Antenna and method for operating an antenna

Abstract

 Antenna comprising a first antenna element (A1), a first feed tab (F1) for feeding a first frequency to the first antenna element (A1), a second feed tab (F2) for feeding a second frequency to the first antenna element (A1), a first shorting tab (S1) arranged between the first feed tab (F1) and the second feed tab (F2) for shorting the first antenna element (A1) to a ground potential (GND), and a tuning slot (T) arranged between the first shorting tab (S1) and the second feed tab (F2). A plurality of switches (SW1, SW2, SW3) are provided with which the inductive behaviour of the tuning slot (T) can be varied.

Description

[0001] The invention relates to an antenna for mobile phones, which are also called cellular phones, and similar wireless devices. Such antennas must be small and have to cover a plurality of frequency bands. Examples of wireless frequency bands are 824 to 960 MHz, 1710 to 2170 MHz and 2300 to 2700 MHz.

[0002] It is an object of the invention to provide an antenna and a method for operating the antenna in which the antenna has small dimensions and covers the above mentioned frequency bands.

[0003] The invention provides an antenna comprising a first antenna element, a first feed tab for feeding a first frequency to the first antenna element, a second feed tab for feeding a second frequency to the first antenna element, a first shorting tab arranged between the first feed tab and the second feed tab for shorting the first antenna element to a ground potential and a tuning slot arranged between the first shorting tab and the second feed tab. A plurality of switches are provided with which the inductive behavior of the tuning slot can be varied. Depending on the inductive behavior of the tuning slot, the first antenna element will either have a resonance at the first frequency or at the second frequency. The first antenna element can thus operate in two frequency bands without requiring an additional antenna that would increase the size of the antenna.

[0004] In an embodiment, the plurality of switches comprises a first switch having a first connecting point coupled to the first feed tab and to a first frequency source and a second connecting point coupled to the ground potential; a second switch having a first connecting point coupled to a second frequency source and a second connecting point coupled to the second feed tab; and a third switch having a first connecting point coupled to the second frequency source and a second connecting point coupled to the ground potential. The switches are used for changing the inductive behavior of the tuning slot and for shorting and connecting the frequency sources.

[0005] In an embodiment, the second switch is coupled to the second feed tab by a matching capacitance. The matching capacitance is used to increase the resonance frequency of the first antenna element.

[0006] In an embodiment, at least the second switch of the plurality of switches is a capacitive radio frequency (RF) micro-electromechanical system (MEMS) switch. Capacitive RF MEMS switches are easier to implement than galvanic RF MEMS switches.

[0007] In an embodiment, the matching capacitance is at least partially provided by the capacitance of the capacitive radio frequency micro-electromechanical system switch used as the second switch. The size of the matching capacitance can then be reduced or the discrete matching capacitance can be completely eliminated by using the capacitance of the MEMS switch.

[0008] In an embodiment, the antenna further comprises a second antenna element corresponding to the first antenna element described previously, wherein the second antenna element is arranged on a side of a printed circuit board that is opposite to the side of the PCB that the first antenna element is arranged on.

[0009] The arrangement of the first and the second antenna elements on opposite sides leads to a reduction of electromagnetic interference between the antennas when both antennas are operated simultaneously. Further, the diversity of the signal paths to the antennas is increased when the antennas are separated as far as is possible.

[0010] In an embodiment, the antenna further comprises a third antenna element which comprises a third feed tab for feeding a third frequency to the third antenna element and a third shorting tab for shorting the third antenna element to the ground potential. The third antenna element can be used for receiving and radiating electromagnetic energy at frequencies which the first antenna element cannot effectively convert.

[0011] In an embodiment, the first frequency lies between 1700 MHz to 2170 MHz, the second frequency lies between 2300 MHz to 2700 MHZ and the third frequency lies between 824 MHz to 960 MHz. These frequencies are commonly used for operating in the GSM, CDMA, UMTS, WiMAX and WiFi systems.

[0012] The invention further provides a method for operating the previously described antenna where the first antenna element is selected for radiating and receiving electromagnetic energy either at the first frequency or at the second frequency by changing the inductive behavior of the tuning slot. The inductive behavior of the tuning slot determines whether the first antenna element resonates at the first frequency or at the second frequency.

[0013] In an embodiment, the first antenna element is configured so that, when operating at the first frequency, the tuning slot acts as a series inductance and the first antenna element is configured so that, when operating at the second frequency, the tuning slot acts as a parallel inductance. The first antenna element is configured by means of a plurality of switches.

[0014] In an embodiment, when operating at the first frequency, the first switch and the second switch are opened and the third switch is closed and, when operating at the second frequency, the first switch and the second switch are closed and the third switch is opened. The first switch and the third switch short the first frequency source and the second frequency source, respectively, so that they do not excite the first antenna element with their respective frequencies. The second switch is used to disconnect the second frequency source. The switches also change the impedance transformation due to the feed and shorting tabs.

[0015] In an embodiment, the capacitance of the second switch is chosen so that the first antenna element has a resonance at the second frequency. The capacitance of the second switch is used as the matching capacitance.

[0016] In an embodiment, when operating at the first frequency, an impedance at the first feed tab is matched to an impedance of the first frequency source by adjusting the relative width of the first feed tab to the width of the first short tab and when operating at the second frequency, an impedance at the second feed tab is matched to an impedance of the second frequency source by adjusting the relative width of the second feed tab to the combined width of the first short tab and the first feed tab. This allows the impedance transformations due to the feed and shorting tabs at the first frequency and the second frequency to be independent from one another.

[0017] In an embodiment, the first antenna element and the second antenna element are operated in a multiple-in/multiple-out (MIMO) or a diversity fashion. The simultaneous use of the first antenna element and the second antenna element is used to improve communication performance.

[0018] In an embodiment, when operating at the third frequency, the first switch and the third switch are closed and the second switch is open. These positions of the switches lead to a better isolation of the first antenna element and the third antenna element.

[0019] The invention will be described using the detailed description provided hereinafter and the accompanying drawings. In the drawings:

FIG. 1

shows an embodiment of an antenna with a first an- tenna element and a third antenna element,

FIG. 2

shows a configuration of switches for operating the first antenna element at a first frequency,

FIG. 3

shows a configuration of switches for operating the first antenna element at a second frequency,

FIG. 4

shows the widths of the first and the second feed tab and of the first shorting tab of the first an- tenna element,

FIG. 5

shows an embodiment of an antenna with a first an- tenna element and a second antenna element for MIMO or diversity operation, and

FIG. 6

shows a configuration of switches for operating the antenna at a third frequency.

[0020] FIG. 1 shows an embodiment of an antenna A which can be used in a mobile phone or other wireless devices. The antenna A comprises a printed circuit board PCB which has metallizations on its opposing main sides. One of the main sides is covered with a conducting ground plane which can be used as a ground potential GND. The other main side has metallizations on it which form part of the first antenna element A1 and the third antenna element A3 for radiating and receiving electromagnetic energy. The first antenna element A1 is operated at a first and a second frequency, the third antenna element A3 at a third frequency. Normal to both main sides of the printed circuit board PCB are feed tabs F1, F2 and F3 and shorting tabs S1 and S2 which connect the radiating and the receiving parts. A person holding the phone is thus less likely to change electrical characteristics by placing a hand on the feed tabs F1, F2 and F3 and on the shorting tabs S1 and S2. While the antenna A shown in FIG. 1 is planar and has a parallel and a normal part with respect to the main sides of the printed circuit board PCB, it should be noted that this is not a prerequisite. The first and third antenna elements A1 and A3 can also be arranged alone, differently and along two dimensions only.

[0021] The first antenna element A1 has a first feed tab F1 for feeding a first frequency, a second feed tab F2 for feeding a second frequency and a first shorting tab S1 for shorting the first antenna element A1 to the ground plane. The first shorting tab S1 is arranged between the first feed tab F1 and the second feed tab F2. Further, the first antenna element A1 has a tuning slot T which is arranged between the first shorting tab S1 and the second feed tab F2. The tuning slot T continues into the metallizations which are parallel to the main sides of the printed circuit board PCB. This radiating and receiving part of the first antenna element A1 has a dimension in one direction which is approximately a quarter of the wavelength of the second frequency.

[0022] The first antenna element A1 can resonate at a first frequency and at a second frequency. The first frequency lies between 1710 to 2170 MHz, while the second frequency lies between 2300 to 2700 MHz. The first or the second frequency is selected by changing the inductive behavior of the tuning slot T. The inductive behavior of the tuning slot T is selected by means of a plurality of switches, which are shown in FIGS. 2 and 3. The switches are further used for supplying the first antenna element A1 with the first frequency and with the second frequency and for changing the impedance transformation due to the feed tab F1 and F2 and the shorting tab S1.

[0023] FIG. 2 shows a configuration of the switches SW1, SW2 and SW3 for operating the first antenna element A1 at a first frequency. The first switch SW1 is open so that a first frequency source U1 is not shorted to a ground potential GND. The ground potential GND can be the ground plane of the antenna A. The signal of the first frequency source U1 is transmitted to the first feed tab F1 and to the radiating part of the first antenna element A1 where it is converted into electromagnetic energy.

[0024] The second switch SW2 is open which disconnects the second frequency source U2 from the second feed tab F2. Further, the third switch SW3 is closed to that the second frequency U2 source is connected to the ground potential GND. By open circuiting the second feed tab F2, the tuning slot T acts as a series inductor, where the inductor is in series to the impedance that the first antenna element A1 would have without the tuning slot T. As a result, the first antenna element A1 has a resonance in the frequency range of 1710 MHz to 2170 MHz.

[0025] FIG. 3 shows a configuration of the switches for operating the first antenna element A1 at the second frequency. The first switch SW1 is closed, so that the signal of the first frequency source U1 is shunted to the ground potential GND. The second switch SW2 is closed, so that the second frequency source U2 is coupled to the second feed tab F2. The third switch SW3 is opened so as not to short the second frequency source U2 to the ground potential GND.

[0026] With the first feed tab F1 being shorted to a ground potential GND and the second feed tab F2 being fed, the series inductance of the tuning slot T is removed. The tuning slot T acts as a parallel inductance, where the inductor is in parallel to the impedance that the first antenna element A1 would have without the tuning slot T. With the removal of the series inductance of the tuning slot T, the first antenna element A1 can resonate at a higher frequency. Further, since both the first shorting tab S1 and the first feeding tab F1 act as parallel shunts to the ground potential, the antenna inductance is reduced. A series matching capacitance C1 is connected to the tuning slot acting as a parallel inductance to further increase the resonance frequency of the first antenna element A1. In summary, the inductive behavior of the tuning slot T is varied by using the first feeding tab F1 for feeding at the first frequency and using the same tab as a shorting tab when operating at the second frequency.

[0027] The first, second and third switch SW1, SW2, SW3 can be any kind of switches. However, it is of advantage to use micro-electromechanical system (MEMS) switches as these have a low loss at radio frequencies and require only a small footprint.

[0028] MEMS switches can be galvanic or capacitive. Galvanic switches make use of metal-to-metal contacts which lead to low losses over a wide bandwidth when closed. However, galvanic MEMS switches have only a reduced number of switching cycles. In contrast to that, capacitive MEMS switches have the advantage that the contacts do not wear out. However, these switches have a significant capacitance when closed which must typically be resonated out by a small series inductance.

[0029] As described above, a series matching capacitance C1 is required for increasing the resonance frequency of the first antenna element A1 to operate it at the second frequency. This matching capacitance C1 can be reduced in value if the capacitance is partially provided by a capacitive MEMS switch which is used for the second switch SW2. If all of the matching capacitance can be provided by the capacitive MEMS switch SW2, the discrete matching capacitance is no longer necessary. In this case, the small series inductance that was used to resonate out the capacitance of the RF MEMS switch is no longer needed. The reduction in the number of parts for the antenna reduces its size and its costs.

[0030] FIG. 4 is a cut-out of the top view of FIG. 1 showing the first and second feed tab F1, F2 and the shorting tab S1 of the first antenna element A1. The first feed tab F1 has a width W1, the second feed tab F2 has a width W2 and the shorting tab S1 has a width WS. When operating at the first frequency, such as is shown in FIG. 2, the impedance transformation of the first feed tab F1 and the first shorting tab S1 is determined by the relative width of W1 to WS. When operating at the second frequency, such as is shown FIG. 3, the impedance transformation of the tabs is determined by the relative width of the second feed tab W2 to the combined width of the first feed tab and the first shorting tab W1 + S1. The impedance transformation for the first frequency and the second frequency are thus independent from each other, which simplifies designing and impedance matching of the first antenna element A1 for operating at both frequencies. The width W1S between the first feed tab F1 and the shorting tab S1 and the width WS2 of the tuning slot T also affect the impedance transformation, however, their effects are difficult to quantify exactly.

[0031] FIG. 5 shows an embodiment of the antenna A which can be used in a multiple input/multiple output (MIMO) or an antenna diversity system. In a MIMO system, multiple antennas at both the transmitter and the receiver are used to increase the data throughput by using a higher spectral efficiency. In antenna diversity systems, the reliability of wireless links is increased by using the independent fading in multiple antenna links. In FIG. 5, the first antenna element A1 is augmented by a second antenna element A2 which is located at an opposite position on the printed circuit board PCB. The first and the second antenna element A1, A2 can be used for cellular MIMO above 1.7 GHz, WiMAX MIMO or WiFi MIMO. They can also be used without MIMO for simultaneous cellular and WiMAX, cellular and WiFi, or WiMAX and WiFi. Here, cellular can mean GSM, CDMA, UTRA (UMTS, TD-SCDMA, etc.) or any other cellular or mobile system.

[0032] FIGs. 1 and 5 further have a third antenna element A3 which is used for receiving and radiating electromagnetic energy at a third frequency. The third antenna element A3 has a third feed tab F3 for feeding a third frequency and a third shorting tab S3 for shorting the third antenna element A3 to the ground plane. The third antenna element A3 is larger than the first and the second antenna element A1, A2 and is designed for resonance at a third frequency between 824 to 960 MHz.

[0033] FIG. 6 shows a configuration of switches for operating the antenna A at the third frequency. The first switch SW1 and the third switch SW3 are closed so that the first frequency source U1 and the second frequency source U2 are shorted to the ground potential GND. The second switch SW2 is opened to disconnect the second frequency source U2 from the first antenna element A1. The third antenna element A3 is coupled to a third frequency source U3 for radiating electromagnetic energy at the third frequency. The first antenna element A1 and the third antenna element A3 show the best isolation when the switches are in the position as shown in FIG. 6 compared with any other position of the switches SW1, SW2 and SW3.

[0034] While FIGs. 2, 3 and 6 are described as having frequency sources U1, U2 and U3 for driving the antenna A, a person skilled in the art knows that the antenna A can also be operated in reverse mode, that is in converting electromagnetic energy into electrical signals. Besides the frequency sources, there would be low noise amplifiers designed for amplifying signals which are received at the antenna A at the corresponding frequencies.

[0035] By using the above described invention, all of the wireless frequency bands within the range of 824 MHz to 2700 MHz can be covered without increasing the antenna size. The first and the second antenna elements A1 and A2 each cover the frequencies 1710 MHz to 2170 MHz and 2300 MHz to 2700 MHz, while the third antenna element A3 covers the frequencies in the range of 824 MHz to 960 MHz.

Reference signs

[0036] 

A

antenna

A1

first antenna element

A2

second antenna element

A3

third antenna element

C1

matching capacitor

F1

first feed tab

F2

second feed tab

F3

third feed tab

GND

ground potential

PCB

printed circuit board

S1

first shorting tab

S3

third shorting tab

SW1

first switch

SW2

second switch

SW3

third switch

T

tuning slot

U1

first frequency source

U2

second frequency source

U3

third frequency source

W1

width of first feed tab

W1S

width between first feed tab and first shorting tab

W2

width of second feed tab

WS2

width between second feed tab and first shorting tab

WS

width of first short tab

Claims

1. Antenna, comprising

- a first antenna element (A1),

- a first feed tab (F1) for feeding a first frequency to the first antenna element (A1),

- a second feed tab (F2) for feeding a second frequency to the first antenna element (A1),

- a first shorting tab (S1) arranged between the first feed tab (F1) and the second feed tab (F2) for shorting the first antenna element (A1) to a ground potential (GND), and

- a tuning slot (T) arranged between the first shorting tab (S1) and the second feed tab (F2),

wherein
a plurality of switches (SW1, SW2, SW3) are provided with which the inductive behaviour of the tuning slot (T) can be varied.

2. Antenna according to claim 1,
wherein the plurality of switches (SW1, SW2, SW3) comprises

- a first switch (SW1) having a first connecting point coupled to the first feed tab (F1) and to a first frequency source (U1) and a second connecting point coupled to the ground potential (GND),

- a second switch (SW2) having a first connecting point coupled to a second frequency source (U2) and a second connecting point coupled to the second feed tab (F2), and

- a third switch (SW3) having a first connecting point coupled to the second frequency source (U2) and a second connecting point coupled to the ground potential (GND).

3. Antenna according to claim 2,
wherein
the second switch (SW2) is coupled to the second feed tab (F2) by a matching capacitance (C1).

4. Antenna according to one of the previous claims,
wherein
at least the second switch (SW2) of the plurality of switches (SW1, SW2, SW3) is a capacitive radio frequency micro-electromechanical system switch.

5. Antenna according to claim 4,
wherein
the matching capacitance (C1) is at least partly provided by the capacitance of the capacitive radio frequency micro-electromechanical system switch used as the second switch (SW2).

6. Antenna according to claim 5,
further comprising
a second antenna element (A2) corresponding to the first antenna element (A1) according to one of the claims 1 to 5, wherein the second antenna element (A2) is arranged on a side of a printed circuit board (PCB) that is opposite to the side of the printed circuit board (PCB) that the first antenna element (A1) is arranged on.

7. Antenna according to one of the previous claims,
further comprising
a third antenna element (A3), comprising

- a third feed tab (F3) for feeding a third frequency to the third antenna element (A3), and

- a third shorting tab (S3) for shorting the third antenna element (A3) to the ground potential.

8. Antenna according to claim 7,
wherein
the first frequency lies between 1710 MHz to 2170 MHz, the second frequency lies between 2300 MHz to 2700 MHz, and
the third frequency lies between 824 MHz to 960 MHz.

9. Method for operating an antenna (A) according to one of claims 1 to 8,
wherein
the first antenna element (A1) is selected for radiating and receiving electromagnet energy either at the first frequency or at the second frequency by changing the inductive behaviour of the tuning slot (T).

10. Method according to claim 9,
wherein
when operating at the first frequency, the first antenna element (A1) is configured so that the tuning slot (T) acts as a series inductance, and
when operating at the second frequency, the first antenna element (A1) is configured so that the tuning slot (T) acts as a parallel inductance.

11. Method according to one of claims 9 or 10,
wherein

- when operating at the first frequency the first switch (SW1) and the second switch (SW2) are opened and the third switch (SW3) is closed, and

- when operating at the second frequency,

the first switch (SW1) and the second switch (SW2) are closed and the third switch (SW3) is opened.

12. Method according to claim 11,
wherein
the capacitance (C1) of the second switch (SW2) is chosen so that the first antenna element (A1) has a resonance at the second frequency.

13. Method according to one of claims 9 to 12,
wherein

- when operating at the first frequency, an impedance at the first feed tab (F1) is matched to an impedance of the first frequency source (U1) by adjusting the relative width (W1) of the first feed tab (F1) to the width (WS) of the first short tab (S1), and

- when operating at the second frequency, an impedance at the second feed tab (F2) is matched to an impedance of the second frequency source (U2) by adjusting the relative width (W2) of the second feed tab (F2) to the combined width (WS, W1) of the first short tab (S1) and the first feed tab (F1).

14. Method according to one of claims 9 to 13,
wherein
the first antenna element (A1) and the second antenna element (A2) are operated in a multiple-in/multiple-out or a diversity fashion.

15. Method according to one of claims 9 to 14,
wherein
when operating at the third frequency, the first switch (SW1) and the third switch (SW3) are closed and the second switch (SW2) is open.

Drawing