# Patent

## Title
Small planar antenna with enhanced bandwidth and small strip radiator

## Abstract

A planar small antenna and a small strip radiator are provided which have increased bandwidth. The small strip radiator has a main strip pattern and a plurality of convoluted strip patterns terminating the main strip pattern at each end. The plurality of convoluted strip patterns are arranged in mirror-symmetrical arrangement with reference to the longitudinal axis of the main strip such that one pair of convoluted strip patterns is convoluted clockwise while another pair is convoluted counterclockwise. As a result, an electrically small antenna radiator requires less metal or conductive material than conventional radiators, and also can operate without adversely affecting the radiation characteristics of the antenna.

## Background
This is a divisional of application Ser. No. 11/207,725 filed Aug. 22, 2005 now U.S. Pat. No. 7,289,076. The entire disclosure of the prior application, application Ser. No. 11/207,725, is considered part of the disclosure of the accompanying divisional application and is hereby incorporated by reference.


BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to RF and microwave antennas, and more particularly, to a small planar antenna and a small conductive strip radiator with improved bandwidth.
2. Description of the Related Art
In L-frequency bandwidth and at UHF frequencies, the size of a half wave dipole antenna presents a restriction in mobile or RFID applications, and therefore, a small antenna with relatively small wavelength is required. However, the size of antenna for a given application is not related mainly to the technology used, but is defined by well-known laws of physics. Namely, the antenna size with respect to the wavelength is the parameter that has the most significant influence on the radiation characteristics of the antenna.
Every antenna is used to transform a guided wave into a radiated one, and vice versa. Basically, to perform this transformation efficiently, the antenna size should be of the order of a half wavelength or larger. Of course, an antenna may be smaller than this size, but bandwidth, gain, and efficiency will decrease. Accordingly, the art of antenna miniaturization is always an art of compromise among size, bandwidth, and efficiency.
In the case of planar antennas, a good compromise may be obtained when most of the given antenna area participates in radiation.
WO 03/094293 discloses an example of miniaturizing the antenna to a size smaller than the size of resonance, while maintaining relatively high gain and efficiency of resonance characteristics. FIG. 1 shows an antenna of WO 03/094293, which is incorporated herein by reference.
Referring to FIG. 1, antenna 1 includes a dielectric substrate 2, a feed line 5, a metal layer 3, a main slot 4 and a plurality of sub slots 6a to 6d which are patterned within the metal layer 3. The metal layer 3 with the main slot 4 and sub slots 6a to 6d form a radiator of the antenna 1.
Meanwhile, FIG. 2 shows a radiator of a conventional antenna which has a vertically-linear slot. FIG. 3 shows a radiator of a conventional antenna with vertically-rotating slot, and FIG. 4 shows a radiator of a conventional antenna with a vertically-spiral slot.
Throughout the description with reference to FIGS. 2 to 4, the common components, that is, main slot and metal layer will be referred to by the same reference numerals. A plurality of sub slots 8a to 8d, 9a to 9d, 10a to 10d of various configurations, are formed at each end of the main slot 4.
A conventional antenna as exemplified above is limited by having narrow bandwidth. Furthermore, the operative frequency bandwidth of a small antenna is a factor in a variety of applications.
Accordingly a need arises for a small antenna, which can operate at an electrically-improved bandwidth, without affecting radiation pattern, gain and radiation efficiency.
Meanwhile, a small antenna requires a large amount of conductive material for a ground layer. Thus, the relatively high weight of conductive material required in antennas also becomes a factor.
SUMMARY OF THE INVENTION
Accordingly, an aspect of the present invention is to provide a planar small antenna which has an improved operative frequency bandwidth, and does not adversely affect radiation pattern, gain and radiation efficiency.
It is another aspect of the present invention to provide a small strip radiator which requires less metal or other conductive material than conventional radiators, and at the same time can operate without adversely affecting radiation characteristics.
The above and other aspects of the present invention can substantially be achieved by providing a planar small antenna, comprising a dielectric substrate, a metal layer formed on the upper part of the dielectric substrate, a main slot patterned within the metal layer, and a plurality of sub slots connected with the main slot, and convoluted in a predetermined direction. The plurality of sub slots may be arranged symmetrically with reference to the longitudinal axis of the main slot.
The predetermined direction may be a clockwise direction or a counterclockwise direction.
Each of the plurality of sub slots which are arranged symmetrically with reference to the longitudinal axis of the main slot, may be convoluted in direction opposite to a counterpart sub slot of said each of the plurality of sub slots.
Respective sectors of the convoluted sub slots may be smaller than 1⁄4 of wavelength which is within the operational frequency range of the antenna.
The plurality of sub slots may include a first right sub slot convoluted clockwise, formed on a upper side of a right side of the main slot, a second right sub slot convoluted opposite to the first right sub slot, formed alongside the inner side of the first right sub slot, a fourth right sub slot convoluted opposite to the first right sub slot, formed on a lower side of the right side of the main slot, and a third right sub slot convoluted opposite to the fourth right sub slot, formed alongside the inner side of the fourth right sub slot.
First to fourth left sub slots may be further provided in a mirror-symmetric arrangement with the first to fourth right sub slots with reference to the main slot, wherein each of the first to fourth left sub slots is convoluted opposite to a counterpart sub slot of the first to fourth right sub slots.
The main slot may have a length smaller than a half wave in the operational frequency of the antenna.
The widths of the sub slots and the main slot may be identical.
The width of the sub slots may be narrower than the width of the main slot.
The width of the sub slots may be wider than the width of the main slot.
A feed line may be further provided at a rear side of the dielectric substrate, having a microstrip line of open-ended capacitive probe.
The widths of the probe and strips of the microstrip line may be identical.
The width of the probe may be narrower than the width of the strips of the microstrip line.
The width of the probe may be wider than the width of the strips of the microstrip line.
According to one aspect of the present invention, a small strip radiator may include a main strip pattern, and a plurality of convoluted strip patterns which terminate the main strip pattern at each end. The plurality of convoluted strip patterns may be arranged in mirror-symmetrical arrangement with reference to the longitudinal axis of the main strip such that one pair of convoluted strip patterns is convoluted in a clockwise direction while another pair is convoluted in a counterclockwise direction.
The main strip may have a centrally placed gap which is a feeding point of the radiator.
The main strip pattern and the plurality of convoluted strip patterns may be formed on the dielectric substrate.
The convoluted strip patterns may be provided in a mirror-symmetric arrangement with reference to the longitudinal axis of the main strip.
A feed may be further provided, with having a direct inlet of an electronic chip into the gap.
A feed may be further provided, with having a planar transmission line placed on the dielectric substrate.
The dielectric substrate, the main strip pattern and the convoluted strip patterns may be substantially planar.
The main strip pattern and the convoluted strip patterns formed as a bulk wire pattern having the same geometry.



BRIEF DESCRIPTION OF THE DRAWINGS
The above aspects of the present invention will be more apparent by describing certain exemplary embodiments of the present invention with reference to the accompanying drawings, in which:
FIG. 1 is a view of a prior art antenna;
FIG. 2 illustrates a radiator of a conventional antenna with a vertically-linear slot;
FIG. 3 illustrates a radiator of a conventional antenna with a vertically-rotating slot;
FIG. 4 illustrates a radiator with a vertically-spiral slot;
FIG. 5 is a perspective view of a planar small antenna according to an exemplary embodiment of the present invention;
FIG. 6 is a detailed plan view of the metal layer of FIG. 5 which has a main slot and a plurality of sub slots therein;
FIG. 7 illustrates distribution of electro-magnetic current in the slot pattern according to an exemplary embodiment of the present invention;
FIG. 8 illustrates radiation pattern on E and H planes of a conventional antenna;
FIG. 9 illustrates radiation patterns on E and H planes of an antenna according to an exemplary embodiment of the present invention;
FIG. 10 is a graphical representation comparing bandwidth characteristics through return loss, between a conventional antenna and an antenna according to an exemplary embodiment of the present invention;
FIG. 11 illustrates small strip radiator according to another exemplary embodiment of the present invention;
FIG. 12 illustrates in detail strip pattern of FIG. 11; and
FIG. 13 illustrates a temporary distribution of electric current density in the strip pattern according to an exemplary embodiment of the present invention.



DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS OF THE INVENTION
Exemplary embodiments of the present invention will be described herein below with reference to the accompanying drawings.
FIG. 5 is a perspective view of a planar small antenna according to an exemplary embodiment of the present invention. Referring to FIG. 5, a planar small antenna 100 according to an exemplary embodiment of the present invention includes a dielectric substrate 20, a metal layer 30 formed on an upper part of the dielectric substrate 20, a main slot 40 and a plurality of sub slots 60a, 60b, 70a, 70b, 80a, 80b, 90a, 90b which are patterned in the metal layer 30, and a feed line 50 which is formed at a lower part of the dielectric substrate 20. The metal layer 30 with the main slot 40 and the plurality of sub slots 60a, 60b, 70a, 70b, 80a, 80b, 90a, 90b form the radiator of the antenna 100.
FIG. 6 is a detailed plan view of the metal layer 30 which has the main slot 40 and sub slots 60a, 60b, 70a, 70b, 80a, 80b, 90a, 90b of FIG. 5. Hereinbelow, the main slot 40 and sub slots 60a, 60b, 70a, 70b, 80a, 80b, 90a, 90b together are referred to as a ‘radiator’.
Referring to FIG. 6, the radiator includes the metal layer 30, a main slot 40 and the plurality of sub slots 60a, 60b, 70a, 70b, 80a, 80b, 90a, 90b which are formed on both sides of the main slot 40.
Each of the sub slots 60a, 60b, 70a, 70b, 80a, 80b, 90a, 90b is connected with the main slot 40. Also, each of the sub slots 60a, 60b, 70a, 70b, 80a, 80b, 90a, 90b are convoluted in clockwise or counterclockwise directions. Additionally, each of the sub slots 60a, 60b, 70a, 70b, 80a, 80b, 90a, 90b are arranged in a mirror-symmetric pattern with reference to the longitudinal axis of the main slot 40.
Accordingly, the first sub slot 60a on the right side and the third sub slot 80a on the right side may be convoluted clockwise, while the second sub slot 70a on the right side and the fourth sub slot 90a on the right side may be convoluted counterclockwise.
Further, the first sub slot 60b on the left side and the third sub slot 80b on the left side may be convoluted counterclockwise, while the second sub slot 70b on the left side and the fourth sub slot 90b on the left side may be convoluted clockwise.
Basically, a radiating part dominates over the electromagnetic properties of every antenna. Thus, when a greater area of the radiator is used for radiation, the operative bandwidth can be improved and antenna miniaturization can be achieved, without diminishing desirable radiation characteristics, such as gain and radiation efficiency.
Unlike the slot pattern of conventional antennas, the radiator according to an exemplary embodiment of the present invention includes four sub slots which are respectively formed on ends of the main slot 40, in a mirror-symmetrical structure with reference to the longitudinal axis of the main slot. The planar small antenna according to this exemplary embodiment has the above rather complicated slot structure for the following reasons.
Generally, the total length of an antenna is smaller than a half wavelength, and may be even smaller than a quarter of the wavelength, which inevitably causes the main slot to have a shortened size. In addition, the radiator of an antenna is required to maintain a half wave resonance characteristic. Accordingly, in order to reduce the size of the antenna, a certain limit voltage may be applied to both ends of the main slot, and therefore, a desired resonance electro-magnetic field distribution is generated at the shortened main shot. In order to provide desired discontinuity of voltage at both ends of the main slot, both terminating ends of a sub slot need termination elements which have an inductive characteristic.
Further, if the length of the termination sub slot is smaller than a quarter of a wavelength, inductive loading is guaranteed. Conventionally, an inductive termination is formed by a pair of linear or spiral slots which are provided at both ends of the main slot 4 (see sub slots 8a to 8d, 9a t 9d, 10a to 10d of FIGS. 2, 3 and 4). Unlike the conventional antennas, in this exemplary embodiment of the present invention, the terminations of the main slot 40 are formed of four sub slots 60a, 70a, 80a, 90a terminating at the right side of the main slot 40 and four sub slots 60b, 70b, 80b, 90b terminating at the left side of the main slot 40, with the respective sub slots 60a, 70a, 80a, 90a and 60b, 70b, 80b, 90b being convoluted in a clockwise or counterclockwise mirror-symmetrical pattern.
FIG. 7 shows the distribution of electro-magnetic currents in the slot pattern according to the above exemplary embodiment of the present invention. Referring to FIG. 7, the direction of electro-magnetic current is schematically indicated by arrows. By the combination of clockwise and counterclockwise-convoluted sub slots 60a, 70a, 80a, 90a, unique electro-magnetic characteristics may be achieved. That is, there are 6 arms 62a, 71a, 75a, 81a, 85a, 92a of convoluted sub slots which have the same electro-magnetic flow as the main slot 40.
In addition, there are two sectors 73a, 83a which have opposite electro-magnetic flow with respect to the flow direction of the main slot 40. The electro-magnetic current has a small amplitude in the two sectors 73a, 83a. 
Meanwhile, an undesirable field coupling effect is initially decreased at the sectors 72a and 74a, 82a and 84a, 61a and 63a, and 91a and 93a, and is further suppressed by the mirror-symmetry arrangement with respect to the longitudinal axis of the main slot 40.
As a result, undesirable phenomenon due to conventional inductive sub slots can be prevented. Additionally, the area which uses electro-magnetic current at the terminating sub slot can be successfully improved, and as a result, increased antenna areas can participate in the radiation efficiently. Therefore, as described above in a few exemplary embodiments of the present invention, a planar small antenna can be provided, which can operate in an improved bandwidth, without adversely affecting the radiation pattern, gain and radiation efficiency.
To compare the performances of the antenna according to an exemplary embodiment of the present invention and the conventional antenna, both antennas were designed to be of an identical size for UHF operation. That is, the metal layer 30 was sized to 0.21λ0×0.15λ0, and the slot is sized to 0.17λ0×0.08λ0, where λ0 denotes waves in free space.
The feed to the antenna may be an open-ended microstrip line with a probe installed at the rear surface of the dielectric substrate or any other transmission line.
FIG. 8 shows a radiation pattern on E and H planes of a conventional antenna, and FIG. 9 shows a radiation pattern on E and H planes of an antenna according to an exemplary embodiment of the present invention.
Referring to FIGS. 8 and 9, it was observed that the forward-directional pattern of both antennas are almost similar. The planar small antenna of the present exemplary embodiment has gain of −1.9 dBi, and the conventional antenna has the gain of −1.8 dBi. Accordingly, advantages of the antenna according to this exemplary embodiment of the present invention may not be remarkable in terms of gain and efficiency.
FIG. 10 is a graphical representation which compares bandwidth characteristics of an antenna according to an exemplary embodiment of the present invention and a conventional antenna based on return loss. Referring to FIG. 10, the return loss of the conventional antenna is indicated by the phantom line, while the return loss of the antenna according to the present exemplary embodiment is indicated by the solid line.
At the return loss of −10 dB level, the antenna according to the exemplary embodiment of the present invention has operation bandwidth of 38 MHz, while the conventional antenna has operation bandwidth of 29 MHz. In other words, the antenna according to the exemplary embodiment of the present invention has approximately 30% wider bandwidth than the conventional antenna. At the same time, the antenna according to the exemplary embodiment of the present invention does not suffer from the influences on the radiation pattern and efficiency, and polarization purity.
Meanwhile, the antenna 100 according to an exemplary embodiment of the present invention as shown in FIG. 5 requires a substantially large amount of conductive material to form a ground metal layer 30. Additionally, the relatively heavy weight of the metal required by the antenna 100 becomes a factor. Accordingly, it is desirable to provide a radiator which requires less metal or other conductive material, and can operate without adversely affecting the radiation characteristic. Such a radiator is suggested below with reference to another exemplary embodiment of the present invention.
Basically, the radiator characteristic is the dominant characteristic of the electro-magnetic characteristics of every antenna. Thus, the maximum area of the radiator should be utilized in the radiation to improve parameters of the antenna. Unlike the radiator with four slot pattern of FIG. 6, a radiator according to another exemplary embodiment of the present invention is based on a strip pattern, because such structure substantially consumes less metal.
The pattern of metal strip geometrically almost duplicates the pattern with four slots as shown in FIG. 6. In other words, according to this particular embodiment of the present invention, the strip replaces the slot on principle of electro-magnetic duality. According to this well-known principle, a dual structure can be formed by replacing the metal with air and replacing air with metal. Dual structures are similar to a positive and negative in photography.
The radiator according to this exemplary embodiment of the present invention can be classified as a ‘complimentary’ radiating structure with respect to the slot pattern-based radiator as shown in FIG. 6. Accordingly, the aspects of the radiator of FIG. 6 are equally applicable to the small strip radiator which will be described below according to another exemplary embodiment of the present invention.
FIG. 11 shows a small strip radiator according to another exemplary embodiment of the present invention.
Referring to FIG. 11, a printed strip radiator 1000 includes a dielectric substrate 200 and a conductive strip pattern 300 which is formed on a surface of the dielectric substrate 200. The dielectric substrate 200 directly forms a small strip radiator 1000.
FIG. 12 shows the strip pattern of FIG. 11 in detail. The strip pattern 300 comprises a main strip 310 and a plurality of strip arms which terminate the main strip 310 at each end. The main strip 310 has a centrally placed gap 360 at feeding point of radiator 1000.
The strip arms 320a, 320b, 330a, 330b, 340a, 340b, 350a, 350b are arranged in pairs which are arranged with respect to the longitudinal axis of the main strip 310. That is, the strip arms 320a, 320b, 330a, 330b, 340a, 340b, 350a, 350b terminate the main strip 310 in such a manner that one arm, for example the arm 320a is convoluted clockwise while another arm, for example, the arm 320b is convoluted counterclockwise. The terminating strip arms are further formed as mirror-symmetrical pairs with respect to the longitudinal axis of the main strip 310.
The size of the metal ground layer 30 of the radiator of FIG. 6 would ideally be infinite. Nonetheless, despite theoretical imperfections of an actual implementation, the radiator 1000 can operate very well, provided that the proper adjustment of the practical strip pattern is taken into account. Of course, the input impedance of the antenna with complimentary radiator would be substantially different and requires proper matching with the particular feeder implementation.
FIG. 13 shows temporary distribution of current density at the strip pattern.
For the case of an electrically small radiator (i.e., small in relation to wavelength), the phase difference of the electro-magnetic field along the structure is small, so instantaneous distribution of the electric current density at the strip pattern can be schematically shown by arrows of proportional length as in FIG. 13. The combination of clockwise and counterclockwise convoluted strip arms provides the termination with unique electro-magnetic features.
Namely, there are six sectors 321b, 331b, 322b, 332b, 314b, 344b in FIG. 13 with the flow of the current being in the same direction as at the main strip 310. The opposite flow of the current with substantially low amplitude exists only on two sectors 325b, 335b. 
The undesirable secondary effect of terminating strip arms is suppressed. Indeed, an undesirable far field coupling effect of pairs of sectors 324b and 323b, 334b and 333b, 312b and 316b, and 342b and 346b is first reduced pair-wise, and then suppressed by the mirror-symmetry with respect to the longitudinal axis of the main strip 310.
Thus, the radiated fields from the strip sectors 324b, 323b, 312b, 316b cancel the radiated fields from the sectors 334b, 333b, 342b, 346b, and they do not contribute to the overall far field. Additionally, the sectors 321b, 331b, 322b, 332b, 314b, 344b of the vertical strip arms using electric current are successfully improved, thereby increasing the area of antenna that effectively participates in the radiation phenomenon.
The radiator thus functions as a basic element of electrically small planar antenna. The feed of the antenna may be realized either through a conventional planar transmission line, or by direct inlet of an electronic chip into the strip pattern.
As a result, exemplary embodiments of the present invention provide a radiator for electrically small antennas that require less metal or other conductive material than conventional radiators, and at the same time, can operate without adversely affecting the radiation characteristics.
The practical method of manufacturing the radiator involves any sort of printed circuit technologies. The substitution of printed strip pattern by bulk wire pattern with the same generic geometry would also not depart from the scope and spirit of the present invention.
As described above in a few exemplary embodiments of the present invention, a planar small antenna may have increased area to effectively participate in the radiation phenomenon, and therefore, provides improved bandwidth, without adversely affecting the radiation pattern, gain and efficiency.
Additionally, with the small strip radiator according to aspects of the present invention, an electrically small antenna radiator can be provided which requires less metal of conductive material than the conventional radiators, and it also can operate without adversely affecting the radiation characteristics of the antenna.
The foregoing exemplary embodiments and aspects of the invention are merely exemplary and are not to be construed as limiting the present invention. The present teaching can be readily applied to other types of apparatuses. Also, the description of the exemplary embodiments of the present invention is intended to be illustrative, and not to limit the scope of the claims, and many alternatives, modifications, and variations will be apparent to those skilled in the art.

## Claims

1. A small strip radiator, comprising:
a main strip pattern; and
a plurality of convoluted strip patterns which terminate the main strip pattern at each end,
wherein the plurality of convoluted strip patterns are arranged in a mirror-symmetrical arrangement with reference to the longitudinal axis of the main strip such that one pair of convoluted strip patterns is convoluted in a clockwise direction while another pair of convoluted strip patterns is convoluted in a counterclockwise direction.

2. The small strip radiator of claim 1, wherein the main strip includes a centrally placed gap which is a feeding point of the radiator.

3. The small strip radiator of claim 2, further comprising a feed which includes a direct inlet of an electronic chip into the gap.

4. The small strip radiator of claim 1, wherein the main strip pattern and the plurality of convoluted strip patterns are formed on dielectric substrate.

5. The small strip radiator of claim 1, further comprising a feed which includes a planar transmission line placed on a dielectric substrate.

6. The small strip radiator of claim 5, wherein the dielectric substrate, the main strip pattern and the convoluted strip patterns are substantially planar.

7. The small strip radiator of claim 1, wherein the main strip pattern and the convoluted strip patterns are formed as a bulk wire.