Patent Publication Number: US-6992630-B2

Title: Annular ring antenna

Description:
FIELD OF THE INVENTION 
   The present invention relates to the field of antennas, and more particularly, this invention relates to a radiating planar or printed antenna that is configured to enhance the gain relative to its area. 
   BACKGROUND OF THE INVENTION 
   Newer designs and manufacturing techniques have driven electronic components to small dimensions and miniaturized many communication devices and systems. Unfortunately, antennas have not been reduced in size at a comparative level and often are one of the larger components used in a smaller communications device. In those communication applications at below 6 GHz frequencies, the antennas become increasingly larger. At very low frequencies, for example, used by submarines or other low frequency communication systems, the antennas become very large, which is unacceptable. It becomes increasingly important in these communication applications to reduce not only antenna size, but also to design and manufacture a reduced size antenna having the greatest gain for the smallest area. 
   In current, everyday communications devices, many different types of patch antennas, loaded whips, copper springs (coils and pancakes) and dipoles are used in a variety of different ways. These antennas, however, are sometimes large and impractical for a specific application. 
   Simple flat or patch antennas can be manufactured at low costs and have been developed as antennas for the mobile communication field. The flat antenna or thin antenna is configured, for example, by disposing a patch conductor cut to a predetermined size over a grounded conductive plate through a dielectric material. This structure allows an antenna with high sensitivity over several GHz RF waves to be fabricated in a relatively simple structure. Such an antenna can be easily mounted to appliances, such as a printed circuit board (PCB). However, none of these approaches focused on reducing the size antenna while providing the greatest gain for the smallest area. 
   SUMMARY OF THE INVENTION 
   In view of the foregoing background, it is therefore an object of the present invention to provide a radiating planar or printed antenna that is configured to enhance the gain relative to its area. 
   This and other objects, features, and advantages in accordance with the present invention are provided by an antenna including a substrate, such as a dielectric material, and an electrically conductive circular ring on the substrate and having an outer diameter and an inner diameter concentrically arranged. The outer diameter is less than 1/10 an operating wavelength, and preferably about 1/20 th , so that the antenna is electrically small relative to the wavelength. The inner diameter is in a range of π/6 to π/2 times the outer diameter, and preferably is π/4 times the outer diameter. 
   The electrically conductive circular ring may have at least one gap therein, and may have first and second circumferentially spaced gaps therein. The first gap defines feed points for the antenna, and a tuning feature, such as a capacitive element, is associated with the second gap. The first and second gaps are preferably diametrically opposed. Alternatively, a magnetically coupled feed ring may be provided within the electrically conductive ring. The magnetically coupled feed ring has a gap therein, to define feed points therefor, and diametrically opposite a gap in the electrically conductive circular ring. Also, an outer shield ring may surrounding the electrically conductive ring and be spaced therefrom. The shield ring has a third gap therein. Furthermore, a feed structure, such as a printed feed line or coaxial feed line, is provided to feed the antenna. 
   A method aspect of the invention includes making an antenna by forming an electrically conductive circular ring on a substrate including forming an outer diameter of the electrically conductive circular ring to be less than 1/10 an operating wavelength so that the antenna is electrically small relative to the wavelength, and forming an inner diameter of the electrically conductive circular ring to be in a range of π/6 to π/2 times the outer diameter. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram of a loop antenna according to a first embodiment of the present invention. 
       FIG. 2  is a schematic diagram of an annular antenna according to another embodiment of the present invention. 
       FIG. 3  is a schematic diagram of an annular antenna including a magnetic coupler according to another embodiment of the present invention. 
       FIG. 4  is a schematic diagram of an annular antenna including a shield ring according to another embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout, and prime notation is used to indicate similar elements in alternative embodiments. 
   The present invention is directed to a thin patch antenna that has the greatest possible gain for the smallest possible area, such as can be used as a wireless local area network (WLAN) antenna in a personal computer or personal digital assistant (PDA). The various embodiments of the antenna can also be used in security, tracking or identification tags, cell phones and any other device that requires a small printed antenna. The antenna is an inductor-type antenna and is planar or “2½ dimensional” as it has some minimal thickness. The antenna is annular or circular in geometry to obtain the maximum area for the minimum diameter while providing the optimal conductor surface. 
   Referring initially to  FIG. 1 , a first embodiment of an antenna  10  according to the present invention will be described. The antenna  10  includes an electrically conductive circular ring  12  on a substrate  14  and can be considered a loop antenna having about a one-half wavelength circumference in natural resonance. An inner diameter is in a range of π/6 to π/2 times the outer diameter, and preferably is π/4 times the outer diameter to enhance antenna gain relative to its area. The outer diameter is about λ/2π. Such an antenna  10  can be used as a radar reflector or proximity sensor, for example. 
   Referring now to  FIG. 2 , another embodiment of an antenna  10 ′ according to the present invention will be described. The antenna  10 ′ again includes an electrically conductive circular ring  12 ′ on a substrate (not illustrated) and is an electrically small antenna that needs to be forced to resonance via a feed structure. In this embodiment, the outer diamter is is less than one-tenth ( 1/10) of the wavelength λ and is preferably about one-twentieth ( 1/20) of the wavelength. Again, the inner diameter is in a range of π/6 to π/2 times the outer diameter, and preferably is π/4 times the outer diameter to enhance its gain relative to its area. 
   The electrically conductive circular ring  12 ′ includes a capacitive element  16 ′ or tuning feature as part of its ring structure and preferably located diametrically opposite to where the antenna is fed, for forcing/tuning the electrically conductive circular ring  12 ′ to resonance. Such a capacitive element  16 ′ may be a discrete device, such as a trimmer capacitor, or a gap, in the electrically conductive circular ring  12 ′, with capacitive coupling. Such a gap would be small to impart the desired capacitance and establish the desired resonance. The electrically conductive circular ring  12 ′ also includes a driving or feed point  18 ′ which is also defined by a gap in the electrically conductive circular ring  12 ′. Furthermore, a feed structure, such as a printed feed line or coaxial feed line, for example a 50 ohm coaxial cable, is provided to feed the antenna, as would be appreciated by the skilled artisan. 
   Alternatively, in reference to  FIG. 3 , another embodiment of the antenna  10 ″ will be described. Here, the antenna  10 ″ includes a magnetically coupled feed ring  20 ″ provided within the electrically conductive ring  12 ″. The magnetically coupled feed ring  20 ″ has a gap therein, to define feed points  18 ″ therefor, and diametrically opposite the capacitive element  16 ″ or gap in the electrically conductive circular ring  12 ″. In this embodiment, the inner magnetically coupled feed ring  20 ″ acts as a broadband coupler and is non-resonant. The outer electrically conductive ring  12 ″ is resonant and radiates. 
   Also, with reference to the embodiment illustrated in  FIG. 4 , an outer shield ring  22 ′″ may surround the electrically conductive ring  12 ′″ and be spaced therefrom. The shield ring  22 ′″ has a third gap  24 ′″ therein. The outer shield ring  22 ′″ and the electrically conductive ring  12 ′″ both radiate and act as differential-type loading capacitors to each other. The distributed capacitance between the outer shield ring  22 ′″ and the electrically conductive ring  12 ′″ stabilizes tuning by shielding electromagnetic fields from adjacent dielectrics, people, structures, etc. Furthermore, additional shield rings  22 ′″ could be added to increase the frequency bands and bandwidth. 
   A method aspect of the invention includes making an antenna  10 ′,  10 ″,  10 ′″ by forming an electrically conductive circular ring  12 ′,  12 ″,  12 ′″ on a substrate  14 ′,  14 ″,  14 ′″ including forming an outer diameter of the electrically conductive circular ring to be less than 1/10 an operating wavelength so that the antenna is electrically small relative to the wavelength, and forming an inner diameter of the electrically conductive circular ring to be in a range of π/6 to π/2 times the outer diameter. 
   Again, the outer diameter is preferably about 1/20 th  of lambda, and the inner diameter is preferably π/4 times the outer diameter. At least one gap  16 ′ may be formed in the electrically conductive circular ring  12 ′. Also, first and second circumferentially spaced gaps  16 ′,  18 ′ may be formed in the electrically conductive circular ring  12 ′, wherein the first gap  18 ′ defines feed points for the antenna  10 ′, and at least one tuning feature is associated with the second gap  16 ′. Here, the first and second gaps  16 ′,  18 ′ are diametrically opposed. 
   A magnetically coupled feed ring  20 ″ may be formed within the electrically conductive ring  12 ″. Here, the magnetically coupled feed ring  20 ″ has the second gap  18 ″ therein diametrically opposite the first gap  16 ″ to define feed points therefor. Additonally, an outer shield ring  22 ′″ may be formed to surround the electrically conductive ring  12 ′″ and spaced therefrom. The shield ring  22 ′″ has a third gap  24 ′″ therein. In each of the embodiments, the substrate  14  preferably comprises a dielectric material, and a feed structure, such as a printed feed line or a coaxial feed line, would be provided to feed the antenna  10  as would be appreciated by the skilled artisan. 
   A non-limiting example of the annular antenna of the present invention is now described. A copper annualr ring antenna of less than 1/20 wavelengths in diameter can operate at a gain of 1 dBi, which is an efficiency of 85 percent. This antenna is implemented in copper at about 1000 MHz. This is the fundamental form of the antenna as a transducer of electromagnetic waves, in that a circle provides the greatest surface area for minimum diameter. 
   This very small and efficient annular antenna design of the present invention can be used in many different wireless products, including radio frequency communications and broadcasts including common consumer electronic applications, such as cell phones, pagers, wide local area network cards, GSM/land mobile communications, TV antennas, and high frequency radio systems. It can also be used in exotic applications, including VLF, GWEN, EMP weapons, ID tags, land mines and medical devices. 
   Many modifications and other embodiments of the invention will come to the mind of one skilled in the art having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is understood that the invention is not to be limited to the specific embodiments disclosed, and that modifications and embodiments are intended to be included within the scope of the appended claims.