 |
Description  |
|
|
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an antenna device, having a
circumferentially variable size, typically embedded in a wristband, for
use with a radio which is generally worn on the arm of a person. More
particularly, the antenna device of the present invention provides a
method and apparatus for automatically compensating for changes in antenna
gain and resonance frequency which result from changes in the antenna
size.
2. Description of the Related Art
FIG. 34 shows an example of a conventional approach to the construction of
portable radio transmitters or receivers. More particularly it shows a
proposed antenna device for an arm-attached type radio apparatus capable
of being carried while worn on a wrist. FIG. 34 shows an arm-attached type
radio apparatus 90 that includes a case body 92 (the main body of the
radio apparatus) accommodating a circuit board for the radio apparatus;
and an arm attaching band 91 connected to both sides of case body 92.
Attaching band 91 has first and second band members 91a, 91b formed of an
insulating material. First and second band members 91a, 91b include,
respectively, first and second band-like conductor plates 93a, 93b that
are embedded within the band members. First and second conductor plates
93a, 93b, are electrically coupled to the radio circuit, and are
electrically coupled at their free ends to band connector portions 91c,
91d of first and second band members 91a, 91b. When first and second band
members 91a, 91b are connected by way of band connector portions 91c, 91d,
first and second conductor plates 93a, 93b form a loop through circuit 94,
thus forming an antenna 95 as shown in FIG. 35. In radio apparatus circuit
94, a high-frequency amplifier circuit 94b is coupled via coupling
capacitor 94a to first conductor plate 93a, and a variable capacitor 94c
is connected between first conductor plate 93a and ground. Note that the
side of second conductor plate 93b is fixed at ground potential.
However, because the thickness of a wearer's wrist varies, there is a
problem with antenna 95. That is, depending on the connecting position of
first and second band members 91a, 9lb, the circumference thereof varies
and consequently, the antenna inductance value changes, and antenna gain
is lowered. In other words, because, the inductance of antenna 95 is
changed as the band size is changed by the wearer, the resonance frequency
is shifted and antenna gain is lowered.
In view of the above problems, an object of the present invention is to
achieve an antenna device for arm-attached type radios capable of
obtaining a stable gain without being affected by the difference in the
band size of the wearer.
SUMMARY OF THE INVENTION
The antenna device of the present invention includes a band connector
portion for bringing the free ends of the insulating band members to a
state where they are separated or to a state where they are connected to
make the band members capable of being attached for example to a wrist; a
conductor plate fixed to the band members, for constructing a loop-like
antenna in the state where the band members are connected by the band
connector portion; and a resonance frequency compensation means for
changing the magnitude of the overlap capacitance formed between the
conductor plate and an electrode unit provided on the band connector
portion according to the connecting position of the free end sides of the
band members. This change in capacitance corresponds with the change in
antenna inductance attributable to a change in the band member connecting
positions.
Resonance frequency compensation can be achieved using: (1) an area
changing portion on the side of the conductor plate, which causes a change
in the magnitude of the overlap capacitance by changing the opposing area
between the electrode unit and the conductor plate according to the
connecting position of the band members; (2) an
effective-dielectric-constant changing portion on the side of the band
member; or (3) a thickness changing portion on the side of the band
member. Thus the product of the inductance and the overlap capacitance is
kept constant and the resonance frequency does not change even when the
inductance of the antenna is changed by the connecting position of the
free end side of the band member, since, in accordance with such change,
the electric coupling capacitance also changes. Thus, a stable antenna
gain may be obtained without being affected by the difference in the band
size of the wearer.
In the present invention, it is preferred that a slit be formed in the
conductor plate in the lengthwise direction so as to make the antenna
function as a slot antenna. Since such antenna is provided with a slit
opening toward the outer periphery, the directivity in the circumferential
direction of the antenna is improved.
Other objects, advantages and attainments together with a fuller
understanding of the invention will become apparent and appreciated by
referring to the following description and claims taken in conjunction
with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows the construction of an arm-attached type radio apparatus
according to Embodiment 1 of the present invention.
FIG. 2 is a cross-sectional view of the apparatus shown in FIG. 1.
FIG. 3 is longitudinal section of the apparatus of FIG. 1.
FIG. 4 is a longitudinal section of a portion around the case body of the
apparatus of FIG. 1.
FIG. 5 shows a portion around the buckle of the apparatus of FIG. 1.
FIG. 6 is an exploded view of buckle shown in FIG. 5.
FIG. 7 is a side view of buckle shown in FIG. 5.
FIG. 8 shows the band members connected by the buckle.
FIG. 9A is a block diagram of the antenna portion of the apparatus shown in
FIG. 1; and 9B is an equivalent circuit diagram.
FIG. 10A is a block diagram of another antenna device embodiment; and (b)
is an equivalent circuit diagram.
FIG. 11 is a circuit block diagram.
FIG. 12A shows an overlap capacitor; and (b) shows an overlap capacitor in
another state.
FIG. 13A shows that the overlap capacitor of the apparatus shown in FIG. 1
changes; and (b) shows the shape of the conductor plate for achieving such
change.
FIGS. 14A-F each show another structure of the conductor plate for making
possible a change in the magnitude of the overlap capacitance.
FIGS. 15A-B illustrate a second embodiment wherein overlap capacitance is a
function of the relative positions of conductor plate and band.
FIGS. 16A-F each show another structure of the conductor plate for making
possible a change in the overlap capacitance.
FIG. 17A shows how the overlap capacitance changes according to Embodiment
3 of the present invention; and 17B shows the shape of the band member for
achieving such change.
FIG. 18 is a schematic cross-sectional view from the back side of the
apparatus according to Embodiment 4 of the present invention.
FIG. 19 is a longitudinal section of the apparatus of FIG. 18.
FIG. 20 is a block diagram of the antenna portion of the apparatus of FIG.
18.
FIG. 21 is a longitudinal section showing the interior of the case body of
the apparatus of FIG. 18.
FIG. 22 is a block diagram of the circuit structure of the apparatus of
FIG. 18.
FIG. 23 is a block diagram of another circuit structure different from the
circuit structure of the apparatus of FIG. 18.
FIG. 24 is a block diagram of a circuit constructed at the interior of the
case body of the apparatus of FIG. 18.
FIG. 25 is a cross-sectional view showing the buckle in an engaged state.
FIG. 26A is a view explanatory of the directivity of the apparatus of FIG.
18; and (b) is a view for explaining the difference from such directivity.
FIG. 27A shows the state where an arm-attached type radio apparatus is left
alone in the appraisal of the directivity; (b) shows the state where the
directivity is appraised by attaching the same to an arm; and (c) is a
graph showing the result of the appraisal of the directivity in these
states.
FIG. 28 is a view explanatory of the state where an arm is extended
horizontally while an arm-attached type radio apparatus is attached to the
arm in the appraisal of directivity.
FIG. 29A shows the state where an arm-attached type radio apparatus is left
alone in the appraisal of the directivity of the arm-attached type radio
apparatus of FIG. 18;29B shows the state where the directivity is
appraised by attaching the same to an arm; and 29C is a graph showing the
result of the appraisal of the directivity in these states.
FIG. 30 is a view explanatory of the state where an arm is bent in front of
the body while an arm-attached type radio apparatus is attached to the arm
in the appraisal of directivity.
FIG. 31 is a view explanatory of the state where an arm is bent at the side
of the body while an arm-attached type radio apparatus is attached to the
arm in the appraisal of the directivity of the arm-attached type radio
apparatus as shown in FIG. 18.
FIG. 32 is a cross-sectional view of an apparatus according to a modified
example of Embodiment 4 of the present invention.
FIG. 33 is a longitudinal section of the apparatus of FIG. 32.
FIG. 34 shows a conventional arm-attached type radio apparatus.
FIG. 35 is a block diagram of the apparatus of FIG. 34.
FIG. 36 shows a cross-section of a solenoid in connection with an
explanation of the Nagaoka coefficient.
FIGS. 37A and 37B are illustrations of solenoids in connection with an
explanation of the Nagaoka coefficient.
DETAILED DESCRIPTION OF THE INVENTION
Embodiment 1
Referring to FIGS. 1-3, a first embodiment of the present invention is
shown wherein an arm band type radio apparatus 1 includes: a receiver body
2 having a circuit board 22 and a circuit block 23 (receiver circuit
block) for the radio apparatus within a casing 21; a band 3 for attachment
to an arm, having a first and second band members 3L, 3R formed of
materials such as leather, silicone resin or urethane resin, connected to
the sides of the receiver body. A metallic buckle (i.e., band connector)
31 is attached to the end portion of second band member 3R, and buckle 31
forms a band connector portion 30 by which the free end of first band
member 3L is connected to the end of second band member 3R.
First and second band members 3L, 3R are formed by sewing together
insulating sheets of material such as leather, silicone resin or urethane
resin or by bonding them together. First and second conductor plates 5L,
5R are fixed inside band members 3L, 3R, respectively. FIG. 1 shows first
and second band members 3L, 3R connected to each other by means of buckle
31. In this way, first and second band members 3L, 3R form a loop. First
and second conductor plates 5L, 5R are electrically connected to the sides
of receiver body 2. Circuit board 22 and circuit block 23 are disposed
inside casing 21. Antenna input terminals 24 are electrically connected to
the pattern of circuit board 22 by such means as soldering. Penetrating
conductors 25 penetrate casing 21 as shown in FIG. 4 to make electrical
connection with antenna input terminals 24. First and second conductor
plates 5L, 5R are electrically connected to penetrating conductors 25.
Insulating members 261, 262 make an airtight insulating seal around
penetrating conductors 25. As shown in FIG. 1, a battery 213 for supplying
power is placed at the interior of casing 21 between a back lid 212 and
circuit board 22. Battery 213 is provided with an electrode plate 214 for
making connection to negative electrode 215 and an electrode plate 217 for
making connection to positive electrode 216. Electrode plate 217 is urged
toward positive electrode 216 by a coil spring 218.
As shown in FIGS. 1-8, a metallic buckle 31 is fixed by means of screws 321
to the free end side of second band member 3R and a metallic electrode
plate 32 is integrally formed on buckle 31. Metallic electrode plate 32 is
conductively connected via screws 321 to second band member 3R. When, as
shown in FIG. 5, the free end of first band member 3L is put through
buckle 31 along arrow A, first and second band members 3L, 3R are
connected to each other at band connector portion 30. At the same time, as
shown in FIG. 8, metallic electrode plate 32 contacts with its surface to
first band member 3L, whereby metallic electrode plate 32 is brought into
a state where it opposes first conductor plate 5L via a resin layer 441 of
first band member 3L. Since resin layer 441 functions as a dielectric
layer, a capacitor 6 is formed between metallic electrode plate 32 and
first conductor plate 5L. In other words, as shown in FIG. 1, when arm
attaching band 3 is formed into a loop by connecting first and second band
members 3L, 3R by means of buckle 31, first and second conductor plates
5L, 5R are also formed into a loop. While being conductively connected to
the sides of receiver body 2, first and second conductor plates 5L, 5R
form an antenna 5 having capacitor 6 placed between first and second
conductor plates 5L, 5R at band connector portion 30.
A block diagram of antenna 5 and its equivalent circuit are shown in FIGS.
9A-9B In antenna circuit 10, having capacitor 6 at band connector portion
30, antenna 5 is connected at the side of receiver body 2 to a receiver
circuit 231 (circuit block 23 for the radio). First and second conductor
plates 5L, 5R are represented by inductors L.sub.1, L.sub.2. Inductors
L.sub.1, L.sub.2 and capacitor 6 are serially connected. Note that, in
FIGS. 9A-B, antenna 5 is serially connected to a variable capacitor 232 of
receiver circuit 231. Variable capacitor 232 is used in adjusting the
resonance frequency by changing its capacitance. Further, while a terminal
234 for connection to a high-frequency circuit is connected via a coupling
capacitor 233 to one side (first conductor plate 5L) of antenna 5, the
other side (second conductor plate 5R) of antenna 5 is grounded to form
receiver circuit 231 of an unbalanced circuit system.
Referring to FIG. 2, at the side of first band member 3L, the end portion
of first conductor plate 5L (serving as one of the electrodes of capacitor
6) forms an area changing portion 15 which tapers off toward the terminal
end thereof. At the side of second band member 3R, metallic electrode
plate 32 is provided, which serves as the other electrode of capacitor 6.
Metallic electrode plate 32 is invariable in its area and has a sufficient
width. Thus, when a wearer with a slender arm uses this, first conductor
plate 5L is mechanically engaged with buckle 31 at a position toward
casing 21 from the free end thereof. That is, since metallic electrode 32
opposes a region having a relatively large width of first conductor plate
5L, capacitance C.sub.1 of capacitor 6 is relatively large. But, when this
is used by a user having a thick arm, first conductor plate 5L is
mechanically engaged with buckle 31 at a position toward the free end
thereof. That is, since metallic electrode plate 32 opposes a region
having a relatively narrow width of first conductor plate 5L, capacitance
C.sub.1 of capacitor 6 becomes smaller. First conductor plate 5L having
such area changing portion 15 is used to form a resonance frequency
compensation structure for compensating the shift in resonance frequency
f. The connection method of receiver circuit 231 to antenna 5 may also be
set as a balanced circuit system by connecting receiver circuit 231, as
shown in FIGS. 10A-B, to both terminals (first and second conductor plates
5L, 5R) of variable capacitor 232. As shown by FIG. 11, receiver circuit
231 includes: a high-frequency amplifier circuit 231bconnected to terminal
234, for amplifying signals passed through antenna circuit 10; a mixer
circuit 231d for mixing the signal passed through high-frequency amplifier
circuit 231b and the signal from a local oscillator circuit 231c to
convert them to an intermediate frequency; an intermediate-frequency
amplifier circuit 231e for amplifying the intermediate frequency; a
detector circuit 231f for detecting the amplified intermediate-frequency
signal; and a regenerating circuit 231g for regenerating the demodulated
signal detected at detector circuit 231f.
In radio apparatus 1, resonance frequency f of antenna circuit 10 as shown
in FIG. 9B is defined, in Eq. (1), by inductance L.sub.1 of first
conductor plate 5L, inductance L.sub.2 of second conductor plate 5R, and
composite capacitance C.sub.t of capacitor 6 and capacitance and variable
capacitor 232.
##EQU1##
Thus, in order to keep a constant resonance frequency f, it suffices to
maintain the relationship between inductance and capacitance indicated in
Eq. (1). This is expressly shown in Eq. (3).
(L.sub.1 +L.sub.2).multidot.C.sub.t =Constant Eq.(3)
Here, inductances L.sub.1 and L.sub.2 are connected in a manner of
high-frequency to each other via buckle 31, where they may be regarded as
a single inductance (antenna inductance) and such antenna inductance
L.sub.t may be expressed by Eq. (4)
L.sub.t =(L.sub.1 +L.sub.2)=K.multidot..mu..sub.0
.multidot.S.multidot.N.sup.2 /M Eq.(4)
where:
K is Nagaoka coefficient (see Appendix II); .mu..sub.0 is permeability in a
vacuum; S is the loop area of antenna 5 when first and second band members
3L, 3R are connected via buckle 31; N is the number of turns of antenna 5;
and M is the width of antenna 5.
The loop area S of antenna 5 may be represented by Eq. 5 when assuming the
opening portion thereof as circular and its radius as "a".
S=.pi..multidot.a.sup.2 Eq.(5)
If the loop length, a, of antenna 5 becomes shorter, then opening area Sa
may be expressed by Eq. (6) if the amount by which it is shortened is
.DELTA..alpha..
##EQU2##
Therefore, by substituting Eq. (6) into Eq. (4), a ratio .DELTA.L.sub.t of
the antenna inductances L.sub.t before shortening and after shortening of
antenna 5 may be expressed by Eq. (7).
##EQU3##
As indicated by Eq. (7), as .DELTA..alpha. becomes larger (i.e., loop
length of the antenna becomes shorter), .DELTA.L.sub.t becomes smaller
than 1. That is, the value of the antenna inductance becomes smaller
compared to the case where the loop length of antenna 5 is not changed
(.DELTA..alpha.=0). On the other hand, when the loop length of antenna 5
becomes longer (.DELTA..alpha.<0), the value of the antenna inductance
L.sub.t becomes larger. Accordingly, in order to keep resonance frequency
f at a constant, it is necessary to increase the composite capacitance
C.sub.t when .DELTA..alpha. is larger (the loop length of antenna 5 is
made shorter) in Eq. (3), since the value of the antenna inductance
becomes smaller. On the other hand, when .DELTA..alpha. becomes smaller to
be negative by passing zero (the loop length of antenna 5 becomes longer),
it is necessary to make smaller the composite capacitance C.sub.t, since
the value of the antenna inductance L.sub.t becomes larger.
Eq. (8) defines the capacitance C.sub.1 of capacitor 6 as:
##EQU4##
where .epsilon. is the dielectric constant of the material between
metallic electrode plate 32 and first conductor plate 5L; A is the overlap
area between opposing metallic electrode plate 32 and first conductor
plate 5L; and d is the distance between metallic electrode plate 32 and
first conductor plate 5L.
Note that, if C.sub.1 <<C.sub.2, antenna capacitance C.sub.t may be
approximated, as shown by Eq. (9), to be C.sub.1
C.sub.t =C.sub.1 Eq.(9)
In order to compensate for the shift in resonance frequency f, it is
necessary, as shown by Eqs. (3) and (4) to make the product of antenna
inductance L.sub.t and composite capacitance C.sub.t a constant. That is,
it suffices to compensate for the value of ratio .DELTA.L.sub.t of the
change in the antenna inductance by ratio .DELTA.C.sub.t of composite
capacitance C.sub.t. This condition is represented by modifying Eq. (3) to
obtain Eqs. (10) and (11).
L.sub.t .multidot..DELTA.L.sub.t .multidot.C.sub.t .multidot..DELTA.C.sub.t
=a constant Eq.(10)
.DELTA.L.sub.t .multidot..DELTA.C.sub.t =1 Eq.(11)
From Eqs. (7) and (11), the condition for compensating resonance frequency
f is that the ratio of change in the ratio .DELTA.C.sub.t of composite
capacitance C.sub.t follows Eq. 12.
##EQU5##
If Eq. (9) holds, the value of .DELTA.C.sub.t may be replaced by the
changing amount of the area A of overlap capacitance C.sub.1 as seen from
Eqs. (8) and (9). That is, Eq. (12) is modified by assuming the area A as
a function of .DELTA..alpha. to obtain Eq. 13.
##EQU6##
where .beta. is the area when .DELTA..alpha.=0. Thus, when the loop length
of antenna 5 becomes shorter (.DELTA..alpha.>0 and its absolute value is
in the increasing direction), the value of A(.DELTA..alpha.) is increased
to increase the value of capacitance C.sub.1 of capacitor 6, whereby the
effect over the resonance frequency caused by a change in the antenna
inductance is compensated for.
Thus, in radio apparatus 1, a shape satisfying Eq. (13), i.e., an area
changing portion 15 tapering off toward the terminal end thereof is
provided, as shown in FIG. 2, on the side of first band member 3L, for the
end portion, of first conductor plate 5L.
To illustrate, take as the reference state the condition in which metallic
electrode 32 opposes a region having a relatively large width of first
conductor plate 5L as shown in FIG. 12A. When the state is changed to the
one shown in FIG. 12B where metallic electrode plate 32 opposes the
terminal end side (a region with a relatively narrow width) of first
conductor plate 5L, while a shift occurs to increase the value of antenna
inductance L.sub.t, the opposing area between first conductor plate 5L and
metallic electrode plate 32 is reduced thereby decreasing capacitance
C.sub.1. Since the amount of reduction (ratio .DELTA.C.sub.t of change) of
the composite capacitance C.sub.t corresponds to satisfy Eq. (11) with
respect to the change (ratio .DELTA.L.sub.t) of antenna inductance
L.sub.t, the effect on resonance frequency f by antenna inductance L.sub.t
is compensated.
An example of the result of such computation will now be described first
with respect to a case where capacitance C.sub.2 of variable capacitor 232
is relatively large, e.g., 1000 pF, so that its effect over the composite
capacitance C.sub.t may be ignored. In order to facilitate the way of
handling area changing portion 15 in the computation, it is regarded, as
shown in FIGS. 13A-B, as a shape wherein its width is narrowed in a
step-like manner toward its terminal end. A change in its width occurs
every 10 mm in the length direction. Further, the opposing position of
metallic electrode plate 32 with respect to first conductor plate 5L is
displaced by 10 mm at a time from the state shown in FIG. 13A. Note that
the width at the reference region A-0 of first conductor plate 5L is 7 mm;
the opposing distance between metallic electrode plate 32 and first
conductor plate 5L, i.e., the thickness of resin layer 441 of first band
member 3L placed between metallic electrode plate 32 and first conductor
plate 5L is 1.8.times.10.sup.-3 m, 1.8.times.10.sup.-4 m, or
1.6.times.10.sup.-5 m; and the dielectric constant .epsilon..sub.r of such
material when the dielectric of air is set to 1 is 3. The thickness of
resin layer 441 being 1.8.times.10.sub.-3 m is the condition for setting
capacitance C.sub.1 of capacitor 6 at the reference region A-0 to about 1
pF; the condition of the thickness being 1.8.times.10.sub.-4 m is the
condition for setting capacitance C.sub.1 at the reference region A-0 to
about 10 pF; and the condition of the thickness being 1.6.times.10.sub.-5
m is the condition for setting capacitance C.sub.1 at the reference region
A-0 to about 100 pF. When the dielectric constant .epsilon..sub.r of first
band member 3L is 5, the above condition setting corresponds to the case
where the thickness is 3.times.10.sup.-3 m, 3.times.10.sup.-4 m, or
2.7.times.10.sup.-5 m, respectively. Under this condition, if the wearer's
arm is relatively thick, metallic electrode plate 32 is slid from the
state as shown in FIG. 13A by 10 mm at a time toward regions A-1, A-2 on
the terminal end side of first conductor plate 5L to reduce the opposing
area A (.DELTA..alpha.). On the other hand, if the wearer's arm is
slender, metallic electrode plate 32 is slid by 10 mm at a time toward
regions A+1, A+2 on the base end side of first conductor plate 5L to
increase the opposing area A (.DELTA..alpha.). The result of computation
for the width of each of the regions A-2.about.A+2 of area changing
portion 15 by which the change in the antenna inductance L.sub.t at that
time may be compensated is shown in Table 1.
TABLE 1
______________________________________
Unit (mm)
Opposing region of First Conductor Plate 5L
Terminal
Reference Base
end side
position end side
A-2 A-1 A-0 A + 1 A + 2
______________________________________
d = 1.8 .times. 10.sup.-3 m
5.44 6.19 7.00 7.96 9.00
d = 1.8 .times. 10.sup.-4 m
5.43 6.19 7.00 7.97 9.02
d = 1.6 .times. 10.sup.-5 m
5.30 6.12 7.00 8.08 9.27
______________________________________
A description will now be given with respect to a case where capacitance
C.sub.2 of capacitance 232 is taken to be 8 pF, for example, and its
effect over the composite capacitance C.sub.t cannot be ignored. In this
case, since it is necessary to include capacitance C.sub.2 of variable
capacitor 232 in the calculation, the computation formula may be expressed
as:
##EQU7##
In this case too, in order to facilitate the handling of first conductor
plate 5L in the computation, it is assumed that, of first conductor plate
5L, the width of area changing portion 15 is narrowed as shown in FIGS.
13A-B toward the terminal end thereof. The state shown in FIG. 13A is
regarded as the reference. From this state, in a similar manner as in the
above-described computation, metallic electrode plate 32 is slid by 10 mm
at a time toward regions A-1, A-2 or toward regions A+1, A+2. The result
of computation of the width of each of the regions A-2.about.A+2 by which
the accompanying change in the antenna inductance L.sub.t may be
compensated for is shown in Table 2. Note that the dielectric constant
.epsilon..sub.r of first band member 3L is 3, and the thickness of resin
layer 441 is 2.0.times.10.sup.-3 m, 9.2.times.10.sup.-4 m,
4.6.times.10.sup.-4 m, 2.3.times.10.sup.-4 m or 1.16.times.10.sup.-4 m.
These are the conditions for setting the composite capacitance C.sub.t at
the reference state to 0.8 pF, 1.6 pF, 2.6 pF, 4.0 pF or 5.3 pF,
respectively. Further, these conditions corresponds to the case where the
thickness is 3.4.times.10.sup.-3 m, 1.5.times.10.sup.-4 m,
7.7.times.10.sup.-4 m, 3.8.times.10.sup.-4 m, and 1.9.times.10.sup.-4 m
when the dielectric constant .epsilon..sub.r is 5.
TABLE 2
______________________________________
Unit (mm)
Overlapping area of
First Conductor Plate 5L
Terminal
Reference Base
end side
position end side
A-2 A-1 A-0 A + 1 A + 2
______________________________________
d = 2.0 .times. 10.sup.-3 m
5.30 6.12 7.00 8.08 9.27
d = 9.2 .times. 10.sup.-4 m
5.13 6.02 7.00 8.24 9.65
d = 4.6 .times. 10.sup.-5 m
4.83 5.86 7.00 8.55 10.38
d = 2.3 .times. 10.sup.-4 m
4.31 5.56 7.00 9.23 12.22
d = 1.16 .times. 10.sup.-4 m
3.45 5.04 7.00 10.99 19.0
______________________________________
In radio apparatus 1, first and second conductor plates 5L, 5R form a
loop-like antenna 5 wherein first and second band members 3L, 3R are
connected to each other by means of buckle 31. Since the loop length of
antenna 5 changes according to the thickness of a wearer's arm, the
inductance value (L) of the antenna varies. However, radio apparatus 1 is
constructed such that the shift in resonance frequency f, caused by a
change in the loop length of antenna 5, is compensated for by changing
overlap capacitance C.sub.1. That is, as the opposing area of first
conductor plate 5L changes on the basis of the change in the loop length
of antenna 5, the product of antenna inductance L and composite
capacitance C.sub.t (C.sub.t =C.sub.1 +C.sub.2), which defines resonance
frequency f, is a constant. Thus, even if the loop length is changed,
resonance frequency f does not change. Therefore, a stable antenna gain is
obtained without being affected by changes in loop length.
Modification of Embodiment 1
As shown in FIG. 14A, in Embodiment 1, the shape of area changing portion
15 of first conductor plate 5L is such that two sides 151, 152, are
shaped, into curves and its width is narrowed toward its terminal end.
Alternatively, as shown in FIG. 14B, its shape may be such that, a side
153, forms a straight line and only another side 154, comprises a
curvilinear shape so that its width is narrowed toward its terminal end.
In addition, its shape may also be: 1) as shown in FIG. 14C where both of
its sides 155, 156 comprise step-like curvilinear shape and its width is
narrowed by steps toward the terminal end edge thereof; 2) as shown in
FIG. 14D having a notch 158 cut into from the center portion of an
terminal end edge 157 thereof; 3) as shown in FIG. 14E where a notch 159
thereof is formed into a stepped mariner; or 4) as shown in FIG. 14F where
one side 160 thereof is a straight line and only another side 161
comprises steps. Further, its shape may also be such that the opposing
area thereof against metallic electrode plate 32 is varied by
intermittently forming, for example, holes on conductor plate 5L.
Embodiment 2
FIG. 15A shows the construction of the terminal end of a first band member
of an antenna device for an arm-attached type radio apparatus according to
Embodiment 2 of the present invention. Since the construction of this
arm-attached type radio apparatus is similar to that of the arm-attached
type; radio apparatus according to Embodiment 1 (with the exception of the
resonance frequency compensation means at the terminal end side of the
first band member thereof), the same reference numerals are given to those
components having corresponding functions.
In radio apparatus 40, in a similar manner as the arm-attached type radio
apparatus according to Embodiment 1 as shown in FIG. 1, its overall
construction includes: a receiver body 2 having within a casing 21 a
circuit board 22 and circuit block 23 for radio apparatus; and an arm
attaching band 3 having a first and second band members 3L, 3R connected
to receiver body 2. Of the arm attaching band 3, the end portion of second
band member 3R has a band connector portion 30 thereon formed of a
metallic buckle 31 attached thereto. Buckle 31 is operable to detachably
connect band members 3L, 3R. Band members 3L, 3R can be made from
materials such as leather, silicone resin or urethane resin. First and
second band members 3L, 3R respectively have first and second conductor
plates 5L, 5R formed therein. By connecting band members 3L, 3R a
loop-like antenna 5 is formed.
Referring to FIGS. 1, 9A and 15A, antenna 5, at band connector portion 30,
has a capacitor 6 formed between a metallic electrode plate 32 formed
integrally with buckle 31 and first conductor plate 5L on the side of
first band member 3L which is mechanically engaged with buckle 31. Thus,
as shown in FIGS. 9A-9B, 10A-10B, the construction of antenna 5 is such
that capacitor 6 and a variable capacitor 232 are serially connected with
respect to inductance L.sub.1 of first conductor plate 5L and inductance
L.sub.2 of second conductor plate 5R.
In radio apparatus 40, as the loop length of antenna 5 changes according to
the thickness of the wearer's arm, antenna inductance L (composed of
inductance L.sub.1 and inductance L.sub.2) is changed and the antenna
resonance frequency is therefore changed. Thus, in radio apparatus 40, a
resonance frequency compensation means is provided, which compensates for
the effect of antenna inductance L on resonance frequency f by changing
capacitance C.sub.1 of capacitor 6. As expressed by Eq. (8), capacitance
C.sub.1 is defined by opposing area A, dielectric constant E and electrode
spacing d. While the width of first conductor plate 5L and the thickness
of first band member 3L are kept constant, an
effective-dielectric-constant changing portion 45 is provided on first
band member 3L serving as the dielectric, where resin layer 451 of the
band is partially missing and an air layer 452 exists. Capacitance C.sub.1
is changed by changing the effective-dielectric-constant of capacitor 6
according to the connecting position on the free end side of arm attaching
band 3. This is done to compensate for the effect of antenna inductance L
on resonance frequency f. Thus, while opposing area A and electrode
spacing are constant (because the width of first conductor plate 5L and
the thickness of first band member 3L are constant), the ratio in which
resin layer 451 and air layer 452 exist as the dielectric is varied
according to the region thereof between metallic electrode plate 32 and
first conductor plate 5L. As a result, the actual capacitance C.sub.1 of
capacitor 6 is expressed by Eq. (14) as it is regarded as the sum of
capacitance C.sub.a1 which is attributable to the capacitance component
which is formed at the portion where resin layer 451 exists and
capacitance C.sub.b2 which is attributable to the capacitance component
which is formed at the position where air layer 452 exists.
##EQU8##
where: .epsilon..sub.1 is the dielectric constant of resin layer 451;
.epsilon..sub.2 is the dielectric constant of the air layer; d is the
thickness of resin layer 451; A.sub.1 is the opposing area of first
conductor plate 5L and metallic electrode plate 32 corresponding to the
portion where resin layer 451 exists; and A.sub.2 is the opposing area of
first conductor plate 5L and metallic electrode plate 32 corresponding to
the portion where resin layer 451 does not exist but air layer 452 exists.
Of thus expressed capacitances C.sub.a1 and C.sub.b1, since the dielectric
constant .epsilon..sub.2 of air layer 452 is extremely small compared to
dielectric constant .epsilon..sub.1 of resin layer 451 and the ratio of
areas A.sub.1, A.sub.2 changes depending on the connecting position of arm
attaching band 3, capacitance C.sub.1 may be regarded as a function of the
connecting position of arm attaching band 3. For example, supposing the
state shown in FIG. 15A as the reference, when the length of antenna 5 has
increased by .DELTA..alpha. from this state, the ratio of the opposing
area A.sub.1 between first conductor plate 5L and metallic electrode plate
32 corresponding to the portion where resin layer 451 exists is reduced to
make capacitance C.sub.1 smaller. That is, since both antenna inductance L
and capacitance C.sub.1 are functions of .DELTA..alpha., regarded as the
shift amount from the reference position, the product of antenna
inductance L and composite capacitance C.sub.t is kept constant even when
the connecting position of arm attaching band 3 is changed by the
thickness of the wearer's arm. As a result, capacitance C.sub.1
compensates for the effect of a change in antenna inductance on resonance
frequency f.
Here, the computation result with respect to the structure of
effective-dielectric-constant changing portion 45 of first band member 3I,
will be described. Note that, in this computation, a description is given
with respect to the case where capacitance C.sub.2 of variable capacitor
232 is taken to be 8 pF and its effect on composite capacitance C.sub.t
cannot be ignored. In order to facilitate the handling of
effective-dielectric-constant changing portion 45 in the computation, it
is supposed that, at effective-dielectric-constant changing portion 45,
the width of the portion at which resin layer 451 exists is narrowed as
displaced by 10 mm toward the terminal end and the opposing position of
metallic electrode plate 32 is displaced by 10 mm at a time from the state
shown in FIG. 15A. Here, the width of first conductor plate 5L is 7 mm and
the width of resin layer 451 is 3.5 mm. Note that it is set so that
capacitance C.sub.1 of capacitor 6 at the reference state shown in FIG.
15A is 2 pF, 8 pF. Of these, for the case of capacitance C.sub.1 being 2
pF, the computation is performed with respect to the case where the
opposing distance between metallic electrode plate 32 and first conductor
plate 5L, is 1.7.times.10.sup.-3 m, 9.2.times.10.sup.-4 m, or
6.1.times.10.sup.-4 m. For the case of capacitance C.sub.1 being 0.8 pF,
the computation is performed with respect to the case where the thickness
of resin layer 451 is 4.26.times.10.sup.-3 m, 2.3.times.10.sup.-3 m, or
1.5.times.10.sup.-3 m. Further, while the computation is performed with
respect to the case where dielectric constant .epsilon..sub.r of resin
layer 451 is 10, 5 or 3, the dielectric constant of the air layer is
treated as 1. Under such condition, if the wearer's arm is relatively
thick, metallic electrode plate 32 is slid from the state shown in FIG.
15A by 10 mm at a time toward regions A-1, A-2 on the terminal end side of
first conductor plate 5L so as to reduce the dielectric constant .epsilon.
thereof. If the wearer's arm is slender, metallic electrode plate 32 is
slid by 10 mm at a time toward regions A+1, A+2 on the base end side of
first conductor plate 5L so as to increase the dielectric constant
.epsilon. thereof. The results of the computation for the width of resin
layer 451 of each of regions A-2.about.A+2 at
effective-dielectric-constant changing portion 45 are shown in Tables 3
and 4.
TABLE 3
______________________________________
When C.sub.t is 2.0pF Unit (mm)
Opposing region of First Conductor Plate 5L
Terminal
Reference Base
end side
position end side
A-2 A-1 A-0 A + 1 A + 2
______________________________________
e.sub.r = 10
| | |
