CA2004365C - Reactance buffered loop antenna and method for making the same - Google Patents

Abstract

ABSTRACT

A reactance buffer maintains a substantially constant resonant frequency for an adjustable size loop antenna having first and second antenna segments. Each segment has first and second ends, the first ends being coupled to a receiver, and the second ends providing loop size adjustment. The reactance buffer comprises a reactance buffer input coupled to the second end of the first antenna segment. A plurality of taps are linearly disposed along a longitudinal axis of the reactance buffer, the distance between the outermost of the plurality of taps providing a predetermined length corresponding to the length of adjustment of the loop circumference. The taps provide selectable reactance buffer outputs for coupling to the second end of the second antenna segment. A plurality of reactance elements are arranged non-serially between the reactance buffer input and each of the plurality of taps, wherein one or more of the reactance elements are coupled between the input and a corresponding one of the taps to provide a substantially constant reactance when measured between the reactance buffer input and each of the plurality of taps.

Description

- X004~65 .

.- REAC~!ANCE BUFli'lZRED LOOP ANT~NNA
AND ~ICTYOD FO~ ~ RING T~lC 8A~IC

FI~ OF 'rHE; TNVEN~
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.

~E~GBIe~IQ~ OF ~HE PRIQ~ ABT
As electronic circuits have been miniaturized, and in particular receivers, it has become possible to package the electronics into housings suitable to be comfortably worn on the wrist. Antennas used with these wrlstworn receivers have often utilized simple single turn loop antennas which have been incorporated into the wristband of the device.
Such an antenna generally used a nonstretchable two-piece wristband i8 shown in FIG. l. Rivets, or similar fasteners, were used to provide a series of regularly spaced holeQ in one of the wristband sections required to accommodate the varying sizes of the human wrist, often providing the electrical connection to close the loop when the wristband was fastened to the wrist. Since the inductance of such a loop antenna is dependent upon the physical geometry of the loop antenna, such as loop diameter or length, the tuning of such a loop antenna varled with the wrist size. Consequently, when the loop antenna was tuned for a particular wrist size, increasing or decreasing the loop diameter by increasing or decreasing the loop length, as would happen when an ad~acent contact point was selected when strapping the device to the wrist, requlted in substantial changes in the antenna's resonant frequency and correspondingly substantial changes in the receiver's sensitivity. As a consequence, factory pretuning of such a wristband loop antenna was not ~-~
po~sible. Commercialization of such wrist worn receivers ':....'. ~ , '~ '' ~" '' ' -. . , ' was consequently limited to retailers employing skilled technicians capable of tuning the antennas on the devices as they were sold. As noted, even thiq did not guarantee antenna performance when the wearer wa~ incon~istent in strapping the device to the wrist.
Other antenna structures have also been proposed for use in wristworn receivers. One such wristband antenna consisted of a number of fexrite 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.

SU~5a~ OF THl;: I~vFNTIQu A reactance buffer is described for maintaining a substantially constant resonant frequency for an adjustable size loop antenna having first and second antenna segments, each segment having first and second ends, the first ends being coupled to a receiver, the second ends providing loop size ad~ustment. The reactance buffer comprises a buffer input coupling to the second end of the first antenna segment. A plurality of taps are linearly disposed along a flat integrated structure, the structure having a predetermined length between the outermost taps corresponding to the loop antenna size ad~ustment required.
The taps provide selectable buffer outputs for coupling to the second end of the second antenna segment. A plurality of reactance elements couple the buffer lnput to each of the plurality of taps and provide a substantially constant reactance measured between the input and each of the plurality of taps.
A wristband loop antenna is descrlbed for a wristworn electronic device which includes a receiver having signal and ground inputs coupled to an antenna resonating capacitor for resonating the loop antenna to a ;~00~365 predetermined frequency. The wristband loop antenna comprises first and second wristband sections. The first wristband section includes a first conductor havlng a flrst end coupled to the receiver slgnal input and a second end.
The first conductor forms a firQt portion of the loop antenna within the first wristband section. A reactance buffer is coupled to the second end of the first conductor, the buffer having a plurality of selectable substantially constant reactance taps linearly disposed along a flat integrated structure. The structure has a predetermined length between the outermost taps corresponding to the wristband loop antenna diameter adjustment is required.
The taps allow adjusting the first wristband length. A
second wristband section includes a second conductor having a first end coupled to the recelver ground and a second end. The second conductor forms a second portion of the loop antenna withln the second wristband section. A
coupling device couples to the second end of the second conductor coupling the conductor to one of the plurality of taps. When the wristband length is ad~usted by selecting one of the plurality of taps, the resonant frequency of the wristband loop antenna remalns substantlally unchanged.
It ls an ob~ect of the present inventlon to provide a loop antenna havlng an ad~ustable slze whlch does not requlre tuning when the size is changed.
It is a further ob~ect of the present lnventlon to provlde a loop antenna which 19 adapted for use with a wristworn devlce.
It 19 a further ob~ect of the present inventlon to provlde a wristband loop antenna whlch can be pretuned.
It 19 a further ob~ect of the present lnvention to provlde a wristband loop antenna whlch when tuned is lnsensltlve to changes ln the wrlstband length.

BRIEF DF~c ~ r~LL ~ L~G~
The features of the lnventlon which are believed to be novel are set forth in particularity in the appended clalms. The lnventlon ltself, together with its further ':"'; ~' :,"
''-''~,','''~' , ,~
~-., Z00~65 objects and advantages thereof, may be best under~tood by reference to the following description when taken in conjunction with the accompanying drawings, in which the several figures of which like reference numeral~ identify identical elements, in which:
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.
10FIG. 2B is an electrical schematic diagram of FIG. 2A.
FIG. 3A ic 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 i9 a diagram of the inductive reactance buffer for the preferred embodiment of the present invention.
20FIG. 53 is an electrical schematic diagram of the inductive reactance buffer of FIG. 5A.
FIG. 6A ls a diagram of a capacitive reactance buffer for an alternate embodiment of the present invention.
FIG. 6B i9 an electrical schematlc diagram of the capacltive reactance buffer of FIG. 6A.
FIG. 7A i9 a diagram of the construction of the capacitive reactance buffer of the alternate embodiment of the present invention.
FIG. 7B i9 a diagram of an alternate construction embodlment of the capacitive reactance buffer.
Table I compares the performance of a loop antenna utlllzlng an lnductlve reactance buffer to the performance of a prlor art loop antenna.
Table II lllustrates the performance of a loop antenna 35 utlllzlng a capacltlve reactance buffer. ;

12ESt~RTP'rIt~N Q~ ~HF P-QF'.l;'ERREn EMROr')TMEN~

200;~365 With respect to the figureq, FIGS. 3 to 6 illuQtrate the preferred embodiment of the present invention, a buffered loop antenna suitable for use with a wristworn electronic device. In order to appreciate the advantages of the present invention, it is beqt to describe in some detail the operation of at least one prior art wristband loop antenna in order to provide an understanding of some of the problems previously encountered. 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 19 and 16 are attached. 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 strapQ 14, a conventional buckle i~ provided which connectQ
to one end of conductor 18. In the other wristband strap 16, a serles of regularly spaced holes are provided to ~-allow for ad~ustment of the wri~tband length. An eyelet is often inserted into each of the holeq to provide electrical connection with conductor 20 within strap 16. This is shown in greater detail in FIG. 2A.
As shown 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 provlde ad~uqtment of the wrlstband are marked T1 through T7 and are evenly qpaced over a length of the wristband, designated ~L. For a typical wristband, ~L is approximately 44 mlllimeters in length for typical variations ln adult wri~t 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 ad~ustable portion of the wristband may be reprosented as a series of inductive elementQ, 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 Tl through T7. It will be appreciated, when the clasp is connected to tap Tl, the wristband size, which is also the relative loop antenna size or diameter ls a m~nimum. When the clasp i9 connected to tap T7, the wristband size, or relative loop antenna size or diameter is a maximum. Thus, it will be appreciated, when the loop antenna is ad~usted and tuned for length Tl, the tuning will be substantially changed at length T7, and for intermediate lengths as well, resulting in reduced receiver 10 sensitivity at lengths other than where originally tuned. -FIGS. 3A and 3B show the general construction of a wristband loop antenna for the preferred embodiment of the present invention. As shown in FIG. 3A, 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 ad~ustability.
A standard two piece clasp, used widely in the watch industry is utillzed ln the constructlon of the preferred embodlment of the present lnvention. The clasp is auitably modlfled, ~uch as wlth plating, to minimize corrosion problems and to maintaln low ohmlc electrlcal contact when the clasp is ~ecured. Platings, such as selective gold platlng of the contact surfaces is preferred, although other platlng techniques may be employed equally as well.
Ad~ustable clasp 212 is slldably positioned along wristband section 202, and provldos electrical contact to first conductor 206. Attached to the end of the second wristband section 204 19 a flxed clasp 214, which couples to one end of second conductor 208, and together with adjustable clasp 212 provldes the means to both electrlcally complete the loop antenna, and to mechanically secure the wrlstband 200 - 2004~65 to the wrist. First wristband ~ection 202 and second wristband section 204 are affixed to the wristworn devlce by an attachment means, such as rigid mounting brackets 216, which are secured to the device housing by faqteners, such as screw~ (not shown). Mounting brackets 216 may be formed from sheet metal, such as stainless steel, or other suitable material which i3 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 a~ the use of watch style spring loaded pins, may be used a~ well.
In the preferred embodiment of the present invention, 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 recelver 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 ~ectlon 204. -FIG. 3B shows the construction details for the first wristband section 202. In the preferred embodiment of the present invention, wristband section 202 is constructed by laminating conductor 206 and reactance buffer 218, which wlll be described ln detail shortly, between top 220 and bottom 222 members whlch are non-stretchable, flexible materials formed by any number of suitable methods, such as by in~ection moldlng or dle cuttlng. Any number of materlals 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 lnclude a recessed area, such as receqs 224, ln which conductor 206, reactance bu~fer 218, and mounting bracket 216 are positioned. Such a recessed area can be formed in the 2004~65 material when the strap i9 molded. As shown in FIG. 3B, conductor 206 has an bent conductor portion 236 which i3 used to retain the conductor in the rece-qs and prevents the conductor from pulling out or moving in the finished wristband section. When it is impractical to provide a recess, adhesives may be utilized to provide the retention of the conductor. Depending on the material of the two members, 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 stamplng, chemical etching, or other suitable process.
FIG. 4 shows a diagram of a wrlstband 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. As previously described, the wristband loop antenna formed by bands A and B are inductive at the operating frequency, lndlcated schematlcally as L~b-X)~ the subscrlpt denotlng the plurallty of inductances as the length of the loop iq ad~usted (x indlcatlng position Tl to T7 and b lndlcatlng the reference end of the second band as shown ln FIG. 4). The reslstance assoclated wlth the conductors 18 shown ~chematlcally as R9. The wristband loop antenna couples to a receiver input and ground as shown, and 19 capacitively tuned, the capacitor shown schematlcally as Co. In the preferred embodlment of the present lnventlon, capacltor C0 couples between the recelver input and ground. The voltage delivered from the loop antenna operatlng ln an electromagnetlc fleld ls shown schematlcally as the voltage source labeled E.
The operatlng frequency of the antenna may be determlned by the followlng well known equation.

200~365 Fant = 1 / 2~ ~ L(b-x) CO
From the previous description of FIGS. 2A and 2B, lt was noted the lnductance at tap Tl, does not equal the inductance at the other taps. Thu~
L(b-1) ~ L(b-2) ~ - L(b-7) where ~(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.
It then follows, if CO is kept conqtant, such a when the capacitor is pretuned at one of the wristband lengths, then Fant(b-1) ~ Fant~b-2) ~ Fant(b-7) which demonstrates, as previously stated, the prior art wristband loop antenna requires retunlng to eliminate -~
variations in ad~usting the wristband to different wrist sizes. This problem is substantially mlnimized wlth the reactance buffer described in FIG. 3B, the operation of which will be described in detail with FIGS. 5A and SB. In practice, the reactance buffer of the present invention provides substantially a constant reactance for each tap position along the wristband, such that L(b~ L(b-2) ~ L(b_ which results in Fant (b-l) ~ Fant (b-2) ~ Fant (b-7) The reactance buffer for the preferred embodlment of the present lnvention, 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 21a for the preferred embodiment of the -present invention. An approximate schematic diagram of reactance buffer 218 i~ shown in FIG. SB. It will be ' -' '" ' , ~00~65 appreciated, that the schematic diagram of FIG. SB ii only a first order approximation for the reactance buffer, in that each conductor in the circuit has an associated inductance value. 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 ad~ustment required.
Referring to FIG. SB, 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 ad~ust the length of the wrlstband, or conversely, the diameter of the wrlstband loop antenna. It will be appreciated, the number of taps provided for the ad~ustment range is for example only, and other numbers may be provided when necessary. Reactance buffer 218 comprises a plurality of reactance elements, shown schematlcally as inductive elements, or inductors, L2-~10. The arrangement, i.e.
series/parallel combinations of these reactance elements, results in a substantially constant reactance when mea~ured between the buffer input 300 and each of the taps T1-T7.
As shown, each inductive element is ln actuality a conductor, the value of the inductance being a function of the geometry of the inductor. Thus, L2 which corresponds to conductor 304, has a substantially equivalent inductance 200~65 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 10 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 TabIe 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 thls difference.
As shown in FIGS. 3B and SA, 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. It will be appreciated, the conductor pattern shown is, for example, only, and any number of conductor patterns may be generated which achieve the qame result, a qubstantially constant reactance measured between the buffer input and each output tap. The conductlve pattern may be formed from sheet metal, such aq copper, beryllium copper and nickel silver.
The material i~ selected to provide the required flexibility, and to withstand the repeated flexing a~sociated with wearing the wristband and repeatedly putting on and removing the wri~tband 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, tln plating.

:
;''. ', `~

2~ 4.~5 Other materials for forming the reactance buffer may also be employed, other than described above. One such material may be a copper foil laminated kapton 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.
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 plurallty of fixed value capacitors to achieve a sub~tantially constant reactance when the length of the wristband is adjusted. As shown in FIG. 6A, a portion of conductor 206 is tapped using conductors 400-412, somewhat in the method of the prior art. However, unlike the prior conductor 206 i9 coupled to each output tap Tl-T7 through a fixed capacitor C1-C7. FIG. 6B shown an approximate schematic diagram of FIG. 6A. In the instance where both inductive and capacitive elements are utilized in the reactance buffer, 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 belng coupled to a tap. The inductive elements are then coupled in series, resulting in the structure shown in FIG. 6B. The values for C1-C7 are -qelected to provide a substantially constant reactance between the input and each output tap, the magnitude of this capacitance being computed as follows:
2~fLCum + 1/2~fCtap - a constant where f lq the frequency of operation, LCum is the cumulatlve inductance a-qqociated with each tap, and Ctap is the partlcular tap capacitance. Thus, LCum would equal L11 + L12, and Ctap would be C2 for tap T2. Thus, 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. ~hile capacltor 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.
One construction method for a reactance buffer utilizing capacitive and inductive elements is shown in FIG. 7A. A flexible circuit 508, such as a kapton film with laminated copper foil i~ first etched to provide a pattern similar to shown in FIG. 6A. Capacitor~ C4-C7, such as leadless, surface mountable chip capacitors, having appropriate values are then soldered, such as uslng reflow -soldering, to attach the capacitors to the conductors. A
molded, or die cut, elastomer or leather band is then 15 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/inductlve reactance buffer. All measurements are referenced to tap T1, and includes a conductor length equlvalent to that found in the first antenna portion. The relative length is the additional length of the wristband, as the wristband is ad~usted from T1 to T7. The total inductance is listed for three tap posltions. Cadded is the computed capacitance required to resonate the total inductance at each tap to a predetermined operating frequency, whlch in the case of thi~ example is 157.7 MHz.
As table II shows, proper selection of fixed value capacitors at each tap can substantially eliminate any changeis ln antenna tuning, as the length of the wristband is changed.
An alternate construction for the capacitive reactance buffer is shown in FIG. 7B. In thls instance, the capacitors are formed during the construction of the wrlstband sectlon 202. As shown in FIG. 7B, 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 con~tant of dlelectrlc 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 capacito~s Cl-C7 i9 provided by conductor 206. In practice, capacitor plate/contacts S00 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 i8 positioned over the contacts, followed by the positioning of conductor 206. Dielectr~c layer 502 may be molded from a suitable dielectric, having a recess in which to position conductor 206. Finally, 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.
As shown in FIG. 7B, wrlstband section 204 may be constructed to provide connectlon to the capacitor/inductor buffer. In thl~ lnstance, conductor 208 may be formed, such as by stamping or colning techniques, to form contacts 506 to be plugged lnto capacitor plate/contacts 500. Two contacts are shown ln this alternate embodiment of the present lnvention. The two contact arrangement provides additional strength to the clasp when the clasp ls secured as well as a more reliable electrical contact. Other methods of forming the contact on conductor 502 may also be employed, such a~ by attachlng separate fixed contacts.
As in the case of the lnductlve buffer of FIG. 5A, 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.
While the description of the buffered loop antenna has been directed primarlly for use in a wristband, it will be appreciated, the reactance buffer of the present inventlon 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 sectlonal 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.
While specific embodiments of the present invention 5 have been shown and described, further modifications and -improvements will occur to those skilled in the art. All -modifications which retain the basic underlying principles disclosed and claimed herein are within the scope and -spirit of the present invention.
We claim:

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B-1 O mm 142 . 4 nEI O pF 1~7 . 7S MHz B-29 . 04 nun ___ ___ _~
B-316. 65 mm ___ ___ B-423 . 05 mm 160 .1 nH39 . 8 pF lS7 . 75 MHz ~-~30 . 07 nan ___ ~__ ___ .
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Claims (20)

1. A reactance buffer, for maintaining a substantially constant resonant frequency for a loop antenna having a plurality of selectable loop circumferences, the loop antenna formed from first and second conductor antenna segments, each antenna segment having first and second ends, the first ends of each antenna segment being coupled to a receiver input, the second ends of each antenna segment being selectably coupled for providing adjustment of the loop circumference, said buffer comprising:
a reactance buffer input coupled to the second end of the first antenna segment;
a plurality of taps, linearly disposed along a longitudinal axis of the reactance buffer, the distance between the outermost of said plurality of taps providing a predetermined length corresponding to the length of adjustment of the loop circumference, said taps providing selectable coupling positions for coupling to the second end of the second antenna segment; and a plurality of reactance elements, arranged non-serially between said reactance buffer input and each of said plurality of taps, wherein one or more of said reactance elements are coupled between said input and a corresponding one of said taps to provide a substantially constant reactance when measured between said reactance buffer input and each of said plurality of taps, whereby the resonant frequency of the loop antenna remains substantially constant when the loop circumference is adjusted.

2. The reactance buffer according to claim 1, wherein said reactance elements are inductive elements.

3. The reactance buffer according to claim 2, wherein the magnitude of the inductance measured between the buffer input and each tap is substantially constant.

4. The reactance buffer according to claim 2, wherein each of said inductive elements is formed from a conductor.

5. The reactance buffer according to claim 4, wherein said conductors are formed from sheet metal.

6. The reactance buffer according to claim 5, wherein said sheet metal is selected from a group of sheet metals consisting of copper, beryllium copper, and nickel silver.

7. The reactance buffer according to claim 1, wherein said reactance elements comprise a plurality of paired inductive and capacitive elements defining inductor/capacitor pairs.

8. A buffered loop antenna, having a plurality of selectable loop circumferences, the loop antenna being coupled to a receiver having signal and ground inputs coupled to an antenna resonating capacitor for resonating the loop antenna to a predetermined frequency, said loop antenna comprising:
a first conductor, having a first end coupled to the receiver signal input and a second end, said first conductor forming a first portion of the loop antenna;
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 loop antenna;
reactance buffer means, comprising a reactance buffer input coupled to said second end of said first conductor, a plurality of taps linearly disposed along a longitudinal axis of said reactance buffer the distance between the outermost of said plurality of taps providing a predetermined length corresponding to the loop circumference adjustment, and a plurality of reactance elements, arranged non-serially between said reactance buffer input and each of said plurality of taps, wherein one or more of said reactance elements are coupled between said input and a corresponding one of said taps to provide a substantially constant reactance when measured between said reactance buffer input and each of said plurality of taps; and coupling means, coupled to said second end of said second conductor, for coupling said second conductor to any of said plurality of taps, whereby the resonant frequency of the loop antenna remains substantially constant when any of said plurality of taps is selected to adjust the loop circumference.

9. The buffered loop antenna according to claim 8 wherein said first and second conductors are sheet metal.

10. The buffered loop antenna according to claim 9 wherein said sheet metal is selected from a group consisting of copper, beryllium copper, and nickel silver.

11. 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 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, said first conductor forming a first portion of the loop antenna within said first wristband section, and reactance buffer means, comprising a reactance buffer input coupled to said second end of said first conductor, a plurality of taps linearly disposed along a longitudinal axis of said reactance buffer the distance between the outermost of said plurality of taps providing a predetermined length corresponding to the loop antenna diameter adjustment, and a plurality of reactance elements, arranged non-serially between said reactance buffer input and each of said plurality of taps, wherein one or more of said reactance elements are coupled between said input and a corresponding one of said taps to provide a substantially constant reactance when measured between said reactance buffer input and each of said plurality of taps; 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 loop antenna within said second wristband section, and coupling means, coupled to said second end of said second conductor, for coupling said second conductor to any of said plurality of taps, whereby when the wristband length is adjusted by selecting any of said plurality of taps, the resonant frequency of said loop antenna remains substantially unchanged.

12. The wristband antenna according to claim 11 wherein said first and second conductors are sheet metal.

13. The wristband antenna according to claim 13 wherein said sheet metal is selected from a group consisting of copper, beryllium copper, and nickel silver.

14. A wrist worn receiving device, comprising:
a receiver located within a housing; and a wrist band, including first and second wrist band sections coupled to said housing, for securing the housing to a users wrist, said first wristband section forming a first portion of a loop antenna, and including a reactance buffer having an input coupled to said receiver, and a plurality of outputs linearly disposed along a longitudinal axis of said first wristband section opposite said housing, said reactance buffer comprising a plurality of reactance elements, arranged non-serially between said reactance buffer input and each of said plurality of taps, wherein one or more of said reactance elements are coupled between said input and a corresponding one of said taps for providing a substantially constant reactance measured between said reactance buffer input and each of said reactance buffer outputs, said second wristband section forming a second portion of the loop antenna, and including a coupling means, coupled to said receiver and affixed to said second wristband section opposite said housing, said coupling means providing selective coupling to said plurality of reactance buffer outputs when securing the housing to the users wrist, whereby the length of said wristband is freely adjustable to fit the users wrist by coupling said coupling means to a corresponding one of said plurality of reactance buffer outputs, and whereby the resonant frequency of said loop antenna remains substantially unchanged when said coupling means is coupled to any of said plurality of outputs.

15. The wrist worn receiving device according to claim 14, wherein said reactance elements are formed from a conductor.

16. The wrist worn receiving device according to claim 15, wherein said conductors are formed from sheet metal.

17. The wrist worn receiving device according to claim 16, wherein said sheet metal is selected from a group of sheet metals consisting of copper, beryllium copper, and nickel silver.

18. The wrist worn receiving device according to claim 14 wherein said first and second wristband sections further include attachment means for coupling said first and second wristband sections to said housing.

19. The wrist worn receiving device according to claim 14 wherein said first wristband section further includes a first conductor coupled between said reactance buffer input and said receiver to form the first portion of the loop antenna, and further wherein said second wristband section further includes a second conductor coupled between said coupling means and said receiver to form the second portion of the loop antenna.

20. The wrist worn receiving device according to claim 14 wherein said coupling means is a conductive clasp for providing adjustment of the wristband length and electrical coupling of the first and second antenna portions.