BACKGROUND In general, antennas are used in base stations for receiving and transmitting signals to and from subscriber units in a cellular communication system.

Antennas are also used as interface means in subscriber units for receiving and transmitting signals to and from base stations. In addition, antennas are used as interface means for repeaters in areas of poor cellular coverage such as underground parking lot, subway, and buildings.

Dipole antenna, Yagi antenna and microstrip antennas are commonly used in wireless or mobile communication systems. The dipole antenna is a half-wave, resonant type antenna having omni-directional radiation characteristic, which is used for antennas of a subscriber unit or a small repeater in a cellular communication system. The Yagi antenna is a linear end-fire antenna, consisting of three or more half-wave dipole antennas, having high directivity and used for array antenna of a small repeater.

Particularly, the microstrip antennas are employed in wireless communication systems including cellular, PCS, WLL, IMT-2000 and satellite

communication systems. The availability of microstrip antenna will be enhanced while the volume of the mobile handset becomes smaller according to the increase in resonant frequencies of mobile communication systems.

Recently, various methods have been suggested to shift the resonant frequency of microstrip patch antennas. Most of them involve changing their geometry, and others involve using varactor-switching diodes to achieve electronic control. However, using those methods, it was difficult to control the resonant frequency of the microstrip patch antennas after they were fabricated.

Fig. 1a to 1c illustrates the structure of the conventional microstrip patch antenna.

In Fig. 1a, a radiation patch (2) made of conductive metal is mounted on the upper surface of a dielectric substrate (1) made of dielectric material such as, Teflon, Rexolite and Polytetrafluoroethylene (PTFE). A ground plane (3) made of conductive metal is mounted on the opposite, bottom surface of the dielectric material (1).

A feed point (21) for supplying power and enabling impedance matching is provided on one side of the radiation patch (2). The radiation patch (2) is electrically coupled with the feed point (21) via a feed line (22).

In the aforesaid configuration, when electric power is applied to the feed point (21), an electromagnetic wave propagates to z-direction in Quasi-TEM mode, TMioo.

As shown in Fig. 1b, the normal components of the electric field at the two edges along the width (w) are in opposite directions and thus out of phase.

As shown in Fig. 1c, since the radiation patch (2) is AJ2 long the normal components of the electric field cancel each other in the broadside direction (L), leaving the tangential components in phase, resulting in maximum radiated field normal to the surface of the structure. Hence the edges along the width can be

represented as two radiating slots, which are A/2 apart and excited in phase and radiating in the half space above the ground plane.

The conventional microstrip patch antenna could be structured on the dielectric substrate of a small size. However, one of the disadvantages of the conventional microstrip patch antenna is the narrow impedance bandwidth. The conventional microstrip patch antenna has 1-2% of impedance bandwidth and various methods for increasing the bandwidth have been suggested.

The U. S. patent Nos. 5,565, 875 and 5,680, 144 disclosed increasing the bandwidth of antennas by changing the shape of the patch into loop or circular arc or laminating parasitic devices. However, they could hardly be embedded in PCS repeater or other mobile communication equipments due to the complicated structure and the thickness of antennas.

In order to increase the resonant frequency of the conventional microstrip antenna, one had to change shape of the antenna. And the available frequency ranges were usually limited by product specification of the antenna.

DETAILED DESCRIPTIONS OF THE INVENTION The present invention proposes a new method to achieve frequency agility using piezoelectric materials without changing the physical formation or substrate of the antennas. When the bias is applied, the thickness of the substrate is varied according to the piezoelectric phenomenon. Because the resonant frequency is shifted according to the thickness, the shift of the resonant frequency could be precisely controlled by the applied electric field.

Several piezoelectric materials such as Quartz, LiNb03 and Pb (Zr, Ti) 03 (PZT) were found to be available as piezoelectric substrates.

Quartz was found to be better material for a substrate because its permittivity

is relatively lower than those of other piezoelectric materials. In addition, the frequency shift characteristic of the quartz substrate was found to be better than that of LiNbO3 or Pb (Zr, Ti) 03.

The microstrip antenna using piezoelectric material of the present invention consists of a piezoelectric substrate, a ground plane, a patch, a feed line, a transformer, and a transmission line.

The piezoelectric substrate of the microstrip patch antennas of the present invention is preferably fabricated using X-cut, Y-cut and Z-cut quartz, and has a disc shape with a diameter of 1-5 inches and a thickness of 0. 1-1 mm.

Preferably, the bottom electrode has a disc shape with a diameter of 1-5 inches and a thickness of 0. 1-5n.

Preferably, the patch has a rectangular shape with a 5x5-15x15mm dimension.

Preferably, the feed line has a rectangular shape with a 0. 1x3-0. 3x8mm dimension.

Preferably, the transformer has a rectangular shape with a 1x3-2x7mm dimension.

Preferably, the transmission line has a rectangular shape with a 0.5x5- 1x5mm dimension.

BRIEF DESCRIPTIONS OF FIGURES Fig. 1a to 1c illustrates the structure of a conventional microstrip patch antenna.

Fig. 2 is an exploded perspective view of the microstrip patch antenna using a piezoelectric material according to the exemplary embodiment of the present invention.

Fig. 3 is a cross-sectional view of the microstrip patch antenna of Fig. 2.

Fig. 4 illustrates a system for measuring the frequency characteristic of the microstrip antenna of the exemplary embodiment of the present invention.

Fig. 5 is a graph showing the shift characteristics of the resonant frequencies versus DC bias on the X-cut quartz substrate of the microstrip antenna according to the exemplary embodiment of the present invention.

Fig. 6 is a graph showing the shift characteristics of the resonant frequencies versus DC bias on the Y-cut quartz substrate of the microstrip antenna according to the exemplary embodiment of the present invention.

Fig. 7 is a graph showing the shift characteristics of the resonant frequencies versus DC bias on the Z-cut quartz substrate of the microstrip antenna according to the exemplary embodiment of the present invention.

An exemplary embodiment of the present invention will be described in detail by referring to the attached figures. The embodiment described in the specification of this application shall not be interpreted as a limitation of the scope of the claims of the present invention.

Fig. 2 is an exploded perspective view illustrating the microstrip patch antenna using a piezoelectric material according to the exemplary embodiment of the present invention. Fig. 3 is a cross-sectional view illustrating the microstrip patch antenna of Fig. 2.

As illustrated in Figs. 2 and 3, the microstrip patch antenna of the present invention comprises a piezoelectric substrate (10), a ground plane (20), a patch (30), a feed line (40), a transformer (50), and a transmission line (60).

The antenna was designed and simulated by Ensemble V. 7.0, which is one of the microstrip antenna simulation tools. The piezoelectric substrate (10) was cleaned by an ultrasonic wave cleaner using acetone for 10 min. After they were

dried using nitrogen gas, an aluminum electrode (20) was deposited on the surface using a thermal evaporator. The feed line (40) was connected to a subminiature A (SMA) connector using silver paste.

The piezoelectric substrate (10) is made of X-cut, Y-cut and Z-cut quartz.

When the bias is applied, the thickness of the substrate is varied by a mechanical strain according to the piezoelectric phenomenon. The resonant frequency and the bandwidth of the microstrip patch antenna could be controlled by the applied electric field without changing the physical formation of the antenna.

Preferably, the piezoelectric substrate (10) has a disc shape with a diameter of 1-5 inches and a thickness of 0.1-1 mm.

A bottom electrode (20) is formed on the bottom surface of the piezoelectric substrate (10). The bottom electrode (20) has the same physical size as the piezoelectric substrate (10), and is evaporated over the bottom surface of the piezoelectric substrate (10).

The patch (30), the feed line (40), the transformer (5), and the transmission line (60) are deposited on the upper surface of the piezoelectric substrate (10) and are connected electrically by thin aluminum plane.

The patch (30) is formed by a thin aluminum plane of rectangular shape with a 5x5-15x15mm dimension, and the feed line (40) is electrically coupled to one side of the patch (30).

The feed line is formed by a thin aluminum plane of rectangular shape with a 0. 1x3-0. 3x8mm dimension, and the X/4 transformer (50) is electrically coupled to one side of the feed line (40).

The X/4 transformer (50) is formed by a thin aluminum plane of rectangular shape with a 1x3-2x7mm dimension, and the transmission line (60) is electrically coupled to one side of the transformer (50).

The transmission line (60) is formed by a thin aluminum plane of rectangular

shape with a 0. 5x5-1x5mm dimension.

Fig. 4 illustrates a system for measuring the frequency characteristic of the microstrip antenna of the exemplary embodiment of the present invention.

While DC bias from 0 to 400V/mm was applied to the microstrip patch antenna by a voltage source (110, Fluke 5100B), the resonant frequencies were measured by the vector network analyzer (100, HP 8722D). The DC bias was supplied through the bias-Tee (70, model 5530A made by Picosecond). The bias- Tee (70) is electrically coupled to the patch (30) formed on the upper surface of the piezoelectric substrate (10).

The X-cut, Y-cut and Z-cut quartz microstrip patch antennas were fabricated using three resonant frequencies, that is, 6,8 and 10 GHz.

Fig. 5 is a graph showing the shift characteristics of the resonant frequencies versus DC bias on the X-cut quartz substrate of the microstrip antenna according to the exemplary embodiment of the present invention.

Afr is defined as the frequency shift from the resonant frequency at zero electric field to the resonant frequency at a specific electric field.

From the piezoelectric equation, the relationship between strain S and electric field E is given as: S . dtE (1) Where dt is the piezoelectric constant. The resonant frequency of the microstrip patch antenna is calculated as below : fr = c / {(L + 2#L)(#reff)1/2} (2) Where L, aL, Ereff. C and tr are length of patch, length extension, effective dielectric constant, speed of light and resonant frequency, respectively. The theoretical calculations are in good agreement with the below experimental results shown in Fig. 5.

As applied electric field increases the resonant frequency of the microstrip

patch antenna increases in the range of 0 to 400V/mm. Up to the electric field of 100V/m, the three resonant frequencies (6,8, 10GHz) increase in the similar way.

When an electric field of 400V/mm was applied, the Afr values of X-cut quartz microstrip patch antennas at 6GHz, 8GHz and 10GHz were 33. 11MHz, 42.13MHz and 55.27MHz, respectively. That is, the three resonant frequencies increase with different resonant frequency against the same external electric field.

The experimental results are in good agreement with the theoretical calculations, showing that the frequency shifts of X-cut microstrip antennas are mainly caused by piezoelectric phenomena.

Fig. 6 illustrates the dependence of the frequency shifts of Y-cut quartz substrate of the microstrip antenna of the present invention on DC bias.

As applied electric field increases from 0 to 400V/mm, the resonant frequency of the microstrip patch antenna increases. When an electric field of 400V/mm was applied, the A fr values of Y-cut quartz microstrip patch antennas at 6GHz, 8GHz and 10GHz were 17.88MHz, 26.75MHz and 34.22MHz, respectively.

Fig. 7 illustrates the dependence of the frequency shifts of Z-cut quartz substrate of the microstrip antenna on DC bias.

When an electric field of 400V/mm was applied, the Afr values of Y-cut quartz microstrip patch antennas at 6GHz, 8GHz and 10GHz were 17. 44MHz, 21.27MHz and 28. 01 MHz, respectively.

The dependence of A fr on electric field at 6GHz, 8GHz and 1 OGHz is shown in Table 1. It is observed that the frequency shifts of X-cut quartz antennas are almost twice the frequency shifts of Y-cut and Z-cut quartz antennas.

[Table 1] Resonant Quartz ß fr [MHz] frequency 100 V/mm 200 V/mm 300V/mm 400V/mm X-cut 8.71 19.17 27.88 33. 11 6GHz Y-cut 6. 38 12 77 15 33 17. 88 Z-cut 8. 35 12. 71 16.26 17. 44 X-cut 10.53 21. 06 34. 75 42. 13 8GHz Y-cut 12 35 20. 06 23. 67 26.75 Z-cut 7. 37 14. 18 18.90 21.27 X-cut 10. 15 27.07 43. 99 55. 27 10GHz Y-cut 14. 66 24. 44 28.11 34. 22 Z-cut 11.20 16.80 22.40 28.01

Therefore, the resonant frequency of the microstrip antenna of the present invention using quartz substrate having piezoelectric phenomenon and mechanical variation, could be controlled by the applied electric field without changing the physical formation of the antenna.

INDUSTRIAL APPLICABILITIES When the bias is applied, the thickness of the substrate is varied according to the piezoelectric phenomenon. Because the resonant frequency is shifted according to the thickness, the shift of the resonant frequency could be precisely controlled by the applied electric field. The microstrip antenna of the present invention could be used as antennas for mobile communication systems.

While this invention has been particularly shown and described with reference to particular embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be effected therein without departing from the spirit and scope of the invention as defined by the appended claims.