Patent Application
20050270238
The present invention is directed generally to an antenna for transmitting and receiving electromagnetic signals, and more specifically to an antenna for integration into a portable or handheld device for receiving and transmitting the electromagnetic signals via the antenna.
It is known that antenna performance is dependent upon the size, shape and material composition of the constituent antenna elements, as well as the relationship between certain antenna physical parameters (e.g., length for a linear antenna and diameter for a loop antenna) and the wavelength of the signal received or transmitted by the antenna. These relationships determine several antenna operational parameters, including input impedance, gain, directivity, signal polarization and radiation pattern.
Generally, an operable antenna should have a minimum physical antenna dimension approximately equal to a half wavelength (or a quarter wavelength above a ground plane) (or a multiple thereof) of the operating frequency to limit energy dissipated in resistive losses and maximize transmitted or received energy. A quarter wavelength antenna (or multiples of a quarter wavelength) operative above a ground plane exhibits properties similar to a half wavelength antenna. Designers of communications products prefer an efficient antenna that is capable of wide bandwidth and/or multiple frequency band operation, electrically matched to the transmitting and receiving components of the communications system, and operable in multiple modes (e.g., selectable signal polarizations and selectable radiation patterns). Quarter wavelength and half wavelength antennas are the most commonly used.
The half-wavelength dipole antenna finds use in many applications. The radiation pattern is the familiar donut shape with most of the energy radiated uniformly in the azimuth direction and little radiation in the elevation direction. Frequency bands of interest for certain communications devices are 1710 to 1990 MHz and 2110 to 2200 MHz. A half-wavelength dipole antenna is approximately 3.11 inches long at 1900 MHz, 3.45 inches long at 1710 MHz, and 2.68 inches long at 2200 MHz. The typical gain is about 2.15 dBi.
The quarter-wavelength monopole antenna disposed above a ground plane is derived from the half-wavelength dipole. The physical antenna length is a quarter wavelength, but interaction of the electromagnetic energy with the ground plane causes the antenna to exhibit half wavelength dipole properties. Thus, the radiation pattern for a monopole antenna above a ground plane is similar to the half-wavelength dipole pattern, with a typical gain of approximately 2 dBi.
Given the advantageous performance of quarter and half-wavelength antennas, conventional antennas are typically constructed so that the antenna length is on the order of a quarter wavelength of the radiating frequency and the antenna is operated over a ground plane or the antenna length is a half wavelength without employing a ground plane. These dimensions allow the antenna to be easily excited and operated at or near a resonant frequency (where the resonant frequency (f) is determined according to the equation c=λf, where c is the speed of light and λ is the wavelength of the electromagnetic radiation). Half and quarter wavelength antennas limit energy dissipated in resistive losses and maximize the transmitted energy. As the operating frequency increases/decreases, the operating wavelength decreases/increases and the antenna element dimensions proportionally decrease/increase. In particular, as the resonant frequency of the received or transmitted signal decreases, the dimensions of the quarter-wavelength and half-wavelength antenna proportionally increase. The resulting larger antenna, even at a quarter wavelength, may not be suitable for use with certain communications devices, especially portable and personal communications devices intended to be carried by a user. Such antennas can protrude from the communications device and thus are susceptible to breakage
The burgeoning growth of wireless communications devices and systems has created a substantial need for physically smaller, less obtrusive and more efficient antennas capable of wide bandwidth or multiple frequency-band operation. It is also desired that the antennas operate in multiple modes (i.e., selectable radiation patterns or selectable signal polarizations). For example, operation in multiple frequency bands may be required for operation of the communications device with multiple communications systems, such as a cellular telephone system and a cordless telephone system. Operation of the device in multiple countries also requires multiple frequency band operation since communications frequencies are not commonly assigned among countries.
Smaller packaging of state-of-the-art communications devices, such as personal handsets, does not provide sufficient space for the conventional quarter and half wavelength antenna elements. Generally, it is not considered feasible to utilize one antenna for each operating frequency or to include multiple matching circuits to provide proper resonant frequency operation at several different frequencies for a single antenna. Thus physically smaller antennas operating in frequency bands of interest and providing the other desired antenna properties (input impedance, radiation pattern, signal polarizations, etc.) are especially sought after.
As is known to those skilled in the art, there is a direct relationship between physical antenna size and antenna gain for a single-element antenna, according to the relationship: gain=(βR){circumflex over ( )}2+2βR, where R is the radius of the sphere containing the antenna and β is the propagation factor or phase constant. Increased gain thus requires a physically larger antenna, while users continue to demand physically smaller antennas. As a further design constraint, to simplify the system design and strive for minimum cost, equipment designers and system operators prefer to utilize antennas capable of efficient multi-band and/or wide bandwidth operation, thus permitting the communications device to access various wireless services operating within different frequency bands or services operating over wide bandwidths. Finally, gain is limited by the known relationship between the antenna operating frequency and the effective antenna length (expressed in wavelengths). That is, the antenna gain is constant for all quarter-wavelength antennas of a specific geometry i.e., at that operating frequency where the effective antenna length is a quarter of the operating frequency wavelength.
To overcome the size limitations of handset and personal communications devices, antenna designers have turned to the use of so-called slow wave structures where the structure's physical dimensions are not equal to the effective electrical dimensions. Recall that the effective antenna dimensions should be on the order of a half wavelength (or a quarter wavelength above a ground plane) to achieve the beneficial radiating and low loss properties discussed above. Generally, a slow-wave structure is defined as one for which the phase velocity of the traveling wave is less than the free space velocity of light. The wave velocity is the product of the wavelength and the frequency and takes into account the material permittivity and permeability, i.e., c/((sqrt(εr)sqrt(μr))=λf Since the frequency remains unchanged during propagation through a slow wave structure, if the wave travels slower (i.e., the phase velocity is lower) than the speed of light, the wavelength within the structure is lower than the free space wavelength. The slow-wave structure de-couples the conventional relationship between physical length, resonant frequency and wavelength.
Since the phase velocity of a wave propagating in a slow-wave structure is less than the free space velocity of light, the effective electrical length of these structures is greater than the effective electrical length of a structure propagating a wave at the speed of light. The resulting resonant frequency for the slow-wave structure is correspondingly increased. Thus if two structures are to operate at the same resonant frequency, as a half-wave dipole, for instance, then the structure propagating a slow wave will be physically smaller than the structure propagating a wave at the speed of light. Such slow wave structures can be used as antenna elements or as antenna radiating structures.
In one embodiment, the present invention comprises an antenna for receiving radio frequency signals. The antenna further comprises a first resonant segment having a shape of a substantially closed curve with an opening defined therein, and a second resonant segment. A third resonant segment extends through the opening into an interior region defined by the closed curve; the third segment is conductively connected to the first segment. The second segment is conductively connected to one of the first segment and the third segment. The first segment is resonant at a first frequency, the second segment is resonant at a second frequency and the third segment is resonant at a third frequency.
The features of the antenna constructed according to the teachings of the present invention will be apparent from the following more particular description of the invention, as illustrated in the accompanying drawings, in which like reference characters refer to the same parts throughout the different figures. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.
Before describing in detail the particular antenna in accordance with the present invention, it should be observed that the present invention resides primarily in a novel and non-obvious combination of elements. So as not to obscure the disclosure with details that will be readily apparent to those skilled in the art, certain conventional elements and steps have been described and illustrated with lesser detail, while other elements and steps more pertinent to understanding the invention have been described and illustrated in greater detail.
A signal terminal 21 of the antenna 10 is connected to a signal source 22 of a communications device (when operative in the transmitting mode). In the receiving mode, the received signal is fed to receiving circuitry (not shown) of the communications device from the signal terminal 21. Although the signal terminal 21 is located at a single point in
The antenna 10 is connected to a ground plane 24, which typically comprises a ground plane in the communications device, via a conductive element 25 extending from a ground terminal 26. In another embodiment, the ground terminal 26 can be moved to another location on the antenna 10.
In the embodiment of
An exemplary communications device operable with the antenna 10 comprises a handset or cellular telephone capable of receiving digital multimedia broadcast signals from a satellite or a terrestrial source. In one application, a satellite transmits two signals to the earth. One signal comprises a direct signal broadcast to handsets (at a frequency of, for example, 2.64 GHz with right-hand circular polarization). A second signal is transmitted to a base-station (at for example, 12 GHz). The base-station (referred to in the communications system as a gap-filler) terrestrially rebroadcasts the received signal to handsets (at for example, 2.64 GHz with linear polarization). Thus, each handset receives two separate signals, one signal directly from the satellite and a second from the gap-filler base station, but both signals have substantially the same information content. The user's handset selects the best-received signal based on a received signal quality metric. In one application, the antenna 10 operates to receive the terrestrial gap-filler signal, as well as a global positioning signal and a cellular telephone signal, each signal received at a different frequency.
In one embodiment the antenna 10 is resonant in three spaced-apart frequency bands (and thus referred to as a tri-band antenna): a first frequency band (f1) of 824-894 MHz (for CDMA communications), a second frequency band (f2) of 1.575 GHz (for global positioning system (GPS) communications) and a third frequency band (f3) of 2.63-2.65 GHz (for digital multimedia broadcast (DMB) communications). A length of the various segments and a distance between segments (identified in
In a preferred embodiment, various components of the antenna 10 are formed from a length of conductive material (such as copper) formed into the illustrated shapes or a shape functionally similar thereto, by simple bending and curve-inducing operations that can be easily performed manually or using a material bending jig. In one embodiment, for example, one or more of the segments 12, 18, 14, 20, 16 and 17 are formed from a single length of conductive material. As explained further below in conjunction with
In yet another embodiment, the segments 12, 18 and 14 comprise linear segments and are oriented to form a linear conductive element, in lieu of the curved element illustrated in
In one embodiment, a camera of the communications device is disposed in a region 60. Thus in one embodiment, the segments of the antenna 10 are curved to encircle the region 60, creating an efficient and compact integration of the antenna 10 into the communications device 49. However, in another embodiment where the antenna 10 is not required to fit into a region of a communications device presenting a curved envelope, the antenna segments can comprise linear elements in a shape as required to fit within the available envelope and provide the desired radiating and receiving properties.
The antenna 80 further comprises a curved region 84 within the segment 14 for increasing the bandwidth and radiation efficiency of the antenna at the low band resonant frequency f1. Use of the curved region 84 may be beneficial in certain embodiments of the present invention, but is not necessarily required in all embodiments.
In
In
The embodiments of
The dimensions, shapes and relationships of the various antenna elements and their respective features as described herein can be modified to permit operation in other frequency bands with other operational characteristics, including bandwidth, radiation resistance, input impedance, radiation efficiency, etc. The antenna is therefore scalable to another resonant frequency by dimensional variation. Those skilled in the art recognize that the interaction and coupling between the elements of a multi-frequency antenna are not susceptible to simple and precise explanation. Further, the affect of these phenomena on antenna performance is complex and not easily determinable. Thus, the description of the various embodiments of the present invention should be interpreted in light of these considerations.
While the invention has been described with reference to preferred embodiments, it will be understood by those skilled in the art that various changes may be made and equivalent elements may be substituted for elements thereof without departing from the scope of the present invention. The scope of the present invention further includes any combination of the elements from the various embodiments set forth herein. In addition, modifications may be made to adapt a particular situation to the teachings of the present invention without departing from its essential scope. For example, different sized and shaped elements can be employed to form an antenna according to the teachings of the present invention. Therefore, it is intended that the invention not be limited to the embodiments taught herein, but that the invention will include all embodiments falling within the scope of the appended claims.
