Patent Application
20090156151
The present invention relates to a wireless device with an internal antenna or antenna system suitable for wireless services requiring a broad bandwidth receiver and/or transmitter. The present invention refers, inter alia, to a portable (for example, handheld) device including an internal antenna system for at least the reception of digital television signals, such as for instance DVB-H (Digital Video Broadcast-Handheld), DMB (Digital Multimedia Broadcasting), T-DMB (Terrestrial Digital Multimedia Broadcasting) or other related digital or analog TV standards or FM. In some embodiments, such a portable device can include means (hardware and software) for wireless (such as mobile and/or cellular) services, whereby the device can be connected to a mobile or wireless network or device. Some embodiments of the present invention can take the form of a handheld device, such as a for instance a handset, a cell phone, a PDA (Personal Digital Assistant), or a smart phone. Such a handheld device can take the form of a single-body compact device, or it can include two or more bodies and a mechanical arrangement to move at least one of those bodies with respect to at least another of said bodies by means of a substantially co-planar displacement, by rotation or pivotation around one or more axes, or by a combination of both.
Wireless communication systems are normally based on a transmission of information between a transmitter and a receiver, whereby the information can be modulated on a carrier signal. In radio communication systems, the signal transmitted by the transmitter is captured by one or more antennas at the receiver end, and converted into an electrical signal, which can then be processed so as to extract the relevant information from the signal.
Many wireless communication systems, such as GSM900, GSM1800, UMTS, etc., operate within narrow frequency bands, centred around a centre frequency (such as 900 MHz in the case of GSM900, 1800 MHz in the case of GSM1800, 2000 MHz for UMTS, etc.). Thus, in order to obtain an adequate gain within the relevant frequency band, antenna systems are used that are tuned to the respective frequency band and that include at least one antenna having a resonant frequency substantially corresponding to the centre frequency of said frequency band.
A problem frequently involved with antennas is their size. For example, a dipole antenna having a resonant frequency f should have a length of about λ/2, where λ is the wavelength corresponding to said resonant frequency (such a dipole is often referred to as half-wavelength dipole). A monopole antenna mounted over a ground-plane should have a length of about λ/4, where λ is the wavelength corresponding to said resonant frequency (such a monopole is often referred to as quarter-wavelength monopole). Even for communication systems based on high frequencies (such as GSM900, GSM1800 and UMTS2000, corresponding to wavelengths of approximately 33 cm, 16 cm and 13 cm, respectively), obtaining sufficiently small and still useful antennas has been considered to be a difficult task (normally, external and/or retractile antennas have been used, or the antennas have been helically arranged, so as to reduce the space they occupy in the handheld devices).
In communication systems using lower frequencies and, thus, longer wavelengths, such as FM (operating in the band of 88-108 MHz) and DVB-H (Digital Video Broadcast-Handheld) (operating in the bands of 470-702 MHz, or 470-770 MHz), the long wavelengths can imply that the typical monopole and dipole antennas can be inappropriate for handsets (such as handsets for mobile radio communication, that normally have quite reduced dimensions). For example, at a typical DVB-H centre frequency of 586 MHz, a typical quarter wavelength (λ/4) monopole antenna would have a length of approximately 12.8 cm. Such a long antenna would not be suitable for a pocket size mobile handset (today, consumers are used to pocket-sized handsets with internal antennas). The size of the external antenna could be reduced by implementing it as a helical wire, but the reduction of the size would imply a reduction of the bandwidth.
However, in many cases, a large bandwidth can be desired. For example, for DVB-H applications, a bandwidth encompassing the band of 470-702 or 470-770 MHz could be desired, together with a gain of not less than −10 dB to −7 dB over said band. Antenna elements are very selective in terms of gain and bandwidth. Although a larger bandwidth can be obtained using lumped components, these components do not provide for an increased gain.
That is, one of the challenges involved with including a TV service in a portable or handheld device relates to the need to cover the wide spectrum that is usually allocated for TV services. For instance, as mentioned above, the DVB-H service in Europe should cover a bandwidth including the 470-770 MHz band (UHF), which implies a relative bandwidth of approximately 50% with respect to the center frequency of said operating band. Other digital and analog TV services and standards, particularly those using the terrestrial broadcast network, would also encompass such a large 50% relative bandwidth at similar frequencies within the VHF-UHF bands.
There is a well known trade-off between antenna size and bandwidth coverage. The smaller the antenna, the smaller the bandwidth. Typical prior art internal antennas for handheld devices feature a 5-15% relative bandwidth at even shorter wavelengths, such as those of cellular, mobile and wireless services (800 MHz-2200 MHz). When an internal antenna is operated outside its typical relative 5-15% bandwidth, the gain, the efficiency and the matching characteristics (VSWR, return-loss) of the antenna become severely degraded, at times to unacceptable levels.
The specifications and characteristics of a digital TV antenna for a portable or handheld device can be very different from those of the internal antennas for conventional mobile services. While a conventional mobile service antenna would require a VSWR<3, a gain better than −2 dBi (dB(isotropic) is the forward gain of an antenna compared to an idealized isotropic antenna) within a 5-15% relative bandwidth (or bandwidths in case of multiband services), a digital TV antenna is usually specified to cover a 50% relative bandwidth with a gain between −10 dBi and 4 dBi or better and a return-loss of −2 dB or better.
Some prior art devices make use of an external antenna, often a mechanically retractable external antenna, to cover TV services. Nevertheless, the use of an external antenna on a small portable device is inconvenient in terms of size (the length of such an antenna is often 7 cm or more), ergonomy, aesthetics, mechanical robustness and durability. It is one of the purposes of the present invention to provide an arrangement for a handheld device including an internal antenna system which is able to provide for the reception of TV services.
An antenna can be characterized by its input impedance, Zin=Rin+jXin (that is, the impedance has a real component (the resistive component or resistance) and an imaginary component or reactance (that can be capacitive or inductive). At the resonant frequency (or, as a resonating element normally has more than one resonant mode, at one of the resonant frequencies, for example, at the lowest resonant frequency) the imaginary component equals 0, that is, Xin=0.
The gain of an antenna system, also referred to as antenna system's efficiency, depends on several features, including the radiating efficiency of the antenna element (which is frequency dependent) and the matching (which is also frequency dependent). “Matching” refers to the reduction of miss match losses that have to be substracted from the radiating efficiency. Normally, for each antenna, a matching network is used that is adapted to the characteristics of the antenna, including its input impedance. However, as the input impedance is frequency dependent, it can be difficult to provide a matching network that provides an adequate gain all over a wide frequency band. That is, in wide-band communication systems, such as FM and DVB-H, it can be difficult to provide a suitable matching network.
It is important to stress that the reception of the TV signal is limited not only by the design of the antenna, but by the design of the whole device. Usually, the device will include one or more printed circuit boards (PCB) embedding one or more ground-planes. Such a ground-plane is also part of the antenna system and its size has an effect on the quality of reception. In general, the smaller the ground-plane, the smaller the covered bandwidth. It is one of the purposes of the present invention to provide an antenna system for a handheld device which includes a comparatively small ground-plane, as this makes it possible to further reduce the size of the handheld device.
Jari Holopainen, et al. “Antenna for Handheld DVB Terminal” 2006 IEEE International Workshop on Antenna Technology: Small Antennas, Novel Metamaterials, pp. 305-308, held at White Plains, N.Y., Mar. 6-8, 2006, discloses an example of an antenna allegedly useful for DVB terminals. Here, the EMC shielding and the printed circuit board (PCB) of a handheld terminal are considered to define a metal box, or chassis, which is utilised as part of the antenna. In accordance with the disclosure, surface currents are induced to the chassis capacitively at one end of the chassis, where the electric field has a maximum. The feed is stated to be a non-resonant and practically non-radiating compact coupling element. It is further stated that because of the non-resonant structure, the resonance needs to be achieved outside the antenna, for example, with a matching circuit.
The coupling element is a substantially L-shaped element which is arranged in correspondence with one of the shorter ends of a rectangular ground-plane constituted by the metal layer of a PCB. This shorter end has a width of 75 mm, and the L-shaped element has the same width. It appears that the non-resonant condition of said L-shaped element is due to its limited length with respect to the DVB-H wavelength. However, a 75 mm long conducting element in a coupled antenna structure such as the disclosed one may still provide for a suitable frequency response, when a suitable matching network is used, as suggested in the above-mentioned document.
However, for many handset applications, a width in the order of 75 mm may be inconvenient or unacceptable. This is, for example, the case in many mobile radio communication handsets, where the size is essential.
Thus, there is a need to find an appropriate antenna arrangement suitable for communication systems requiring a large bandwidth and that allows the antennas to be conveniently housed inside small size devices such as handsets, while at the same time providing for the necessary gain.
A first aspect of the invention relates to a wireless portable device for radio communication (such as a mobile handset or hand-held terminal), comprising
When said imaginary part of said input impedance of the antenna element is positive, the input impedance is said to be “inductive”, and when it is negative, it is said to be “capacitive”. At resonance, the input impedance is “purely resistive”.
In this text, the expression bandwidth is to be interpreted as referring to a frequency band over which the device and antenna system complies with certain specifications, depending on the service for which the devise is adapted. For example, for a device adapted to receive and process a digital television signal, an antenna system having a relative bandwidth of at least 40% (preferably not less than 45% or 50%) together with a gain of not less than −10 dB (preferably not less than −7 dB, more preferably not less than 4 dB) can be preferred. Also, a return-loss of −2 dB or better within the corresponding frequency band can be preferred.
The gain of an antenna depends on factors such as its directivity, its radiating efficiency and its miss match losses. Both the radiating efficiency of the radiating antenna element and the matching characteristics are frequency dependent (even directivity is strictly frequency dependent, although for these cases, directivity remains almost constant across the band, typically changing only 2 to 3 dB). An antenna is normally very efficient at its resonant frequency and maintains a similar performance within the frequency range defined by its bandwidth around its resonant frequency (or resonant frequencies). Outside said frequency range, the efficiency and other relevant antenna parameters deteriorate with an increasing distance to said resonant frequency. This is normally not a substantial problem in narrow-band communication systems, but it can be a big problem in wide-band communication systems, such as FM and DVB-H. A low antenna efficiency, that is the result of the radiation efficiency and the miss match losses, can be compensated or partly compensated, especially when the antenna is severely mismatched by a matching network with high miss match losses. However, also the matching quality is frequency dependent, and the matching network should also be adapted to the input impedance of the antenna. Thus, obtaining a suitable matching quality with low miss match losses all over the relevant wide frequency band, or over the relevant parts thereof, can be a very complex task. One reason for this is that the real (resistive) component of the input impedance of the antenna changes very rapidly with the frequency in a frequency range close to the resonant frequency of the antenna (cf., for example,
It has been found that this problem can, surprisingly, be substantially solved or reduced if an antenna or antenna element is used that has no resonant frequency within the relevant frequency band, as per the invention. This makes it possible to use an antenna element the input impedance of which can be relatively constant (both in terms of resistance and reactance) within the frequency band, which, in turn, makes it possible to obtain an adequate gain by means of high matching quality (that is, with low miss match losses), with only one matching circuit or with a limited amount of matching circuits: the substantially constant input impedance of the antenna, throughout the relevant frequency band, makes this possible. Thus, the surprising result is that by intentionally using a non-resonant antenna element (which, at a first look, would imply a loss of gain) and by taking advantage of the fact that a high quality matching (that is, low miss match losses) can easily be obtained, it becomes possible to comply with the overall gain requirements of the communication system.
Said at least one matching network can comprise a plurality of different matching networks and switching means arranged so as to selectively operatively connect one of said matching networks to the antenna or between the antenna and the communication circuitry, in accordance with a selected frequency sub-band within said lower frequency limit (fmin) and upper frequency limit (fmax). Thus, reception of the signal can be made using the one of said matching networks that is most appropriate at a certain stage, for example, for receiving a certain TV channel or similar. The switch can be implemented in many ways, for example, a switch can be used that is continuously changing its state so as to provide for a continuous “scanning” of the matching networks and of the corresponding “sub-bands”. Alternatively, the switch can be selectively set to a fixed state in which it remains until the user decides to change its state, for example, by selecting a different TV channel, or similar.
Another aspect of the present invention refers to a portable or handheld device including radio frequency communication circuitry, an antenna system comprising at least two antenna elements tuned around two or more different central frequencies (within a frequency band having a lower frequency limit (fmin) and un upper frequency limit (fmax)), a ground plane, a switch for selectively connecting (only) one of the at least two antenna elements to the communication circuitry, and, optionally, one or more matching networks. In some embodiments, one or more of the at least two antenna elements is acting as a parasitic element for at least one driven antenna element which is operating within its frequency range around its central frequency.
In some embodiments each of the antenna elements covers a certain portion of the required bandwidth, whereby the antenna elements complement each other. In some embodiments, when the antenna element tuned at the higher frequency is under operation (receiving), the antenna element tuned at the lower frequency is acting as a parasitic element, causing the gain curve of the antenna system when only the antenna element tuned to the higher frequency is under operation to feature an additional local maximum at a lower frequency. On the other hand, in some cases where the antenna element tuned at the lower frequency is active, the antenna element tuned at the higher frequency is typically not acting as a parasitic element, although it is also possible to arrange it to act as a parasitic element, if necessary or convenient. In any case, this arrangement, that is, the use of at least one antenna element to act as a parasitic element for another antenna element, can create one or more local maxima of gain of the antenna system while only the other antenna element is under operation, thereby improving the gain of said antenna system in a certain sub-band.
More specifically, said at least two antenna elements can comprise a first antenna element having a first electrical length and a second antenna element having a second electrical length, said first electrical length being larger than said second electrical length, wherein, for said frequency band having a lower frequency limit (fmin) and un upper frequency limit (fmax), said first antenna element is arranged to have a maximum of gain at a first frequency (f1), and said second antenna element is arranged to have a maximum of gain at a second frequency (f2), said second frequency being higher than said first frequency, both said first frequency and said second frequency being frequencies within said frequency band (fmin<f1<f2<fmax), wherein at least one of said first and second antenna elements is arranged to act as a parasitic element for another one of said first and second antenna elements so that the antenna system has a local maximum of gain at a third frequency (f3) substantially different from said first frequency (f1) and second frequency (f2), said third frequency being a frequency within said frequency band.
Thus, by “creating” this (further) local maximum at the third frequency, the corresponding portion of the “gain” curve can be raised, thus lifting the gain curve over the specified level at and around said third frequency. This can help to increase the bandwidth of the antenna system.
Said third frequency can be lower than said first frequency.
Said “another one” of said first and second antenna elements can be the second antenna element, so that said first antenna element acts as a parasitic element for said second antenna element at said third frequency. This arrangement may be easier to implement than the other way around, and may thus be preferred by some antenna designers.
At least one of said first and second antenna elements can be connected to ground through a matching network including, at least, one inductance or capacitance or both. A matching network can be as simple as made of passive components (such as an inductance or a capacitance) or more complex (for example, comprising active components). A simple matching network including one inductance increases the electrical length of the antenna element and therefore reduces the natural resonant frequency of said antenna element. On the other hand a matching network including one capacitance reduces the electrical length of the antenna element and therefore increases the natural resonant frequency of said antenna element.
In some embodiments of the invention, said at least “another one” of said first and second antenna elements may not be connected to ground through a matching network.
In some embodiments of the invention, which may be more laborious from an antenna designer's point of view (for example, when choosing the relevant antenna parameters for a specific case), said second antenna element can be arranged to act as a parasitic element for said first antenna element, so that said antenna system has a local maximum of gain at a fourth frequency (f4) different from said first frequency (f1), said second frequency (f2) and said third frequency (f3), said fourth frequency (f4) being a frequency within said frequency band. This can even further help to increase the relevant bandwidth of the antenna system.
The combination of two or more antenna elements according to the present invention makes it possible to obtain a gain that could not be easily obtained by a single antenna. At the same time the PCB (ground) can be kept small, even as small as, for example, 90×40 mm or smaller, which makes it possible to reduce the overall size and weight of the portable or handheld device.
The two aspects described above can be combined in the same device.
A further aspect of the invention, which can be combined with one or more of the aspects described above, and which has been found to be especially useful for handheld devices for digital video services, especially for digital video reception within an operating band encompassing the frequency band of 470 MHz-770 MHz (or, for example, 470 MHz-702 MHz) (such as the DVB-H operating band), is based on devices having two ground-planes, or, rather, one ground-plane having two portions. Basically, this aspect of the invention relates to a wireless portable device for radio communication, comprising radio frequency communication circuitry (for example, for DVB-H) and an antenna system comprising at least one antenna element and a ground-plane comprising two electrically interconnected conductive portions that are interconnected by at least two conductive strips (2230, 2240).
It is known to use one conductive strip to interconnect two portions of a ground-plane, in, for example, clam-shell devices for radio communication. However, it has been found that, at least when receiving in an operating band of 470 MHz-770 MHz, the gain can be improved over a substantial portion of the operating band when using two strips to interconnect the portions of the groundplane.
Typically in a clam-shell device for radio communication having two ground-planes, or one ground-plane having two portions, said ground-planes or portions are interconnected by means of a flexfilm for at least data communication and signaling purposes. Said flexfilm can feature an electrical connection between said ground-planes or portions to establish a common grounding. It has been found that by adding a second connection between said ground-planes or portions of the ground-plane the gain can be improved. Said second connection can be implemented as a conducting strip connecting those ground-planes or portions (said conducting strip can comprise a conductive metal strip and, optionally, some lumped elements such as capacitors and/or inductors).
The at least two conductive strips can interconnect a first end portion of a first one of said electrically conductive portions and a second end portion of a second one of said electrically conductive portions. For example, if said electrically conductive portions are substantially rectangular, each of said electrically conductive portions having two shorter sides and two longer sides, said first end portion can correspond to a shorter side of said first one of said electrically conductive portions and said second end portion corresponding to a longer side of said second one of said electrically conductive portions.
The two conductive portions can be pivotally arranged with respect to each other, for example, each portion can be housed in a separate body portion of a device comprising at least two pivotally arranged body portions.
The antenna element can be arranged at one end of a first one of said electrically conductive portions, and said at least two conductive strips can be arranged at an opposite end of said first one of said electrically conductive portions, for examples, near to respective opposite ends of said opposite end of said first one of said electrically conductive portions.
The use of at least two or preferably two conductive strips for interconnecting the ground-plane portions have proved to be especially advantageous at least when a non-resonant antenna system is used, as the one described above.
The antenna system arrangement according to the present invention is compatible with the use of other antenna elements for the coverage of cellular mobile services (such as for instance GSM850, GSM900, GSM1800, GSM1900, UMTS, CDMA, W-CDMA, CDMA2000, . . . ) since it can provide for the reception of TV signals with minimum coupling or disturbance of the cellular/mobile antenna.
In some embodiments of the invention, at least one antenna element can comprise at least one conductive portion and a plurality of switches arranged in said conductive portion, said switches being arranged for selectively setting the effective electrical length of the antenna element to one of a plurality of values, for tuning the antenna element. This arrangement makes it possible to electronically tune an antenna element and system already incorporated into a device, such as a handset.
At least one antenna element can be arranged (or at least substantially arranged) over a ground-plane free area, that is, over an area where there is no ground plane in correspondence with the foot-print of the antenna or, at least, in correspondence with part of said foot-print. This has been found to make it possible to increase the gain of the antenna system.
The abovementioned lower and upper frequency limits will depend on the service to be covered by the device. For example, for DVB-H services, said lower frequency limit can be around 400-500 MHz, for example, (approximately) 470 MHz, and said upper frequency limit can be around 650-800 MHz, for example, around 700-800 MHz, for example, around 702 or 770 MHz. In some embodiments suitable for FM communication, said lower frequency limit can be less than or around 88 MHz, and said upper frequency limit can be above or around 108 MHz. In some embodiments suitable for T-DMB, said lower frequency limit can be around 180-186 MHz and said upper frequency limit can be around 204-210 MHz.
The device can be a handheld device, such as a handset for cellular telecommunication. It can also be any other kind of portable device including signal processing circuitry, such as a portable computer with means for wireless communication, etc.
The device can include means for processing digital television signals and for displaying corresponding video images.
The device can be a device adapted for receiving digital television signals.
In accordance with some embodiments of the invention, the antenna elements (or at least one of them) can include a portion shaped as a space-filling curve. This can further help to reduce the size of the antenna.
Said curve can, for example, comprise at least five segments, wherein each of said at least five segments forms an angle with each adjacent segment in said curve, wherein at least three of the at least five segments of said curve are shorter than one-fifth of the longest free-space operating wavelength of the antenna, wherein each angle between adjacent segments is less than 180°, and at least two of the angles between adjacent sections are less than approximately 115°. This is considered to be helpful to reduce the size of the antenna. The curve can, for example, be arranged such that at least two of the angles are defined respectively in the clockwise and counter-clockwise directions at opposite sides of the curve. Said at least two angles can be smaller than 180°, for example, smaller than 115°.
In some embodiments of the invention, at least three of the at least five segments of said curve are shorter than one-tenth of the longest free-space operating wavelength of the antenna. In some embodiment of the invention, a majority of the at least five segments of said curve are shorter than one-fifth of the longest free-space operating wavelength of the antenna.
In accordance with some embodiments of the invention, at least one of said antenna elements can include a portion shaped as a box-counting curve. Said curve can, for example, have a box-counting dimension larger than 1.15, 1.5 and/or 2.
In accordance with some embodiments of the invention, at least one of said antenna elements can include a portion shaped as a grid dimension curve; this curve can have a grid dimension larger than 1.15, 1.5 and/or 2.
The curve can be fitted over a flat or curved surface.
In accordance with some embodiments of the invention, the curve can comprise segments that are arranged in a self-similar way with respect to the entire curve, so that the curve is a self-similar curve.
In accordance with some embodiments of the invention, said curve can be a curve selected from the group consisting essentially of the Hilbert, Peano, SZ, ZZ, HilbertZZ, Peanoinc, Peanodec, and PeanoZZ curves (cf. WO-A-01/54225 which describes these curves and which is incorporated herein by reference):
In accordance with some embodiments of the invention, the segments of the curve can be arranged in a dissimilar way with respect to the entire curve, whereby the curve is not a self-similar curve.
In accordance with some embodiments of the invention, each antenna element fits in a rectangle the largest side of which has a length that does not exceed one-fifth of the longest free-space operating wavelength of the antenna element, or even one-twentieth of the longest free-space operating wavelength of the antenna.
In accordance with some embodiments of the invention, at least one of said antenna elements includes a portion having a multi-level structure.
Of course, although the different aspects of the invention disclosed herein can be applied or implemented separately, it is also possible to combine them, when appropriate.
In some embodiments, the antenna system of the portable or handheld device can be re-used by the cellular, wireless or mobile service to enhance the transmission or reception of wireless or mobile signals. This can be achieved by switching off the TV service while connecting the TV antenna subsystem to the mobile/cellular subsystem. Such a connection can be made through a radio-frequency (RF) ground-plane embedded on the PCB of the device.
One of the advantages of certain embodiments of the present invention is that the device can be able to keep its performance under normal operating conditions, also when the user is holding the device with his/her hand and/or close to his/her body. The particular arrangement of the antenna inside the device, according to certain aspects of the present invention, makes it possible to minimize the effect of the human body on the signal reception, and in some cases, even to enhance the reception of the TV signal. It has been observed that a direct contact between the user and the ground-plane can improve reception. Thus, it is considered that, at least in certain cases, it can be preferred to provide the wireless device with a conductive or metal external surface or casing, for example, by painting a plastic casing with a metallic or other type of conductive paint or coating, so that when the user touches the device during use of the device, the user will be in contact with the ground-plane. This has been found to improve DVB-H performance, especially at the medium and lower frequencies of the 470 MHz-770 MHz band.
Although one of the main objects of the present invention is a device, antenna system and means to receive digital TV signal, the person skilled in the art will notice that the present invention might be useful for the reception of other analog or digital signals with similar requirements in terms of bandwidth and gain. As well, a device, antenna system and method according to the present invention may be useful for transmission of digital or analog signals in a portion or even the whole bandwidth (with a lower frequency limit (fmin) and an upper frequency limit (fmax)) provided that the efficiency is sufficient in said portion or in the whole band.
FIG. 1—Example of how to calculate the box counting dimension.
FIG. 2—Examples of space filling curves for antenna design.
FIG. 3—Example of how to calculate the box counting dimension using a grid of rectangular cells to divide the smallest possible rectangle enclosing the curve.
FIG. 4—Example of how to calculate the box counting dimension using a grid of substantially square cells.
FIG. 5—Example of a curve featuring a grid-dimension larger than 1, referred to herein as a grid-dimension curve.
FIG. 6—The curve of
FIG. 7—The curve of
FIG. 8—The curve of
FIG. 9—Perspective view of an antenna system comprising a ground plane and two antenna elements and detailed perspective view of the two antenna elements.
FIG. 10A—Perspective top view and perspective bottom view of an antenna system comprising a ground plane and three antenna elements. Two antenna elements can be seen in the top view and the third antenna element can be seen in the bottom view.
FIG. 10B—shows the feeding means for the three antenna elements of the antenna system of
FIG. 11—shows an arrangement for tuning one or more of the antenna elements.
FIG. 12—is a schematic perspective view of an arrangement including a non-resonant antenna mounted in correspondence with an area in which the ground-plane has been removed.
FIG. 13—schematically illustrates some components of a circuit including a non-resonant antenna.
FIG. 14—shows an impedance diagram schematically illustrating how the real and imaginary parts of the input impedance of the antenna vary according to the frequency.
FIGS. 15A-15C—show some alternative non-resonant antenna designs, as well as their corresponding Smith charts.
FIG. 15D—illustrates a radiation efficiency vs. frequency diagram for the antennas of
FIG. 16—illustrates an alternative antenna design, wherein an antenna element 1601 is substantially shaped in accordance with the Hilbert curve.
FIG. 17—illustrates how the “at least one” matching network can comprise a plurality of matching networks.
FIG. 18—illustrates the simulated frequency response of an antenna system based on a Hilbert antenna substantially as illustrated in
FIGS. 19A-19B—illustrates the gain using different matching networks.
FIGS. 20A-20B—schematically illustrates an arrangement including two antenna elements tuned to different central frequencies, and the corresponding gain curves for said antenna elements, including local maxima produced due to parasitic effects, respectively.
FIGS. 21A-21B—schematically illustrate the analogous aspects of an alternative arrangement,
FIGS. 22A-22H—schematically illustrates a comparison of two different ways of interconnecting two ground-plane portions, according to prior art and according to the invention, respectively, as well as the computed current distribution when a non-resonant antenna is used.
FIG. 23A-23B—schematically illustrate the gain curves at the horizontal plane when using one and two strips for interconnecting the two ground-plane portions, respectively.
Although the illustrated examples relate to cases in which the input impedance of the antenna is capacitive within the relevant frequency band (that is, the imaginary part of the input impedance is negative for said frequency band), the invention could also be implemented with an inductive input impedance of the antenna (that is, featuring a positive imaginary part of the input impedance for the relevant frequency band).
It has been found that when trying to reduce the size of a non-resonant antenna while substantially maintaining the basic antenna shape, there is a reduction in efficiency, especially at low frequencies, for example, at frequencies close to the lower end of the above-mentioned 470-770 MHz band.
This can be useful in order to improve the received signal within a certain “sub-band”. Thus, this arrangement can compensate a certain loss of efficiency of the antenna, and thus allow for further miniaturisation of the antenna, while maintaining the general performance of the antenna system above the minimum levels specified for a certain application, for example, for DVB-H services.
Matching network 1321 (470-500 MHz): Z1=9.1 nH, Z2=36 nH
Matching network 1322 (550-590 MHz): Z1=6.2 nH, Z2=22 nH
Matching network 1323 (600-650 MHz): Z1=7.5 nH, Z2=15 nH
Matching network 1324 (720-770 MHz): Z1=8.7 nH, Z2=4.7 nH
Line 2010 schematically illustrates a standard specification for minimum gain along a frequency band fmin−fmax (which could be the interval of 470-770 MHz). Now, if the two antenna elements would be completely independent, it would be difficult to obtain sufficient gain all over the frequency spectrum. However, this can be overcome by means of letting at least one of said antenna elements act as a parasitic element for another one of the antenna elements. Here, the first antenna element has been arranged to act as a parasitic element for the second antenna element, so that the second antenna element has a local maximum of gain at a third frequency (f3) different from said first frequency (f1) and second frequency (f2). In this way, the gain of the second antenna element improves at the lower end of the frequency band (this has been schematically illustrated in
Also, in accordance with one possible arrangement, the second antenna element can also be arranged to act as a parasitic element for the first antenna element at certain frequencies, which could imply a local maximum of gain of the first antenna element at a fourth frequency (f4), as schematically illustrated in
Normally, it can be enough that one of the antenna elements acts as a parasitic element for the other one, as illustrated in
Each of said two antenna elements in
Each of said two antenna elements is tuned to a different frequency. In some embodiments, said frequencies are within the desired TV band (such as for instance the 470-770 MHz band), while in some embodiments, one or both are tuned to a center frequency outside said band.
The antenna system including said two antennas can be mounted close to one edge of the PCB. In case of elongated PCBs and devices, the antenna system can, for example, be placed substantially close to a shorter edge of said PCB.
At least one portion of the ground layer or ground plane 2004 within the footprint of the antenna system on the PCB can be removed, leaving a clearance 900 on the ground layer of the PCB (as shown in
On the other hand,
It has been observed that using the two-strip connection, a better gain can be obtained at least in the higher range of the 470 MHz-770 MHz frequency band.
In the embodiment illustrated in
In some examples, one or more of the antenna elements may be miniaturized by shaping at least a portion of the antenna element (e.g., a part of an arm in a dipole or in a monopole, a perimeter of the patch of a patch antenna, the slot in a slot antenna, the loop perimeter in a loop antenna or in a gap-loop antenna, or other portions of the antenna) as a space-filling curve (SFC). Examples of space filling curves (including for instance the Hilbert curve or the Peano curve) are shown in
A space-filling curve can be fitted over a flat surface, a curved surface, or even over a surface that extends in more than one plane, and due to the angles between segments, the physical length of the curve is larger than that of any straight line that can be fitted in the same area (surface) as the space-filling curve. Additionally, to shape the structure of a miniature antenna, the segments of the SFCs should be shorter than at least one fifth of the free-space operating wavelength, and possibly shorter than one tenth of the free-space operating wavelength. Moreover, in some further examples the segments of the SFCs should be shorter than at least one twentieth of the free-space operating wavelength. The space-filling curve should include at least five segments in order to provide some antenna size reduction; however a larger number of segments may be used, such as for instance 10, 15, 20, 25 or more segments. In general, the larger the number of segments and the narrower the angles between them, the smaller the size of the final antenna. An antenna shaped as a SFC is small enough to fit within a radian sphere (e.g., a sphere with a radius equal to the longest free-space operating wavelength of the antenna divided by 2π). However, the antenna features a resonance frequency lower than that of a straight line antenna substantially similar in size.
A SFC may also be defined as a non-periodic curve including a number of connected straight, substantially straight and/or curved segments smaller than a fraction of the longest operating free-space wavelength, where the segments are arranged in such a way that no adjacent and connected segments form another longer straight segment and wherein none of said segments intersect each other.
Alternatively, a SFC can be defined as a non-periodic curve comprising at least a minimum number of bends, wherein the distance between each pair of adjacent bends is shorter than a tenth of the longest free-space operating wavelength. Possible values of said minimum number of bends include 5, 10, 15, 20 and 25. In some examples, the distances between pairs of consecutive bends of the SFC are different for at least two pairs of bends. In some other examples, the radius of curvature of each bend is smaller than a tenth of the longest operating free-space wavelength.
Yet another definition of a SFC is that of a non-periodic curve comprising at least a minimum number of identifiable cascaded sections. Each section of the SFC forms an angle with other adjacent sections, and each section has a diameter smaller than a tenth of the longest free-space operating wavelength. Possible values of said minimum number of identifiable cascaded sections include 5, 10, 15, 20 and 25.
In one example, an antenna geometry forming a space-filling curve may include at least five segments, each of the at least five segments forming an angle with each adjacent segment in the curve, at least three of the segments being shorter than one-tenth of the longest free-space operating wavelength of the antenna. Preferably each angle between adjacent segments is less than 180° and at least two of the angles between adjacent sections are less than 115°, and at least two of the angles are not equal. The example curve fits inside a rectangular area, the longest side of the rectangular area being shorter than one-fifth of the longest free-space operating wavelength of the antenna. Some space-filling curves might approach a self-similar or self-affine curve, while some others would rather become dissimilar, that is, not displaying self-similarity or self-affinity at all (see for instance 210, 211, 212).
In other examples, one or more of the antenna elements may be miniaturized by shaping at least a portion of the antenna element to have a selected box-counting dimension. For a given geometry lying on a surface, the box-counting dimension is computed as follows. First, a grid with rectangular or substantially squared identical boxes of size L1 is placed over the geometry, such that the grid completely covers the geometry, that is, no part of the curve is out of the grid. The number of boxes N1 that include at least a point of the geometry are then counted. Second, a grid with boxes of size L2 (L2 being smaller than L1) is also placed over the geometry, such that the grid completely covers the geometry, and the number of boxes N2 that include at least a point of the geometry are counted. The box-counting dimension D is then computed as:
For the purposes of this document, the box-counting dimension may be computed by placing the first and second grids inside a minimum rectangular area enclosing the conducting trace of the antenna and applying the above algorithm. The first grid in general has n×n boxes and the second grid has 2n×2n boxes matching the first grid. The first grid should be chosen such that the rectangular area is meshed in an array of at least 5×5 boxes or cells, and the second grid should be chosen such that L2=½ L1 and such that the second grid includes at least 10×10 boxes. The minimum rectangular area is an area in which there is not an entire row or column on the perimeter of the grid that does not contain any piece of the curve. Further the minimum rectangular area preferably refers to the smallest possible rectangle that completely encloses the curve or the relevant portion thereof.
An example of how the relevant grid can be determined is shown in
Alternatively the grid may be constructed such that the longer side (see left edge of rectangle in
If the value of D calculated by a first n×n grid of identical rectangular boxes (
The desired box-counting dimension for the curve may be selected to achieve a desired amount of miniaturization. The box-counting dimension should be larger than 1.1 in order to achieve some antenna size reduction. If a larger degree of miniaturization is desired, then a larger box-counting dimension may be selected, such as a box-counting dimension ranging from 1.5 to 2 for surface structures, while ranging up to 3 for volumetric geometries. For the purposes of this patent document, curves in which at least a portion of the geometry of the curve or the entire curve has a box-counting dimension larger than 1.1 may be referred to as box-counting curves.
Alternatively a curve may be considered as a box counting curve if there exists a first n×n grid of identical square or identical rectangular boxes and a second 2n×2n grid of identical square or identical rectangular boxes where the value of D is larger than 1.1, 1.15, 1.2, 1.25, 1.3, 1.35, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, or 2.9.
In any case, the value of n for the first grid should not be more than 5, 7, 10, 15, 20, 25, 30, 40 or 50.
For very small antennas, for example antennas that fit within a rectangle having a maximum size equal to one-twentieth the longest free-space operating wavelength of the antenna, the box-counting dimension may be computed using a finer grid. In such a case, the first grid may include a mesh of 10×10 equal cells, and the second grid may include a mesh of 20×20 equal cells. The grid-dimension (D) may then be calculated using the above equation.
In general, for a given resonant frequency of the antenna, the larger the box-counting dimension, the higher the degree of miniaturization that will be achieved by the antenna.
One way to enhance the miniaturization capabilities of the antenna (that is, reducing size while maximizing bandwidth, efficiency and gain) is to arrange the several segments of the curve of the antenna pattern in such a way that the curve intersects at least one point of at least 14 boxes of the first grid with 5×5 boxes or cells enclosing the curve. If a higher degree of miniaturization is desired, then the curve may be arranged to cross at least one of the boxes twice within the 5×5 grid, that is, the curve may include two non-adjacent portions inside at least one of the cells or boxes of the grid. The relevant grid here may be any of the above mentioned constructed grids or may be any grid. That means if any 5×5 grid exists with the curve crossing at least 14 boxes or crossing one or more boxes twice the curve may be said to be a box counting curve.
The terms explained above can be also applied to curves that extend in three dimensions. If the extension in the third dimension is rather small the curve will fit into an n×n×1 arrangement of 3D-boxes (cubes of size L1×L1×L1) in a plane. Then the calculations can be performed as described above. Here the second grid will be a 2n×2n×1 grid of cuboids of size L2×2×L1.
If the extension in the third dimension is larger an n×n×n first grid and a 2n×2n×2n second grid will be taken into account. The construction principles for the relevant grids as explained above for two dimensions apply equally in three dimensions.
In yet other examples, one or more of the antenna elements may be miniaturized by shaping at least a portion of the antenna element to include a grid dimension curve. For a given geometry lying on a planar or curved surface, the grid dimension of the curve may be calculated as follows. First, a grid with substantially square identical cells of size L1 is placed over the geometry of the curve, such that the grid completely covers the geometry, and the number of cells N1 that include at least a point of the geometry are counted. Second, a grid with cells of size L2 (L2 being smaller than L1) is also placed over the geometry, such that the grid completely covers the geometry, and the number of cells N2 that include at least a point of the geometry are counted again. The grid dimension D is then computed as:
For the purposes of this document, the grid dimension may be calculated by placing the first and second grids inside the minimum rectangular area enclosing the curve of the antenna and applying the above algorithm. The minimum rectangular area is an area in which there is not an entire row or column on the perimeter of the grid that does not contain any piece of the curve.
The first grid may, for example, be chosen such that the rectangular area is meshed in an array of at least 25 substantially equal preferably square cells. The second grid may, for example, be chosen such that each cell of the first grid is divided in 4 equal cells, such that the size of the new cells is L2=½ L1, and the second grid includes at least 100 cells.
Depending on the size and position of the squares of the grid the number of squares of the smallest rectangular may vary. A preferred value of the number of squares is the lowest number above or equal to the lower limit of 25 identical squares that arranged in a rectangular or square grid cover the curve or the relevant portion of the curve. This defines the size of the squares. Other preferred lower limits here are 50, 100, 200, 250, 300, 400 or 500. The grid corresponding to that number in general will be positioned such that the curve touches the minimum rectangular at two opposite sides. The grid may generally still be shifted with respect to the curve in a direction parallel to the two sides that touch the curve. Of such different grids the one with the lowest value of D is preferred. Also the grid whose minimum rectangular is touched by the curve at three sides (see as an example
The desired grid dimension for the curve may be selected to achieve a desired amount of miniaturization. The grid dimension should be larger than 1 in order to achieve some antenna size reduction. If a larger degree of miniaturization is desired, then a larger grid dimension may be selected, such as a grid dimension ranging from 1.5-3 (e.g., in case of volumetric structures). In some examples, a curve having a grid dimension of about 2 may be desired. For the purposes of this patent document, a curve or a curve where at least a portion of that curve is having a grid dimension larger than 1 may be referred to as a grid dimension curve. In some cases, a grid dimension curve will feature a grid dimension D larger than 1.1, 1.15, 1.2, 1.25, 1.3, 1.35, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, or 2.9.
In general, for a given resonant frequency of the antenna, the larger the grid dimension the higher the degree of miniaturization that will be achieved by the antenna.
One example way of enhancing the miniaturization capabilities of the antenna is to arrange the several segments of the curve of the antenna pattern in such a way that the curve intersects at least one point of at least 50% of the cells of the first grid with at least 25 cells (preferably squares) enclosing the curve. In another example, a high degree of miniaturization may be achieved by arranging the antenna such that the curve crosses at least one of the cells twice within the 25 cell grid (of preferably squares), that is, the curve includes two non-adjacent portions inside at least one of the cells or cells of the grid. In general the grid may have only a line of cells but may also have at least 2 or 3 or 4 columns or rows of cells.
For a more accurate calculation of the grid dimension, the number of square cells may be increased up to a maximum amount. The maximum number of cells in a grid is dependent upon the resolution of the curve. As the number of cells approaches the maximum, the grid dimension calculation becomes more accurate. If a grid having more than the maximum number of cells is selected, however, then the accuracy of the grid dimension calculation begins to decrease. Typically, the maximum number of cells in a grid is one thousand (1000).
For example,
It should be understood that a grid-dimension curve does not need to include any straight segments. Also, some grid-dimension curves might approach a self-similar or self-affine curves, while some others would rather become dissimilar, that is, not displaying self-similarity or self-affinity at all (see for instance
The terms explained above can be also applied to curves that extend in three dimensions. If the extension in the third dimension is rather small the curve will fit into an arrangement of 3D-boxes (cubes) in a plane. Then the calculations can be performed as described above. Here the second grid will be composed in the same plane of boxes with the size L2×L2×L1.
If the extension in the third dimension is larger an m×n×o first grid and a 2m×2n×2o second grid will be taken into account. The construction principles for the relevant grids as explained above for two dimensions apply equally in three dimensions. Here the minimum number of cells preferably is 25, 50, 100, 125, 250, 400, 500, 1000, 1500, 2000, 3000, 4000 or 5000.
In another example, at least a portion of one or more of the antenna elements may be coupled, either through direct contact or electromagnetic coupling, to a conducting surface, such as a conducting polygonal or multilevel surface. Further, the antenna element may include the shape of a multilevel structure. A multilevel structure is formed by gathering several identifiable geometrical elements such as polygons or polyhedrons of the same type or of different type (e.g., triangles, parallelepipeds, pentagons, hexagons, circles or ellipses as special limiting cases of a polygon with a large number of sides, as well as tetrahedral, hexahedra, prisms, dodecahedra, etc.) and coupling these structures to each other electromagnetically, whether by proximity or by direct contact between elements.
At least two of the elements may have a different size. However, also all elements may have the same or approximately the same size. The size of elements of a different type may be compared by comparing their largest diameter. The polygons or polyhedrons of a multilevel structure may comprise straight, flat and/or curved peripheral portions. Some polygons or polyhedrons may have perimeter portions comprising portions of circles and/or ellipses.
The majority of the component elements of a multilevel structure have more than 50% of their perimeter (for polygons) or of their surface (for polyhedrons) not in contact with any of the other elements of the structure. In some examples, the said majority of component elements would comprise at least the 50%, 55%, 60%, 65%, 70% or 75% of the geometric elements of the multilevel structure. Thus, the component elements of a multilevel structure may typically be identified and distinguished, presenting at least two levels of detail: that of the overall structure and that of the polygon or polyhedron elements which form it. Additionally, several multilevel structures may be grouped and coupled electromagnetically to each other to form higher level structures. In a single multilevel structure, all of the component elements are polygons with the same number of sides or are polyhedrons with the same number of faces. However, this characteristic may not be true if several multilevel structures of different natures are grouped and electromagnetically coupled to form meta-structures of a higher level.
A multilevel antenna includes at least two levels of detail in the body of the antenna: that of the overall structure and that of the majority of the elements (polygons or polyhedrons) which make it up. This may be achieved by ensuring that the area of contact or intersection (if it exists) between the majority of the elements forming the antenna is only a fraction of the perimeter or surrounding area of said polygons or polyhedrons. The elements (polygons or polyhedrons) are identifiable by their exposed edges and, when there is contact or overlapping between elements, by the extension of their exposed edges (such as for example through projection) into said region of contact or overlapping.
One example property of a multilevel antenna is that the radioelectric behavior of the antenna can be similar in more than one frequency band. Antenna input parameters (e.g., impedance) and radiation patterns remain substantially similar for several frequency bands (i.e., the antenna has the same level of impedance matching or standing wave relationship in each different band), and often the antenna presents almost identical radiation diagrams at different frequencies. Such a property allows the antenna to operate simultaneously in several frequencies, thereby being able to be shared by several communication devices. The number of frequency bands is proportional to the number of scales or sizes of the polygonal elements or similar sets in which they are grouped contained in the geometry of the main radiating element.
In a multilevel antenna operating in several frequency bands, different subsets of geometrical elements of the multilevel structure are associated with the different frequency bands of the antenna. In some cases for example, the overall structure can be responsible for one frequency, and different subsets of geometrical elements within the structure be responsible for other frequency bands. In some examples, a first subset of geometrical elements can comprise at least some of the geometrical elements of a second subset, while in other cases the first subset may comprise a majority of the geometrical elements of the second subset (i.e., the second subset is substantially within the first subset).
In addition to their multiband behavior, multilevel structure antennae may have a smaller than usual size as compared to other antennae of a simpler structure (such as those consisting of a single polygon or polyhedron) operating at the same frequency. The empty spaces defined within the multilevel structure provide a long and winding path for the electrical currents, making the antenna resonate at a lower frequency than that of a radiating structure not including said empty spaces. Additionally, the edge-rich and discontinuity-rich structure of a multilevel antenna may enhance the radiation process, relatively increasing the radiation resistance of the antenna and/or reducing the quality factor Q (i.e., increasing its bandwidth).
A multilevel antenna structure may be used in many antenna configurations, such as dipoles, monopoles, patch or microstrip antennae, coplanar antennae, reflector antennae, aperture antennae, antenna arrays, or other antenna configurations. In addition, multilevel antenna structures may be formed using many manufacturing techniques, such as printing on a dielectric substrate by photolithography (printed circuit technique); dying on metal plate, repulsion on dielectric, or others.
| Number | Date | Country | Kind |
|---|---|---|---|
| 06113473.0 | May 2006 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2006/007782 | 8/2/2006 | WO | 00 | 11/14/2008 |
| Number | Date | Country | |
|---|---|---|---|
| 60788857 | Apr 2006 | US |
