The present invention relates generally to antenna arrangements and more particularly, but by no means exclusively, to antenna arrangements for use in wireless devices such as Radio Frequency Identification Transponders (RFID).
In the field of wireless communications, the efficiency of a wireless circuit is largely governed by the performance of the antenna to which it is coupled. Antenna efficiency is dependent not only upon physical dimensions (ie size, shape and material composition) but also based on operational parameters. These may include input impedance, gain, directivity, radiation pattern and signal polarity.
In Radio Frequency Identification (RFID) transponder systems, the choice of antenna for an RFID tag is primarily limited by the small physical dimensions of the tag's substrate, the impedance of the integrated circuitry and the operational bandwidth. For an operable Ultra High Frequency (UHF) RFID tag antenna, the minimum physical dimension of the antenna is in the order of a quarter wavelength (or a multiple thereof) of the desired operating frequency and quarter-wavelength dipole antennas are commonly used.
Designated bands for RFID communication exist in most countries. Typically these bands will differ from country to country and as such, RFID tag antennas are designed to be operable in the frequency band corresponding to the country in which they are to be sold. For example, in the United States short range communication may be limited to a frequency band of between 902 to 928 MHz, while in Australia the specified operational band for short range communication may be 860 to 888 MHz. As a result, RFID tags which are designed to be operable in one country may not be operable in another country due to the different operational bands.
An antenna arrangement is provided that includes two or more electrically coupled antenna elements having different operational characteristics. The two or more antenna elements are both arranged to resonate generally about a first operational frequency in order to widen the operational bandwidth of the antenna arrangement.
In general, in one aspect, two antenna elements having different operational characteristics may be combined such that their resonances overlap for widening the effective bandwidth. A wider bandwidth may allow the antenna to operate over a large range of signal frequencies, thereby making it suitable for use in different applications/systems operating in different frequency bands.
Implementations of the invention may include one or more of the following. The operational characteristics of the antenna elements may comprise an antenna gain, input impedance, effective length, input return loss, directivity, radiation pattern, quality factor and/or bandwidth. One of the antenna elements may be a loop antenna and another of the antenna elements may be a dipole antenna. The loop antenna may extend outwardly from the dipole antenna. The loop antenna may extend in the same plane as the dipole antenna and may be centred about the dipole antenna. The loop antenna may incorporate a side wall which runs parallel to the dipole antenna. In an alternative embodiment, both of the antenna elements are dipole antennas.
In general, in another aspect, the invention features a radio frequency communication device which comprises a processing unit and an antenna arrangement. The antenna arrangement includes two or more electrically coupled antenna elements having different operational characteristics and which are both arranged to resonate generally about a first operational frequency. In this manner, the antenna elements may effectively widen the operational bandwidth of the antenna arrangement.
In accordance with this aspect, the antenna elements may be arranged to receive/transmit signals over a widened band of frequencies making the radio frequency communication device suitable for use in different regions of the world that have different standards in frequencies. The radio frequency communication device may be a radio frequency identification transponder (RFID) tag. The processing unit may be responsive to radio frequency signals centred about the first operational frequency at the antenna.
In general, in another aspect, the invention provides a radio frequency communication system which comprises a radio frequency device including a processing unit and an antenna arrangement. The antenna arrangement includes two or more electrically coupled antenna elements having different operational characteristics and which are both arranged to resonate about a first operational frequency to widen the operational bandwidth of the antenna arrangement. The radio frequency communication system also includes a remotely positioned interrogator which is arranged to transmit a signal at the first operational frequency and the processing unit is responsive to the signal.
Operation of a typical prior art RFID system is described with reference to
An interrogation device 110 is provided for interrogating the RFID tag 102. The interrogation device includes communication electronics (not shown), memory 112 and an interrogation antenna 114. As used herein, the term “interrogation device” may comprise any device which is capable of wirelessly communicating with the RFID tag 102. Further, the signal used to communicate with the RFID tag 102 may be any one of a variety of different types of signal fields including electric, magnetic, or electro-magnetic. In the depicted embodiment, the interrogation device 110 communicates with the RFID tag 102 by emitting a modulated radio frequency signal through the interrogation antenna 114 using a carrier frequency corresponding to the resonant frequency of the RFID tag antenna 104, specified above.
When the RFID tag 102 is positioned in the presence of the radio frequency signal field emitted from the interrogation device 110, the antenna 104 resonates and a portion of the received energy is absorbed and subsequently transferred to the integrated circuit 106, where it is converted into electrical potential energy and stored. The electrical potential energy appears as a voltage at the internal power supply connections (not shown) of the integrated circuit 106. If the signal strength is above a predetermined threshold, the RFID tag 102 will respond to the reception of the signal by modulating the signal to impart information about the RFID tag 102 (which may be pre-programmed into the integrated chip 106) onto a back-scattered signal field which propagates back to the interrogation device 110. In embodiments where the RFID tag 102 is a passive tag, the signal strength must be sufficient for the integrated circuit 106 to rectify the signal and use the signals energy for the tag's power source.
With reference to
With reference to
According to the embodiments described above, it can be seen that antenna arrangements in accordance with the present invention exhibit a greatly widened operational bandwidth over typical RFID tag antennas, due to the two antenna elements having different operational characteristics resonating close together where maximum transfer of energy is occurring over a widened range of frequencies. This is particularly advantageous where communication over a widened range of frequencies is desirable. For example, the antenna arrangements according to embodiments of the present invention are suitable for use in devices which are sold in different geographical regions employing different operational spectrums.
It will be understood to those skilled in the art that the antenna elements may be made of any type of suitable conductive material, such as metal foils, etched aluminium, plated copper printed conductive patterns made from conductive inks, or any other conductive materials known in the art.
Furthermore, it is envisaged that other types of antenna which are well-known in the art may be used in combination to achieve widened bandwidth operation. For example the antenna elements may comprise of any one or more of folded dipole antennas, slot antennas, circular loop antennas etc; being limited only by the desired use and properties of the devices to which they are coupled.
It should also be understood to those skilled in the art that stubs may be added to physically alter the effective length (ie operational characteristic) of the antenna elements, rather than by altering their physical length as described in preceding paragraphs.
In the preceding description, an embodiment of the present invention was described in the context of an antenna arrangement for a passive RFID transponder tag operable in the UHF band and ranging generally between 850 MHz to 960 MHz. However, it should be understood that the present invention is not limited to the embodiment described herein and is equally applicable to other forms of wireless communication devices operable over different frequency bands.