Patent Application
20100149067
The present invention relates to an antenna, for example an ultra-wideband antenna, and in particular to a parasitic antenna element for use in an antenna.
Ultra-wideband is a radio technology that transmits digital data across a very wide frequency range, 3.1 to 10.6 GHz. It makes use of ultra low transmission power, typically less than −41 dBm/MHz, so that the technology can literally hide under other transmission frequencies such as existing Wi-Fi, GSM and Bluetooth. This means that ultra-wideband can co-exist with other radio frequency technologies. However, this has the limitation of limiting communication to distances of typically 5 to 20 metres.
There are two approaches to UWB: the time-domain approach, which constructs a signal from pulse waveforms with UWB properties, and a frequency-domain modulation approach using conventional FFT-based Orthogonal Frequency Division Multiplexing (OFDM) over Multiple (frequency) Bands, giving MB-OFDM. Both UWB approaches give rise to spectral components covering a very wide bandwidth in the frequency spectrum, hence the term ultra-wideband, whereby the bandwidth occupies more than 20 per cent of the centre frequency, typically at least 500 MHz.
These properties of ultra-wideband, coupled with the very wide bandwidth, mean that UWB is an ideal technology for providing high-speed wireless communication in the home or office environment, whereby the communicating devices are within a range of 20 m of one another.
The fourteen sub-bands are organized into five band groups: four having three 528 MHz sub-bands, and one having two 528 MHz sub-bands. As shown in
The basic timing structure of a UWB system is a superframe. A superframe consists of 256 medium access slots (MAS), where each MAS has a defined duration, for example 256 μs. Each superframe starts with a Beacon Period, which lasts one or more contiguous MASs. The start of the first MAS in the beacon period is known as the “beacon period start”.
The technical properties of ultra-wideband mean that it is being deployed for applications in the field of data communications. For example, a wide variety of applications exist that focus on cable replacement in the following environments:
Current UWB designs utilize omni-directional antennas. In future systems, which are targeted at very high data rate applications, there are benefits in using a number of higher gain elements, each of which covers a specific angular sector. Although travelling wave elements can be selected which offer the wide bandwidth required, an array of such elements is relatively large.
There is therefore a need for an antenna which can be easily adapted to radiate in a specific direction, and over a broad range of frequencies.
According to a first aspect of the invention, there is provided a parasitic antenna element comprising: a first switch located near the base of the antenna element for selectively coupling the antenna element to a ground plane for quenching a first current mode; and a second switch located along the antenna element for selectively partitioning the antenna element into first and second portions, thereby quenching higher order current modes.
The parasitic antenna element has the advantage of being selectively reflective or transparent over a wide range of frequencies. Furthermore, the parasitic antenna element has the advantage of being low cost, since it may be printed on a PCB.
According to another aspect of the invention, there is provided an antenna array comprising at least one parasitic antenna element as defined in the appended claims.
For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the following drawings, in which:
a and 4b are schematic diagrams showing current modes on a resonant monopole;
The biasing is achieved by providing a switch 16 at the base of each parasitic element 14.
The parasitic element 14 comprises a monopole 18 on a printed circuit board (PCB). The monopole 18 is electrically connected to the ground plane 11 of the antenna array via a switch 16, for example a PIN diode. The PIN diode 16 is controlled by the application of control voltages V1 and V2.
Thus, by opening and closing the switch 16, the monopole 18 can be made to reflect or be transparent to different frequencies. This is shown in more detail in
a shows a monopole 18 connected to a ground plane 11, where the switch 16 (not shown) is closed. In this arrangement, the monopole 18 is highly reflective at frequencies close to the quarter-wave resonant frequency. This is illustrated by the dotted line in
b shows the same monopole 18 connected to the ground plane 11, but where the switch 16 (not shown) is open. In this arrangement, although reflection of the quarter-wave resonance is quenched, thus making the monopole 18 transparent at such frequencies, the monopole 18 is highly reflective at much higher frequencies (for example twice the frequency of the resonant frequency in
As can be seen from the above, the bandwidth offered by an antenna of this type is limited by the resonant nature of the parasitic elements; this affects their reflectivity and (more importantly) transparency characteristics.
The present invention allows quenching of the higher orders described with reference to
Referring to
Alternatively, the second switch 19 may be positioned at a different location along the antenna element 18. This will lead to different higher orders being quenched. The positioning of the second switch 19 is therefore chosen according to which frequencies need to be quenched.
It is noted that the bias element connecting the control signal V3 to the second switch 19 may be a wire having a high resistance, thereby reducing the coupling of RF currents into the control circuitry.
The element 20 comprises a first switch, for example a PIN diode 22, located near the base of the element, connecting the ground plane to two outer conductors 24, 26 connected in parallel. First and second control voltages V1 and V2 are applied to either side of the PIN diode 22 and thus control the switching of the PIN diode 22.
The two outer conductors 24, 26 connect to a second switch, for example a PIN diode 28. The second PIN diode 28 is connected in turn to a monopole element 30.
The antenna element 20 further comprises a central conductor 32 positioned between the two outer conductors 24, 26, such that the combination of the outer conductors 24, 26 and the central conductor 32 forms a coplanar waveguide. A third control voltage V3 is applied to the central conductor 32, one end of which is connected between the second PIN diode 28 and the monopole element 30, thus enabling the central conductor 32 to bias the second switch 28. In this way, the second PIN diode 28 can be controlled using the voltages V2 and V3. The coplanar waveguide has electrical characteristics that are similar to a short length of coaxial cable, and is designed such that the RF currents are primarily supported by the outer two conductors 24, 26, while the dc bias currents flow in the central conductor 32.
It can therefore be seen that the parasitic antenna element 20 has first and second switches: one at the base (PIN diode 22) and one approximately half way along the parasitic element (PIN diode 28). In this embodiment, the lower half of the parasitic element is realized as a coplanar waveguide with a central conductor 32. This not only provides a convenient method for biasing the second PIN diode 28, but the outer conductors 24, 26 provide shielding and reduce coupling of the RF energy into the bias circuitry. In other words, the bias signal V3 is protected from the RF signals radiating around the structure.
The central conductor 32 may be realized using a meander line 32 as the central conductor of the coplanar waveguide. The meander line 32 is preferably highly resistive. This has the effect of further reducing the coupling of the RF energy into the bias circuitry. Furthermore, the inductance of the meander line decouples the bias circuitry from the mid point of the antenna, thus eliminating any loading effect the bias may have on the performance of the antenna.
Although not shown in
Although PIN diodes have been used to illustrate the switches throughout the description, one skilled in the art will appreciate that alternative switches could be used. For example, microelectromechanical systems (MEMS) switches or GaAs FET switches could be used as alternatives.
In operation, the parasitic antenna element will be provided as part of an antenna array substantially as described with reference to
There is therefore described a parasitic antenna element that is selectively reflective or transparent at a wide range of frequencies. This is accomplished by providing a first switch located near the base of the antenna element for selectively coupling the antenna element to a ground plane for quenching a first current mode, and a second switch located along the antenna element for selectively partitioning the antenna element into first and second portions, thereby quenching higher order current modes.
The antenna element is low cost, as it may be printed on a PCB. An antenna incorporating such parasitic antenna elements may be operated over an increased range of frequencies, and thus bandwidth. As such, the antenna element is particularly suited for use in ultra-wideband systems.
Although the preferred embodiment is described in relation to the parasitic element being switched between ground (to act as a reflector) and open circuit (to become transparent), it is noted that the parasitic element may also be switched to a signal source in order to act as a radiator.
It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. The word “comprising” does not exclude the presence of elements or steps other than those listed in a claim, “a” or “an” does not exclude a plurality, and a single processor or other unit may fulfil the functions of several units recited in the claims. Any reference signs in the claims shall not be construed so as to limit their scope.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0706327.4 | Mar 2007 | GB | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/GB2008/001067 | 3/27/2008 | WO | 00 | 2/17/2010 |
