i.e. RADIO AERIALS
    • H01Q7/00—Loop antennas with a substantially uniform current distribution around the loop and having a directional radiation pattern in a plane perpendicular to the plane of the loop
    • H01Q7/02—Collapsible antennas; Retractable antennas
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01Q—ANTENNAS, i.e. RADIO AERIALS
    • H01Q1/00—Details of, or arrangements associated with, antennas
    • H01Q1/27—Adaptation for use in or on movable bodies
    • H01Q1/273—Adaptation for carrying or wearing by persons or animals

Definitions

  • This invention relates generally to the field of loop antennas, and more particularly to a reactance buffered loop antenna suitable for use as a wristband antenna for a wrist worn electronic device.
  • One prior method of overcoming impedance variation for a loop antenna is to design a loop antenna to have a maximum length of one-tenth of the wavelength at the resonant frequency.
  • the loop antenna so designed comprises a loop conductor and a variable capacitor connected in series for resonating the antenna.
  • the loop area of the conductor, the circumferential length and the equivalent radius are adjusted so that the ratio of the resonant frequency f o of the antenna and the resonant frequency fm at which the input admittance is a minimum is maintained within the range 0.5-f o /f m -3.0.
  • One such wristband antenna consisted of a number of ferrite antenna links affixed to a rigid wristband.
  • Another wristband antenna consisted of conductors incorporated into a wristband so as to allow a stretchable wristband. Both types of antennas exhibited the same tuning problems as the non-stretchable wristband antenna. As the geometry of the loop was changed, and depending upon the position on the wrist, detuning and reduced receiver sensitivity would occur.
  • a wristband loop antenna for a wrist worn electronic device, the device including a receiver having signal and ground inputs coupled to an antenna resonating capacitor for resonating the wristband loop antenna to a predetermined frequency
  • said wristband loop antenna comprising: a first wristband section, including: a first conductor, having a first end for coupling to the receiver signal input and a second end, the first conductor forming a first portion of the wristband loop antenna within said first wristband section; and a plurality of taps linearly disposed along a longitudinal axis of the first wristband section, the distance between an outermost tap of said plurality of taps providing a predetermined length corresponding to the loop antenna diameter adjustment; and a second wristband section, including: a second conductor, having a first end coupled to the receiver ground and a second end, said second conductor forming a second portion of the wristband loop antenna within the second wristband section; and coupling means, coupled to said second end of said second conductor
  • the present invention advantageously provides a loop antenna having an adjustable size which does not require tuning when the size is changed, and which may be adapted for use with a wristworn device. Furthermore, the invention beneficially allows a wristband loop antenna which can be pre-tuned and which, when tuned, is insensitive to changes in the wristband length.
  • FIG. 1 is a diagram of a prior art wristworn device utilizing a wristband loop antenna.
  • FIG. 2A is an exploded view of one half of the adjustable strap section of FIG. 1.
  • FIG. 2B is an electrical schematic diagram of FIG. 2A.
  • FIG. 3A is a diagram of a wristband loop antenna for the preferred embodiment of the present invention.
  • FIG. 3B is a diagram of the construction of an inductive reactance buffer for the preferred embodiment of the present invention.
  • FIG. 4 is an diagram of a typical wristband loop antenna and an equivalent electrical schematic diagram.
  • FIG. 5A is a diagram of the inductive reactance buffer for the preferred embodiment of the present invention.
  • FIG. 5B is an electrical schematic diagram of the inductive reactance buffer of FIG. 5A.
  • FIG. 6A is a diagram of a capacitive reactance buffer for an alternate embodiment of the present invention.
  • FIG. 6B is an electrical schematic diagram of the capacitive reactance buffer of FIG. 6A.
  • FIG. 7A is a diagram of the construction of the capacitive reactance buffer of the alternate embodiment of the present invention.
  • FIG. 7B is a diagram of an alternate construction embodiment of the capacitive reactance buffer.
  • Table I compares the performance of a loop antenna utilizing an inductive reactance buffer to the performance of a prior art loop antenna.
  • Table II illustrates the performance of a loop antenna utilizing a capacitive reactance buffer.
  • FIGS. 3 to 6 illustrate the preferred embodiment of the present invention, a buffered loop antenna suitable for use with a wristworn electronic device.
  • a typical prior art wristband loop antenna arrangement 10 is shown in FIG. 1.
  • the receiver is located in housing 12 to which two non-stretchable straps 14 and 16 are attached.
  • a conductor 18 and 20 Within each strap 14 and 16 is located a conductor 18 and 20 respectively.
  • This conductor may be either a round or a flat conductive wire.
  • Attached to one of the wristband straps 16, a conventional buckle 22 is provided which connects to one end of conductor 20.
  • a series of regularly spaced holes 24 are provided to allow for adjustment of the wristband length.
  • An eyelet is often inserted into each of the holes to provide electrical connection with conductor 20 within strap 16. This is shown in greater detail in FIG. 2A.
  • a wide flat sheet-metal conductor 100 is located within strap 102. Eyelets 104 provide contact to conductor 100.
  • the holes used to provide adjustment of the wristband are marked T1 through T7 and are evenly spaced over a length of the wristband, designated ⁇ L.
  • ⁇ L is approximately 44 millimeters in length for typical variations in adult wrist size.
  • a loop antenna constructed as shown in FIGS. 1 and 2A is an electrically small loop antenna, approximately one-quarter wavelength in size at VHF frequencies. Such a loop antenna is inductive at most frequencies of interest, and is capacitively tuned. Consequently, the adjustable portion of the wristband may be represented as a series of inductive elements, as shown in FIG. 2B.
  • the particular magnitude of the inductance of each element is a function of the geometry, or size, of the conductor, in this instance, the conductor geometry between each tap T1 through T7.
  • the wristband size which is also the relative loop antenna size or diameter is a minimum.
  • the wristband size, or relative loop antenna size or diameter is a maximum.
  • FIGS. 3A and 3B show the general construction of a wristband loop antenna for the preferred embodiment of the present invention.
  • the wristband loop antenna 200 includes two non-stretchable, but flexible straps, or wristband sections 202 and 204.
  • the first wristband section 202 includes a first conductor 206 which forms a first portion of the loop antenna, while the second wristband section 204 includes a second conductor 208 and forms the second portion of the loop antenna.
  • the first wristband section 202 further includes a series of regularly spaced apertures 210, such as holes or slots, linearly disposed along the wristband to provide adjustability.
  • a standard two piece clasp, used widely in the watch industry is utilized in the construction of the preferred embodiment of the present invention.
  • the clasp is suitably modified, such as with plating, to minimize corrosion problems and to maintain low ohmic electrical contact when the clasp is secured.
  • Platings such as selective gold plating of the contact surfaces is preferred, although other plating techniques may be employed equally as well.
  • Adjustable clasp 212 is slidably positioned along wristband section 202, and provides electrical contact to first conductor 206. Attached to the end of the second wristband section 204 is a fixed clasp 214, which couples to one end of second conductor 208, and together with adjustable clasp 212 provides the means to both electrically complete the loop antenna, and to mechanically secure the wristband 200 to the wrist.
  • First wristband section 202 and second wristband section 204 are affixed to the wristworn device by an attachment means, such as rigid mounting brackets 216, which are secured to the device housing by fasteners, such as screws (not shown).
  • Mounting brackets 216 may be formed from sheet metal, such as stainless steel, or other suitable material which is generally unaffected by contact with the skin. Stainless steel is advantageous in not requiring any plating for providing corrosion resistance. It will be appreciated, the rigid mounting of the wristband sections is exemplary and that other attachment means, such as the use of watch style spring loaded pins, may be used as well.
  • conductor 208 is a flat sheet-metal conductor formed from half hard beryllium copper material which is 3-4 mils thick. Other materials such as copper, nickel silver, and other conductive materials may be used as well. Conductor 208 is generally continuous through the length of wristband section 204, coupling on one end to the fixed clasp 214 and to a receiver input, such as the receiver ground input, at the device housing. Conductor 208 may be formed in a manner shown in FIG. 3B to provide positive retention of the conductor within the body of wristband section 204.
  • FIG. 3B shows the construction details for the first wristband section 202.
  • wristband section 202 is constructed by laminating conductor 206 and reactance buffer 218, which will be described in detail shortly, between top 220 and bottom 222 members which are non-stretchable, flexible materials formed by any number of suitable methods, such as by injection molding or die cutting. Any number of materials may be used for the top 220 and bottom 222 members, such as a urethane rubber, leather and the like.
  • the bottom member 222, or the top member 220 may include a recessed area, such as recess 224, in which conductor 206, reactance buffer 218, and mounting bracket 216 are positioned.
  • conductor 206 has an bent conductor portion 236 which is used to retain the conductor in the recess and prevents the conductor from pulling out or moving in the finished wristband section.
  • adhesives may be utilized to provide the retention of the conductor.
  • the two members may be joined by such processes as chemical bonding, including solvents and adhesives; mechanical bonding, including thermal, and ultrasonic bonding; and stitching or adhesive bonding, as in the case of a leather wristband. Insert molding of complete wristband sections may also be used, thereby eliminating many of the secondary wristband assembly operations described.
  • Conductors 206 and 208 are formed from flat sheet metal using such methods as stamping, chemical etching, or other suitable process.
  • FIG. 4 shows a diagram of a wristband antenna and an equivalent electrical schematic diagram which is useful in describing the operation of both the prior art wristband loop antenna, and the buffered loop antenna of the present invention.
  • the wristband loop antenna formed by bands A and B are inductive at the operating frequency, indicated schematically as L (b-x) , the subscript denoting the plurality of inductances as the length of the loop is adjusted (x indicating position T1 to T7 and b indicating the reference end of the second band as shown in FIG. 4).
  • the resistance associated with the conductors is shown schematically as R s .
  • the wristband loop antenna couples to a receiver input and ground as shown, and is capacitively tuned, the capacitor shown schematically as C o . In the preferred embodiment of the present invention, capacitor C o couples between the receiver input and ground.
  • the voltage delivered from the loop antenna operating in an electromagnetic field is shown schematically as the voltage source labeled E.
  • the operating frequency of the antenna may be determined by the following well known equation.
  • F ant 1 / 2 ⁇ ⁇ L (b-x) C o
  • L (b-1) , etc. represents the magnitude of the total inductance measure at each tap position.
  • the total inductance of the loop antenna is the sum of the inductance of band A and band B, corrected for the differential inductance associated with varying the length of the loop in the adjustable zone.
  • the reactance buffer for the preferred embodiment of the present invention by providing a substantially constant reactance at each tap position, allows the wristband loop antenna to be tuned only once at any of the selectable wristband lengths, and thereafter the wristband loop antenna remains tuned, even when the diameter of the antenna loop is changed.
  • FIG. 5A shows a diagram of the physical layout of the reactance buffer 218 for the preferred embodiment of the present invention.
  • An approximate schematic diagram of reactance buffer 218 is shown in FIG. 5B.
  • the schematic diagram of FIG. 5B represents inductance values associated with horizontal conductors. While the vertical conductors also have inductance values associated with them, they are shown schematically as conductors, or conductive elements. It will be appreciated, this first order approximation is sufficient to one of ordinary skill in the art to understand the operation of the reactance buffer 218 to be described.
  • Reactance buffer 218 is an integrated structure, as shown in FIG. 5A in that the buffer input, the taps, and the reactance elements are formed from a flat sheet metal strip.
  • the taps are linearly disposed along the integrated structure providing buffer outputs to select the wristband size.
  • the outermost taps, T1 and T7, are spaced a predetermined length, corresponding to the amount of wristband size adjustment required.
  • first conductor 206 is shown schematically as inductor L1.
  • Reactance buffer 218 input is shown generally as conductor 300.
  • Reactance buffer 218 includes a plurality of taps T1-T7 which are used to adjust the length of the wristband, or conversely, the diameter of the wristband loop antenna. It will be appreciated, the number of taps provided for the adjustment range is for example only, and other numbers may be provided when necessary.
  • Reactance buffer 218 comprises a plurality of reactance elements, shown schematically as inductive elements, or inductors, L2-L10. The arrangement, i.e. series/parallel combinations of these reactance elements, results in a substantially constant reactance when measured between the buffer input 300 and each of the taps T1-T7.
  • each inductive element is in actuality a conductor, the value of the inductance being a function of the geometry of the inductor.
  • L2 which corresponds to conductor 304, has a substantially equivalent inductance value to L3 which corresponds to conductor 306.
  • Inductance values at other taps are combinations of inductances corresponding to a number of series and parallel inductors, as shown.
  • Table I illustrates the relative performance of the inductive reactance buffer compared to the prior art loop antenna design. All measurements are referenced to tap T1, and includes a conductor length equivalent to that found in the first antenna portion. The relative length is the additional length of the wristband, as the wristband is adjusted from T1 to T7. The inductance change is the change in inductance value associated with each tap relative to the inductance reference measure at T1. The total inductance and change in inductance for the prior art antenna are tabulated in the last two columns of Table I. As Table I shows, the change in inductance for the prior art antenna was measure at 59.1 nanohenries, compared to a maximum change of 4.3 nanohenries. It will be appreciated that further optimization of the conductor geometries in the reactance buffer can be made to reduce this difference.
  • reactance buffer 218 may be advantageously and economically formed from a single flat sheet metal conductor which has been formed, such as by die stamping or chemical etching.
  • the conductor pattern shown is, for example, only, and any number of conductor patterns may be generated which achieve the same result, a substantially constant reactance measured between the buffer input and each output tap.
  • the conductive pattern may be formed from sheet metal, such as copper, beryllium copper and nickel silver. The material is selected to provide the required flexibility, and to withstand the repeated flexing associated with wearing the wristband and repeatedly putting on and removing the wristband from the wrist.
  • the conductor may be plated to enhance the solderability, and durability of the conductor, with a plating such as a copper, nickel, tin plating.
  • reactance buffer may also be employed, other than described above.
  • One such material may be a copper foil laminated KAPTONTM material, wherein the reactance buffer pattern is formed using convention printed circuit etching techniques. Coupling of the pattern to the tap areas would be the same, or similar to the stamped metal reactance buffer, such as with rivets.
  • FIGS. 6A/6B and 7A/7B Alternate construction methods for the reactance buffer is shown in FIGS. 6A/6B and 7A/7B.
  • the reactance buffers of FIGS. 6A/6B and 7A/7B utilize a plurality of fixed value capacitors to achieve a substantially constant reactance when the length of the wristband is adjusted.
  • a portion of conductor 206 is tapped using conductors 400-412, somewhat in the method of the prior art.
  • the prior conductor 206 is coupled to each output tap T1-T7 through a fixed capacitor C1-C7.
  • FIG. 6B shown an approximate schematic diagram of FIG. 6A.
  • the reactance elements may be considered to include a plurality of paired inductive and capacitive elements, such as L11 and C1.
  • Each inductive and capacitive element has an input and an output, the input of the capacitive element being coupled to the output of the inductive element, and the output of the capacitive element being coupled to a tap.
  • the inductive elements are then coupled in series, resulting in the structure shown in FIG. 6B.
  • L cum would equal L11 + L12
  • C tap would be C2 for tap T2.
  • C1 when used, would have the smallest capacitance value for resonating with inductor L11, whereas C7 would have the largest capacitance value for resonating with the series combination of L11-L17. While capacitor C1 is shown, it will be appreciated C1 can be omitted with the buffer retaining the same electrical characteristics previously described, in which case C2 would have the smallest inductance value resonating with 111 and 112.
  • FIG. 7A One construction method for a reactance buffer utilizing capacitive and inductive elements is shown in FIG. 7A.
  • a flexible circuit 508, such as a KAPTONTM film with laminated copper foil is first etched to provide a pattern similar to shown in FIG. 6A.
  • Capacitors C4-C7, such as leadless, surface mountable chip capacitors, having appropriate values are then soldered, such as using reflow soldering, to attach the capacitors to the conductors.
  • a molded, or die cut, elastomer or leather band is then assembled enclosing the flexible circuit using one of more of the procedures previously described for the inductive reactance buffer of FIGS. 3A and 3B.
  • Table II illustrates the relative performance of the capacitive/inductive reactance buffer. All measurements are referenced to tap T1, and includes a conductor length equivalent to that found in the first antenna portion. The relative length is the additional length of the wristband, as the wristband is adjusted from T1 to T7. The total inductance is listed for three tap positions. Cadded is the computed capacitance required to resonate the total inductance at each tap to a predetermined operating frequency, which in the case of this example is 157.7 MHz. As table II shows, proper selection of fixed value capacitors at each tap can substantially eliminate any changes in antenna tuning, as the length of the wristband is changed.
  • FIG. 7B An alternate construction for the capacitive reactance buffer is shown in FIG. 7B.
  • the capacitors are formed during the construction of the wristband section 202.
  • one plate of capacitors C1-C7 is coupled with a contact 500.
  • the size of the plate 500 is a function of the capacitance required at each tap, the thickness of dielectric layer 502, and the dielectric constant of dielectric layer 502. Computation of the size of the capacitor plate is well known to one of ordinary skill in the art.
  • the second plate of each of the capacitors C1-C7 is provided by conductor 206.
  • capacitor plate/contacts 500 are placed in a molded wristband half 504. Each capacitor plate/contact has a different geometry corresponding to the required capacitance at each tap.
  • Dielectric layer 502 is positioned over the contacts, followed by the positioning of conductor 206.
  • Dielectric layer 502 may be molded from a suitable dielectric, having a recess in which to position conductor 206.
  • the top wristband half 510 is positioned on the stack, and the combination laminated by one or more appropriate techniques previously described for the inductive reactance buffer construction.
  • wristband section 204 may be constructed to provide connection to the capacitor/inductor buffer.
  • conductor 208 may be formed, such as by stamping or coining techniques, to form contacts 506 to be plugged into capacitor plate/contacts 500.
  • contacts 506 are shown in this alternate embodiment of the present invention. The two contact arrangement provides additional strength to the clasp when the clasp is secured as well as a more reliable electrical contact. Other methods of forming the contact on conductor 502 may also be employed, such as by attaching separate fixed contacts.
  • the capacitive buffers of FIGS. 7A and 7B may be described as a flat integrated structure which includes the buffer input, taps and reactance elements.
  • the reactance buffer of the present invention can be used in other loop antenna applications as well.
  • examples of such applications include any variable size loop antenna, either electrically small or electrically large and having any cross sectional configuration, such as circular, square, rectangular or other.
  • Other applications include such special purpose variable size loop antennas, such as could be located in belts, rigid bracelets, ankle straps, and the like.

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  • Details Of Aerials (AREA)
  • Input Circuits Of Receivers And Coupling Of Receivers And Audio Equipment (AREA)
  • Variable-Direction Aerials And Aerial Arrays (AREA)
  • Aerials With Secondary Devices (AREA)
  • Support Of Aerials (AREA)
  • Microscoopes, Condenser (AREA)
  • Investigating, Analyzing Materials By Fluorescence Or Luminescence (AREA)
  • Luminescent Compositions (AREA)
EP90901381A 1989-01-23 1989-12-20 Reactance buffered loop antenna Expired - Lifetime EP0454694B1 (en)

Applications Claiming Priority (3)

Application Number Priority Date Filing Date Title
US29927689A 1989-01-23 1989-01-23
US299276 1989-01-23
PCT/US1989/005684 WO1990008404A1 (en) 1989-01-23 1989-12-20 Reactance buffered loop antenna and method for making the same

Publications (3)

Publication Number Publication Date
EP0454694A1 EP0454694A1 (en) 1991-11-06
EP0454694A4 EP0454694A4 (en) 1992-06-03
EP0454694B1 true EP0454694B1 (en) 1995-04-12

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Application Number Title Priority Date Filing Date
EP90901381A Expired - Lifetime EP0454694B1 (en) 1989-01-23 1989-12-20 Reactance buffered loop antenna

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EP (1) EP0454694B1 (fi)
JP (1) JP2588063B2 (fi)
KR (1) KR930008833B1 (fi)
AT (1) ATE121225T1 (fi)
CA (1) CA2004365C (fi)
DE (1) DE68922221T2 (fi)
DK (1) DK136291A (fi)
ES (1) ES2070311T3 (fi)
FI (1) FI913520A0 (fi)
MY (1) MY104488A (fi)
WO (1) WO1990008404A1 (fi)

Families Citing this family (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2908248B2 (ja) * 1993-07-16 1999-06-21 日本電気株式会社 腕時計型の選択呼出受信機
GB2280065B (en) * 1993-07-16 1997-05-14 Nec Corp Wristwatch-type selective calling receiver
JP3586929B2 (ja) * 1995-05-10 2004-11-10 カシオ計算機株式会社 携帯無線機器用アンテナおよび携帯無線機器
CN112993534B (zh) * 2019-12-16 2023-04-07 RealMe重庆移动通信有限公司 穿戴式电子设备
DE102022134855A1 (de) 2022-12-27 2024-06-27 microsynetics GmbH Befestigungsband für eine tragbare Computervorrichtung

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* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS57142002A (en) * 1981-02-27 1982-09-02 Toshiba Corp Small-sized loop antenna
US4713808A (en) * 1985-11-27 1987-12-15 A T & E Corporation Watch pager system and communication protocol

Also Published As

Publication number Publication date
ES2070311T3 (es) 1995-06-01
MY104488A (en) 1994-04-30
KR910700550A (ko) 1991-03-15
EP0454694A4 (en) 1992-06-03
DE68922221T2 (de) 1995-11-09
CA2004365A1 (en) 1990-07-23
JP2588063B2 (ja) 1997-03-05
FI913520A0 (fi) 1991-07-23
DK136291D0 (da) 1991-07-16
KR930008833B1 (ko) 1993-09-15
WO1990008404A1 (en) 1990-07-26
CA2004365C (en) 1994-05-03
ATE121225T1 (de) 1995-04-15
EP0454694A1 (en) 1991-11-06
DK136291A (da) 1991-07-16
DE68922221D1 (de) 1995-05-18
JPH04504491A (ja) 1992-08-06

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