Tunable slot antenna with capacitively coupled slot island conductor for precise impedance adjustment

A coaxial resonant slot antenna includes a flat rectangular conductive box having its top plate with a slot being defined therein, and a strip conductor disposed inside the box and electrically insulated from the box while high frequency or RF power is fed to the strip. An island conductor is provided in the slot for defining a capacitance between itself and the frame. This capacitance is rendered variable in value by use of a variable circuit.

BACKGROUND OF THE INVENTION 
The present invention relates generally to antenna architectures and, more 
particularly, to slot antenna structures utilizing a coaxial resonator for 
use with mobile communication units including, but not limited to, 
cellular radiotelephone handsets in wireless communications systems. 
Wireless telecommunication systems are known to employ portable or handheld 
mobile communication units such as for example cellular radiotelephone 
handsets operatively associated with a limited number of wireless 
communication resources and a remote system resource controller. As the 
cellular handsets require smaller size and less thickness, miniaturization 
or "down-sizing" of the antenna module adapted for use with such units has 
become more critical in recent years. Until today, several approaches to 
the small-size antenna have been proposed and developed. One previously 
known approach is to use a slot antenna incorporating a coaxial resonator. 
An exemplary coaxial-resonator slot antenna has been disclosed in U.S. 
Pat. No. 5,914,693 filed Sep. 5, 1996. The slot antenna disclosed is 
structurally designed so that its centrally disposed elongate or "strip" 
conductor is kept in non-contact with a flat rectangular box-like 
conductive frame body of the resonator to miniaturize the antenna. This 
antenna also features in absence of any particular outer projections 
facilitating mounting of the antenna into the enclosure of a cellular 
radiotelephone handset. 
Since the prior art small-size slot antenna has such resonator structure, 
the volume thereof is proportional to its impedance matching 
bandwidth--that is, the smaller the volume, the less the bandwidth. 
Accordingly, in cases where the antenna is practiced at communication 
units of a broad-band wireless communication system with an increased 
capacity by use of a plurality of different carrier frequencies allocated 
thereto, the impedance matching frequency band to be achieved by the 
antenna might be widened enlarging the antenna in size and in volume. 
As readily appreciated by a person skilled in the art, those frequencies 
used for telephone interconnect calls between one particular base station 
and its associated wireless communication units, such as cellular 
handsets, are much less than the frequency band inherently allocated to 
the entire communication system. Accordingly, adaptively changing the 
antenna's impedance matching center frequency to a presently selected 
frequency for a telephone call attempt may render narrower the antenna's 
inherent frequency band, which in turn downsizes the antenna. One typical 
antenna module incorporating this approach is a tunable slot antenna as 
disclosed in copending U.S. patent application (Ser. No. 09/035,848, filed 
Mar. 6, 1998. This antenna is a coaxial resonator-used slot antenna 
including a variable capacitive element connected between a selected 
location at or near one end of a built-in strip conductor, which end is 
far from a connection point of the strip being supplied with 
high-frequency electric power, and an opposing plate of a rectangular box. 
The antenna may vary in impedance matching center frequency by varying the 
capacitance value of the variable capacitance element. An exemplary 
structure of the tunable slot antenna is shown in FIGS. 10a and 10b. 
The tunable slot antenna shown in FIGS. 10a-10b includes a conductive flat 
box 1. The box 1 has therein an elongate strip-like conductor 3 that is 
electrically insulated from the box 1 and extends along the axis of 
resonance. The box 1 has a top plate surface in which a slot 2 is formed 
overlying and crossing the strip 3. Strip 3 has one end at which a 
connection point 10 is disposed and its opposite end has a small opening 
11 defined for disposal of an island conductor 4. The connection point 10 
is operatively coupled to a high frequency or radio frequency (RF) power 
supply circuit 7, which operates to supply RF power between the connection 
point 10 and its opposing part of the bottom plate of the box 1. RF power 
supply 7 is associated with a certain element for elimination of unwanted 
high frequency current drain, and a variable direct current (DC) power 
supply 9. A variable capacitive element 6 is connected between a selected 
location at or near the far end of strip 3 with small hole 11 and its 
opposing part of the top plate of the box 1. Variable capacitor 6 receives 
a DC voltage from DC power supply 9 via strip 3 and RF current drain 
eliminator 8. 
In the antenna structure of FIGS. 10a-10b, the variable capacitor 6 may 
vary in capacitance value in response to receipt of a DC voltage applied 
from variable DC power supply 9 thereby causing a current flowing in strip 
3 just beneath slot 2 to likewise change in phase. Such strip current 
phase change may in turn serve to permit strip 3 to change in length 
equivalently or "virtually," which length closely relates to the resonant 
frequency of the tunable slot antenna shown. This makes it possible for 
the antenna to change or modify the impedance matching center frequency, 
that is, resonant frequency. 
BRIEF SUMMARY OF THE INVENTION 
The coaxial resonator-based slot antenna taught by U.S. patent application 
Ser. No. 08/708,563 and the tunable slot antenna proposed in the prior 
U.S. patent application based on Japanese Patent Application No. 9-54825 
are such that the matching condition is determinable depending on both the 
strip current phase and the slot length. In this respect, suppose in the 
FIG. 10 antenna that the capacitor 6 is varied in capacitance altering the 
current phase of strip 3. If this is the case, as the resonant frequency 
varies, so does the resultant matching condition, thereby rendering it 
difficult to efficiently supply the antenna with RF power. The prior art 
approaches are also faced with a problem: the inability to permit the 
resonance frequency to vary over a wide range while achieving such 
efficient RF power supply to the antenna. This can be said because the 
variable capacitor for suppressing the variation range of the matching 
state to the extent that RF power is efficiently supplied to the antenna 
remains extremely less in both absolute capacitance value and changeable 
quantity. 
It is therefore an object of the present invention to provide a new and 
improved slot antenna structure capable of avoiding the problems 
encountered with the prior art. 
It is another object of the invention to provide an improved tunable slot 
antenna capable of permitting the resonant frequency to vary over an 
extended range while maintaining the antenna matching condition required. 
It is a further object of the invention to provide a tunable slot antenna 
capable of forcing both the length of a slot and the length of a 
strip-like conductor immediately underlying the slot to equivalently vary 
or change at a time, thereby widening the resonant frequency variable 
range while maintaining the antenna matching condition required. 
According to one aspect of the present invention, a tunable slot antenna 
includes a conductive box with a slot formed in one principal surface 
thereof, and a conductor insulatively disposed or "embedded" inside the 
box to spatially intersect the slot. Alternating current (AC) power is fed 
between a connection point of the conductor and the box. The box also 
includes an island-like conductor which is formed in the slot to be 
electrically isolated from the box, and electrical circuitry connected 
between the island and a wall plate of the box for permitting the 
capacitance therebetween to vary in value. 
In accordance with another aspect of the invention, a tunable slot antenna 
is provided which includes a flat conductive box of a generally 
parallelepipedic shape or rectangular prism shape, and an elongate 
conductor or "strip" member insulatively embedded inside the box. The box 
has in its upper plate surface a slot overlying the conductive strip to 
spatially cross the same. The strip has one end where a connection point 
is disposed and connected thereto, permitting high frequency or radio 
frequency (RF) power to be supplied between the connection point and a 
wall plate of the box. The box also includes an elongate island conductor 
as disposed within the slot. The island conductor is electrically 
insulated from the box. Variable capacitance circuitry is provided and 
connected between the island conductor and the wall plate of the box for 
allowing the capacitance therebetween to vary in value. With such an 
arrangement, varying the capacitance between the island conductor and the 
wall plate of the box may cause the antenna to widely vary or change in 
impedance matching center frequency, i.e. resonant frequency, without 
affecting the inherent matching condition of the tunable slot antenna. 
It should be noted that scheme for letting the resonant frequency of 
coaxial resonator-based slot antenna to vary by equivalently or 
"virtually" altering the physical length of the strip conductor has also 
been employed in the structure shown in FIGS. 10a-10b. One significant 
difference of the invention over this structure is that the latter is 
designed to directly couple its variable capacitive element between the 
end of such strip and a wall plate of the box whereas the former 
incorporates a specific variable capacitance circuit capable of varying 
the value of a capacitance between the island conductor and a wall plate 
of the box, the island conductor being disposed within the slot and 
capacitively coupled to the strip. Thereby, even where the capacitance 
value is greatly altered by the variable capacitance circuit, it is 
possible to insure fine or precise capacitance value variation between the 
strip and the box, which may in turn enable the antenna to change its 
impedance matching center frequency with increased accuracy and enhanced 
reliability. 
It is also noted that a minimal configuration required to attain the 
intended virtual slot length variability or adjustability stated supra may 
be a slot antenna having a first conductor with a slot formed therein, and 
a second conductor while AC power is supplied between the first and second 
conductors, wherein the antenna further includes a third conductor 
disposed inside the slot to be electrically insulated from the first 
conductor, and circuitry connected between the first and third conductors 
for permitting a capacitance therebetween to vary in value. 
Additionally, the prescribed island conductor capacitively coupled to the 
strip need not always be provided in the slot in order to achieve the 
objective of precisely changing the antenna impedance matching center 
frequency by creation of a minute or fine capacitance value variation 
between the "internal" conductor embedded inside the box and a wall plate 
of the box. In some cases a slot antenna is employable which includes a 
conductive rectangular box with a slot formed in its one principal 
surface, and a conductor insulatively disposed inside the box and 
spatially crossing the slot while letting AC power supply be fed between a 
connecting point of the conductor and the box, wherein the antenna further 
includes an island conductor capacitively coupled to the conductor inside 
the box, and circuitry connected between the island and the box for 
varying or changing the value of a capacitance between the two. 
The invention should not exclusively be limited to the slot antennas, and 
may alternatively be applicable to those antenna modules of the type which 
may include a first conductor and a second conductor with AC power being 
supplied therebetween, wherein a third conductor is disposed opposing the 
second conductor while circuitry is connected between the first and third 
conductor for varying the capacitance in value therebetween. In this case 
also, it is possible to attain a fine or precise capacitance value 
variation between the first and second conductors. 
The tunable slot antenna's matching condition is determinable by both the 
current phase on the conductive strip underlying the slot and the length 
of such slot. Where the variable capacitance circuit operates to change or 
vary the capacitance value between the island conductor and a grounded 
wall plate of the box, if for example the resulting capacitance is 
sufficiently large in value, the island conductor is substantially equal 
in potential to the wall plate--namely, ground potential. This causes the 
slot to equivalently or "virtually" decrease in width to the extent that 
such reduction corresponds to the size of island conductor. This partial 
decrease in slot width may be equivalent to an increase in slot length. 
Thus, varying the capacitance value of the variable capacitance circuit 
enables the slot to virtually vary in length. Since an increase in 
capacitance value results in an virtual increase in both strip length and 
slot length, it becomes possible to maintain the intended matching 
condition of the antenna. 
The variable capacitance circuit for use with the tunable slot antenna 
incorporating the principles of the invention may be a device or element 
variable in capacitance value upon application of a DC voltage thereto, 
including but not limited to a capacitance variable diode. When employing 
such DC voltage-controlled capacitance-variable element, one end of it is 
electrically connected to the island conductor whereas the other end 
thereof is coupled to a grounded wall plate of a flat conductive box. This 
makes it possible to permit the capacitance between the island conductor 
and the wall plate of the box to vary upon application of a DC voltage to 
the island conductor. 
Supplying a control signal to the variable capacitance circuit is 
attainable by providing a control signal transmission lead wire as 
embedded inside the box and is electrically insulated from the box, which 
lead has one end connected to the circuit via a small hole formed in a 
selected plate of the box and an opposite end coupled to a control circuit 
through another small hole in a box plate. 
The use of such antenna for communication units in wireless communications 
systems makes it possible to properly tune the antenna's resonant 
frequency at any selected one of radio frequencies updatable every time a 
connection is done for telephone interconnect calls. In this case, the 
frequency band allocated to the antenna per se may be narrowed to cover a 
mere bandwidth required for such telephone calls. This renders the 
resulting antenna frequency band considerably narrower than the frequency 
band allocated to the wireless communications system. Consequently, the 
antenna module may be less in volume than prior art antennas designed to 
cover the whole part of the system frequency band, which may in turn 
facilitate mounting the antenna to wireless communication units such as 
for example handheld radiotelephone handsets. Further, since the antenna 
offers widened or extended resonant-frequency variable range, the 
applicability thereof may likewise expand covering those radiotelephone 
handset units for use in broad-band wireless communications systems. 
Furthermore, the prescribed variable capacitance circuit for use with the 
tunable slot antenna in accordance with the invention may be certain 
circuitry responsive to receipt of a control signal applied at a certain 
terminal thereof for performing a switching operation to selectively 
change between two or more preset capacitance values. Typically, the 
circuitry may be a combination of a high frequency or RF switch device and 
more than one capacitive elements operatively coupled thereto. In this 
case the RF switch has its control node, and also input and output nodes 
one of which is connected to the frame plate and the other of which is 
coupled to an island conductor via a capacitive element. With such an 
arrangement, the capacitance between the island conductor and the wall 
plate of the box may be varied or modified in value by supplying a control 
signal to the control node of RF switch thereby attaining the intended 
turn-on/off control thus causing the switch to be in either the open state 
or close state between its input and output nodes. 
The use of a plurality of such capacitive elements and a multi-input/output 
RF switch may achieve multi-value capacitance variation scheme. Where 
appropriate, the plural capacitors and multi-node RF switch may be 
implemented into a single integrated circuit (IC) chip set. 
These and other objects, features and advantages of the invention will be 
apparent from the following more particular description of preferred 
embodiments of the invention, as illustrated in the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
Tunable slot antenna in accordance with some preferred embodiments of the 
present invention will be described in detail with reference to FIGS. 1a 
to 9b below. Note that the same reference numerals are used to designate 
the same or similar parts or components. 
First Embodiment 
A tunable slot antenna in accordance with one embodiment of the invention 
is shown in FIGS. 1a and 1b, wherein FIG. 1a is a perspective view whereas 
FIG. 1b is a sectional view taken along line IB--IB of FIG. 1a (not drawn 
to scale). The slot antenna shown is arranged to include a flat conductive 
box 1 of a rectangular or parallelepipedic shape. The conductive box 1 has 
a top wall plate, a bottom wall plate, two opposite end wall plates and 
two opposite side wall plates, all of the wall plates being electrically 
conductive, as shown in FIG. 1a. The box 1 has in its top plate a 
generally "U"-shaped slot opening 2. The shape of the slot opening 2 is 
not limited to the illustrated one. The interior space of the box 1 is 
filled with a dielectric material (not shown), in which an elongate 
strip-like conductor 3 is securely embedded. The strip 3 has at its one 
end a connection point or a coupler section 10, and has its opposite end 
acting as a free or open end, which immediately underlies and spatially 
intersect the U-shaped slot 2. The box 1 is coupled to ground potential. 
The box 1 has in the bottom plate an island conductor 14 in its 
corresponding hole formed at a location near one of the end wall plates of 
the box 1. The island conductor 14 is electrically insulated from the box 
1. The island conductor 14 is positioned just beneath the connection point 
or the coupler section 10 of the strip 3. Island conductor 14 is 
interconnected to the strip 3 at its connection point 10 via a conductive 
vertical through hole conductor (hereafter, simply referred to as "through 
hole" for simplicity) 13 inside box 1. A high frequency or radio frequency 
(RF) power supply circuit 7 is connected between island 14 and the bottom 
plate of the box 1 as shown in FIG. 1b. Island conductor 14 may thus 
function as an RF power feed port of the illustrative antenna. 
As best depicted in FIG. 1a, an island conductor 20 is disposed within the 
slot 2 at the base of the "U" on the top plate of the box 1. A variable 
capacitance circuit 16 is mounted on the top wall plate of the frame 1 in 
a way such that circuit 16 is connected between the island conductor 20 
and the top plate of the box 1. The variable capacitance circuit 16 has a 
control terminal connected via a lead wire 30 to its associated control 
circuit 50, which is also mounted on the top plate of the box 1. The 
control circuit 50 is operable to supply a DC voltage of a selected 
potential to the control terminal of the variable capacitance circuit 16 
through the control lead 30. 
The variable capacitance circuit 16 is variable in capacitance between its 
pair of terminals under control of the control circuit 50, one of which 
terminals is connected to the island conductor 20 and the other of which 
is to the top plate of the box 1 at a selected location thereon. The 
control node of the variable capacitance circuit 16 is for receiving a DC 
voltage control signal used to vary the terminal-to-terminal capacitance. 
The control lead 30 has one end connected to the control terminal of 
circuit 16 and the other end coupled to the controller 50. The control 
signal from controller 50 is variable in potential level thus rendering 
the value of a capacitance between island 20 and box 1 likewise variable. 
The capacitance between the island conductor 20 and flat rectangular box 1 
is combinable with the capacitance between strip conductor 3 and island 
conductor 20 into a synthetic capacitance which may provide an additive or 
extra capacitance between a certain location at or near the open edge of 
strip 3 spaced far from the connection point 10 thereof and a box plate 
coupled to ground, whereby the density of electric flux lines at the 
location at or near the open end of strip 3 far from the connection point 
10 increases, as compared to the case where the island conductor 20 and 
variable capacitance circuit 16 are absent, so that the density of current 
as induced on the strip 3 increases accordingly so as to compensate for an 
increase in electric flux line density. This results in the strip 3 being 
virtually changed in length, which in turn permits the impedance matching 
center frequency, namely resonant frequency, to change or vary 
accordingly. Since the synthetic capacitance is made of series-connected 
capacitances, it is possible by reducing the capacitance value between the 
strip 3 and island conductor 20 to attain fine or precise capacitance 
value variation even where the capacitance between the island conductor 20 
and box 1 is greatly changed in value by the variable capacitance circuit 
16. This in turn enables the illustrative antenna to vary in resonance 
frequency with enhanced precision. 
In addition, the island conductor 20 residing within the slot 2 is 
capacitively coupled to the top plate of the box 1; accordingly, part of a 
current generated near or around the slot 2 behaves to flow into the 
island conductor 20 depending on the actual capacitance value selected. 
Therefore, permitting the variable capacitance circuit 16 to vary or 
modify the capacitance value between the island conductor 20 and box 1 
makes it possible for the current generatable around slot 2 to change in 
flow path--that is, enabling the slot 2 to virtually or equivalently 
change its physical length. 
One of the significant advantages of the slot antenna shown in FIGS. 1a-1b 
lies in capability to permit both the strip conductor 3 and slot 2 to 
equivalently change or vary in length at a time by causing the variable 
capacitance circuit 16 to appropriately modify or alter the capacitance 
value between the island conductor 20 and box 1. Since the antenna 
matching condition employable in the coaxial resonator-based slot antenna 
is determinable depending on both the current phase on the strip 3 just 
beneath the slot 2 and the length of slot 2, the use of the illustrative 
structure capable of simultaneously altering the both parameters makes it 
possible for the antenna impedance-matching center frequency, i.e. 
resonant frequency, to widely vary or change over an extended region 
without having to badly affect the matching condition per se. 
Another advantage of the embodiment of FIGS. 1a-1b is that the 
modifiability or changeability of the capacitance between the island 
conductor 20 and box 1 may render the resonant frequency likewise variable 
under control of the control circuit 50 which is for use in supplying the 
variable capacitance circuit 16 with a control signal for the intended 
capacitance variation or adjustment. This in turn allows the control 
signal from control circuit 50 to vary in conformity with a variable RF 
frequency as selectively allocated per wireless communication attempt, 
such as a telephone interconnect call over public telephone network 
channels, thereby letting the RF frequency be identical to the resonance 
frequency. It is thus possible to force the antenna's bandwidth at a 
certain resonance frequency to be limited to the bandwidth required for a 
presently desired communication event which must be extremely less than 
the entire frequency band coverage the system requires, thereby enabling 
the slot antenna to decrease in volume. In addition, the slot antenna with 
the structure stated supra is wide in variable range of resonance 
frequency, and is thus applicable to mobile radio, portable radio, or 
radio/telephone terminals for use in wireless communication systems with 
increased system frequency band. 
The antenna of FIGS. 1a-1b may be designed to have a thickness-reduced or 
"thin" planar structure that is as flat as currently available coaxial 
resonant slot antenna, which leads to the applicability to built-in 
antenna configurations with no particular outer projections by mounting 
the illustrative antenna on a mother board of RF circuitry in 
communication terminals such as for example cellular radiotelephone 
handsets. 
It should be noted that the prescribed "antenna dimension reduction" 
concept per se--i.e. the antenna dimension is reduced due to fulfillment 
of a dielectric material inside the flat frame body as compared to the 
case of no such dielectric materials--may be similar in principle to that 
disclosed in the above-identified copending U.S. Patent Application based 
on Japanese Patent Application No. 9-54825. 
Second Embodiment 
A tunable slot antenna in accordance with another embodiment of the 
invention is shown in FIGS. 2a and 2b, which is generally similar to that 
of FIGS. 1a-1b with the control circuit 50 being replaced by a variable DC 
power supply circuit 9. In this configuration the variable capacitance 
circuit 16 is responsive to receipt of a DC voltage applied from DC power 
supply 9 via control lead 30 for varying or changing the capacitance value 
between the island conductor 20 and flat rectangular box 1. 
An advantage of this embodiment is that the transmit/receive or 
"transceive" characteristics may be enhanced because of the fact that 
control lead 30 provided near or around the antenna structure is kept 
substantially constant in potential with time (DC in nature) so that 
unnecessary noises are hardly given to the antenna. 
It is noted here that the variable DC power supply circuit 9 is arranged to 
vary the voltage potential under control of its associative voltage 
controller circuitry (not shown), which is operable to generate a control 
signal for use in identifying an appropriate voltage value corresponding 
to a presently established radio frequency while permitting the DC power 
supply 9 to produce a predefined DC voltage in response to such control 
signal. DC power supply 9 has its control signal receive terminal (not 
shown) for input of the control signal. 
Third Embodiment 
Referring now to FIGS. 3a and 3b, a tunable slot antenna in accordance with 
still another embodiment of the invention is shown which is similar to 
that of FIGS. 2a-2b with the variable capacitance circuit 16 being 
replaced with a specific variable capacitor element 6. This element may 
continuously vary in capacitance value upon receipt of a DC voltage. 
Element 6 may typically be a variable capacitance diode, which is called 
the "vari-cap" diode in some cases. The diode 6 has its one node connected 
to the slot island conductor and the other node coupled to the grounded 
top plate of the flat rectangular box 1. Island conductor 20 and box 1 
define a specified capacitance therebetween whose value is variable or 
changeable by applying a selected DC voltage from variable DC power supply 
9 to the island 20 via control lead 30. 
An advantage of the slot antenna structure shown in FIGS. 3a-3b lies in the 
capability to reduce complexity of circuit configuration thus reducing the 
cost penalty of parts used. This can be said because the variable 
capacitance circuit consists essentially of a single variable capacitance 
diode 6. Another advantage is that the resonant frequency may be 
continuously adjustable to have any desired values by appropriately 
determining the value of a DC voltage used. This is true because diode 6 
is of the device capable of continuously varying its capacitance value. 
It is to be noted that a voltage controller circuit (not shown) operatively 
associated with such variable capacitance diode 6 is designed to prestore 
therein the relation of a DC application voltage versus capacitance value 
of diode 6, and also capable of permitting the antenna's resonant 
frequency to be identical or "tuned" at any desired radio frequency by 
"notifying" variable DC power supply 9 of an appropriate voltage value for 
production of the intended capacitance value corresponding to the radio 
frequency. 
Fourth Embodiment 
Turning now to FIGS. 4a and 4b, a tunable slot antenna in accordance with 
yet another embodiment of the invention is shown which is designed so that 
the DC voltage feed part for supplying a DC voltage to the variable 
capacitor element is coupled to the connection point of strip conductor 3. 
More specifically, as best illustrated in FIG. 4b, a variable capacitance 
diode 6 is associated with a resistive element 21. The resistor 21 has one 
end connected to the island conductor 20 and the other end coupled to an 
island conductor 4, which is provided in a small opening defined in the 
top plate of flat rectangular box 1 in a way such that the island 
conductor 4 is electrically insulated from the top wall plate of the box 
1. The round island 4 is in turn connected via a conductive though-hole 5 
to strip conductor 3 at a specified location at or near the free or open 
end of the strip 3 far from the connection point 10. 
Resistor 21 has its resistance value large enough to be negligible relative 
to the RF impedance at the far opposite end of strip 3 distant from the 
connection point 10 while at the same time being sufficiently less than 
the impedance of a DC voltage application node of a variable capacitance 
element 6, thereby enabling island conductor 20 to be substantially equal 
in DC potential to the connection point 10 without deteriorating the RF 
power fed to the strip 3. More practically, the prescribed condition is 
achievable by setting the resistance of the resistor 21 at a value falling 
within a range of from several kilo-ohms (k.OMEGA.) to several hundreds of 
k.OMEGA.. 
Coincidence of the DC voltage feed part of the variable capacitance element 
6 to the connection point 10 of the strip 3 may avoid the necessity of 
employing the control lead 30, thus further reducing complexity of circuit 
configuration. The elimination of control lead 30 disposed near the slot 2 
leads to the capability of further suppression of affection to radiation 
patterns of the antenna. 
The connection point 10 is fed with RF current and DC voltage from the 
island conductor 14, which is insulatively disposed in the small hole in 
the bottom plate of frame 1 and is electrically connected to the 
connection point 10 via through hole 13. The power feed scheme using an RF 
power supply circuit 7 and variable DC power supply 9 as well as a 
specific device or element 8 used for elimination of RF current drain 
toward DC power supply 9 may be similar in principle to that taught by he 
above-identified copending U.S. Application based on Japanese Patent 
Application No. 9-54825. 
Fifth Embodiment 
Referring now to FIGS. 5a-5b, a tunable slot antenna in accordance with a 
further embodiment of the invention is shown which employs a variable 
capacitance circuit capable of switching the interterminal capacitance 
between two or more values in response to a control signal supplied 
thereto. More specifically, the antenna module shown is similar to that of 
FIGS. 1a-1b with the variable capacitance circuit 16 being replaced by a 
multiple capacitance-value changeable capacitance circuit 51. As best 
shown in FIG. 5b, this circuit 51 consists essentially of a serial 
combination of a multi-node RF switch device and a preselected number of 
parallel capacitors coupled thereto. In this embodiment the switch may be 
a three-node switch operable to selectively change its output capacitance 
value among three different values of the capacitors. As shown in FIG. 5b, 
multi-variable capacitance circuit 51 has its common switch node connected 
to the slot island conductor 20 while the three capacitively variable 
terminals thereof are electrically coupled to the grounded top plate of 
the box 1, through three parallel capacitors of predefined capacitance 
values different from one another. Variable capacitance circuit 51 is 
responsive to a control signal supplied from control circuit 50 via 
control lead 30 to a control terminal of circuit 51, for performing a 
switching operation to let the capacitance between the island conductor 20 
and box 1 be set at a desired value as selected from among the three 
preset capacitance values. 
Preferably, respective capacitors of capacitance circuit 51 are designed so 
that the antenna resonant frequency determinable depending on the 
capacitance value between the island conductor 20 and box 1 is exactly the 
same as any one of desired antenna resonance frequencies. It is thus 
possible, by supplying circuit 51 with a control signal permitting 
generation of respective capacitance values, to cause the antenna 
resonance frequency to be identical or "tuned" at any desired frequency. 
In this case the slot antenna of FIGS. 5a-5b might come with a limitation 
as to the attainability of limited resonant frequency values as compared 
to the second embodiment shown in FIGS. 2a-2b with continuous 
capacitance-value changeability due to DC voltage application; 
fortunately, the presence of such limitation will never raise any serious 
problems when reduction to practice for application to mobile 
radiotelephone handsets because of the fact that the carrier frequency for 
use therein must set at a series of discrete values. 
Additionally, the control signal being supplied to the variable capacitance 
circuit 51 of FIG. 5b may be a digital signal that exhibits differences in 
potential level and/or variable pattern with time for use in enabling 
execution of the intended capacitance-value switching. Such digital signal 
is inherently durable against the signal interference as applied from 
other circuits used, which may in turn enable achievement of enhanced 
resonant frequency stability--that is, permitting the antenna to be stably 
set at its required fixed resonance frequency in an extended time. 
Sixth Embodiment 
A tunable slot antenna in accordance with a still further embodiment of the 
invention is shown in FIGS. 6a-6b, which is similar to that shown in FIGS. 
5a-5b with the control circuit 50 being replaced by the variable DC power 
supply circuit 9 of FIG. 2b and with the variable capacitance circuit 51 
of FIG. 5b being replaced by a combination of a capacitor 22 and a high 
frequency or RF switch 23. The series connection of capacitor 22 and RF 
switch 23 functions as the variable capacitance circuit capable of 
switching its output capacitance between two or more preset interterminal 
capacitance values, the latter being such that the impedance between 
certain terminals is changeable in response to receipt of a DC voltage at 
a selected terminal. Capacitor 22 has two nodes one of which is connected 
to island conductor 20 and the other of which is to one of input and 
output terminals of the RF switch 23, which has its other terminal coupled 
to the top plate of rectangular box 1. The RF switch 23 also has a control 
terminal tied to control lead wire 30. Upon application of a DC voltage 
from variable DC power supply 9 via the control lead 30, the RF switch 23 
may change its impedance between the input and output terminals so that 
the resulting impedance is changeable between the high and low states 
depending on the potential value of the DC voltage applied. 
With such an arrangement, the resultant value of a capacitance between the 
island conductor 20 and the flat box 1 is equal to the value of an 
conductor-to-conductor capacitance as inherently present between the 
island conductor 20 and frame 1 in cases where the RF switch 23 is in the 
high impedance state between the input and output terminals thereof; 
alternatively, where the input/output impedance is low, the resulting 
capacitance value equals the interconductor capacitance value plus a 
capacitance value of the capacitor 22. 
The variable DC power supply circuit 9 is variable in potential under 
control of its associated voltage controller circuit (not shown). This 
voltage controller is designed to generate a control signal for 
determination of an appropriate voltage value corresponding to a presently 
selected RF frequency, whist variable DC power supply 9 is responsive to 
receipt of the control signal for producing a predefined DC voltage. 
Variable DC power supply 9 may have its control signal input terminal (not 
shown) for receiving the control signal. 
An advantage of the tunable slot antenna of FIGS. 6a-6b is that two 
different resonant frequencies may selectively be established in a 
switchable fashion in response to the DC voltage as applied from variable 
DC power supply 9 under control of the voltage controller. 
While this embodiment is designed to make use of a serial combination of 
single RF switch 23 and one capacitor 22, a parallel combination of a 
plurality of such similar switch/capacitor serial connections may 
alternatively be employable between the island conductor 20 and the top 
plate of the box 1, thereby enabling achievement of multiple capacitance 
values and thus plural resonant frequency values on a case-by-case basis. 
Still alternatively, such multiple serial switch/capacitor combinations 
may be replaced with circuitry including plural capacitors and an RF 
switch with multiple input/output nodes which are implemented together 
into a single IC chip package. With such an arrangement also, similar 
advantages are obtainable. 
Seventh Embodiment 
A tunable slot antenna shown in FIGS. 7a-7b in accordance with a yet 
further embodiment of the invention is similar to that of FIGS. 6a-6b with 
an extra island conductor 24 being added to the top plate of the box 1 and 
also with a common node between the capacitor 22 and RF switch 23 being 
electrically connected to island 24. More specifically, the island 
conductor 24 is provided within a small hole formed in the frame top plate 
in a way such that island conductor 24 is electrically isolated from the 
box 1. The switch/capacitor common node is conducted by a lead wire to 
round island 24 as depicted in FIG. 7b. As shown, the capacitor 22 has one 
end connected to the elongate island conductor 20 and the other end 
coupled to island conductor 24. The RF switch 23 has one of its 
input/output terminals coupled to the island conductor 20 and the opposite 
end conducted to the grounded top plate of the box 1. 
With such an arrangement, it becomes possible to potentially fix or settle 
the capacitor 22 and the input/output terminals of RF switch 23 to 
respective conductors on the top of the box 1, thus increasing the 
reliability of circuitry concerned. Another advantage of this embodiment 
is that reflow techniques or equivalents thereto for use in mounting 
electronics parts on standard printed circuit boards (PCBs) may be 
employed to integrally mount respective necessary parts or components on 
the slot antenna frame body 1, thereby greatly reducing assembly costs in 
the manufacture of the antenna module. 
Eighth Embodiment 
A tunable slot antenna shown in FIGS. 8a-8b is similar to that of FIGS. 
7a-7b with the control lead wire being partly placed in the interior of 
the flat box 1. More specifically, the box 1 has further island conductors 
25 and 29 on its top and bottom plates, respectively. Upper island 
conductor 25 is provided within a small hole formed in the top plate of 
the box 1 so as to be electrically insulated from the box 1. Similarly, 
lower island conductor 29 is in a small hole in the bottom plate of the 
box 1 and insulated therefrom. The box 1 includes vertical conductive 
through holes 26 and 28 which are electrically connected to island 
conductors 25, 29, respectively. A control lead 27 is formed or "embedded" 
inside the box 1 to horizontally extend for interconnection between 
through holes 26, 28 as best shown in FIG. 8b. Another control lead 30 has 
its one end electrically connected to the control terminal of RF switch 
23. The other end of the switch 23 is coupled to the island conductor 25, 
which is insulatively disposed within the hole in the top plate of the box 
at a selected location near switch 23. The island conductor 25 is tied to 
the internal control lead 27 via through hole 26 in the box 1. Control 
lead 27 is in turn coupled via through hole 28 to an island conductor 29 
on the bottom plate of the box 1. The island conductor 29 is connected to 
the variable DC power supply 9 for receiving a DC voltage therefrom to 
thereby control an operation of the RF switch 23. 
An advantage of the structure of FIGS. 8a-8b lies in the capability to 
greatly suppress influence upon the antenna's radiation patterns, which 
influence can otherwise occur due to the presence of "external" control 
leads outside the box 1. Such suppression is attainable because certain 
affectable part of the control lead configuration used for application of 
DC voltage to the variable capacitance circuit is moved or "interplanted" 
to inside of box 1 so that radiation-pattern affectability decreases 
accordingly. 
Ninth Embodiment 
A tunable slot antenna shown in FIGS. 9a-9b is similar to that shown in 
FIGS. 8a-8b with the strip conductor 3 and the internal control lead 27 
inside the flat box 1 being modified in electrical connection with respect 
to their associated external parts or components of the antenna. More 
specifically, strip 3 is connected to its associated RF power supply 
circuit 7 via an end-face through hole 15 with a semicircular cylindrical 
profile. Through hole 15 extends vertically along one of the end wall 
plates of rectangular the box 1 of FIG. 9a, and is electrically insulated 
from the box 1. In other words, the connection point 10 of strip 3 is 
coupled to through hole 15 on the end wall plate of the antenna. Internal 
control lead 27 is connected at its one end to the control terminal of 
"external" RF switch 23 via island conductor 25 and through hole 26 in a 
way similar to that shown in FIG. 8b. Lead 27 is connected at its opposite 
end to variable DC power supply circuit 9 via the opposite semicircular 
cylindrical through hole 35 that is vertically elongated along the other 
end wall plate of the box 1 as shown in FIG. 9a. The through holes 13 and 
35 may be circular cylindrical as in the other embodiments or may have any 
other shape. 
In this embodiment of FIGS. 9a-9b, the through hole 15 functions as a 
coupler-section extension (or leading) terminal whereas the through hole 
35 acts as a control-lead power feed node while permitting the lower parts 
of the through holes 15, 35 to be substantially the same in level as the 
bottom surface of the box 1--namely, flush with the ground potential plate 
thereof. This may facilitate mounting of the slot antenna onto a printed 
circuit board (PCB) used. One preferable antenna mount procedure is as 
follows: prepare a PCB with a conductive lead pattern and a ground 
conductor plane being formed on one surface; then, mount antenna structure 
of FIGS. 9a-9b with its bottom surface contacting the PCB. When this is 
done, the bottom surface of the antenna structure is contacted to the 
ground conductor plane while simultaneously causing the lead pattern to 
come into direct contact with the end-face through holes 15, 35. This may 
allow utilization of currently available standard automated assembly 
techniques without the need for any additional modifications thereto. The 
antenna module of this embodiment is advantageous in reducing production 
costs of cellular radiotelephone handsets when reduction to practice. 
Any one of the foregoing tunable slot antenna structures incorporating the 
principles of the invention may be manufactured using presently available 
standard multilayer substrate/PCB fabrication technologies, as in the 
tunable slot antenna as disclosed in the above-identified copending U.S. 
Patent Application based on Japanese Patent Application No. 9-54825. This 
may ensure that forming or mounting the antenna and RF circuitry on the 
same substrate or PCB makes it possible to further reduce parts costs and 
manufacturing costs of handheld communication terminals including, but not 
limited to, cellular radiotelephone handsets. 
It has been described that the tunable slot antenna modules embodying the 
present invention stated supra are capable of varying or altering the 
antenna's impedance matching center frequency, i.e. resonant frequency, in 
a wide bandwidth without having to adversely affecting the inherent 
matching condition of the antenna. This may be achievable due to one 
unique feature that enables both the slot and the strip conductor 
immediately underlying the same to equivalently vary in length 
simultaneously. The enhanced resonant frequency variability makes it 
possible for the antenna modules disclosed herein to be preferably 
applicable to mobile radiotelephone handsets with a wide system frequency 
range. Applying the antenna to such handheld communication units enables 
the antenna's resonant frequency to accurately keep track of radio 
frequencies selectively updated every time a telephone interconnection is 
established, which in turn makes it possible to reduce the frequency band 
the antenna must cover, thus reducing the volume of antenna. When applying 
the antenna modules, resultant cellular radiotelephone handsets are 
capable of elimination of external projections thereby increasing 
portability and hand-carriability while reducing the size thereof. 
While the invention has been described with reference to specific 
embodiments, the description is illustrative of the invention and is not 
to be construed as limiting the invention. Various modifications and 
applications may occur to those skilled in the art without departing from 
the true spirit and scope of the invention as defined by the appended 
claims.