Abstract:
A miniaturized microstrip antenna with variable broadband operation comprised of a ground patch, an isosceles-triangular patch with a base side, two isosceles sides, a top angle opposite the base side and two base angles. A pair of primary slots extending from the top angle sides toward the base angles are embedded in the isosceles-triangular patch. At least one pair of secondary slots extended from each primary slot. A substrate is located between the ground patch and the isosceles-triangular patch. The primary slots are approximately parallel to the sides of the isosceles-triangular patch. The second and the third slots branching from the primary slots are approximately perpendicular to the base side of the isosceles-triangular shaped patch. It is found that by selecting a proper dimension, the good broadband operation can be obtained. Lastly, inclusion of the slots and adjustment of the size of the slots on the microstrip antenna allows for a reduction in overall size and area of the microstrip antenna.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a microstrip antenna. Specifically, it relates to a miniaturized microstrip antenna with variable broadband operation. 
     2. Description of the Related Art 
     The size of a conventional microstrip antenna is determined by half of the operating wavelength. However, when the conventional microstrip antenna is operates at VHF or UHF band, the size of a conventional microstrip antenna is increased to enhance reception. Consequently, the size of a conventional microstrip antenna can become unduly large when operating at a low resonant frequency. 
     Examples of existing, conventional microstrip antennea are disclosed as follows: TAIWAN patent no.364228 “Miniaturized broadband microstrip antenna”, U.S. Pat. No. 5,453,752 “Compact broadband microstrip antenna” and U.S. Pat. No. 5,680,144 “Wideband, stacked doubled C-patch antenna having gap-coupled parasitic elements”; or Euro patent no. EP0624578 “Compact broadband microstrip antenna”, etc. 
     In the prior art, a single probe-fed microstrip antenna is proposed and the dual frequency operation is achieved by embedding slots to the microstrip patch. Moreover, since that the frequency ratio of the two operating frequencies is not necessary to be very close, the dual-band design is more simple than the proposed broadband design. By using slots to change the surface current distribution of the resonant modes, dual-frequency operation with a variable ratio of the two frequencies can be obtained. However, to obtain a broadband performance, the two resonant frequencies must be relatively close to one another and the frequency ratio of the two resonant frequencies must meet certain limits. 
     Furthermore, the current trend of integrated circuit design is for virtually all communication products to become miniaturized in size. Apart from the broadband operation incorporated into the system, the design of the antenna needs to allow for the miniaturization of antenna size according to the overall circuit size. 
     However, in the conventional art disclosed above, there is currently no such design utilizing slots to both increase the operating bandwidth of the antenna while simultaneously minimizing the antenna size. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to provide a simple, miniaturized variable bandwidth broadband microstrip antenna with variable broadband operation. 
     To achieve the objective described above, the present invention provides a microstrip antenna comprised of a ground patch and an isosceles-triangular patch with a pair of primary slots extending from the top angle towards the base angles with a second pair of slots (hereinafter referred to as the second and third slots) connected to and extending downward from each of the primary slots. The primary slots are approximately parallel to the sides of the isosceles-triangular patch while the second and third slots are approximately perpendicular to the base side of the triangle. A substrate connects the ground patch and the isosceles-triangular patch. 
     The proposed microstrip antenna has a simple structure, low prime cost, is easy to manufacture and achieves size reduction at wide operating bandwidth. The microstrip antenna of the present invention thus has good application value for the manufacturing industry. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is hereinafter described in detail by reference to the accompanying drawings in which: 
     FIG. 1 is a side view of the structure of the microstrip antenna of the present invention; 
     FIG. 2A is a top view of the structure of the microstrip antenna of the present invention; 
     FIGS. 2B and 2C show the patch surface current distributions of the two resonant modes for the present invention in FIG. 1; 
     FIG. 3 shows the measured result of the input resistive experiment according to the size of the slots of the microstrip antenna of the present invention; 
     FIG. 4 shows the measured result of the return loss according to the size of the slots (modified) of the microstrip antenna of the present invention; 
     FIG. 5 shows the measured result of the return loss according to the size of the slots (again, modified) of the microstrip antenna of the present invention; 
     FIGS. 6A and 6B represent the measured results of the E-plane and the H-plane radiation patterns of the microstrip antenna at the first resonant mode; 
     FIGS. 6C and 6D represent the measured results of the E-plane and H-plane radiation patterns of the microstrip antenna at the second resonant mode. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is a reduced-size antenna with variable broadband operation. In the following description of the embodiment, a probe-fed method is adopted as the example. However, it shall be understood this method is for illustrative purposes only. Therefore, this demonstrated methodology should not limit the scope of the present invention. Any other feed methods may also be adopted under the same application. Additionally, only the essential components of the present invention are introduced herein. Other components generally known to those skilled with the art have been omitted to keep the description concise. As for the values of the sizes designated to the embodiment of the present invention described below, the values are for illustrative purpose only. The practical values should depend upon the actual application or practice. It should also be noted that the shapes of the slots and the microstrip patch, their respective sizes and configurations assigned are specific, demonstrative examples only. They also shall not limit the scope of the present invention. 
     As shown in FIG.  1  and FIG. 2, the microstrip antenna of the present invention is primarily composed of a substrate and two patches. In the embodiment of the present invention, microstrip antenna  30  contains a ground patch  10  and a microstrip patch  20 . In addition, a substrate  11  is located between the two patches. Also, a first terminal such as connector  14  penetrating through the substrate  11  and the ground patch  10  has a second terminal such as a positive terminal  12  connected to a feed point  26  of the microstrip patch  20 . Furthermore, the ground patch  10  is electrically linked to the ground. 
     Since substrate  11  is made from insulating materials, the resonant frequency and the operating bandwidth of the antenna are varied under the influences of the dielectric constant. The shape of microstrip patch  20  is an equilateral triangle with a pair of bent slots embedded in the equilateral-triangular patch  20 . In the embodiment of the present invention, the microstrip patch  20  has been designed as an equilateral triangle comprised of three sides,  21 , 22  and  23 . Sides  22  and  23  represent respectively the first and second sides, while side  21  represents the third or base side of the triangle. Each side of the triangle  20  is about 5 cm in length. The triangle also has a pair of slots,  24  and  25 , symmetrical to the Y-axis. Slots  24  and  25  comprise three sections of slots of different lengths: slot  24  is comprised of slots  24 A,  24 B and  24 C, while slot  25  is comprised of slots  25 A,  25 B and  25 C. 
     Slots  24 A and  25 A are parallel to sides  22  and  23  respectively of equilateral triangle  20 . Slots  24 A and  25 A are offset from their respective sides of equilateral triangle  20  approximately 0.3 to 0.5 cm thereby providing improved broadband performance. Slots  24 B and  25 B are connected to slots  24 A and  25 A near the base side  21  of the equilateral-triangular patch  20  at an angle of 150 degrees to slots  24 A and  25 A respectively and are parallel to the Y-axis. The two slots  24 B and  25 B are approximately 0.04 to 0.06 cm away from the base side  21  of the equilateral-triangular  20 . Furthermore, slots  24 C and  25 C are located between the center line (Y-axis) of the equilateral-triangular patch  20  and slots  24 B and  25 B respectively. The two slots  24 C and  25 C are parallel to slots  24 B and  25 B respectively. The feed point  26  of the connecting terminal  12  is located at approximately the center line (Y-axis) of the isosceles-triangular patch  20 , as shown in FIG.  2 A. In the present design, by selecting a proper dimension of such a pair of slots, the first two broadband radiation modes of TM 10  and TM 20  of the microstrip antenna can be perturbed such that these two modes of similar radiation characteristics can be exited at frequencies close to each other. Consequently, the microstrip antenna bandwidth can be enhanced as well as antenna size is greatly reduced. 
     As shown in FIG. 2B and 2C, the two excited resonant modes demonstrate a first resonant mode (TM 10 ) and a second resonant mode (TM 20 ) of the equilateral-triangular microstrip antenna. Wherein, the corresponding excited patch surface current of the first resonant mode (TM 10 )is 1 and the corresponding excited patch surface current of the second resonant mode (TM 20 ) is 2. The corresponding exited patch surface current 1 flows along the Y dimension toward the top angle  27  whereas the corresponding excited patch surface current 2 flows from the center of the triangular patch toward the top angle  27  and the base angles  28  and  29 . In the microstrip antenna of the present invention, slots  24 B,  24 C,  25 B and  25 C are parallel to the Y-axis. Therefore, they do not perturb the excited patch surface current 1 of the TM 10  mode, and the resonant frequency of the TM 10  mode will not be affected by the slots described above. On the other hand, the exited patch surface current path of the TM 20  mode well be increased by the slots described above. The resonant frequency of the TM 20  mode is lowered significantly by increasing the dimension of the slots  24 B,  24 C,  25 B and  25 C. 
     In addition, since slots  24 A and  25 A are not parallel to the excited patch surface current of the TM 10  mode, the resonant frequency of the TM 10  mode can be changed by adjusting the lengths of the slots described. In the embodiment of the present invention, slots  24 A and  25 A are extended toward the center of the isosceles-triangular microstrip patch  20  along the dimension parallel to the equilateral sides  22  and  23  of the equilateral triangle causing the resonant frequency of the TM 10  mode to decrease progressively. Consequently, by decreasing the resonant frequencies of the TM 10  and TM 20  mode, the microstrip antenna of the present invention can achieve broadband operation while effectively minimizing the size of the antenna. 
     The relevant testing result of the embodiment of the present invention is presented in FIGS. 3 thru  6 . The improvement made by the present invention can thus be proved by the numerical experiment results described below. 
     The First Embodiment 
     FIG. 3 represents the measured result of the return loss of the microstrip antenna apparatus of the present invention. To achieve the objectives of miniaturization and bandwidth enhancement of the microstrip antenna of the present invention, the lengths of the slots  24 A,  24 B and  24 C are adjusted to 23 mm, 7 mm and 15.5 mm respectively, and the distance between slots  24 B and  24 C is adjusted to 4 mm. Slot  25  is symmetrical to slot  24  and is configured with the same principle. After measuring, it was found that the impedance bandwidth W 1 , determined from 10 dB return loss, of microstrip antenna apparatus configured can achieve 5.0% (96 MHz) which is approximately 3 times more bandwidth than a conventional microstrip antenna. 
     The Second Embodiment 
     FIG. 4 represents the measuring result of the return loss relative to the slot size(s) of the microstrip antenna apparatus of the present invention. In the second embodiment, the slot lengths of the first embodiment are extended. The lengths of slots  24 A,  24 B and  24 C are adjusted to 26 mm, 7 mm and 18 mm respectively, and the distance between slots  24 B and  24 C is adjusted to 5 mm. Slot  25  is symmetrical to slot  24  and is configured with the same principle described above. After measuring, it is found that the impedance bandwidth W 2 , determined from 10 dB return loss, of the microstrip antenna can achieve 5.2% (92 MHz) which is approximately 3.25 times that of a conventional microstrip antenna. 
     The Third Embodiment 
     FIG. 5 represents the measured result relative to the slot sizes of the microstrip antenna apparatus of the present invention. In the third embodiment, the slot lengths of the second embodiment are again extended. The lengths of slots  24 A,  24 B and  24 C are adjusted to 27 mm, 7.2 mm and 18.5 mm respectively, and the distance between slots  24 B and  24 C is adjusted to 6 mm. Slot  25  is symmetrical to slot  24  and is configured with the same principle described earlier. After measuring, it is found that the impedance bandwidth, determined from 10 dB return loss, of the microstrip antenna can achieves 5.3% (90 MHz) which is approximately 3.5 times that of a conventional microstrip antenna. 
     From the experimental results described above, it is demonstrated that the bandwidths (determined from 10 dB return loss) of the three embodiments respectively are: 1786 MHz˜1882 MHz for the first embodiment, 1734 MHz˜1827 MHz for the second embodiment and 1668 MHz˜1758 MHz for the third embodiment. It is noted that the bandwidths decrease sequentially. Compared with a conventional isosceles and/or equilateral-triangular microstrip antenna, the area reduction rates achieved are approximately 8.2%, 14.9% and 24.9% respectively. In other words, when the design parameters described are used in the third embodiment, the size of the equilateral-triangular patch with operating bandwidth of 5.3% can be reduced to about 75% of a conventional equilateral-triangular microstrip antenna. The contrast is even greater when compared with a conventional circular microstrip antenna whereby size can be reduced to about 60% that of the conventional circular microstrip antenna. 
     Please refer to FIGS. 6A,  6 B,  6 C and  6 D, wherein, FIGS. 6A and 6B are the measured E-plane and the H-plane radiation patterns of the microstrip antenna at the first resonant mode MT 10  shown in FIG.  3 . FIGS. 6C and 6D represent the measured results of the E-plane and H-plane radiation patterns of the microstrip antenna at the second resonant mode TM 20  shown in FIG.  3 . 
     As demonstrated by FIGS. 3,  6 A and  6 B, the resonant frequency of the first resonant mode is 1804 MHz. The bold lines E 1  and H 1  represent the measured results of the copolarized radiation patterns in the E-plane and the H-plane respectively, while the lines E 2  and H 2  represent the measured results of the crosspolarized radiation patterns in the E-plane and the H-plane respectively. FIGS. 3,  6 C and  6 D demonstrate the resonant frequency of the second resonant mode TM 20  is 1882 MHz. The bold lines represent the measured results of the copolarized radiation patterns in the E-plane and the H-plane respectively whereas the lines E 20  and H 20  represent the measured results of the crosspolarized radiation patterns in the E-plane and the H-plane respectively. 
     It can be concluded from the comparisons between FIGS. 6A,  6 B,  6 C and  6 D that the resonant mode TM 10  and the resonant mode TM 20  have similar radiation characteristics and same polarization planes. Additionally, by comparing the measured results of the crosspolarized radiation patterns of both the E-plane and the H-plane for the two resonant modes, the radiation intensities are similar. The cross-polarization levels for the two resonant modes are larger than 15 dB. 
     Therefore, from the experimental results of the embodiment herein described, the structure of the microstrip antenna of the present invention does achieve the objective of broadband operation while also achieving size reduction. The present invention can be applied to a variety of a personal mobile communication devices such as Digital Enhanced Cordless Telephones (DECT) 1800, Personal Communication Systems (PCS) 1900, or the 2.45 GHZ wireless communication modules of home RF applications. 
     While the invention has been described by way of example and in terms of the preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiments. On the contrary, it is intended to cover various modifications and similar arrangements as would be apparent to those skilled in the art. Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements, which is defined by the following claims and their equivalents.