Wideband slot antenna

A wideband slot antenna according to the present invention is a ¼ effective wavelength slot antenna in which a ground conductor 103 having a finite area is allowed to function as a dipole at lower frequencies. An inductive region 123 is provided within an feed line 113 in a region intersecting a slot 111, and an antenna feed point 117 for connection to an external unbalanced feed circuit is provided at a position which satisfies high impedance conditions for an unbalanced ground conductor current.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an antenna with which a digital signal or an analog high-frequency signal, e.g., that of a microwave range or an extremely high frequency range, is transmitted or received.

2. Description of the Related Art

For two reasons, wireless devices are desired which are capable of operating in a much wider band than conventionally. A first reason is the need for supporting short-range wireless communication systems, for which the authorities have given permission to use a wide frequency band. A second reason is the need for a single terminal device that is capable of supporting a plurality of communication systems which use different frequencies.

For example, a frequency band from 3.1 GHz to 10.6 GHz, which has been allocated by the authorities to short-range fast communication systems, corresponds to a bandwidth ratio as wide as 109.5%. As used herein, “a bandwidth ratio” is a bandwidth, normalized by the center frequency f0, of a band. On the other hand, patch antennas and ½ effective wavelength slot antennas, both of which are known as basic antenna structures, have operating bands (as converted to bandwidth ratios) of less than 5% and less than 10%, respectively, and thus cannot realize the above-described widebandness. To take for example the frequency bands which are currently used for wireless communications around the world, a bandwidth ratio of about 30% is required in order to cover from the 1.8 GHz band to the 2.4 GHz band with the same antenna. In order to simultaneously cover the 800 MHz band and the 2 GHz band, a bandwidth ratio of about 90% must be similarly realized. In order to simultaneously cover from the 800 MHz band to the 2.4 GHz band, a bandwidth ratio of 100% or more is required. Thus, as the number of systems to be supported by the same terminal device increases, and as the frequency band to be covered becomes wider, the need will increase for a small-sized wideband antenna.

The open ended ¼ effective wavelength slot antenna, shown in schematic diagrams inFIGS. 22A to 22C, is one of the most basic planar antenna structures (Conventional Example 1).FIG. 22Ais an upper schematic see-through view;FIG. 22Bis a schematic cross-sectional view taken along line AB; andFIG. 22Cis a schematic see-through rear view, as seen through the upper face side. As is shown inFIGS. 22A to 22C, a feed line113exists on the upper face of a dielectric substrate101. A recess is formed in a depth direction109afrom an outer edge105aof an infinite ground conductor103, which in itself is provided on the rear face of the dielectric substrate101. Thus, it functions as a resonator composed of a slot111having an open leading end at an open point107. The slot111is a circuit which is obtained by removing the conductor completely across the thickness direction in a partial region of the ground conductor103, and which resonates near a frequency fs such that its slot length Ls corresponds to a ¼ effective wavelength. The feed line113, which partially intersects the slot111, electromagnetically excites the slot111. The feed line113is connected to an external circuit via an input terminal. Note that, in order to establish input matching, a distance Lm from an open end point119of the feed line113to the slot111is typically set to about a ¼ effective wavelength at the frequency fs. Moreover, a line width W1of the feed line113is typically designed so that the characteristic impedance of the feed line113is set to 50 Ω, in accordance with the substrate thickness H and a dielectric constant of the substrate.

As shown inFIG. 23, Japanese Laid-Open Patent Publication No. 2004-336328 (hereinafter “Patent Document 1”) discloses a structure for operating the ¼ effective wavelength slot antenna shown in Conventional Example 1 at a plurality of resonant frequencies (Conventional Example 2). Although the band can be expanded through operation at a plurality of resonant frequencies, characteristics which are as ultrawideband as currently desired cannot be obtained with the frequency characteristics shown in Patent Document 1.

Non-Patent Document 1 (“A Novel Broadband Microstrip-Fed Wide Slot Antenna With Double Rejection Zeros” IEEE Antennas and Wireless Propagation Letters, vol. 2, 2003, pages 194 to 196) discloses a method for realizing a wideband operation of a slot resonator with short-circuited both ends, which is a ½ effective wavelength slot antenna (Conventional Example 3). One input matching method for a conventional slot antenna has been to intersect and excite the slot resonator14at a point where a ¼ effective wavelength at the frequency fs is obtained, beginning from the open end point119of the feed line113. However, in Conventional Example 3, as shown inFIG. 24(which shows an upper schematic see-through view), the region spanning a distance Lind from the open end point119of the feed line113is replaced by a transmission line having a characteristic impedance higher than 50 Ω. The resultant inductive region121is coupled to the slot111in a substantial center thereof. Herein, Lind is set to a ¼ effective wavelength at the frequency f0, so that the inductive region121functions as a separate ¼ wavelength resonator from the slot resonator. This increases the number of resonators within the equivalent circuit structure (which is one in usual slot antennas) into two, and since resonators that are resonating at close frequencies are coupled to each other, a multiple resonance operation is obtained. In the example shown in FIG. 2(b) of Non-Patent Document 1, reflected impedance characteristics as good as −10 dB or less are obtained with a bandwidth ratio 32% (from near 4.1 GHz to near 5.7 GHz). As shown in comparison with respect to the measured characteristics in FIG. 4 of Non-Patent Document 1, the bandwidth ratio of the antenna of Conventional Example 3 are much more wideband than the bandwidth ratio of 9% of a usual slot antenna which is supposedly produced under the same substrate conditions.

Moreover, Non-Patent Document 2 (“Impedance Measurement of the Antenna on the Portable Telephone using Fiber-optics”, 2003 Grand Meeting of the Institute of Electronics, Information, and Communication Engineers, B-1-206 2003, page 206; Conventional Example 4) reports that, in a small-sized communication terminal in which the ground conductor area that can be secured for antenna operation is finite, use of an unbalanced feed circuit for feeding will allow an unbalanced ground conductor current occurring in the ground conductor to flow back to the ground conductor of the feed circuit, thus affecting the measurement accuracy of radiation characteristics and impedance characteristics itself. For this reason, Non-Patent Document 2 does not use a high-frequency unbalanced feed circuit for feeding. Rather, as shown inFIG. 25, Non-Patent Document 2 takes the trouble of employing optical fibers to ensure that the ground conductor in the communication terminal is fed in an isolated manner from the feeding system, thus adopting a measurement technique which avoids unfavorable influences of an unbalanced ground conductor current in the small-sized antenna.

As described above, conventional slot antennas not only lack sufficient widebandness but also have a problem in that, even if widebandness is realized within a small shape, their radiation characteristics and reflected impedance characteristics may not remain stable depending on the state of connection with an external unbalanced feed circuit, thus making it difficult to know their characteristics when mounted in terminal devices.

Firstly, as in Conventional Example 1, the operating band of a usual open ended slot antenna, which only has a single resonator structure within its structure, is restricted by the resonation mode band, so that the frequency band in which good reflected impedance characteristics can be obtained only amounts to a bandwidth ratio of less than about 10%.

Although Conventional Example 2 realizes a wideband operation because of a capacitive reactance element being introduced in the slot, it is well conceivable that an additional part such as a chip capacitor is required as the actual capacitive reactance element, and that variations in the characteristics of the newly-introduced additional part may cause the antenna characteristics to vary. Moreover, judging from the examples disclosed inFIG. 14andFIG. 18of this document, it is difficult to realize low-return input matching characteristics across an ultrawide band.

In Conventional Example 3, the bandwidth ratio characteristics are only as goods as about 35%. Moreover, use of a slot resonator with short-circuited both ends (which is a ½ effective wavelength resonator) is disadvantageous in terms of downsizing as compared to the antennas of Conventional Example 1 and Conventional Example 2, in which an open ended slot resonator (which is a ¼ effective wavelength resonator) is used.

Even if the principles of multiple resonance operation of Conventional Example 3 are introduced into the ¼ effective wavelength slot antenna design of Conventional Example 1 or Conventional Example 2, when the small-sized antenna operates as shown in Conventional Example 4, an unbalanced ground conductor current will flow back to the ground conductor of the unbalanced feed circuit which is connected to the antenna. Depending on the shape of the unbalanced feed circuit in which an unbalanced ground conductor flows, e.g., the length of a coaxial cable which is connected to the antenna for the purpose of knowing its characteristics, the radiation characteristics and reflected impedance characteristics will change. In particular, the radiation characteristics may drastically change depending on the state of the external circuit.

SUMMARY OF THE INVENTION

The present invention solves the aforementioned conventional problems, and an objective thereof is, in a small-sized wideband slot antenna whose basic construction is an open ended slot antenna, to realize an operation which is more wideband than conventionally and eliminate causes for unstable radiation operation due to connection with an external circuit, thus realizing a stable operation.

A wideband slot antenna according to the present invention comprises: a dielectric substrate having a front face and a rear face; a ground conductor provided on a rear face of the dielectric substrate, the ground conductor having a finite area; a slot recessing into the ground conductor, in a depth direction from an open point at a portion of an outer edge of the ground conductor, one end of the slot being an open end; and a feed line for feeding a high-frequency signal to the slot, the feed line being formed on the front face of the dielectric substrate and at least partially intersecting the slot. On the front face of the dielectric substrate, an antenna feed point for connecting an external unbalanced feed circuit to the feed line is provided at a position facing an outer edge of the ground conductor opposite from the open point; and the feed line is bent by at least 90° within a plane which is parallel to the front face of the dielectric substrate so as to reach the antenna feed point. The slot and the antenna feed point are each disposed at a center of the ground conductor along a width direction which is orthogonal to the depth direction. In an inductive region spanning a length of a ¼ effective wavelength at a resonant frequency fs of the slot from the open end point, a characteristic impedance of the feed line is prescribed to be higher than 50 Ω; and the feed line and the slot intersect each other at a center of the inductive region. A distance from the open point to each outer edge at an end of the ground conductor along the width direction corresponds to a length equal to or less than a ¼ effective wavelength at the frequency fs, and the ground conductor has a lowest-order resonant frequency at a frequency which is lower than the frequency fs.

In a preferred embodiment, the dielectric substrate further has a dielectric layer covering the feed line.

A wideband slot antenna according to the present invention comprises: a dielectric substrate having a front face and a rear face; a ground conductor provided on a rear face of the dielectric substrate, the ground conductor having a finite area; a slot recessing into the ground conductor, in a depth direction from an open point along an outer edge of the ground conductor, one end of the slot being an open end; and a feed line for feeding a high-frequency signal to the slot, the feed line being formed on the front face of the dielectric substrate and at least partially intersecting the slot. On the front face of the dielectric substrate, an antenna feed point for connecting an external unbalanced feed circuit to the feed line is provided at a position facing an outer edge of the ground conductor opposite from the open point. The feed line is bent by at least 90° within a plane of the dielectric substrate so as to reach the antenna feed point; and the slot and the antenna feed point are each disposed at a center of the ground conductor along a width direction which is orthogonal to the depth direction. In an inductive region spanning a length of a ¼ effective wavelength at a resonant frequency fs of the slot from the open end point, a characteristic impedance of the feed line is prescribed to be higher than 50 Ω. The feed line and the slot intersect each other at a center of the inductive region; at a first point near the slot, the feed line branches into a group of branch lines including at least two branch lines, and at least branch line pair in the group of branch lines is connected at a second point near the slot, thus forming at least one loop line in the feed line. The loop line at least partially intersects a border line between the slot and the ground conductor, and the slot is excited at two or more feed points which are at different distances from the open point along the depth direction. A maximum value of a loop length of the entire loop line is prescribed to be a length less than 1× effective wavelength at an upper limit frequency of an operating band; in the group of branch lines, any branch line that does not constitute a part of the loop line but is left open-ended at a leading end has a branch length which is less than ¼ effective wavelength at an upper limit frequency of the operating band. A distance from the open point to each outer edge at an end of the ground conductor along the width direction is prescribed to a length equal to or less than a ¼ effective wavelength at the frequency fs, and the ground conductor has a lowest-order resonant frequency at a frequency which is lower than fs.

In a preferred embodiment, the dielectric substrate further has a dielectric layer covering the feed line.

In accordance with a wideband slot antenna of the present invention, a wideband operation can be realized which has been difficult to realize with conventional slot antennas. Moreover, instability in radiation characteristics occurring due being connected with an external unbalanced feed circuit which is connected to the antenna is eliminated, whereby stable operation is made possible.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Embodiments

FIG. 1is an upper schematic see-through view illustrating the structure of a wideband slot antenna according to a first embodiment of the present invention.

A ground conductor103having a finite area is formed on a rear face of the dielectric substrate101. A slot111recesses into the ground conductor103in a depth direction109a, from an open point107which is provided at an outer edge105aextending along a width direction109bof the ground conductor103, with one end of the slot111being left open. The slot111functions as a ¼ effective wavelength open ended slot resonator. Assuming that the slot width Ws is negligible relative to the slot length Ls, a resonant frequency fs of the slot111is a frequency such that the slot length Ls corresponds to a ¼ effective wavelength.

In the case where the above assumption does not hold true, the resonant frequency fs of the slot111is a frequency such that a slot length which takes the slot width into consideration (Ls×2+Ws)÷2 corresponds to a ¼ effective wavelength.

Preferably, the resonant frequency fs according to the present invention is prescribed to be approximately a center frequency f0of the operating frequency band. A feed line113which at least partially intersects the slot111is formed on a front face of the dielectric substrate101.

FIG. 2Ashows a schematic cross-sectional view of the wideband slot antenna, taken along a dotted line AB inFIG. 1. The present specification illustrates a structure in which the feed line113is disposed on the frontmost face of the dielectric substrate101and the ground conductor103is disposed on the rearmost face of the dielectric substrate101. However, as illustrated inFIG. 2B, by methods such as adopting a multilayer substrate in which a plurality of dielectric layers and conductive layers are stacked, either or both of the feed line113and the ground conductor103may be disposed at an inner layer plane of the dielectric substrate101. Moreover, it is not a limitation that there is one conductor wiring surface functioning as a ground conductor103for the feed line113. As shown inFIG. 2C, a structure may be adopted in which opposing ground conductors103sandwich a layer in which the feed line113is formed. In other words, the wideband slot antenna according to the present invention can provide similar effects in a circuit structure in which a strip line circuit structure, as well as a microstrip line structure, is adopted in at least a portion thereof. The same is also true of a coplanar waveguide and a grounded coplanar waveguide structure.

As used in the present specification, the term “dielectric substrate” broadly encompasses a dielectric layer or a dielectric multilayer substrate having a ground conductor formed on one face (rear face) and a feed line formed on another face (front face) thereof. Alternatively, to a “dielectric substrate” having a feed line formed on a surface thereof, another dielectric layer may be added so as to cover the feed line. In other words, the wideband slot antenna according to the present invention encompasses all of the constructions ofFIGS. 2A to 2C.

Note that, in the present specification, a “slot” is defined as a portion of the conductor layer composing the ground conductor103where the conductor is completely removed along the thickness direction thereof. In other words, any portion of the ground conductor103which is merely ground off its surface in a partial region to result in a reduced thickness is not a “slot”.

The ground conductor103is a conductor structure having a finite region, which, along the outer edge105a, extends over lengths Wg1and Wg2on both sides from the open point107along the width direction109b. Herein, Wg1and Wg2each has a value which is equal to or greater than a length Lsw that corresponds to a ¼ effective wavelength at the frequency fs. This is a necessary condition for stabilizing the antenna radiation characteristics of the slot mode.

On the other hand, because of having a circuit area which is limited within a finite region, the ground conductor according to the present invention operates also as a ground conductor dipole antenna which makes use of the entire ground conductor structure. A ground conductor dipole antenna and an open ended slot antenna are similar in that high-frequency currents concentrate to flow at a short-circuiting point of a slot, both antennas can provide radiation characteristics with common polarization characteristics, while sharing the same circuit board.

If the resonant frequency fd of the ground conductor dipole antenna can be prescribed to be slightly lower than the resonant frequency fs of the open ended slot mode, rather than being equal to the resonant frequency fs, the operating band of the wideband slot antenna can be expanded toward the lower frequency side.

Since the ground conductor103has a slot portion in a substantially central portion thereof, the resonator length of the ground conductor dipole antenna is effectively extended. As a result, in a wideband slot antenna according to the present invention in which Wg1and Wg2are set to values equal to or greater than Lsw, the frequency fd will always be lower than the resonant frequency fs, whereby a wideband operation is guaranteed. From the standpoint of downsizing, it would not be practical for the frequency fd to have a much lower value than the operating band frequency of the slot mode. In other words, by prescribing both Wg1and Wg2to the least necessary value, it becomes possible to bring the frequency fd close to the operating band of the slot mode while in the form of a small-sized antenna.

A characteristic impedance of the feed line113in the region spanning a distance Lind from the open end point119is prescribed to be higher than 50 Ω. As a result, the aforementioned region of the feed line113constitutes an inductive region121, such that the distance Lind is approximately equal to a ¼ effective wavelength at the frequency fs. In other words, the inductive region121constitutes a ¼ wavelength resonator, and couples to a ¼ effective wavelength resonator that is constituted by the slot111. This results in multiple resonance, whereby the antenna operating band of the slot111in the slot mode is effectively increased.

At the substantial center of the inductive region121along its longitudinal direction, the feed line113intersects the slot111. In Conventional Example 1, even if the ground conductor were to be limited to a finite area, it would be difficult to obtain continuity with the band of the ground conductor dipole mode if the band of the slot mode itself were limited, thus making it impossible to obtain effects similar to those of the present invention. According to the present invention, since the slot mode operating band expands toward the lower frequency side as described above, antenna operation is realized in a wide operating band which is continuous with the operating band of the ground conductor dipole.

The inductive region121is connected to the usual region of the feed line113having a characteristic impedance of 50 Ω. The feed line113is bent by at least 90° within a plane which is parallel to the front face of the dielectric substrate101, and reaches an antenna feed point117which is provided at a position facing an outer edge105bof the ground conductor103.

The antenna feed point117is set at a position near the outer edge105b, opposite from the outer edge105aof the ground conductor which extends along the width direction and along which the open point107is set. The open point107and the antenna feed point117are both provided near the center of the ground conductor103along the width direction109b.

In an antenna mode that emerges when the feed line113excites the slot111, a high-frequency current occurs at the slot short-circuiting point125.

FIG. 3schematically shows, with arrows, high-frequency currents131flowing through the ground conductor103. The high-frequency currents131occurring due to excitation of the slot111flow along the border line between the slot111and the ground conductor103and reaches the open point107, and thereafter flows along the outer edges of the ground conductor103. When another conductor is connected to an outer edge of the ground conductor103, it becomes very difficult to prevent the high-frequency currents from flowing into the other connected conductor because conductors have a very low impedance. However, by providing an antenna feed point at the aforementioned high-symmetry position, it becomes possible to realize a very high input/output impedance for this high-frequency current, which flows through the ground conductor103in an unbalanced manner.

As shown inFIGS. 4A and 4B, the ground conductor103in the wideband slot antenna according to the present embodiment can be regarded as a conductor structure in which a highly-symmetrical pair of finite ground conductors103aand103bare combined at the slot short-circuiting point.FIGS. 4A and 4Beach schematically show how high-frequency currents flow in the ground conductor103, in relation to the power-feeding structure in each mode;FIG. 4Aillustrates the balanced mode, whereasFIG. 4Billustrates the unbalanced mode.

Under the balanced mode illustrated inFIG. 4A, it is as if out-of-phase high-frequency currents131aand131bwere being fed to the ground conductor pair103aand103bin opposite directions from the feed point15. This amounts to a strongest in-phase high-frequency current flowing at the connection point between the ground conductor pair, i.e., at the slot short-circuiting point. On the other hand, under the unbalanced mode illustrated inFIG. 4B, it is as if in-phase high-frequency currents131awere being fed to the ground conductor pair103aand103bin opposite directions from the center. Consequently, the high-frequency currents at the connection point between the ground conductor pair103aand103bare canceled. This means that, as the symmetry between the ground conductor pair103aand103bincreases, and as the antenna feed point becomes closer to the symmetrical point between the ground conductors, there is a higher input/output impedance of the unbalanced ground conductor mode from the antenna feed point according to the present invention. Therefore, even when an external unbalanced feed circuit is connected to the ground conductor103, the antenna feeding conditions adopted according to the present invention can prevent any unbalanced ground conductor current from flowing back to the ground conductor103of the external unbalanced feed circuit.

Note that, in the ½ effective wavelength slot antenna of Conventional Example 3, high-frequency currents occurring at the short-circuiting points at both ends of the slot resonator will flow along the outer edges of the slot, and no electric currents will occur that flow along the outer edges of the ground conductor103. Thus, the problems of unbalanced ground conductor currents flowing along the outer edges of the ground conductor103are unique to the case of performing unbalanced feeding by adopting an open ended slot resonator, which is advantageous for downsizing and widebandness.

In the wideband slot antenna according to the present invention, the slot shape does not need to be rectangular, and may be replaced by any arbitrary shape. In particular, by providing many fine and short slots in parallel connection to the main slot, an inductance which is in series to the main slot can be added to the circuitry and thus the slot length of the main slot can be reduced, which is preferable in practice. Also, under the condition where the main slot is made narrow in slot width and folded into a meandering shape or the like for downsizing, the wideband-realizing effect of the wideband slot antenna according to the present invention can be similarly obtained.

Next, a second embodiment of the wideband slot antenna according to the present invention will be described. In the second embodiment shown inFIG. 5, at the position which is designated as the inductive region121in the first embodiment, at least a partial region of the feed line113is replaced by a loop line123. In the present embodiment, the loop line123realizes characteristics which are even more wideband than in the first embodiment.

A loop length Lp of the loop line123is prescribed to less than 1× effective wavelength at an upper limit frequency fH of the operating band. In other words, the resonant frequency flo of the loop line123is prescribed to be higher than the frequency fH. Other than the loop line123, a part of the feed line113may also branch out to form an open stub, but its stub length must be prescribed to less than a ¼ effective wavelength at the upper limit frequency fH of the operating band. In other words, the resonant frequency fst of the open stub is prescribed to be higher than the frequency fH. Thus, in the second embodiment, wiring lines are allowed to branch out from the feed line113in the inductive region123, thereby improving the band characteristics of the wideband slot antenna. This improvement in characteristics is not to be confused with an active utilization of the resonance phenomenon of each branching wiring line alone; instead, it utilizes a phenomenon which is exhibited only because of the combination of a slot antenna and a loop line.

The loop line123of a wideband slot antenna according to the embodiment of the present invention increases the number of places where the slot resonator is excitable to more than one, and also adjusts the electrical length of the input matching circuit, whereby an ultrawideband antenna operation is realized.

Hereinafter, the functions of the loop line123will be specifically described.

First, high-frequency characteristics in the case where a loop line structure is adopted in a traditional high-frequency circuit will be described, assuming that an infinite ground conductor is provided on a rear face thereof.

FIG. 6Ashows a schematic diagram of a circuit in which a loop line123, composed of a first path205(having a path length L1) and a second path207(having a path length L2), is connected between an input terminal201and an output terminal203. The loop line resonates under the conditions where a sum of the path length Lp1of the first path115aand a path length Lp2of the second path115bequals 1× effective wavelength of the transmission signal, and thus may sometimes be employed as a ring resonator. However, when Lp1and Lp2are shorter than the effective wavelength of the transmission signal, the loop line123does not exhibit a steep frequency response, and therefore it has not been particularly necessary to employ such a loop line123in a usual high-frequency circuit. The reason is that, in a traditional high-frequency circuit having a uniform ground conductor, in a non-resonant band, fluctuations in the local high-frequency current distribution due to the introduction of a loop line will be averaged out in terms of macroscopic high-frequency characteristics.

On the other hand, as shown in the upper schematic see-through view ofFIG. 5, introduction of the loop line123into a slot antenna according to the present invention provides a unique effect which cannot be obtained in the aforementioned traditional high-frequency circuit. A high-frequency current on the ground conductor can flow in a direction131calong the first path205, or in a direction131dalong the second path207. As a result, different paths131cand131dcan be created in the flow of the high-frequency currents at the ground conductor side, thus enabling the slot111to be excited at a plurality of places. Local changes in the high-frequency current distribution in the ground conductor near the slot modulate the slot mode resonance characteristics, and drastically expand the antenna operating band in this mode.

This will be described with reference toFIGS. 7A and 7B, which schematically show cross-sectional structures of transmission lines. In a traditional transmission line as shown inFIG. 7A, it is at ends403and405of the wiring line that a concentrated distribution of the high-frequency current occurs at the signal conductor401side, and it is in a region407opposing the signal conductor401that the same occurs at the ground conductor103side. Therefore, by merely increasing the width of the feed line113in the slot antenna, no substantial changes can be caused in the distribution of the high-frequency currents at the ground conductor side. On the other hand, branching the signal conductor into the two paths205and207as shown inFIG. 7Binnovatively allows the high-frequency currents to be separately distributed into different ground conductor regions413and415respectively opposing the paths205and207.

Moreover, the loop line newly introduced in the wideband slot antenna according to the present invention not only functions to increase the number of places where the slot antenna is excitable to more than one, but also functions to adjust the electrical length of the feed line113. Fluctuations in the electrical length of the feed line due to the introduction of the loop line allows the resonance conditions of the feed line113to further shift to multiple resonance conditions, thus further enhancing the effect of expanding the operating band according to the present invention. Specifically, by introducing the loop line near the slot, based on a difference in electrical length (i.e., the path with the shorter electrical length VS the path with the longer electrical length, among the two paths composing the loop line), it is ensured that a resonance phenomenon which is obtained as the slot resonator couples to the inductive region occurs at a plurality of (two or more) frequencies. Thus, the matching condition which has already been wideband is made even more wideband.

To summarize the above, a wideband slot antenna according to the second embodiment of the present invention is capable of operation in a wider band than that of a conventional slot antenna, based on the combination of a first function of enhancing the resonance phenomenon of the slot itself into multiple resonance and a second function of enhancing the resonance phenomenon of the feed line that couples to the slot into multiple resonance. The location of the antenna feed point is similar to the location of the antenna feed point in the wideband slot antenna according to the first embodiment of the present invention.

However, in order to maintain wideband matching characteristics, it is required that the loop line is not used under conditions where the loop line may resonate by itself. To take the loop line123ofFIG. 6Afor example, the loop length Lp, which is a sum of the path length Lp1and path length Lp2, is prescribed to be less than 1× effective wavelength at the frequency fH. In the case where a plurality of loop lines exist within one wideband slot antenna, it is necessary that the largest of the loop lines in the antenna satisfies the aforementioned conditions.

One high-frequency circuit which is more commonplace than a loop line is an open stub shown inFIG. 6B. As shown in an upper schematic see-through view ofFIG. 8, some of the wiring lines branching from the feed line of the wideband slot antenna of the present embodiment may have an open stub structure213. However, from the standpoint of wideband characteristics, use of a loop line is more advantageous than use of an open stub for the purpose of the present invention. Since the open stub213is a ¼ effective wavelength resonator, its stub length Lp must be, at the most, prescribed to be less than ¼ effective wavelength at the frequency fH.

With reference toFIG. 6Cshowing an exemplary loop line which is characterized by an extremely small Lp2, the advantages of a loop line against an open stub will be described. In the loop line123, as Lp2is made extremely small, the loop line123will apparently become closer to an open stub. However, the resonant frequency of the loop line in the case where Lp2approximates zero is a frequency for which Lp1equals the effective wavelength, and the resonant frequency of an open stub is a frequency for which Lp3equals a ¼ effective wavelength. If the two structures are compared under conditions where half of the Lp1is equal to Lp3, the lowest-order resonant frequency of the loop line will correspond to twice the lowest-order resonant frequency of the stub line.

As can be seen from the above description, when converted into frequency band, a loop line is twice as effective a structure, as an open stub, to be adopted for a feed line which must avoid any resonance phenomenon in a wide operating band. Moreover, since an open-end point119of the open stub ofFIG. 6Bis “open” in the circuitry, no high-frequency current will flow through the open-end point119. Therefore, even if an open-end point119is provided near the slot, it will be difficult to attain electromagnetic coupling with the slot. On the other hand, as shown inFIG. 6C, a point213cof the loop line123is by no means “open” in the circuitry, and therefore a high-frequency current is certain to flow therethrough. Thus, when provided near the slot, it is easy to attain electromagnetic coupling with the slot. Also from this standpoint, it is more advantageous for the purpose of the present invention to adopt a loop line than to adopt an open stub.

The above description should make it clear that, in order to realize a wideband operation in the wideband slot antenna according to the present invention, it is most effective to introduce a loop line, rather than a line having a thick line width or an open stub.

FIG. 9is an upper schematic see-through view of an embodiment in which three branch lines extend from the feed line113. Although the number of branch lines extending from the feed line113may be prescribed to be three or more, not as drastic an expansion of the operating band will be obtained as in the case where there are two branch lines. Within the group of branch lines including a plurality of branches, it is only the paths205and207at both ends that has a high distribution intensity of high-frequency current, and therefore the high-frequency current flowing through a path209lying therebetween does not become very intense. However, inserting the path209in between the paths205and207can improve the resonant frequency of the loop line composed of the paths205and207, which will be effective from the standpoint of expanding the operating band.

The effects of the present invention can be obtained so long as the loop line123is provided near the slot. As shown inFIG. 5, it is preferable that the first path205and the second path207composing the loop line123each intersect at least either one of border lines237and239between the slot111and the ground conductor103.

Moreover, as shown inFIG. 10andFIG. 11, it is not impossible to obtain the effects of the present invention with a construction where the loop line123intersects neither of the border lines237and239between the slot111and the ground conductor103along the depth direction109a. The reason is that, a phase difference corresponding to the path difference between the first path205and the second path207occurs in the high-frequency currents that excite the slot, thus resulting in an effect of shifting the input matching condition toward the more wideband side. Strictly speaking, it suffices if the gap between the outermost point141of the loop line123and the border line237(or239) is less than 1× line width of the feed line113. The reason is that, when the aforementioned gap is prescribed to be shorter than the line width of the feed line113, the phase difference occurring between the local high-frequency currents flowing at the ground conductor side will not disappear, correspondingly to the phase difference between the high-frequency currents flowing at both ends of the signal conductor.

The loop line123is formed in the inductive region121. It is preferable that the line width is prescribed to be equal to or less than the line width of the feed line in the inductive region121. A plurality of loop lines may be formed. Such a plurality of loop lines may be connected in series or in parallel to one another. Two loop lines may be directly interconnected, or indirectly connected via a transmission line of an arbitrary shape.

In the wideband slot antenna according to the present invention, bandpass filters or band elimination filters (which are unbalanced input/output circuits), switch ICs, amplification ICs, and the like, or an integrated module accommodating the same may be inserted at any point between the antenna feed point117and the inductive region121.

In the wideband slot antenna according to the present invention, the connection between the ground conductor103and an external unbalanced feed circuit to be made at the antenna feed point117is not limited to being achieved on the rear face of the dielectric substrate101. Specifically, via a through-conductor near the connection point, the ground conductor may be guided onto the front face of the dielectric substrate, and a connection may then be made on the front face of the dielectric substrate, based on a coplanar waveguide structure. The advantageous effects of the present invention will not be lost in such a construction. Rather, since both connections, i.e., one for the signal conductor and another for the signal conductor, ground conductor, are realized on the front face of the dielectric substrate, surface mounting of the wideband slot antenna according to the present invention on an external mounting substrate will become possible.

EXAMPLE

In order to clarify the effects of the present invention, input impedance characteristics and radiation characteristics of four slot antennas as respectively shown in the upper schematic see-through views ofFIG. 12(Example 1),FIG. 13(Example 2), andFIG. 14(Comparative Examples 1 and 2) were analyzed by a commercially-available electromagnetic field simulator. The parameters of the respective circuit boards are summarized in Table 1.

Conditions were set for the antennas on the premise that all of the antennas were to be produced on the same size of circuit board. The conductor pattern was assumed to be a copper line having a thickness of 40 microns, and it was ensured that the precision range would fall within what can be obtained through wet etching. At the point shown as the antenna feed point117in each figure, a virtual feeding was assumed in which a coaxial connector (not shown) was connecting between the antenna and the coaxial cable135. As the coaxial cable length Lc, two lengths of 50 mm and 150 mm were assumed, and an ideal feeding was performed at the tip of the coaxial cable. In other words, the operation stability and widebandness of each antenna, including the influences which the coaxial cable having the length Lc to be connected as an unbalanced feed circuit would exert on the characteristics, were analyzed.

An analysis was also performed which assumed that Lc=zero, i.e., an ideal high-frequency feeding was occurring at the antenna feed point117. In the Comparative Examples, it was assumed that the feed line was not bent, so that the direction in which the coaxial cable was oriented was the Y axis direction in the coordinate axes ofFIG. 14. On the other hand, in the Examples, the feed line was bent within the plane so as to extend toward the antenna feed point117. Therefore, the direction in which the coaxial cable was oriented was the X axis direction inFIGS. 12 and 13.

FIG. 15shows the frequency dependence of return loss in Comparative Example 1 and Comparative Example 2, in the case where Lc=150 mm. In Comparative Example 1, in a 20% bandwidth ratio range from 3.04 GHz to 3.73 GHz, the return loss was less than −10 dB; and from 2.9 GHz to 4.3 GHz, the return loss was less than −7.5 dB. At 6.3 GHz, the return loss reached −4.9 dB, and thus wideband characteristics were not obtained. In Comparative Example 2, the return loss was about −3 dB to about −4 dB from 2.5 GHz to 8 GHz, and thus low-return characteristics were not obtained.

On the other hand,FIG. 16shows the frequency dependence of return loss in Example 1 and Example 2, in the case where Lc=150 mm. Example 1 retained low-return characteristics of −7.5 dB or less, from 3.2 GHz to above 11 GHz. Furthermore, Example 2 exhibited wideband low-return characteristics with a return loss of −10 dB or less, in the entire band from 3.1 GHz to above 11 GHz. As will be clear from a comparison against the Comparative Examples shown inFIG. 15, both Examples achieved wide operating bands. Note that there was hardly any influence of changing Lc on the input impedance, both in the Examples and in the Comparative Examples.

As for the radiation characteristics of Comparative Examples 1 and 2, a tendency was observed that their characteristics greatly varied depending on Lc.FIGS. 17A and 17Bshow the radiation characteristics on the YZ plane at 3 GHz in Comparative Example 1, in the cases where Lc=50 mm and Lc=150 mm, respectively. The data shown by a thin line in the figures is the characteristics in the case where Lc=zero, which is presented for comparison. If the unfavorable influences of an unbalanced ground conductor current are successfully avoided, which is the objective of the present invention, the three sets of characteristics will coincide; however, entirely different sets of characteristics are obtained depending on Lc. Similarly,FIGS. 18A and 18Bshow radiation characteristics at 6 GHz. As is clear fromFIGS. 17A and 17BandFIG. 18, in the Comparative Examples, at all frequencies, a tendency was confirmed that the radiation characteristics depended strongly on the cable length.

FIGS. 19A and 19Bshow the radiation characteristics on the YZ plane at 3 GHz in Example 2, in the cases where Lc=50 mm and Lc=150 mm, respectively. Similarly, the radiation characteristics at 6 GHz and 9 GHz are shown inFIGS. 20A and 20BandFIGS. 21A and 21B, respectively. The data shown by a thin line in the figures is the characteristics in the case where Lc=zero, which is presented for comparison. In Example 2, stable radiation characteristics which hardly depended on Lc were realized, whereby attainment of the objective of the present invention was confirmed. Similarly in Example 1, stable radiation characteristics which hardly depended on Lc were obtained. Moreover, in Examples 1 and 2, in the entire operating band, similar effects were obtained with respect to all radiation characteristics, including radiation characteristics on the XZ plane.

Without an increase in circuit footprint and production cost, the matching band can be expanded by a wideband slot antenna according to the present invention. Thus, with a simple construction, it is possible to realize a multi-functional terminal device which would conventionally have required mounting a plurality of antennas. It is also possible to contribute to the realization of a short-range wireless communication system, which exploits a much wider frequency band than conventionally. Moreover, since it is possible to expand the operating band without using chip parts, it is also useful as an antenna which is highly immune to productional variations. Moreover, at frequencies lower than the frequency band of the slot antenna, the slot antenna undergoes the operation of a ground conductor dipole antenna that has the same polarization characteristics as those of the slot antenna, and therefore the antenna can be utilized as a small wideband slot antenna. It can also be used as a small-sized antenna in a system which requires ultrawideband frequency characteristics where digital signals are transmitted or received wirelessly. In either case, when mounted in a terminal device, the present antenna makes it possible to provide characteristics free of instabilities in radiation operation due to being connected to an unbalanced feed circuit which is connected to the antenna.