Patent Grant
7616163
The present invention relates generally to antennas and antenna systems and more specifically to embedded antennas and antenna systems operative at certain frequencies, including digital video broadcast frequencies.
It is known that antenna performance is dependent on the size, shape, and material composition of the antenna elements, the interaction between elements and 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 physical and electrical characteristics determine several antenna operational parameters, including input impedance, gain, directivity, signal polarization, resonant frequency, bandwidth and radiation pattern. Since the antenna is an integral element of a signal receive and transmit path of a communication device, antenna performance directly affects device performance.
Generally, an operable antenna should have a minimum physical antenna dimension on the order of a half wavelength (or a multiple thereof) of the operating frequency to limit energy dissipated in resistive losses and maximize transmitted or received energy. Due to the effect of a ground plane image, a quarter wavelength antenna (or odd integer multiples thereof) operative above a ground plane exhibits properties similar to a half wavelength antenna. Communications device product designers prefer an efficient antenna that is capable of wide bandwidth and/or multiple frequency band operation, electrically matched (e.g., impedance matching) to the transmitting and receiving components of the communications system, and operable in multiple modes (e.g., selectable signal polarizations and selectable radiation patterns).
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. But as the operational frequency increases/decreases, the operational wavelength decreases/increases and the antenna element dimensions proportionally decrease/increase. In particular, as the frequency of the received or transmitted signal decreases, the dimensions of the quarter wavelength and half wavelength antenna proportionally increase to maintain a resonant condition. 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. Since these antennas tend to be larger than the communications device with which they operate, the antenna is typically mounted with a portion of the antenna protruding from the communications device. Such mounting schemes subject the antenna to possible damage.
The burgeoning growth of wireless communications devices and systems has created a substantial need for physically smaller, less, obtrusive, and more efficient antennas that are capable of wide bandwidth or multiple frequency-bank operation, and/or operation 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 or signal protocols within different frequency bands. For example, a cellular telephone system transmitter/receiver and a global positioning system receiver operate in different frequency bands using different signal protocols. Operation of the device in multiple countries also requires multiple frequency band operation since communication frequencies are not commonly assigned in different countries.
Smaller packaging of state-of-the-art communications devices, such as personal communications handsets and laptop computers, does not provide sufficient space for the conventional quarter and half wavelength antenna elements. Physically smaller antennas operable in the frequency bands of interest (i.e., exhibiting multiple resonant frequencies and/or wide bandwidth to cover all operating frequencies of communications device) and providing the other desired antenna-operating properties (input impedance, radiation pattern, signal polarizations, etc.) are especially sought after.
To overcome the antenna size limitations imposed by handset and personal communications devices, antenna designers have turned to the use of slow wave or meanderline 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 in which the phase velocity of the traveling wave is less than the free space velocity of light. The wave velocity (c) is the product of the wavelength and the frequency and takes into account the material permittivity and permeability, i.e., c/((sqrt(εr)sqrt(μt))=λf. Since the frequency does not change 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.
Current antenna solutions for digital video broadcast (DVB) or digital television broadcast utilize external dongle antenna assemblies that are unwieldy, connected by wire to the television receiver and in most embodiments offer poor performance over a broad bandwidth. DVB systems may operate at the traditional television broadcast carrier frequencies, as well as cellular, PCS, DCS, and UMTS carrier frequencies. Efficient antenna operation is desired over all operative frequency bans to permit a portable or mobile receiving device to receive multiple DB signals. The use of multiple antennas within the receiving device is generally discouraged due to the space requirements for multiple antennas.
Reception of video signals by mobile or portable receivers is further complicated by signal fading and multi-path interferences. The problem of acceptable performance is exacerbated by the use of relatively simple receivers operating with a low gain antenna to receive the video signal.
Prior art television and video antennas include passive and active devices. Passive antennas may comprise a whip antenna or a loaded whip antenna having a length substantially less than ¼ wavelength at the operating frequency. Generally the whip (monopole) may exhibit fundamental resonance at one frequency somewhere in the desired spectral range covered by the receiver, with a maximum bandwidth limit governed by the well known Chu-Harrington relation. The Chu-Harrington Limit establishes the minimum volumetric antenna size for a given bandwidth and radiometric efficiency; or conversely the maximum bandwidth the antenna will present for a given volumetric size and efficiency/ At frequencies outside this bandwidth, the antenna becomes less efficient at converting received wave energy into a usable electrical signal. Nevertheless, whip antennas have been used for many years for portable television signal reception, albeit with non-optimal results.
An active solution for improving the bandwidth limitations of receive-only antennas is to incorporate an amplifier at the antenna terminals. The amplifier can be designed to match the impedance of the antenna over a broad frequency range, as is known. This approach has several drawbacks: 1) the amplifier mush have a broad bandwidth and low noise contribution over the entire received signal frequency range, and 2) the amplifier must exhibit high linearity and low distortion even at high signal levels to prevent mixing of signals appearing in our out of band. With respect to item 1), the noise performance of the antenna amplifier combination is seldom as good as that achievable over a narrower bandwidth. Regarding item 2), proximity to high power transmitter widespread in urban environments can cause interference in even the best receiver designs. Also, signal mixing can produce spurious signals in the desired passband.
Very small antennas, as required in video-receiving laptop computers and handheld or portable video receivers, are particularly sensitive to noise interference from on-board digital circuits. This noise may be broadband or within the passband of the receiver's “front end” amplifier.
One embodiment of the invention comprises an antenna providing a tunable resonant frequency within a low frequency band and further providing a high resonant frequency. The antenna comprising a first radiating structure of a first effective electrical length, a second radiating structure of a second effective electrical length having a fractional integer relationship to a wavelength related to the high resonant frequency and a variable reactance element connecting the first and the second radiating structures, wherein varying a reactance of the variable reactance element tunes the antenna within the low frequency band.
The present invention can be more easily understood and the advantages and uses thereof more readily apparent when the following detailed description of the present invention is read in conjunction with the figures briefly described below. In accordance with common practice, the various described features are not drawn to scale, but are drawn to emphasize specific features relevant to the invention. Like reference characters denote like elements throughout the figures and text.
Before describing in detail the particular method and apparatus related to antennas and antenna systems of the present invention, it should be observed that the present invention resides primarily in a novel and non-obvious combination of elements and process steps. 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 presented with lesser detail, while the drawings and the specification described in greater detail other elements and steps pertinent to understanding the invention.
The following embodiments are not intended to define limits as to the structure or method of the invention, but only to provide exemplary constructions. The embodiments are permissive rather than mandatory and illustrative rather than exhaustive.
The antennas and antenna systems of the present inventions advantageously presents a narrower bandwidth than prior art antennas and antenna systems and can therefore improve the signal-to-noise ratio of the received signal. Prior art antenna system do not optimize antenna performance by tuning the antenna resonance to specific frequencies or frequency bands according to tuning of the receiver, resulting in suboptimal antenna performance (efficiency). The present invention teaches antenna systems having tuning capabilities to improve signal reception, and tunable multiband antenna structures to alleviate certain propagation challenges encountered with typical video receivers.
The present invention provide efficient antenna system and antenna operation on one (or several) channels (i.e., where a channel comprises a carrier frequency and a frequency band above and below the carrier bandwidth) to which the receiver is tuned. The invention therefore provides a smaller antenna than prior art antennas with similar functionality, since in one embodiment the receiver actively and automatically commands the antenna to operate over a prescribed frequency region or at a prescribed resonant frequency. Tuning the antenna systems and/or the antennas according to the present invention may also reduce interference problems experienced in multiple signal urban environments.
The teachings of the present inventions provide improved reception of DVB (or any other received signals) where the transmitted signal bandwidth is within the passband of the antenna and where the receiver must tune over a larger bandwidth than is efficiently achievable from a single fix-tuned antenna. Advantageously, the present invention reduce interference from proximate strong radiators or on-board noise sources and improve received signal strength. These beneficial features result from the inherent selectivity provided by the antenna's relatively smaller bandwidth compared to the prior art antennas and from the improved radiation efficiency of the antenna resulting from active control of the resonant frequency to match the desired received signal at its nominal band center.
The selectivity offered by the present antenna systems and antennas when operating in the receiving mode also allows interoperability with communications devices that include a transmitter, such as a cellular telephone.
In one embodiment, the antenna systems of the present inventions are self-contained (for example, an antenna module comprising the radiating structures and operative electronics elements) and internal to the wireless device, thereby improving the durability of the wireless device with respect to prior art devices that incorporate clumsy whip antennas.
One embodiment of an antenna system 30 constructed according to the teachings of the present inventions is depicted in a block diagram of
The antenna system 30 is characterized by two inputs (control signals supplied by the DVB (for example receiving system (not shown) and one output. A first input signal (provided on an input line 34) comprises a serial data stream from a microprocessor or other digital device (e.g., a radio frequency controller) that contains information as to the channel or frequency to which the DVB receiving system is tuned. The information in digital from may be contained in one or more data bytes. A clock pulse or other synchronizing signal (provided on an input line 38) commands a serial to parallel converter 42 to sample the serial bit stream at the appropriate time to capture the serial data indicating the frequency or channel of the receiving system.
The data is latched and parallel data (shown schematically as a double-line arrowhead 44 in
In another embodiment, an antenna system 60 of
In a configuration of
An embodiment of
Terminals 218 supply signals to receiving circuits when the antenna structure 200 operates in a receive mode (and receive signals for transmitting when the antenna structure 200 operates in a transmit mode). Preferably, the antenna 200 is operative proximate a ground plane (not shown in
When properly dimensioned, the antenna 200 presents tunable resonant frequencies in a band extending from about 470 MHz to about 860 MHz and a resonant frequency at about 1675 MHz. In this embodiment the antenna structure 200 can be tuned to a desired resonant frequency in the 470-860 MHz DVB band by changing the capacitance of the variable capacitor 212 and can be controlled to receive a DVB broadcast at 1675 MHZ. Thus the antenna 200 can be used with a communications device for receiving DVB signals in these two primary DVB broadcast bands/frequencies.
The conductive bridge 208 and the meanderline sections 204 and 210 cooperate to form a half wave dipole antenna (referred to as a primary antenna) with a resonant frequency of about 1675 MHz. Thus the effective electrical length of the bridge 208 and the meanderline sections 204 and 210 is about a half wavelength at about 1675 MHz.
Preferably an effective electrical length of the extension 214 is about equivalent to an effective electrical length of the radiating structure formed by the meanderline sections 204 and 210 and the bridging section 208. In one embodiment both effective electrical lengths are about a half wavelength (or a different fractional integer relationship) at about 1675 MHz. Therefore the resonance of the extension 214 does not adversely affect the resonance properties of the radiating structure formed by the meanderline sections 204 and 210 and the bridging section 208.
The low band resonance of the antenna structure 200 between 470 and 860 MHz is achieved by changing the capacitance of the variable capacitor 212. When the variable capacitor 212 is implemented as a varactor diode, the presented capacitance is responsive to the applied DC reverse-bias voltage. In one embodiment, a capacitance of about 10 pf provides a resonant frequency of about 470 MHz. A capacitance of about 1 picofarad causes the antenna 200 to be resonant at about 830 MHz. Resonant values between 470 and 860 MHz are achievable responsive to the different capacitance values.
The capacitance value presented by the variable capacitor 212 does not appreciably affect the high band resonant frequency of 1675 MHz. When the antenna 200 is operative in a communications receiving device, a signal indicating a desired receiving frequency may be provided to the antenna 200 to affect the capacitance of the variable capacitor 212 and thereby tune the antenna to the receiving frequency within the 470-860 MHz band. The antenna presents a resonant frequency of about 1675 MHz irrespective of the value of the capacitor 212.
In another embodiment (not illustrated) the extension 214 comprises a meanderline having an effective electrical length of about a half wavelength at the desired resonant frequency.
In another embodiment (not illustrated) the variable capacitor 212 is replaced by a fixed-value capacitor. Such an antenna is resonant in two spaced-apart frequency bands.
With reference to the antenna system 80 of
The inventors have determined that if the effective electrical length of the extension 214 is different from the effective electrical length of the radiating structure formed by the meanderline sections 204 and 210 and the bridging section 208 at a given frequency, for example at 1675 MHz, then as the capacitance of the variable capacitor 212 is changed to tune the low frequency resonance, the resonance at 1675 MHz also shifts.
In yet another embodiment, the fixed-value capacitors 222 are replaced with variable capacitors (e.g., varactor diodes) to provide additional tuning capabilities for the antenna 220. Further, each variable capacitor can provide a different capacitance range. Thus variable capacitance valued can be presented responsive to the closure of one or more switches and further responsive to the value of the capacitance selected for any of the closed switches. Such an embodiment provides additional tuning capabilities for the antenna, including tuning to and within different frequency bands than the exemplary DVB bands discussed herein.
In yet another embodiment (not illustrated) switches can be located to switchably connection the meanderline region 210A and the extension region 214A (see
With reference to the antenna system 70 of
In one application, the antenna 200 or 220 of respective
Exemplary results for an antenna structure constructed according to the teachings of the present invention, such as the antenna structure 200 of
In one embodiment the switches 364 are implemented by connecting one or more of the taps 360 to ground through an inductor (not shown) to establish a DC ground for each tap 360.
The antenna structure 399 further comprises an extension 408 capacitively coupled to a terminal region 410 of the radiating structure 400 via a variable capacitor 412, with the capacitance value selected responsive to a desired resonant frequency. In one embodiments the capacitor 412 comprises a varactor diode as described above. The extension 408 comprises a conductive rectangular shape, a meanderline or another shape that presents a half wavelength resonating element at the frequency of interest.
If the capacitor is an effective short at the desired frequency, the combination of the radiating structure 400 and the extension 408 presents a three-quarter wavelength structure. Thus if the resonant frequency of the structure/counterpoise combination 400/404 is f0, then the resonant frequency with a shorted capacitor is about f0/3. As in the embodiments described above, the frequency f0 remains relatively fixed as the lower resonant frequency is tuned by varying the capacitance of the capacitor 412. Specifically, the lower resonant frequency increases as the reactance presented by the capacitor is varied through a range from the short circuit to an open circuit.
In one application, the antenna structure 399 is embedded in a handset communication device, where conductive elements (e.g., a printed circuit board ground plane, conductive material of the device case) may serve as the counterpoise 404.
The antenna structure 399 may present a broader bandwidth above and below 1675 MHz than other antenna embodiments described herein according to the teachings of the inventions.
In another embodiment, a segment of the radiating structure 400 between the feed 405 and the capacitor 412 is replaced with a meanderline appropriately dimensioned to provide the desired resonance characteristics.
In another embodiment, the antenna structure 399 of
If the quarter wavelength radiating structure/counterpoise combination 400/404 of
In yet another embodiment, the antenna structure of
Another antenna system 550 is illustrated in
The various presented embodiments comprising the tuning capacitor (e.g., a varactor) also provide the capability to tune the antenna to overcome the affect of the user's hand (for an antenna incorporated into a handset device) on the antenna resonance. The affect of the user's body (for an antenna incorporated into a laptop computer) or proximate objects can also be avoided by proper tuning of the antenna according to the teachings of the present invention.
The embodiments of the inventions employing balanced antenna structures have better noise immunity, from internal or external noise sources, over other prior art antenna structures.
To design an antenna according to the present invention, it is first necessary to empirically determine an antenna's resonant frequencies responsive to the use of different capacitance values between the meanderline segment 210 and the extension 212 (see the embodiment of
In other embodiments, other radiating structures can be substituted for the depicted high-band radiating structures (e.g., the meanderline 204/210 and the conducting bridge 208 of
While the present 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 functionally equivalent elements may be substituted for the elements thereof without departing from the scope of the invention. For example, although the invention has been described in the context of an antenna for receiving DVB signals, the teachings of the invention can be applied to receiving (and transmitting) signals at different frequencies. The scope of the present invention further includes any combination of 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. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.
The present application claims the benefit of under Section 119(e) of the provisional patent application field on Jan. 25, 2006 assigned application No. 60/762,196.
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| Number | Date | Country | |
|---|---|---|---|
| 20070216590 A1 | Sep 2007 | US |
| Number | Date | Country | |
|---|---|---|---|
| 60762196 | Jan 2006 | US |
