WO1996003783A1

WO1996003783A1 - Double ring microstrip antennas

Info

Publication number: WO1996003783A1
Application number: PCT/US1995/009284
Authority: WO
Inventors: Mohamed S. Sanad
Original assignee: Wireless Access Incorporated
Priority date: 1994-07-27
Filing date: 1995-07-24
Publication date: 1996-02-08
Also published as: AU3142495A

Abstract

A double ring microstrip antenna (200) having a reduced size produces substantially isotropic radiation patterns while showing minimal degradation in performance when operated in close proximity to other electronics or the human body. The microstrip antenna (200) includes a ground plane (212) and a radiating patch (216), each made of conductive material, and a dielectric layer (214) positioned between the ground plane (212) and the radiating patch (216), which together form an antenna structure having two side edges (220, 222), a main radiating edge (220), and a short circuit edge (222). According to the invention, a short circuit provided along one of the edges (222) causes a reflection which creates a 'mirror' image of the radiating patch (216) relative to the shorted edge (222), resulting in a double ring effect. Different ring shapes are disclosed.

Description

DOUBLE RING MICROSTRIP ANTENNAS

REFERENCE TO RELATED APPLICATION This application is a continuation-in-part of the commonly assigned, co-pending patent application Serial No.

08/049 , 560 , entitled "A SMALL , DOUBLE RING MICROSTRIP ANTENNA" , by Mohamed S. Sanad, filed on April 19, 1993, the content of which is incorporated herein by reference and co-pending patent application Serial No. 08/049,514, entitled "A SMALL

MICROSTRIP ANTENNA HAVING A PARTIAL SHORT CIRCUIT", by Mohamed S.

Sanad, filed on April 19, 1993, and incorporated herein by reference.

BACKGROUND OF THE INVENTION The present invention relates to small microstrip antennas for use in electronic devices. Particularly, the invention relates to microstrip antennas suitable for use in portable electronic devices which operate in close proximity to humans or other electronic equipment while positioned in a variety of orientations, yet are small enough to fit within a credit card size pager, a portable telephone, or a portable computer.

Advances in digital and radio electronics have resulted in the production of a new breed of personal communications equipment posing special problems for antenna designers. These systems are typically small in size and may be easily carried by a user. Some, such as pagers, are designed to fit inside a user's pocket and are expected to reliably receive signals while in that position. Such placement can be a problem, as proximity of the human body to various types of antennas results in degraded radiation of the antenna. This phenomenon is sometimes termed the "body effect" and can alter an antenna's resonant frequency as well as its gain. Antennas used in such an environment need to take into account the signal-degrading effects present near a human body or the like.

Many personal communications systems are also intended to receive signals while positioned in a variety of orientations. The pager, again, is an example of such a system. To be useful, a pager should be fully capable of receiving signals while in any orientation. Thus, antennas with isotropic radiation characteristics are most effective in such environments. Such operating environments also demand that an antenna be multi-polarized in order for it to function without degradation in a wide range of orientations.

In addition, the small size of personal communications equipment dictates that any antenna used must have a small overall size and a low profile. Further, it is desirable to utilize an antenna that is inexpensive to manufacture and that has characteristics which allow it to be tuned for use in a variety of applications.

A microstrip antenna comprises a dielectric sandwiched between a conductive ground plane and a planar radiating patch. Microstrip antennas are useful alternatives for applications requiring a small and particularly thin overall size. Microstrip antennas are commonly produced in half wavelength sizes, in which there are two primary radiating edges parallel to one another. It is known that the size may be further reduced if all of one of the primary radiating edges of a half wavelength antenna is short circuited, permitting the length to be reduced to approximately a quarter wavelength. These antennas are recognized to be useful in applications requiring a low-cost small antenna which is generally linearly-polarized.

It has been found that the radiation pattern of microstrip antennas may be changed by varying the shape or "geometery" of the antenna. Many different geometries for microstrip antennas have been developed with resulting differences in radiation patterns and resonant frequencies. For example, numerous geometries are shown in the HANDBOOK OF MICROSTRIP ANTENNAS, James and Hall, eds., Peter Peregrinus Ltd., London, UK, 1989, pp. 24-39. Although conventional microstrip antennas have many advantageous characteristics, they do not meet the special demands of miniature personal communications electronics for several reasons: they are vulnerable to human body effects; they are difficult to tune without changing the overall shape or size of the dielectric or radiator; they generally do not exhibit isotropic radiation patterns; they are generally linearly-polarized; and they are typically too large to fit within small communications devices when designed for operation at frequencies of interest, typically below 1 GHz.

Accordingly, what is needed is a microstrip antenna of a design having a reduced size which has a low resistance to human body effects while exhibiting substantially isotropic radiation patterns. It would be a further advantage to provide such an antenna with advanced tuning capabilities while remaining relatively inexpensive to manufacture.

SUMMARY OF THE INVENTION According to the invention, a microstrip antenna having a reduced size is disclosed which produces substantially isotropic radiation patterns while showing minimal degradation in performance when operated in close proximity to other electronics or the human body. Specifically, according to the invention, a microstrip antenna includes a ground plane and a radiating patch, each made of conductive material, and a dielectric layer positioned between the ground plane and the radiating patch, which together form a rectangular antenna structure having two side edges, a main radiating edge, and a short circuit edge. According to the invention, a short circuit provided along one of the edges creates a "mirror" image of the radiating patch relative to the shorted edge. The short-circuited edge may be fully shorted, or in accordance with the invention may be shorted along only a portion or selected portions of the short circuit edge, relative to the wavelength, as is discussed in the co- pending patent application Serial No. 08/049,514, entitled "A SMALL MICROSTRIP ANTENNA HAVING A PARTIAL SHORT CIRCUIT", by Mohamed S. Sanad, filed on April 19, 1993, and incorporated herein by reference.

The radiating patch also includes a ring area cut¬ out from the patch, exposing a portion of the dielectric material underneath the radiating patch. The ring may have a rectangular shape, as disclosed in the co-pending patent application, Serial No. 08/049,560, entitled "A SMALL, DOUBLE RING MICROSTRIP ANTENNA", by Mohamed S. Sanad, filed on April 19, 1993, and incorporated herein by reference. The ring may also have a triangular, circular, diamond, or other shape.

In one embodiment, the ring is offset so that it is closer to the shorted edge than to the radiating edge. However, in other embodiments the ring could be centered, or shifted closer to the radiating edge. The distance between the radiating edge and the edge of the ring proximate thereto is maximized in order to reduce the length of the antenna. The desired input impedance can be selected by varying the length between the edge of the ring and the shorted side, and by varying the two dimensions between the edges of the rings and the two nonradiating edges. Adjustment of other features, such as trimming the nonradiating edges closely to the ground plane, can further enhance the isotropic nature of the radiation pattern.

In another embodiment, the antenna is formed in a shape that is compatible with Personal Computer Memory Card International Association (hereinafter, "PCMCIA") standards, including size, shape and radiation characteristics, allowing it to fit within a PCMCIA card for easy installation in a variety of types of electronic equipment. Other shapes and sizes of the antenna will allow it to be used in a wide range of communication equipment.

For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing description taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective view of a prior art quarter wavelength microstrip antenna;

Fig. 2 is a perspective view of one embodiment of a double triangular ring microstrip antenna; Fig. 3 is a perspective view of another embodiment of a double triangular ring microstrip antenna;

Fig. 4 is a perspective view of a double diamond ring microstrip antenna;

Fig. 5 is a perspective view of a double circular ring microstrip antenna;

Figs. 6A, 6B, and 6C are partial perspective views of alternative shorting schemes which may be utilized with the antennas of the present invention;

Fig. 7 is a perspective view of a microstrip antenna installed within a pager.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Fig. 1 is a view of a prior art quarter-wavelength microstrip antenna 100. The antenna includes a dielectric layer 110 sandwiched between a conductive ground plane 120 and a conductive radiating patch 130. The ground plane 120 and the radiating patch 130 are electrically connected by short circuit 140. The radiating patch 130 is energized by connection through a coaxial cable 150 to feed point 160. For the purpose of providing example dimensions, one typical configuration of an existing quarter wavelength microstrip antenna requires the use of a radiating patch 130 which is approximately 57 mm in length in order to achieve a resonant frequency of 863 MHz in free space. This resonant frequency was achieved using a dielectric 110 having a dielectric constant of 2.2, and which is 2.286 mm thick.

The size requirements of the type of antenna shown in Fig. 1 are further increased by the use of a section of exposed dielectric layer 112 which extends an additional 12 mm beyond the radiating patch 130. Also, two side edges of exposed dielectric 114a and 114b are provided, each of which is 6 mm in width. Thus, a typical quarter wavelength microstrip antenna, designed for operation at 863 MHz, is 69 mm long by 52 mm wide.

Laboratory testing of such a typical quarter wavelength antenna 100 revealed that operation of the antenna near a human body can reduce the resonant frequency from 863

MHz to approximately 854 MHz. In further testing, it has been found that the body effect causes a mismatch between the antenna and receiver, resulting in a return loss increase from -29 dB in free space to -8 dB when beside the human body. It has been found that a double rectangular ring geometery exhibits superior performance characteristics while occupying a smaller area as compared with the typical microstrip antenna configuration of Fig. 1. Such an antenna is disclosed in the co-pending parent patent application, Serial No. 08/049,560, entitled "A SMALL, DOUBLE RING MICROSTRIP ANTENNA", by Mohamed S. Sanad, filed on April 19, 1993, the content of which is incorporated herein by reference. Due to the complexity added by the mirror image formed relative to the shorted edge, no acceptable modelling methods are currently known which can accurately predict the performance of such a double ring antenna. Therefore, extensive laboratory testing and experimentation was required to develop the antennas of the present invention. Although specific embodiments are disclosed, it will become apparent to those skilled in the art that acceptable performance characteristics may be attained by choosing different dimensions and shapes. Features of antennas of the present invention will now be described by first referring to Fig. 2, in which a quarter wavelength microstrip antenna 200 having a double triangular ring 250 is shown. In a specific embodiment, the dimensions of which will be described, the microstrip antenna 200 has a resonant frequency of approximately 931.5 MHz, a frequency which is allocated for use by personal pagers. As will become apparent to those skilled in the art, the dimensions given may be scaled in order to achieve other resonant frequencies.

The microstrip antenna 200 includes a ground plane 212 formed of a conductive material. A dielectric layer 214 is positioned proximate the ground plane 212. A radiating patch 216 is formed on the side of the dielectric layer 214 opposite the ground plane 212. In a currently preferred embodiment, the dielectric layer 214 is 0.43 mm thick, and a dielectric such as Duroid 5880 is used. Duroid 5880 has a relative permittivity of approximately 2.2 and is available from Rogers Corporation, Chandler, Arizona. The radiating patch 216 preferably has a thickness of 1.0 oz./m² of copper foil. The microstrip antenna 200 includes four sides: a primary radiating edge 220 opposite a shorted edge 222; and a first side edge 224 opposite a second side edge 226. Even though the side edges 224 and 226 may be termed "nonradiating" edges, an amount of radiation will normally be emitted from them. However, that amount is small in comparison to the radiation emitted from the primary radiating edge 220. The primary radiating edge 220 is open circuited along its entire length, as is the first side edge 224 and the second side edge 226. In the embodiment shown, shorted edge 222 is short circuited along its entire length. Specifically, the shorted edge 222 couples the radiating patch layer 216 to the ground plane 212. The shorted edge 222 may comprise any conductive material.

In the embodiment shown in Fig. 2, a wrapped copper foil is utilized. The copper foil may be similar to the foil that is used to form the radiating patch 216. In another embodiment, a plurality of shorting posts is utilized. This embodiment is discussed infra in conjunction with Fig. 6. A more detailed discussion is given in co-pending patent application Serial No. 08/281,875 , entitled "SHORTING POSTS

FOR MICROSTRIP ANTENNA", by Mohamed S. Sanad, filed on 27 July 1994 , and incorporated herein by reference. In another embodiment, discussed infra in conjunction with Figs. 3 and 6, the shorted edge may comprise one or more partial short circuits.

A feed point 240 is positioned proximate to the shorted edge 222, and approximately centered within the width of the antenna L^. The position of the feed point 240 is selected so that the input impedence is 50 ohms in the intended environment of operation. The feed point 240 is, in one particular embodiment, connected to a conventional coaxial cable 242. The coaxial cable 242 is attached so that its outer conductor is coupled to the ground patch 212, and its center conductor is coupled to the radiating patch 216 at the feed point 240.

In an alternative embodiment, a microstrip feed may be utilized. This embodiment is shown and discussed infra in conjunction with Fig. 6, and is described more fully in co- pending patent application, Serial No. 08/283,064 _t entitled "MICROSTRIP ANTENNA HAVING A MICROSTRIP FEED", by Mohamed S. Sanad, filed on 29 July 1994 and incorporated herein by reference. Other conventional techniques may also be utilized in connecting the antenna.

In a preferred embodiment, the input impedence of the antenna 200 is selected to match 50 ohms, typically the characteristic impedence of the coaxial transmission line 242. A number of antenna characteristics apparent to one skilled in the art, including placement of the feed point 240 and others that will be discussed, affect the input impedence, all of which should be taken into account in tuning the antenna.

An aperture or ring 250 is positioned proximate the feed point 240. The ring 250 in one embodiment is shaped as an equilateral triangle having an edge length L . The triangle 250 is preferably positioned so that one edge is parallel to the radiating edge 220. The width 1^ of the microstrip antenna 200 is greater than the edge of the ring L.. In a preferred embodiment, for a resonant frequency of 931.5 MHz, the width 1^ is approximately 40 mm. The triangular ring 250 is positioned apart from the first side edge 224 by a length I^s, which is preferably equal to 11 mm. The triangular ring 250 is preferably centered between side edges 224 and 226. However, the relative position of the ring 250 within the radiating patch 216 can be varied to change the input impedence and size of the antenna. For example,. the ring 250 may be offset towards the shorted edge 222 to reduce the total length of the antenna. The length L_A of the microstrip antenna 200 is greater than the size of the ring 250. Preferably, the length L_A is 41.5 mm. It is presently preferred that 1>^_S1 and 1^₅₂ are each approximately 7.5 mm in length. However, any of these lengths can be varied to change the input impedence at the frequency of interest. Further, the ring 250 may be positioned anywhere within the radiating patch 216 subject to constraints as hereinafter explained. It is understood that varying the size and position of the ring within the patch will affect certain antenna operating characteristics, including the radiation pattern and the input impedence.

According to the invention, modifications in the microstrip antenna 200 reduce the size substantially with little or no reduction in gain, as compared to conventional microstrip antennas such as the one shown in Fig. 1. In the preferred embodiment, the thickness of the dielectric layer 214 (W_D) at the frequencies of interest, is between .4 mm and 1 mm. It will be apparent to those skilled in that art that the thickness _D can be increased in order to realize an increase in gain while retaining the overall size of the antenna. A technique used to increase gain is to provide an exposed dielectric section 270. The radiating patch 216 does not extend over this section 270. The ground plane layer 212 extends to the edge 220. In previous antennas, such as the one shown in Fig.

1, a large section of exposed dielectric was thought to increase gain. However, in the preferred embodiment, the length L_Q of the exposed dielectric section 270 is substantially smaller than conventional teaching would suggest. It is believed that the double ring geometry of the antenna 200 allows the length L_Q to be made much smaller without any reduction in gain. In the preferred embodiment, the length Lp is equal to 0.5 mm for an antenna designed to operate at 931.5 MHz. Conventional references suggest that this length should be substantially larger, e.g., up to 35% of the total length L_A which, if followed, would require the exposed dielectric section 270 to be about 15 mm (more than 30 times larger) in the preferred embodiment. In an alternative embodiment, the radiating patch 216 may also extend to the primary radiating edge 220, that is, Lr_j may equal zero.

Performance testing in a laboratory has shown that the preferred embodiment of the antenna 200, while occupying a smaller space than previous antennas, is particularly resistant to human body effects and produces generally isotropic radiation patterns.

In a typical quarter wavelength microstrip antenna, such as the one shown in Fig. 1, the electrical distance between the short circuited edge and the radiating edge is approximately equal to one quarter of the wavelength of the resonant frequency in the dielectric material. However, in the present invention as described in connection with the embodiment of Fig. 2, the length (L_A) of the antenna from the short circuited edge 222 to the primary radiating edge 220 is less than the quarter wavelength of a given resonant frequency. It is believed that the double ring mirroring effect and the partial or fully short-circuited edge 222, as well as the triangular shape of the ring 250, operate to allow the length L_A of the antenna 200 to be less than a quarter wavelength for a given resonant frequency. In a specific embodiment, for a resonant frequency of 931.5 MHz, the physical length L_A of the antenna is 41.5 mm, which is approximately 40% less than any known conventional microstrip antenna with an equivalent gain.

In the preferred embodiment, the width L^ is made approximately equivalent to the length L_A so that an almost square structure is provided. If the width 1^ were to be made larger, gain would increase somewhat. However, it has been found that the width 1^ could be reduced below that of the length L_A without a large reduction in gain. The relatively square structure provides advantages including convenience in installation and a reduced size. In other embodiments, the width I-v, could be made wider for more gain, but this would naturally increase the overall size of the microstrip antenna 200. Also, the width L^ could be made narrower to decrease the overall size of the microstrip antenna 200, but this would reduce gain. In the preferred embodiment, the first and second side edges 224, 226 are formed close to the edge of the ground plane and radiating patch so that the edges of the ground plane 212, the dielectric layer 214 and the radiating patch 216 are approximately flush, or evenly lined up. It is believed that when those edges are closely aligned a more isotropic radiation pattern is achieved. It has been found that the dielectric layer 214 may extend beyond the radiating patch 216 and ground plane 212 without adversely affecting the radiation pattern. However, the edges 224, 226 of the radiating patch 216 and ground plane 212 should be flush. Extending the dielectric layer 214 may be advantageous in certain installations.

Referring now to Fig. 3, a perspective view of another specific embodiment of a double triangular ring microstrip antenna 300 is shown. This embodiment is designed to operate at 940 MHz. The basic structure of the antenna is similar to the antenna shown in Fig. 2. The antenna 300 includes three layers comprising a dielectric 314 sandwiched between a ground plane 312 and a radiating patch 316.

Preferably, the dielectric is Duroid 5880 and is 0.60 mm thick. The layers are substantially rectangular in shape, having a length L_A and a width h^ which, in a specific embodiment designed for operation at 940 MHz, are equal to 43.5 mm and 40 mm respectively. The antenna 300 has four outside edges: a primary radiating edge 320; first and second side edges 324, 326; and a short circuited edge 322. In the embodiment shown in Fig. 3, the short circuited edge 322 comprises a partial short circuit utilizing a length of wrapped copper foil. As will be discussed in more detail in conjunction with Fig. 6, other shorting approaches may also be utilized.

The antenna of Fig. 3 includes a triangular shaped aperture or ring 350 centrally positioned in the radiating patch 316. The triangular shaped ring 350 exposes a portion of dielectric layer 314. Antenna 300 operates in a manner similar to the operation of the antenna of Fig. 2 in that a "mirror" image of the radiating patch 316 is created relative to the short circuited edge 322. Electrically, the antenna 300 appears to be an antenna with a double triangular ring.

In this embodiment, the short circuit edge 322 is a partial short circuit made of wrapped copper foil. Preferably, the shorted section has a length L_s greater than

1/3 of the total width of the antenna L^ and is centered along the short circuit edge 322. This leaves two open circuit exposed sections of dielectric 360, 380 having lengths of Lo_S1 and I-os_2' These exposed sections preferably have a depth Lo_D of 2 mm.

In one embodiment, the triangular ring 350 is an equilateral triangle, having side lengths of approximately 25 mm. The triangular ring 350 is positioned on the radiating patch such that one of the sides of the triangle is substantially parallel to the shorted edge 322. Satisfactory results have been attained by positioning the equilateral triangle about 11 mm from the primary radiating edge 320. It is understood that the size and relative position of the triangle may be varied in order to achieve different resonant frequencies and gains as well as to increase or decrease the input impedance of the antenna 300. For instance, the triangle may be positioned such that one edge is parallel to the primary radiating edge 320. Tests have shown that the double triangular ring antenna 300 of Fig. 3 achieves results similar to the double triangular ring antenna 200 of Fig. 2 but at a different resonant frequency.

While an equilateral triangle ring shape with one side parallel to the shorted edge is considered preferable, other shapes have heretofore unappreciated advantages. For example, referring now to Fig. 4, a perspective view of a double diamond ring microstrip antenna 400 is shown. Again, the primary dimensions of the antenna are, preferably, similar to those given in conjunction with the discussion of Figs. 2 and 3. In this embodiment, however, a diamond shaped ring 450 is formed in the radiating patch 416, exposing a portion of the dielectric layer 414. The diamond shaped ring 450 has a height Lr_jH and a width L^, and is preferably centered between side edges 424 and 426. In a specific embodiment, the diamond shaped ring 450 has an edge length L^ of about 16.5 mm, and a corner 451 spaced apart from the primary radiating edge 420. Experimentation has shown that the area of the ring 450 should be approximately the same as the area of the triangular ring 350 in order to achieve the best results. Those skilled in the art will recognize that varying the size, shape, orientation and position of the diamond shaped ring 450 will affect the performance characteristics of the antenna 400.

Fig. 5 is a perspective view of a microstrip antenna 500 with a circular ring 550 according to the invention. In a preferred embodiment, the external shape and dimensions of the antenna are similar to those of antennas 200, 300, and 400 of Figs. 2, 3 and 4. In the antenna of Fig. 5, however, a circular ring 550 is formed in the radiating patch 516 exposing a portion of the dielectric layer 514. The circular shape of ring 550 has no straight boundaries, a center 560 and a radius R_c. In one particular embodiment, the center is positioned midway between side edges 524 and 526, and about 20 mm from the shorted edge 522. Preferably, the radius R_c of the circle is about 9.5 mm. Tests have shown that this embodiment of a microstrip antenna performs satisfactory when the area of the circle 550 approaches the area of the triangular ring 350.

Each of the double ring microstrip antennas described herein may be fabricated with an insulative solder mask which serves to insulate and protect the antenna. Particularly, the conductors on the antennas are covered with a thin (e.g., ≤l mm thick) plastic insulator that isolates the antenna from direct contact with surrounding circuit components such as batteries. The solder mask also serves to protect the copper in the conductive layers from oxidizing or otherwise corroding.

Each of the double ring microstrip antennas described herein may also be fabricated with a variety of shorting and feed schemes, shown in Figs. 6A-6C. Referring now to Fig. 6A, an alternative approach to shorting edge 522 is shown. In this embodiment, the short circuited edge 522 comprises a partial short circuit utilizing two partially shorted sections 523 and 524. Each section has a length L_S1 and L_S2, the sum of which is less than the width of the antenna 1^ by an amount L_QS. Preferably, the lengths L_S1 and L_S2 are equal. In the embodiment shown, a plurality of shorting posts 560a-560d are used to short the radiating patch 516 to the ground plane 512. The shorting posts are formed of a conductive material, preferably copper, and may be formed by drilling or other means well known in the art. In a preferred embodiment, the lengths L_S1, L_S2 of short circuit sections 523 and 524 are equal to about 8 mm on each side, where the overall width of the antenna L^ is 40 mm. The relative sizes of the shorted sections 523 and 524 may be selected to establish a selected resonant frequency and gain of the antenna. In one embodiment, two shorting posts are evenly spaced within short circuit sections 523 and 524. It has been found that the use of a greater number of shorting posts directly impacts the resonant frequency of the antenna.

Referring now to Fig. 6B, a partial perspective view of an alternative shorting scheme is shown. In this embodiment, a single partial short circuit section 523 is provided. Short circuit section 523 is constructed, in one embodiment, from a wrapped section of copper sheeting which electrically connects the radiating patch 516 with the ground plane 512. Preferably, section 523 is about 13 mm in length when the width of the antenna (1^) is 40 mm, or about 1/3 the length of the edge. In each of Figs. 6A and 6B, a coaxial feed 542 is shown. An alternative embodiment is shown in Fig. 6C, where a microstrip feed 543 is utilized in conjunction with two partial short circuit sections 523 and 524. Those skilled in the art will recognize that the double ring effect may be achieved utilizing any combination of the above- mentioned shorting and feed schemes. It is believed that the total length of all shorted section(s) , whether connected directly or not, must be sufficient to provide a mirror image of the radiating patch in the ground plane. Therefore, the total length of the shorted section(s) should not be reduced below that length which provides such an adequate mirror image. The width of the shorted section(s) is chosen primarily to satisfy the required input impedence of 50 ohms. It has been observed that changing the width of the shorted section(s) affects the input impedence, and therefore varying the width L_s as a percentage of the total width 1_*^ can be useful in tuning the antenna. However, it has been experimentally observed that if the width L_s is decreased below, for example 10% of 1^, then the antenna does not perform efficiently, or it may not work at all. If the length L_s of the short is reduced to zero, the antenna's properties will shift to those of a half wavelength antenna. On the other hand, if the length L_s is increased above, for example, 90% of 1^, then the partially shorted microstrip antenna will begin to assume the properties of a fully shorted quarter wavelength antenna. Therefore, the length L_s should be between 10% and 90% of the value of 1^. It is currently preferred that the length L_s of the short circuit is within the range of 20% to 50% of the length 1^ of the entire short circuited edge 522. The currently preferred length L_s is approximately 30% of 1^.

Referring now to Fig. 7 a microstrip antenna 600 is shown installed within a pager 602. Fig. 7 illustrates a pager 602 having a microstrip antenna 600 with a first side edge 606 facing outward. Other installation configurations may be utilized depending upon the particular circuit configuration of the pager 602 and other factors. For example, the antenna 600 may be positioned in an orientation which provides the best radiation pattern, or which produces the least amount of interference with surrounding electronics. As will be appreciated by those familiar with the art, the present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. For example, a double ring microstrip antenna may be constructed using a full or a partial short. The short may be implemented using shorting posts or wrapped copper foil. Further, any size or shape of ring or antenna may be used. Any of the antennas developed according to the present invention may utilize a microstrip feed rather than a coaxial feed. Those skilled in the art will also appreciate the wide ability to tune the antennas of the present invention by modifying certain features such as the size or shape of the ring, or the number or placement of shorting posts. It is believed that, based upon the forgoing disclosure, those of skill in the art will now be able to produce double ring microstrip antennas having different performance characteristics by modifying the dimensions and scaling of the preferred embodiment. Further, It is apparent that the present invention may be utilized in a wide range of applications, from pagers to portable computers.

Accordingly, the disclosure of the invention is intended to be illustrative, but not limiting, of the scope of the invention which is set forth in the following claims.

Claims

WHAT IS CLAIMED IS;

I Claim: 1. A microstrip antenna comprising: a ground plane of an electrically-conductive material; a radiating patch of an electrically-conductive material; a dielectric layer positioned between the ground plane and the radiating patch; shorting means positioned adjacent an edge of the ground plane and the radiating patch for directly electrically coupling a selected segment of the radiating patch to the ground plane; said ground plane and said radiating patch defining a substantially co-extensive overlaid rectangular structure including a base edge, a radiating edge positioned opposite the first edge, a first side edge, and a second side edge positioned opposite said first side edge; and wherein said radiating patch defines a ring of a shape formed within a perimeter defined by said radiating patch, a portion of said dielectric layer being exposed by said ring.

2. The microstrip antenna of claim 1 wherein said shape of said ring is a triangle having a vertex nearest the radiating edge.

3. The microstrip antenna of claim 1 wherein said shape of said ring is a circle.

4. The microstrip antenna of claim 1 wherein said shape of said ring is a diamond having a corner nearest the radiating edge.

5. The microstrip antenna of claim 2 wherein said triangle is an equilateral triangle, positioned so that a base of said triangle is substantially parallel to said base edge of said antenna, and is spaced apart therefrom.

6. The microstrip antenna of claim 1 wherein said shorting means comprises a copper wrap extending the length of said base edge.

7. The microstrip antenna of claim 1 wherein said shorting means comprises a copper wrap extending a portion of the length of said base edge; wherein a partial short circuit is formed.

8. The microstrip antenna of claim 1 wherein said shorting means comprises a plurality of shorting posts electrically connecting said radiating patch with said ground plane.

9. A partially shorted double ring microstrip antenna comprising: a ground plane comprising a conductive material; a radiating patch comprising a conductive material and having a base edge; a dielectric layer positioned between the ground plane and the radiating patch; shorting means positioned adjacent said base edge for directly electrically coupling only a portion of the radiating patch to the ground plane; wherein said double ring microstrip antenna defines a rectangular structure including said base edge, a radiating edge positioned opposite said base edge, a first side edge, and a second side edge positioned opposite said first side edge; and wherein said radiating patch defines a ring of a shape formed within a perimeter defined by said radiating patch, a portion of said dielectric layer being exposed by said ring.

10. The microstrip antenna of claim 9 wherein said portion of said base edge has a length between 10% to 90% of the total length of said first edge.

11. The microstrip antenna of claim 9 wherein said shorting means comprises copper wrap.

12. The microstrip antenna of claim 9 wherein said shorting means comprises a plurality of shorting posts.

13. The microstrip antenna of claim 9 further comprising feed means, coupled to said radiating patch, for feeding a signal to said radiating patch.

14. The microstrip antenna of claim 13 wherein said feed means comprises a coaxial cable.

15. The microstrip antenna of claim 13 wherein said feed means comprises a microstrip.

16. The microstrip antenna of claim 9 wherein said shape of said ring is a triangle.

17. The microstrip antenna of claim 9 wherein said shape of said ring is a circle.

18. The microstrip antenna of claim 9 wherein said shape of said ring is a diamond shape.