The present invention relates to antenna devices, and, more particularly to dual or multiple band antenna devices adapted for mounting on glass or similar dielectric surfaces.
The invention has been developed primarily for use in PDS, CDMA, TDMA, AMPS and GSM telecommunication system and will be described herein after with respect to that application. However, the invention is not limited to that particular field of use and is also applicable to in-vehicle and other portable and stationary applications, other telecommunications systems as well as GPS navigation systems.
Vehicle mounted antenna devices of various sorts have been used to receive UHF and other band signals for many years. One known device utilises external whip antennas which are capacitively or inductively linked to a coupling unit inside the glass, which in turn is linked to a cellular phone or other transmitting and/or receiving apparatus.
Another known device involves a generally planar antenna adhered to the inside of the window. Most previously known devices were designed to function with only a single frequency bandxe2x80x94for example, the prevailing cellular telephone frequency, typically about 800-900 MHz.
Increasingly, however, demand exists for such an antenna which is capable of dual band and/or multiple band operation. One aspect of this demand relates to dual band GSM telephone standards, which operate on 900 and 1800 MHz in Austrialia, most of Europe and much of Asia. In addition, many nations (including Australia, the Americas and much of Asia) also operate cellular telephony in the 800 MHz and 1900 MHz bands. Such cellular telephony systems also include PDS, TDMA, CDMA and AMPS systems.
Some dual band vehicle and non-vehicle mounted antenna devices have been proposed, providing varying degrees of suitability. However, the dual bands to which the antenna is resonant are generally related, for example, as multiples or harmonics. It is important to appreciate that in many of these applications it is necessary not just to receive but also to transmit.
Typically, known dual band devices have operated by exciting two resonant frequencies by means of either a single feed or a power divider/phase shifter combining the frequencies into a single port.
A further issue with many existing dual band devices is that they are relatively complicated and expensive. Some known antennas in use have used planar or similar circuits which are inherently broadband, however, their effectiveness in the field has been poor with the antennas exhibiting poor performance and, as a result, limited market acceptance.
It is an object of the present invention to provide an effective dual band and/or multiple band antenna, in which the separate bands need not have a particular relationship to one another. A further object of the present invention is to provide a simple and economical construction for such a dual band or multiple band antenna.
According to a first aspect of the invention there is provided an antenna device including:
a first conductive element having a slot which forms a closed path and includes a section of conductive material disposed in a portion of the slot;
a second conductive element disposed adjacent the first element and having a central opening and a strip opening extending from the central opening to some predetermined length beyond the edge of the opening, a conductive strip feed element disposed in the strip opening and at least two patch elements of predetermined dimensions which function as notch circuits and are generally radially inwardly extending into the central opening;
a dielectric insulator disposed intermediate the conductive elements such that, in use, the device has at least two frequency bands to receive and/or transmit RF signals.
Preferably, the first and second conductive elements and the dielectric insulator are generally planar and include a pair of major faces. More preferably, the first and second conductive elements are bonded to respective major faces of the dielectric insulator. More preferably, the first and second conductive elements are integrally formed on respective major faces of the dielectric insulator.
Preferably the slot is substantially circular and defines an annulus. Also preferable, the slot has a substantially uniform thickness and may be assymmetrical.
Preferably, the dielectric insulator is a PCB material.
Preferably, the central opening is substantially circular..
More preferably, the device is for use in CDMA, TDMA, PCS, third generation mobile and/or AMPS telecommunications systems, UHF and/or VHF television systems, and/or GPS satellite navigation systems.
Preferably, the device is removably encased in a dielectric material.
According to another aspect of the invention there is provided an antenna device including:
a slot antenna having a closed path slot; a patch element; and a dielectric material disposed intermediate the slot antenna and patch element wherein the patch element is selectively configured such that the resonant frequency of the slot antenna is altered allowing the device to transmit and/or receive RF signals over at least two frequency bands.
Preferably, the patch element functions as a notch circuit. Preferably, the patch element includes at least four notches, preferably of different shapes. Preferably, the device, in use, transmits and/or receives RF signals over four frequency bands.
Preferably, the notch circuit, slot antenna and dielectric material are all generally planar and may be integrally formed. Preferable also, the notch circuit and slot antenna are respectively bonded to the dielectric material.
Preferably, the slot is circular and defines an annulus. More preferably, the slot has non-uniform thickness.
The principles of annular slot antennas are a well known in the art and are particularly common in satellite and airborne applications because of their low profile. The typically provide medium gain, omnidirectional performance for a single frequency or single band of frequencies. The simplest form of such an antenna consists of an extended thin flat sheet of metal with the slot adapted to radiate electromagnetic radiation.
The slot is excited by a voltage source such as a balanced parallel transmission line connected to the opposite edges of the slot, or a coaxial line. A typical bandwidth of a slot antenna might be as much as one octave but not more.
In one form they can typically consist of a circuit board with simply a connector fitted to the board and a randome covering the circuit board. The circuit board has a ground plane on one side of the board. The slot element is cut into the ground plane which is from where the radiating wave is propagated.
The slot element according to preferred embodiments of the invention is typically circular with its diameter (both inside and outside diameter and, amongst other things, the ratio between the two) determining the frequency of operation of the slot antenna element.
In general terms, the resonant frequency of an annular slot antenna is determined by
xcexg=2xcfx80R/n 
where R is the median radius of the slot, n is an integer and xcexg is the wavelength on the microstrip line.
The patch element, or notch circuits as employed in some preferred embodiments of the invention, may be altered geometrically whilst achieving largely the same result. The nature of the patch elements is that they provide a loading and are generally dependent on a specific application. It ahs been found that a particular geometry and/or componentry results in the tuning of the frequency bands of interest. For example, providing a substitute bonding tape to adhere the device to a surface will generally provide a different dielectric effect and some small perturbations to the geometry will be required.
Accordingly, the geometry of the patch elements may be altered depending upon the components employed. For example, a change in the binding means of material, housing material or dimensions, or even the dielectric insulator may alter the response of the antenna. Equally, an alternative geometry may be arrived at in accordance with aspects of the present invention but with a different patch geometry. That is, there is no inherent ratio at play between the number of patch elements, or notches, and the number of bands over which the device may operate. Each patch has an effect on the tuning, and its is the combined load provided by the patches and other elements of the device which results in the desired tuning response.
For details of specific aspects of dimensions and design, reference is made to standard citations dealing with these topics which include, for example:
S. K. Palit, xe2x80x9cDesign and development of wideband microstrip antenna,xe2x80x9d IEEE Proceedings, 146, No. 1, pp. 35-39, February 1999;
Yongxi Qian, xe2x80x9cA microstrip patch antenna using novel photonic band-gap structuresxe2x80x9d Microwave Journal 40, No. 1, January 1999;
Jasik, xe2x80x9cAnnular slotxe2x80x9d, Antenna Engineering Handbook, 2nd Edn, pp. 8/12-15, 1984;
Jean-Francois Zurcher, xe2x80x9cMicrostrip antennasxe2x80x9d, Broadband patch antennas, pp. 19-40, 1995;
Fujimoto, xe2x80x9cEssential techniques in mobile antenna systems designxe2x80x9d, Mobile Antenna Systems Handbook, pp. 17-111, 1994;
K. Y. See, xe2x80x9cRigorous approach to modelling electromagnetic radiation from finite-size printed circuits structuresxe2x80x9d IEEE Proceedings, 146, No. 1, pp. 29-34, February 1999;
L. Zaid Kossiavas, xe2x80x9cDual-frequency and broadband antennas with stacked quarter wavelength elementsxe2x80x9d, IEEE Transaction, 47, No.4, pp. 654-658, April 1999; and
John Kraus, xe2x80x9cSlot antennaxe2x80x9d Antennas, 2nd Edn, pp. 624-628, 1988.