COMPACT WIDEBAND ANTENNA

Abstract: A broadband, compact, omnidirectional antenna (10) usable in the 800-1000 MHz frequency range incorporates an elongated, symmetrical, radiating stud (26) which terminates in a cylindrical metallic radiating cap (22). A cylindrical, non-conductive hollow housing (32) with first and second ends (30a, 30b) surrounds the stud and is closed at one end by the radiating cap. Over all, the length of the antenna is less than a corresponding quarter wave monopole while providing better performance characteristics, and, greater structural stability then metal or whip-type antennas.

Description: COMPACT WIDEBAND ANTENNA Field of the Invention: The invention pertains to antennas usable at 800MHz and 900 MHz radiating frequencies More particularly, the invention pertains to compact, omnidirectional, broadband antennas usable in multiple bands encompassing 800-1000 MHz service. Background of the Invention: Various types of mobile, radio telephone-type service in the 800-900 MHz frequency spectrum have become very popular with numerous subscribers world- wide These systems have produced several different omnidirectional antenna designs Known antennas often have an exposed metal rod or whip which might be shortened by a loading coil Such antennas have either a direct connection to an RF feed, necessitating a hole in the respective vehicle, if vehicle mounted, or a "through the glass" capacitive-type coupling between an RF feed and a radiator One such antenna has been disclosed in US Patent No 5,742,255 owned by the assignee hereof While generally useful for their intended purposes, known whip antennas tend to be bendable or breakable even when the vehicle is only driven through a car wash This has been a long term inconvenience to users Another issue involves bandwidth, and impedance characteristics in view of the proliferating number of services operating in different bands It is inconvenient and expensive to have to manufacture, stock and install different antennas for different services There continues to be a need for structurally stable, omnidirectional antennas usable in the 800- 1000 MHz frequency range Preferably, such antennas would provide better multi-band performance than is currently available while at the same time alleviating some of the structural weaknesses of known whip-type antennas Summary of the Invention: A mobile or fixed station antenna has a compact profile and a wide instantaneous operating bandwidth The antenna enhances performance by enabling the transmission of signals over a very broad frequency bandwidth In most cases, a single compact wideband antenna in accordance with the present invention provides supeπor antenna performance across multiple communication system bands, such as cellular AMPS, 800 MHz special mobile radio (SMR), 900 MHz ISM, and 900 MHz GSM Enhanced performance is achieved as a result of a compact, rugged, physical package that may be shorter than a standard quarter wave monopole The antenna does not have an exposed metal rod, or whip, as is standard for mobile antennas In one embodiment, the conductive portion of the antenna is either enclosed in a protective housing, or is itself part of the housing enclosure The antenna includes a non-conductive protective housing which encloses, at least in part, a vertical conductive mast In another embodiment, a contact at the antenna's base transfers energy from the base connector to the vertical conductive assembly Various types of RF connectors could be used The antenna can be coupled to industry standard mobile antenna mounts This allows for easy replacement of existing hardware The antenna radiator includes a conductive, elongated element of appropriate length for the desired operating frequency A conductive disc is connected to a distal end thereof In one embodiment, the element is rod-like In another embodiment, the element includes a helical coil with axially extending leads In either embodiment, the element has an over-all length less than one-quarter wavelength of a selected operating frequency In either embodiment, the respective radiator is substantially self- supporting with the radiator extending from an RF-type connector In neither embodiment does the radiator need axial support from an axially extending support such as an axially oriented printed circuit board The conductive disc serves dual electrical and mechanical functions Electrically, the disc functions as the top portion of the radiating antenna assembly Mechanically, it serves as the top cap for the compact wideband antenna assembly In another aspect, a metal ring at the base of the non-conductive housing can be used to fine-tune antenna performance In yet another embodiment, antenna impedance can be tuned using a fixed capacitor coupled between the radiator and a ground Numerous other advantages and features of the present invention will become readily apparent from the following detailed description of the invention and the embodiments thereof, from the claims and from the accompanying drawings Brief Description of the Drawings: Fig 1 illustrates an exploded view of a compact wideband antenna in accordance with the present invention, Fig 2 is a side view of certain structural component of the preferred embodiment of the present invention, Fig 3 is a side view of an alternate embodiment of antenna of the present invention, Fig 4 A, 4B together illustrate exploded and assembled views of another embodiment of the present invention, Fig 5 illustrates measured comparative VSWR plots vs frequency over a 900-2900 MHz range, Fig 6 is a plot of measured VSWR over an 800-1000 MHz range, Fig 7 illustrates comparative plots of antenna gain over an 770-970 MHz range, Fig 8 illustrates measured VSWR over a 400-600 MHz range, Fig 9 illustrates an assembled view of a dualband embodiment of the present invention, and Fig 10 is a plot of measured VSWR of the dualband embodiment over an 800-2000 MHz range Detailed Description of the Preferred Embodiments: While this invention is susceptible of embodiment in many different forms, there are shown in the drawing and will be described herein in detail specific embodiments thereof with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the invention to the specific embodiments illustrated Referring to Figs 1 -2 of the drawings, a compact wideband antenna assembly 10 is illustrated located on a conductive ground plane 12 The compact antenna assembly can be threaded onto one of the standard mobile antenna mounts 16 used by the mobile industry In addition, the assembly 10 can also be mounted on other connector bases for use as a high performance fixed station antenna A signal source feedline 18, typically a coaxial cable, can be used to energize the assembly 10 The assembly 10 includes a disc shaped metal radiator 22 which contains a central bore 22a which receives an end 26a of elongated radiator 26. A proximal end 26b engages mount 16. A non-conductive, hollow, molded housing 30 surrounds radiator 26. A distal end 30a abuts adjacent cap or radiator 22. A proximal end 30b slidably engages an optional, annular metal ring 32 which can be incorporated to improve performance. The non-conductive housing 30 serves to encapsulate the vertical conductive mast 26 and to provide mechanical support to the assembly 10 The vertical conductive mast 26 is attached to the cap 22. For ease and simplicity of assembly, one attachment method is to screw the threaded mast 26 into a tapped hole 22a in the bottom of the cap 22 In the preferred embodiment shown in Figs 1, 2, the metal base ring 32 provides fine tuning of the antenna impedance As illustrated in the embodiment 10a of Fig. 3, the ring 32 can also be made from a non-conductive material such as PVC In this embodiment, the entire housing 30', as shown in Fig 3 can be formed of a single molded member. A washer 34 supports the conductive mast 26 and provides the correct physical alignment In the preferred embodiment, a spring-loaded contact 26c provides contact between the mount 16 and the vertical mast 26 The contact 26c could also be formed as a leaf spring, or other conductive device The conductive cap 22 serves a dual mechanical and electrical function The mechanical function of the cap 22 is to act as the top enclosure of the assembly 10 and to provide mechanical strength to the assembly 10. In its electrical capacity, the metallic cap 22 provides a capacitive load, thereby reducing the physical height of the assembly 10 The electrical loading created by the cap 22 provides a more stable antenna impedance as a function of frequency than standard vertical monopole antennas In comparison to a standard vertical monopole, the impedance bandwidth of the wideband antenna 10 is more than twice as great In one alternate embodiment 10b illustrated in Fig. 4A, 4B, the cylindrical vertical conductive mast 26 is replaced by a helical wound coil 46 with top and bottom leads 46a, b In the embodiments of Figs 1 -4B, the length of the vertical element is less than one quarter wavelength at the center of the frequency band A tuning capacitor 48 may be used to match the antenna impedance to fifty ohms. The tuning capacitor 48 leads are connected between a vertical element contact ring 50a and a ground contact ring 50b As noted above, the effective length of the coil 46 and leads 46a, b is less than one-quarter wavelength of a selected frequency The diameter of coil 46 is selected to optimize antenna performance In comparison to a vertical monopole, the impedance bandwidth of the antenna 10, 10a, 10b, can be significantly greater Fig 5 illustrates comparative VSWR plots of the antenna 10 and a quarter wave monopole The antenna 10 has a VSWR less than 1 6- 1 over a frequency range from 990 to 2880 MHz, greater than 97% bandwidth The quarter wave monopole 1 6 1 VSWR bandwidth is only in a range of 1950 MHz to under 2300 MHz The antenna 10 tuned for the cellular AMPS frequency band has an impedance bandwidth that ranges from 775 MHz to 960 MHz at a VSWR of better than 1.6 1, see Fig 6 This antenna is less than 6 1 cm tall, 30% shorter than a quarter wavelength monopole (quarter wavelength equals 8 72 cm at 860 MHz) The present antenna is also a more efficient wideband radiator than a quarter wave monopole, translating into a flatter gain response versus frequency Fig 7 illustrates comparative measured relative antenna gain for the antenna 10 and a quarter wave monopole The gain of antennas in accordance with the present invention is substantially constant over 805-935 MHz band As illustrated in Fig 8, an antenna 10 tuned for the UHF frequency band exhibited an impedance bandwidth of 465 MHz to 512 MHz with a VSWR less than 1 7 1 This wide bandwidth performance was achieved with an antenna 8 3 cm tall, or 0 13 wavelengths The antenna 10 can be optimized for dualband cellular AMPS (824-896 MHz) and PCS (1850-1900 MHz) operation Fig 9 illustrates the dimensions of one dualband implementation of the antenna 10c The overall height of the antenna less than 4 6 cm. A 100 pf capacitor 51 , coupled between radiator 26 and ground, is used to optimize the antenna impedance in the two frequency bands Fig 10 illustrates the measured VSWR of the antenna 10c The peak VSWR is less than 1 85 1 in the AMPS band, and less than 1 7 1 in the PCS band Overall antenna height is less than one eighth of one wavelength a frequency of 824 MHz In summary, an antenna assembly, such as antenna 10 can be expected to achieve superior wideband performance in both radiation efficiency and impedance bandwidth Furthermore, antennas such as antenna 10, exhibit the following performance features 1) Radiation efficiency exceeding a quarter wave monopole, 2) A rugged compact antenna assembly, 3) A physical height less than 1/6 of a wavelength in the cellular AMPS band, 4) A radiation pattern well suited for mobile or fixed station applications where the primary antenna radiation is directed along the horizon, with a minimum concentration of energy at the zenith, and 5) Low Q, wideband resonant operation, without the requirement for lumped electrical components such as capacitors, inductors, or resistors From the proceeding, it is clear that a wideband antenna as disclosed, provides a novel means for a mobile or fixed station antenna with extremely wideband performance in a very rugged, durable package From the foregoing, it will be observed that numerous variations and modifications may be effected without departing from the spirit and scope of the invention It is to be understood that no limitation with respect to the specific apparatus illustrated herein is intended or should be inferred It is, of course, intended to cover by the appended claims all such modifications as fall within the scope of the claims

Claims: What is Claimed: I An antenna comprising an RF feed element; a metal stud of a selected length coupled to the feed element at one end, and having a second, displaced, end; and a radiating cap, coupled to the second end and oriented substantially perpendicular to the stud. 2. An antenna as in claim 1 wherein the stud has a length parameter selected in accordance with a predetermined radiating frequency 3 An antenna as in claim 1 wherein the cap is symmetrical about a centerline 4 An antenna as in claim 3 wherein the cap is cylindrical 5 An antenna as in claim 1 which includes a housing which surrounds the stud and which has an end closed by the cap 6 An antenna as in claim 5 wherein the cap comprises a metal radiator and the housing comprises a molded enclosure formed of a resin 7 An antenna as in claim 6 which includes a metallic ring carried at a second, displaced, end of the housing 8 An antenna as in claim 3 wherein the stud is symmetrical about the centerline 9 An antenna as in claim 8 wherein the stud and the cap are substantially cylindrical 10 An antenna as in claim 1 wherein the stud includes an axially oriented helical coil I I An antenna as in claim 1 wherein the stud is self-supporting 12 An antenna as in claim 1 wherein the stud is substantially rigid and not flexible. 13 An omnidirectional, broadband, low profile antenna comprising a non-conductive, hollow, cylindrical body having first and second spaced apart ends, an elongated, self-supporting radiator element extending axially within the body, and a symmetrical radiating cap, coupled to an end of the element and to the body closing an end thereof 14. An antenna as in claim 13 which includes an RF port, coupled to another end of the element adjacent to another end of the body 15. A antenna as in claim 13 wherein the cap is substantially cylindrical. 16. An antenna as in claim 13 wherein the body, the element and the cap are symmetrical relative to a common centerline 17 An antenna as in claim 13 wherein the radiator element has a length selected such that an overall antenna height selected in accordance with a predetermined radiating frequency is less than a height parameter of a quarter wavelength monopole antenna of the same frequency 18. An antenna as in claim 13 wherein the radiator element has a length selected such that an overall height selected in accordance with a predetermined radiating frequency provides a bandwidth that extends across at least a selected 150 MHz wide radio communications band 19 An antenna as in claim 18 wherein the radiator element includes a helical coil. 20 An omnidirectional antenna comprising a mounting element for attachment with a substantially horizontal orientation to a mounting surface wherein the mounting element carries an RF connector; an elongated radiator, carried by and oriented so as to be perpendicular to at least part of the mounting element, a cylindrical, non-conductive housing carried by the mounting element, which surrounds the radiator and extends from the mounting element with a distal open end, a disc-shaped radiator section, coupled to an end of the elongated radiator, oriented so as to be substantially perpendicular thereto, covering the distal open end of the housing wherein a substantially constant antenna gain is exhibited over a bandwidth on the order of 150 MHz 21 An antenna as in claim 20 which exhibits a VSWR < 1 5 1 over the bandwidth 22 An antenna as in claim 20 wherein the elongated radiator includes a helical coil, having first and second ends, located within the housing 23 An antenna as in claim 20 which includes a tuning capacitor 24. An antenna as in claim 20 wherein the elongated radiator is self- standing and has a length less than one-quarter of a selected wavelength 25 An antenna as in claim 22 wherein the coil includes first and second leads with each lead attached to a respective coil end and wherein the coil and associated leads have an over-all length less than one-quarter of a selected wavelength 26 An antenna as in claim 23 wherein a value of the capacitor is selected to optimize an antenna impedance characteristic in accordance with a selected criterion 27 An antenna as in claim 20 wherein the elongated radiator has a diameter selected to optimize performance relative to a selected criterion at a selected frequency 28 An antenna as in claim 22 wherein the elongated radiator has a diameter selected to optimize performance relative to a selected criterion at a selected frequency. 29 An antenna as in claim 22 wherein the radiator is self- supporting 30 A dualband antenna comprising a non-conductive, hollow cylindrical housing having first and second open ends, an RF feed element carried by the housing adjacent to one of the ends; an elongated metal radiator, coupled to the feed element and extending axially within the housing, a radiating disk, coupled to a distal end of the radiator and oriented to close the other end of the housing 31. An antenna as in claim 30 which includes a variable impedance element, coupled at least to the radiator. 32. An antenna as in claim 31 wherein the element comprises an impedance optimizing capacitor. 33. An antenna as in claim 30 sized to provide a peak VSWR less than 1.95:1 in one band and less than 1.8: 1 in a higher frequency band.

Inventors: MANNION MARTIN J, MANNION, MARTIN, J.
Applicants: MAXRAD INC, MAXRAD, INC.
Application year: 2000
Main classification: H01Q1
IPC codes: H01Q1, H01Q5, H01Q9