WO2004057697A2

WO2004057697A2 - Antenna with rapid frequency switching

Info

Publication number: WO2004057697A2
Application number: PCT/IL2003/001061
Authority: WO
Inventors: Asher Peled; Ehud Heyman; Ben-Zion Steinberg; Rafael Kastner; Amir Boag
Original assignee: Xellant Mop Israel Ltd.
Priority date: 2002-12-19
Filing date: 2003-12-11
Publication date: 2004-07-08
Also published as: AU2003285741A8; AU2003285741A1; WO2004057697A3

Abstract

An antenna assembly (20) for a communication device (21) includes an antenna (22) and a tuning circuit (32). The tuning circuit includes a network of impedance elements (42, 44, 52, 54), which are coupled to load the antenna with a complex impedance such that the antenna assembly has first and second resonant frequencies in distinct first and second bands. At least one switch (56) is coupled to switch a configuration of the impedance elements so as to vary the impedance of the network, thus tuning the first and second resonant frequencies simultaneously.

Description

ANTENNA WITH RAPID FREQUENCY SWITCHING

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Patent Application 60/434,582, filed December 19, 2002, whose disclosure is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to antennas, and specifically to antennas whose frequency response can be switched over multiple bands. BACKGROUND OF THE INVENTION

Tunable antennas are known in the art. For example, U.S. Patent 6,614,399, whose disclosure is incorporated herein by reference, describes a multi-band compact tunable directional antenna for wireless communication devices. The antenna provides resonance over at least two frequency bands with a directional radiation pattern and reduced specific absorption rate (SAR) . The antenna structure includes a conducting plate resonator, which is spaced from a larger, rectangular ground plane conductor. The size and shape of the resonator and ground plane are compatible with the dimensions of wireless communications devices such as cell phones.

The antenna structure described in U.S. Patent 6,614,399 includes two or more tuning capacitors, which cause the resonator to resonate over two or more frequency ranges. The capacitors can be made variable by techniques such as switched fixed capacitors, which are selected by PIN diodes, or by using voltage-controlled capacitors ( "varactors") . In either case, the capacitance value may be controlled electrically or by a digital command signal. The use of variable capacitors is said to facilitate seamless roaming across cellular service regions having different frequency allocations for particular bands.

As another example, U.S. Patent 6,255,994, whose disclosure is incorporated herein by reference, describes an inverted-F antenna, which is capable of coping with changes in available frequency bands. The antenna comprises a radiating element and a ground conductor, which is arranged to be opposite to the radiating element with a specific gap. A feeding terminal and a grounding terminal are electrically connected to the radiating element. At least one impedance element (an inductance or capacitance) is provided in a line connecting the grounding terminal to the ground conductor. A switch is used for selectively inserting the impedance element into the line. The resonant frequency of the antenna is changed by operating the switch.

Other types of antennas with switchable frequency response are described in U.S. Patents 6,300,909 and 5,585,810, whose disclosures are incorporated herein by reference.

SUMMARY OF THE INVENTION Embodiments of the present invention provide improved methods and devices for switching the resonant frequency of an antenna. A tuning circuit, which comprises a network of impedance elements, such as lumped capacitors and inductors, is arranged to load the antenna. One or more switches are coupled to engage or disengage selected impedance elements from the antenna. Each different setting of the switches changes the configuration of the impedance elements loading the antenna, and thus varies the resonant frequency of the antenna. In some embodiments, the impedance elements are arranged so that the antenna resonates in two different bands, and so that changing the setting of the switches tunes the frequencies of both resonant bands simultaneously.

In some embodiments of the present invention, the switches that are used for frequency tuning comprise field effect transistors (FETs) . These switches consume very low power, in contrast to the PIN diodes and varactors that are used in tunable antennas known in the art. Therefore, antenna assemblies produced in accordance with these embodiments are useful particularly (although not exclusively) in handheld, battery-powered mobile communication devices, such as cellular handsets and wireless computing devices. In some of these embodiments, a novel voltage divider circuit is provided in order to prevent radio frequency (RF) voltage overload of the switches during transmission.

Fast switching of antenna resonance, as described above, permits the use of narrowband, high-gain antennas, whose instantaneous bandwidth is less than the total bandwidth that the antenna must be capable of serving. To give the requisite band coverage, the antenna frequency may be switched as necessary between different sub-bands within the total bandwidth. For example, the antenna may be switched between the frequencies of different cells in a cellular network or different channels in a wireless local area network (WLAN) . When the communication device to which the antenna is coupled transmits and receives signals in different time slots, it is even possible to switch the antenna between the transmit and receive bands in the appropriate time slots. The use of a narrowband, high-gain antenna in this manner reduces the power consumption of the communication device and enhances signal quality. This approach also relaxes the design constraints on the antenna itself, since it is often difficult to achieve high bandwidth in the types of small antennas that are typically used in mobile communication devices, such as cellular telephones .

There is therefore provided, in accordance with an embodiment of the present invention, an antenna assembly for a communication device, including: an antenna; and a tuning circuit, including: a network of impedance elements, which are coupled to load the antenna with a complex impedance such that the antenna assembly has first and second resonant frequencies in distinct first and second bands; and at least one switch, which is coupled to switch a configuration of the impedance elements so as to vary the impedance of the network, thus tuning the first and second resonant frequencies simultaneously. In disclosed embodiments, , the at least one switch includes a field-effect transistor (FET) , and the impedance elements include one or more capacitors and one or more inductors. Typically, the network includes a resonant LC circuit, including at least a first one of the capacitors and at least one of the inductors, which is coupled in a series connection with at least a second one of the capacitors. The at least one switch may be coupled to switch the second one of the capacitors into and out of the series connection. In one embodiment, the series connection includes the second one of the capacitors and a third one of the capacitors, which is arranged in series with the second one of the capacitors, and the at least one switch is coupled to the network at a point intermediate the second one and the third one of the capacitors.

Additionally or alternatively, the resonant LC circuit is coupled in the series connection with the second one of the capacitors and with at least a third one of the capacitors, which is arranged in parallel with the second one of the capacitors, and the at least one switch includes at least first and second switches, which are coupled respectively to switch the second one and the third one of the capacitors into and out of the series connection.

In some embodiments, the antenna includes a surface having a slot opening therethrough, and at least one of the impedance elements is disposed across the slot. Typically, the surface has at least first and second slots opening therethrough, and the impedance elements include at least a first impedance element that is disposed across the first slot, and at least a second impedance element that is disposed across the second slot, wherein the first and second impedance elements are coupled to one another in series. Additionally or alternatively, the surface having the slot opening therethrough is a first surface of the antenna, and the antenna further includes a second surface, spaced from the first surface so as to define a cavity therebetween, wherein the slot opens into the cavity.

In a disclosed embodiment, the impedance elements include at least first and second impedance elements, which are arranged in series, and the at least one switch is coupled to the network at a point intermediate the first and second impedance elements.

In some applications of the present invention, the first resonant frequency is in a band between about 800 and 1000 MHz, and the second resonant frequency is in a band between about 1700 and 1900 MHz.

There is also provided, in accordance with an embodiment of the present invention, an antenna assembly for a communication device, including: an antenna; and a tuning circuit, including: a network of impedance elements, which are coupled to load the antenna with an impedance, which determines a resonant frequency of the antenna assembly, the network including at least first and second impedance elements arranged in series; and a switch, which is coupled to the network at a point intermediate the first and second impedance elements, and which is operative to switch a configuration of the impedance elements so as to vary the impedance of the network, thus tuning the resonant frequency.

Typically, the first and second impedance elements are configured to operate as a radio frequency (RF) voltage divider, so as to prevent a RF voltage overload on the switch while the communication device is transmitting a signal via the antenna.

In a disclosed embodiment, the first and second impedance elements include first and second capacitors, and the switch is configured to couple the second capacitor into and out of the series with the first capacitor. In one embodiment, the communication device is configured to transmit and receive signals via the antenna assembly in respective transmit and receive time slots at different transmit and receive frequencies, and the switch is operative to switch the configuration of the impedance elements so as to tune the resonant frequency between the transmit and receive frequencies, and the assembly further includes a controller, which is adapted to actuate the switch in synchronization with the time slots, so that the resonant frequency corresponds to the transmit frequency during the transmit time slots and to the receive frequency during the receive time slots.

Typically, the switch is operative to ground the point intermediate the first and second impedance elements during the receive time slots and to decouple the point from ground during at least some of the transmit time slots.

There is additionally provided, in accordance with an embodiment of the present invention, an antenna assembly for a communication device, which is configured to transmit and receive signals in respective transmit and receive time slots at different transmit and receive frequencies, the assembly including: an antenna; a tuning circuit, including: a network of impedance elements, which are coupled to load the antenna with an impedance, which determines a resonant frequency of the antenna; and at least one switch, which is coupled to alter a configuration of the impedance elements so as to vary the impedance, in order to tune the resonant frequency between the transmit and receive frequencies; and a controller, which is adapted to actuate the at least one switch in synchronization with the time slots, so that the resonant frequency corresponds to the transmit frequency during the transmit time slots and to the receive frequency during the receive time slots. There is further provided, in accordance with an embodiment of the present invention, a method for tuning an antenna assembly, including: coupling a network of impedance elements to load an antenna with a complex impedance such that the antenna assembly has first and second resonant frequencies in distinct first and second bands; and switching a configuration of the impedance elements so as to vary the impedance of the network, thus tuning the first and second resonant frequencies simultaneously.

There is moreover provided, in accordance with an embodiment of the present invention, a method for tuning an antenna assembly, including: coupling a network of impedance elements to load an antenna with an impedance, which determines a resonant frequency of the antenna assembly, the network including at least first and second impedance elements arranged in series; coupling a switch to the network at a point intermediate the first and second impedance elements; and actuating the switch so as to vary the impedance of the network, thus tuning the resonant frequency.

There is furthermore provided, in accordance with an embodiment of the present invention, a method for wireless communication, including: configuring a communication device to transmit and receive signals via an antenna in respective transmit and receive time slots at different transmit and receive frequencies; coupling a network of impedance elements to load the antenna with an impedance, which determines a resonant frequency of the antenna; and switching a configuration of the impedance elements so as to vary the impedance, in order to tune the resonant frequency between the transmit and receive frequencies in synchronization with the time slots, so that the resonant frequency corresponds to the transmit frequency during the transmit time slots and to the receive frequency during the receive time slots. The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1A is a schematic, pictorial illustration of a dual- band antenna assembly with switchable resonant frequencies, in accordance with an embodiment of the present invention;

Fig. IB is a schematic side view of the antenna assembly of Fig. 1A; Fig. 2 is a schematic frontal view of a tuning circuit used in a dual-band antenna assembly, in accordance with an embodiment of the present invention;

Fig. 3 is an electrical schematic diagram showing elements of an antenna tuning circuit, in accordance with an embodiment of the present invention;

Figs. 4A and 4B are schematic plots of the predicted reflection coefficient of a switchable antenna assembly as a function of frequency for different settings of a tuning circuit used in the antenna assembly, in accordance with an embodiment of the present invention; and

Figs. 5A and 5B are schematic plots of the measured reflection coefficient of a switchable antenna assembly as a function of frequency for different settings of a tuning circuit used in the antenna assembly, in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS Reference is now made to Figs. 1A and IB, which schematically illustrate an antenna assembly 20, in accordance with an embodiment of the present invention. Fig. 1A is a pictorial view, showing the antenna assembly mounted on a communication device 21, while Fig. IB presents a side view. Device 21 typically comprises a cellular handset or, alternatively, a wireless computing device, such as a personal digital assistant (PDA) with communication capability or a WLAN transceiver embedded in a computer. Alternatively, the principles of the present invention may be used in communication devices of other types, as well.

Assembly 20 comprises an antenna 22, which may protrude outside communication device 21, as shown in the figures, or may be mounted inside the case of the communication device. In the present example, antenna 22 is a cavity antenna, having a front conductive surface 24 and a rear conductive surface 26, which define a cavity therebetween. In an exemplary embodiment for cellular applications, antenna 22 is about 40 mm wide by 20 mm high, with a separation of about 5 mm between the front and rear surfaces. Alternatively, larger or smaller dimensions may be used. A RF feed 28, surrounded by a coaxial shield 30, couples front surface 24 to a transceiver circuit (not shown) in device 21. A switched tuning circuit 32 is connected in series between front surface 24 and ground. The tuning circuit is described in detail hereinbelow.

Alternatively, antennas of other types, as are known in the art, may be tuned using the approach embodied in assembly 20. For example, antenna 22 may comprise an inverted F or planar inverted F antenna. Other suitable antenna types for this sort of switched tuning include meander antennas, slot antennas, monopole and dipole antennas, and microstrip antennas (including patch antennas) . Modification required in each of these antenna designs to accommodate tuning circuit 32 will be apparent to those skilled in the art upon reading the present disclosure.

Fig. 2 is a schematic, frontal view of tuning circuit 32, in accordance with an embodiment of the present invention. Circuit 32 comprises a network of inductors and capacitors, coupled across two slots that open into the cavity of assembly

20: an upper slot 34 between the lower edge of front surface 24 and an intermediate conductor 36, and a lower slot 38 between conductor 36 and a ground 40. A set of fixed inductors 42

(having inductance Li) and capacitors 44 (with capacitance Ci) are connected in parallel across slot 34. Switched impedance units 45, 46, 47 and 48 are connected in parallel across slot 38, along with a fixed capacitor 52 (C₄) . Units 45 and 46 each

comprise a series capacitor 52 (C2) and a shunt capacitor 54

(C₅) , with a field-effect transistor (FET) 56 coupled between a point intermediate the capacitors and ground. Units 47 and 48 each comprise only series capacitor 42 (C3) and FET 56.

Each FET 56 is actuated (opened or closed) by changing the bias voltage that is applied to the gate (G) of the FET. When the gate is unbiased, the FET is switched on, so that its source and drain are galvanically engaged in an effective short circuit. When a suitable bias, typically +5 volts, is applied to the gate, the FET is switched off, so that there is effectively an open circuit between the source and drain.

The state of FETs 56 determines the capacitance of each of units 45, 46, 47 and 48, and thus determines the overall capacitance Op of the portion of circuit 32 that is coupled across lower slot 38. Table I below presents this impedance for a set of different states of the FETs:

TABLE I - LOWER SLOT CAPACITANCE STATES

In the table above, C₂R = C₂C₅/ (C₂+C₅) .

The total load impedance Zj_, of circuit 32 is obtained by

taking the capacitance C in series with the resonant LC impedance of the upper portion of circuit 32, which is coupled across slot 34 :

jω ιτ

ZL = ^~ + (1) ^ω T 1 - ω^zCι_TLι_T

Here LI and Cι are the total inductance and capacitance of the parallel inductors 42 and capacitors 44 in the upper portion of the circuit, and ω is the frequency in rad/sec.

The resonant frequencies of antenna assembly 20 are given by the roots CUQ of the following resonance condition:

Im Zχ,⁽-^y(j⁾

⁼ - 0Md ( 2 ]

Here Z^ is the wave impedance of the metallic body of antenna

22, which is given in the present case by the effective length d of the antenna, together with the permeability μ of the material filling the antenna (μo if the antenna is filled with

air) . Changing the states of FETs 56 changes CT, as

illustrated in Table I above, and therefore changes ZL, as

given by equation (1) , and with it the resonant frequencies CDQ of the antenna assembly. From the form of equation (1) , it can be seen that when circuit 32 is used, equation (2) has two roots, so that assembly 20 has two resonant frequencies. By suitable choice of the values of capacitance and resistance, these frequencies can be made to fall in two distinct operating bands of communication device 21. For example, for a cellular handset, one of the resonant frequencies can be set in the GSM band (880-900 MHz) , while the other is in the GPRS band (1710-1880 MHz) . Other frequency combinations will be apparent to those skilled in the art. A three-band antenna assembly may be designed using similar principles, using two parallel LC circuits with switched capacitors in series.

Fig. 3 is a schematic electrical diagram showing details of units 45 and 46 in tuning circuit 32, in accordance with an embodiment of the present invention. A bias voltage supply 60 is connected to the gate of FET 56 through resistors 62 and 64. A tuning control circuit 68 comprises a switch 66, which turns off the gate bias when closed. Similar switches are provided to control the other FETs in circuit 32. Alternatively, other means, as are known in the art, may be used by the control circuit to actuate FETs 56. Control circuit 68 is typically contained in communication device 21, and is operative to turn the gate bias to each of FETs 56 on and off selectively in order to set the state of tuning circuit 32. Typically, control circuit 68 sets switches 66 according to the sub-band on which device 21 is to communicate, and changes the settings when the sub-band selection is to change. A change in the switch settings generally changes the sub-band selection simultaneously in both the upper and lower operating bands served by antenna assembly 20. In some applications, such as cellular communications according to GSM and TDMA standards, communication device 21 transmits signals in one frequency band and receives signals in another. Furthermore, the communication device transmits and receives signals only in certain time slots. Assuming the transmit and receive time slots do not overlap, control circuit 68 may toggle FETs 56 rapidly, in synchronization ' with the time slots, so that assembly 20 is tuned to the assigned transmit sub-band during transmit slots and to the assigned receive sub-band during receive slots. This rapid band switching will work even when communicating in full duplex mode, since the data transmitted and received by device 21 are typically compressed so that continuous voice data can be transmitted and received within the assigned time slots.

As an alternative, tuning circuit 32 may be designed so that one of the resonant frequencies of antenna assembly 20 is in the transmit band, and the other resonant frequency is in the receive band. As long as the transmit and receive sub- bands change together in a predetermined way (when device 21 moves from one cell to another in a cellular network, for example) , FETs 56 may be operated to switch both the transmit and receive sub-bands to the proper settings simultaneously.

The arrangement of capacitors 52 and 54 in series across slot 38 serves as a RF voltage divider when FET 56 is switched off (and thus behaves as an open circuit) . The reason for this arrangement is that the RF voltage that develops over slot 38 during transmission, in small antennas such as antenna 22, is typically higher than the total voltage that the FET can sustain in the off position. (This sort of phenomenon is sometimes referred to as RF voltage overflow.) The series arrangement of capacitors 52 and 54 reduces the voltage across FET 56 when the FET is turned off. This voltage reduction circuitry is not needed when the FET is turned on (short circuit) . In the specific design of circuit 32 that is shown in Fig. 2, the choice of transmit and receive frequencies is such that only the FETs in switched impedance units 45 and 46 are ever turned off during transmission. The FETs in units 47 and 48 are always on during transmission, and are turned off in order to select receive bands. Therefore, there is no need for a voltage divider to protect the FETs in units 47 and 48. In other designs, of course, the voltage divider arrangement may be applied to other FETs, or to all of the FETs (or none of the FETs) , as required.

Figs. 4A and 4B are schematic plots of the predicted reflection coefficients Sn of antenna assembly 20, when the antenna assembly is fed by a 50Ω coaxial cable, in accordance with an embodiment of the present invention. Curves 70, 72, 74, 76 and 78 show the reflection coefficients in the lower (900 MHz) band, while curves 80, 82, 84, 86 and 88 show the reflection coefficients in the upper (1700-1800 MHz) band. These curves correspond respectively to states #1 through #5 of tuning circuit 32 that are shown above in Table I, and correspond to the resonant sub-bands of assembly 20 in each state. The curves were calculated based on the antenna dimensions given above and the following values of the circuit components:

Li = 7.15 nH Ci = 1.2 pF C₂ = 0.6 pF

C₃ = 0.4 pF C₄ = 1.4 pF C₅ = 2.2 pF.

Changing these values or other aspects of the circuit configuration will, of course, change the band selection. Figs. 5A and 5B are schematic plots of actual, measured reflection coefficients of another antenna assembly (not shown in the figures) , which was constructed in accordance with an alternative embodiment of the present invention, and was fed by a 50Ω coaxial cable. The antenna in this embodiment was similar to antenna 22, as described above, while the tuning circuit had eight switched impedance units across slot 38, created by duplicating each of units 45, 46, 47 and 48. Switching off the FET in each of these units in turn, in a manner similar to that illustrated in Table I above, gives nine different states of the tuning circuit. Accordingly, Figs. 5A and 5B each show nine different sub-bands served by the antenna assembly in the lower and upper bands, respectively.

Although certain particular tuning circuit configurations are described above, alternative switched tuning circuits based on the principles of the present invention will be apparent to those skilled in the art and are considered to be within the scope of the present invention. Such tuning circuits may be designed to cause the antenna assembly in which they are used to have two simultaneous, switchable resonant frequencies, as in the embodiments described above, or to have more than two switchable resonant frequencies, or only a single resonant frequency. The resonant frequencies may be in the cellular service range, as shown in Figs. 4A/B and 5A/B, or they may alternatively be at lower frequencies or at higher, multi- gigahertz frequencies.

It will thus be appreciated that the embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.

Claims

1. An antenna assembly for a communication device, comprising: an antenna; and a tuning circuit, comprising: a network of impedance elements, which are coupled to load the antenna with a complex impedance such that the antenna assembly has first and second resonant frequencies in distinct first and second bands; and at least one switch, which is coupled to switch a configuration of the impedance elements so as to vary the impedance of the network, thus tuning the first and second resonant frequencies simultaneously.

2. The assembly according to claim 1, wherein the at least one switch comprises a field-effect transistor (FET) .

3. The assembly according to claim 1, wherein the impedance elements comprise one or more capacitors and one or more inductors .

4. The assembly according to claim 3, wherein the network comprises a resonant LC circuit, comprising at least a first one of the capacitors and at least one of the inductors, which is coupled in a series connection with at least a second one of the capacitors.

5. The assembly according to claim 4, wherein the at least one switch is coupled to switch the second one of the capacitors into and out of the series connection.

6. The assembly according to claim 5, wherein the series connection comprises the second one of the capacitors and a third one of the capacitors, which is arranged in series with the second one of the capacitors, and wherein the at least one switch is coupled to the network at a point intermediate the second one and the third one of the capacitors.

7. The assembly according to claim 4, wherein the resonant LC circuit is coupled in the series connection with the second one of the capacitors and with at least a third one of the capacitors, which is arranged in parallel with the second one of the capacitors, and wherein the at least one switch comprises at least first and second switches, which are coupled respectively to switch the second one and the third one of the capacitors into and out of the series connection.

8. The assembly according to any of claims 1-7, wherein the antenna comprises a surface having a slot opening therethrough, and wherein at least one of the impedance elements is disposed across the slot.

9. The assembly according to claim 8, wherein the surface has at least first and second slots opening therethrough, and wherein the impedance elements comprise at least a first impedance element that is disposed across the first slot, and at least a second impedance element that is disposed across the second slot .

10. The assembly according to claim 9, wherein the first and second impedance elements are coupled to one another in series.

11. The assembly according to claim 8, wherein the surface having the slot opening therethrough is a first surface of the antenna, and wherein the antenna further comprises a second surface, spaced from the first surface so as to define a cavity therebetween, wherein the slot opens into the cavity.

12. The assembly according to any of claims 1-7, wherein the impedance elements comprise at least first and second impedance elements, which are arranged in series, and wherein the at least one switch is coupled to the network at a point intermediate the first and second impedance elements.

13. The assembly according to any of claims 1-7, wherein the communication device is configured to transmit and receive signals via the antenna assembly in respective transmit and receive time slots at different transmit and receive frequencies, and wherein the at least one switch is operative to switch the configuration of the impedance elements so as to tune at least one of the resonant frequencies between the transmit and receive frequencies, and wherein the assembly further comprises a controller, which is adapted to actuate the at least one switch in synchronization with the time slots, so that the resonant frequency corresponds to the transmit frequency during the transmit time slots and to the receive frequency during the receive time slots.

14. The assembly according to any of claims 1-7, wherein the first resonant frequency is in a band between about 800 and 1000 MHz, and wherein the second resonant frequency is in a band between about 1700 and 1900 MHz.

15. An antenna assembly for a communication device, comprising: an antenna; and a tuning circuit, comprising: a network of impedance elements, which are coupled to load the antenna with an impedance, which determines a resonant frequency of the antenna assembly, the network comprising at least first and second impedance elements arranged in series; and a switch, which is coupled to the network at a point intermediate the first and second impedance elements, and which is operative to switch a configuration of the impedance elements so as to vary the impedance of the network, thus tuning the resonant frequency.

16. The assembly according to claim 15, wherein the first and second impedance elements are configured to operate as a radio frequency (RF) voltage divider, so as to prevent a RF voltage overload on the switch while the communication device is transmitting a signal via the antenna.

17. The assembly according to claim 15, wherein the switch comprises a field-effect transistor (FET) .

18. The assembly according to claim 15, wherein the first and second impedance elements comprise first and second capacitors.

19. The assembly according to claim 18, wherein the switch is configured to couple the second capacitor into and out of the series with the first capacitor.

20. The assembly according to any of claims 15-19, wherein the antenna comprises a surface having a slot opening therethrough, and wherein the first and second impedance elements are disposed across the slot.

21. The assembly according to claim 20, wherein the surface having the slot opening therethrough is a first surface of the antenna, and wherein the antenna further comprises a second surface, spaced from the first surface so as to define a cavity therebetween, wherein the slot opens into the cavity.

22. The assembly according to any of claims 15-19, wherein the communication device is configured to transmit and receive signals via the antenna assembly in respective transmit and receive time slots at different transmit and receive frequencies, and wherein the switch is operative to switch the configuration of the impedance elements so as to tune the resonant frequency between the transmit and receive frequencies, and wherein the assembly further comprises a controller, which is adapted to actuate the switch in synchronization with the time slots, so that the resonant frequency corresponds to the transmit frequency during the transmit time slots and to the receive frequency during the receive time slots.

23. The assembly according to claim 22, wherein the switch is operative to ground the point intermediate the first and second impedance elements during the receive time slots and to decouple the point from ground during at least some of the transmit time slots.

24. An antenna assembly for a communication device, which is configured to transmit and receive signals in respective transmit and receive time slots at different transmit and receive frequencies, the assembly comprising: an antenna; a tuning circuit, comprising: a network of impedance elements, which are coupled to load the antenna with an impedance, which determines a resonant frequency of the antenna; and at least one switch, which is coupled to alter a configuration of the impedance elements so as to vary the impedance, in order to tune the resonant frequency between the transmit and receive frequencies; and a controller, which is adapted to actuate the at least one switch in synchronization with the time slots, so that the resonant frequency corresponds to the transmit frequency during the transmit time slots and to the receive frequency during the receive time slots.

25. The assembly according to claim 24, wherein the at least one switch comprises a field-effect transistor (FET) .

26. The assembly according to claim 24, wherein the impedance elements comprise one or more capacitors and one or more inductors .

27. The assembly according to claim 26, wherein the network comprises a resonant LC circuit, comprising at least a first one of the capacitors and at least one of the inductors, which is coupled in a series connection with at least a second one of the capacitors.

28. The assembly according to claim 27, wherein the at least one switch is coupled to switch the second one of the capacitors into and out of the series connection in synchronization with the time slots so as to tune the resonant frequency.

29. The assembly according to any of claims 24-28, wherein the antenna comprises a surface having a slot opening therethrough, and wherein at least one of the impedance elements is disposed across the slot.

30. The assembly according to claim 29, wherein the surface having the slot opening therethrough is a first surface of the antenna, and wherein the antenna further comprises a second surface, spaced from the first surface so as to define a cavity therebetween, wherein the slot opens into the cavity.

31. A method for tuning an antenna assembly, comprising: coupling a network of impedance elements to load an antenna with a complex impedance such that the antenna assembly has first and second resonant frequencies in distinct first and second bands; and switching a configuration of the impedance elements so as to vary the impedance of the network, thus tuning the first and second resonant frequencies simultaneously.

32. The method according to claim 31, wherein switching the configuration comprises actuating a field-effect transistor (FET) in order to switch the configuration.

33. The method according to claim 31, wherein the impedance elements comprise one or more capacitors and one or more inductors.

34. The method according to claim 33, wherein coupling the network comprises coupling at least a first one of the capacitors and at least one of the inductors to define a resonant LC circuit, and coupling the resonant LC circuit in a series connection with at least a second one of the capacitors.

35. The method according to claim 34, wherein switching the configuration comprises switching the second one of the capacitors into and out of the series connection.

36. The method according to claim 31, wherein the antenna comprises a surface having a slot opening therethrough, and wherein coupling the network comprises disposing at least one of the impedance elements across the slot .

37. The method according to claim 31, wherein coupling the network comprises arranging at least first and second impedance elements, and wherein switching the configuration comprises coupling at least one switch to the network at a point intermediate the first and second impedance elements, and actuating the at least one switch.

38. The method according to any of claims 31-37, and comprising coupling a communication device to transmit and receive signals via the antenna assembly in respective transmit and receive time slots at different transmit and receive frequencies, wherein switching the configuration comprises tuning at least one of the resonant frequencies between the transmit and receive frequencies in synchronization with the time slots, so that the resonant frequency corresponds to the transmit frequency during the transmit time slots and to the receive frequency during the receive time slots.

39. The method according to any of claims 31-37, wherein the first resonant frequency is in a band between about 800 and 1000 MHz, and wherein the second resonant frequency is in a band between about 1700 and 1900 MHz.

40. A method for tuning an antenna assembly, comprising: coupling a network of impedance elements to load an antenna with an impedance, which determines a resonant frequency of the antenna assembly, the network comprising at least first and second impedance elements arranged in series; coupling a switch to the network at a point intermediate the first and second impedance elements; and actuating the switch so as to vary the impedance of the network, thus tuning the resonant frequency.

41. The method according to claim 40, wherein coupling the network comprises configuring the first and second impedance elements to operate as a radio frequency (RF) voltage divider, so as to prevent a RF voltage overload on the switch while the communication device is transmitting a signal via the antenna.

42. The method according to claim 40, wherein the switch comprises a field-effect transistor (FET) .

43. The method according to claim 40, wherein the first and second impedance elements comprise first and second capacitors.

44. The method according to any of claims 40-43, and comprising coupling a communication device to transmit and receive signals via the antenna assembly in respective transmit and receive time slots at different transmit and receive frequencies, wherein actuating the switch comprises operating the switch so as to tune the resonant frequency between the transmit and receive frequencies in synchronization with the time slots, so that the resonant frequency corresponds to the transmit frequency during the transmit time slots and to the receive frequency during the receive time slots.

45. The method according to claim 44, wherein operating the switch comprises grounding the point intermediate the first and second impedance elements during the receive time slots, and decoupling the point from ground during at least some of the transmit time slots.

46. A method for wireless communication, comprising: configuring a communication device to transmit and receive signals via an antenna in respective transmit and receive time slots at different transmit and receive frequencies; coupling a network of impedance elements to load the antenna with an impedance, which determines a resonant frequency of the antenna; and switching a configuration of the impedance elements so as to vary the impedance, in order to tune the resonant frequency between the transmit and receive frequencies in synchronization with the time slots, so that the resonant frequency corresponds to the transmit frequency during the transmit time slots and to the receive frequency during the receive time slots.

47. The method according to claim 46, wherein switching the configuration comprises actuating a field-effect transistor (FET) in order to switch the configuration.

48. The method according to claim 46 or 47, wherein coupling the network comprises coupling a resonant LC circuit in a series connection with a capacitor, and wherein switching the configuration comprises switching the capacitor into and out of the series connection in synchronization with the time slots so as to tune the resonant frequency.

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eo

CO CO r

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FIG. 4B

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(GHz)

FIG. 5B FREQUENCY (GHz)