Abstract:
There is provided a phased array antenna comprising a plurality of antenna elements ( 12 ) and switching circuitry configured to switch the phased array antenna to an inactive mode. The switching to the inactive mode comprises the switching circuitry connecting random or pseudo-random impedance elements ( 20 ) to the antenna elements to reduce the peak backscatter level of the phased antenna array.

Description:
TECHNICAL FIELD OF THE INVENTION 
       [0001]    The invention relates to a phased array antenna. 
       BACKGROUND TO THE INVENTION 
       [0002]    In backscatter communications systems such as passive Radio Frequency Identification (RFID) tags, a device transmits a signal towards an antenna, and then measures the signal that is reflected back from the antenna. Each antenna must backscatter the frequency differently in order for the device to identify a particular antenna. 
         [0003]    However, there are only a finite number of different backscattered signals that can be backscattered from the antennas and detected by the device. Antennas that have the same backscattering characteristics are difficult to distinguish from one another. 
         [0004]    It would therefore be desirable to control the amount of backscatter emitted by an antenna, for example so that the backscatter of a particular antenna could be minimised to prevent it from being detected, or to prevent it from interfering with backscatter from a nearby antenna having similar backscattering characteristics. 
       SUMMARY OF THE INVENTION 
       [0005]    According to an embodiment of the invention, there is provided a phased array antenna comprising a plurality of antenna elements and switching circuitry configured to switch the phased array antenna to an inactive mode. The switching to the inactive mode comprises the switching circuitry connecting random or pseudo-random impedance elements to the antenna elements to reduce the peak backscatter level of the phased antenna array. 
         [0006]    The inventor has realised that antenna backscattering could be controlled by using a phased array antenna. Phased array antennas typically comprise a plurality of antenna elements configured in a periodic manner. The direction in which the phased array antenna is most sensitive for receiving signals can be controlled by applying appropriate impedance matching circuitry to the antenna elements, in order to control the signal phase at each antenna element. In particular, the inventor has realised that the impedance matching circuitry of a phased array antenna could alternatively be used to control the amount of backscattering of the antenna. 
         [0007]    When energy is backscattered from phased array antennas, the energy reflected from each of the antenna elements interferes constructively or destructively depending upon the spacing of the antenna elements and the phase and frequency of the backscattered energy. Accordingly, the total amount of backscatter at any one point is the phasor sum of the backscatter from each of the individual antenna elements. Normally, the backscattered energy reaches a high peak in a direction from the antenna in which the energy reflected from each of the antenna elements interferes constructively. 
         [0008]    Applying a random or pseudo-random pattern of matching impedances to the antenna elements can remove the backscattering peak(s) that are normally present in particular direction(s) from the antenna. Specifically, the random impedance elements cause the antenna elements to emit energy at random phases, and so there are no particular directions in which the backscattered energies from the antenna elements all add constructively. Therefore, switching to the inactive mode helps minimise the backscatter of the antenna, helping to prevent it from being detected and/or reducing interference between it and the backscatter of other antennas. 
         [0009]    Preferably, the random or pseudo-random impedance elements consist of impedance elements having at least one of capacitive, inductive, and resistive components. This is because short-circuit impedance elements typically backscatter strongly and so it can be advantageous to avoid using these. 
         [0010]    Furthermore, the switching circuitry may be configured to vary the values of the impedance elements, for example by using variable impedance components or by switching between impedance components. The impedance elements may be varied each time the phased array antenna is switched to the inactive mode, or may be varied periodically, for example at regular time intervals. This varying between different random or pseudorandom values may help prevent the lobes from two identical phased array antennas having the same random or pseudorandom patterning from effectively adding together to increase interference at any particular frequency. Alternatively, the impedance elements may be permanently fixed at predetermined random or pseudorandom values to save costs. 
         [0011]    Advantageously, the phased array antenna may be switched to a receive mode, where the antenna can receive signals. The switching to the receive mode may comprise the switching circuitry connecting impedance matching elements to the antenna elements for receiving energy at particular frequencies and/or from particular directions. 
         [0012]    The switching circuitry may be further configured to switch the phased antenna array to a transmit mode, for example by connecting an output signal to each of the antenna elements. The antenna elements may be driven with the output signal at varying phases to transmit the output signal from the antenna in one or more specific directions. 
         [0013]    The impedance elements are referred to as random or pseudorandom impedance elements, as they are either randomly selected, or are selected to according to a pre-determined pseudorandom arrangement which does not have any significant regular features that would cause large peaks of constructive interference in the phased array antenna&#39;s spatial backscattering response. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    Embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings, in which: 
           [0015]      FIG. 1  shows a schematic diagram of a phased array antenna according to an embodiment of the invention; 
           [0016]      FIG. 2  shows part of a switching circuitry of the phased array antenna of  FIG. 1 ; and 
           [0017]      FIG. 3  shows a graph of backscattered power according to another embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    An embodiment of the invention will now be described with reference to  FIGS. 1 and 2 .  FIG. 1  shows a schematic plan diagram of a phased antenna array  10  having thirty-six periodically spaced antenna elements  12 . In this embodiment, the antenna elements  12  are square conductive patches formed on an insulative substrate. 
         [0019]    The thirty-six antenna elements  12  are connected to thirty-six respective T/R modules and thirty-six corresponding impedances, which collectively form a switching circuitry for controlling the phased antenna array  10 .  FIG. 2  shows a diagram of one T/R module  15  and one corresponding impedance  20  that are associated with a respective antenna element  12 . 
         [0020]    The antenna element  12  is connected to a PIN diode switch within the respective T/R module  15 . The PIN diode switch is Configured to switch the respective antenna element  12  to one of three output connections. One output connection leads to a low noise amplifier LNA for receiving signals, one output connection leads to a solid state power amplifier SSPA for transmitting signals, and the other output connection leads to the corresponding impedance  20  for when the antenna element is inactive. 
         [0021]    The low noise amplifier LNA and solid state power amplifier SSPA each include impedance matching circuitry for matching to the respective antenna element  12 . The value of the impedance element  20  is set by an input signal S 11 . 
         [0022]    Considering the phased array  10  as a whole, when the thirty-six PIN diode switches connect the thirty-six antenna elements  12  to the respective thirty-six LNA&#39;s in receive mode, or to the respective thirty-six SSPA&#39;s in transmit mode, the phased array  10  will produce a main lobe and grating lobes according to the signal frequency. Increasing the spacing between the antenna elements  12  towards the wavelength λ of the signal frequency would result in increased directivity of the main lobe, but also in an increase of the grating lobes. Increasing the spacing between the antenna elements  12  beyond the wavelength λ of the signal frequency would result in multiple unwanted grating lobes. 
         [0023]    When the thirty-six PIN diode switches connect the thirty-six antenna elements  12  to the thirty-six respective impedances  20  in inactive mode, the random values of the impedances  20  result in a much flatter spatial response than the main and grating lobes that are present in the transmit or receive modes. This is due to the random impedances  20  producing random phase shifts in incoming signals that are reflected from the phased array, thereby preventing any directions of strong constructive or destructive interference for the phased array as a whole when the contributions from each of the elements  12  are added together. 
         [0024]    The values of the impedance elements  20  are randomly set by respective signals S 11 . The signals S 11  may vary the value of the impedance element  20  by varying inductive/capacitive components, or by switching between various inductivecapacitive components of the impedance element  20 . Alternatively each impedance element  20  could be permanently fixed at a predetermined random/pseudorandom value such that the signals S 11  are not required. 
         [0025]    In this embodiment, each antenna element has a respective T/R module and corresponding impedance, although alternatively the antenna elements could be grouped into groups with one T/R switch and corresponding impedance per group. 
         [0026]    An example of how randomly selecting the impedances that are connected to phased array antenna elements can affect the spatial response of the antenna will now be illustrated with reference to  FIG. 3 . A notional two-dimensional phased array of 900 antenna elements on a square grid of 530 mm×530 mm was simulated at 9 GHz. 
         [0027]      FIG. 3  shows a graph of the spatial backscattering response of the simulated array, with the direction Theta from the antenna being plotted against the x-axis in degrees, and the reflection back from the antenna being plotted against the y-axis in an arbitrary dB scale. 
         [0028]    The first trace  30  was taken with the antenna elements properly matched for transmit/receive modes, and shows a large main lobe of reflected power reaching up to −4 dB at 0 degrees. The trace  30  also shows twenty-three grating lobes gradually reducing in power as the angle Theta increases from 0 degrees to 90 degrees. 
         [0029]    The four traces  35 ,  36 ,  37 , and  38  were taken with four respective random configurations of phase shift applied to each antenna element, as may be applied by using random impedance elements. For each of these traces, each antenna element was randomly set with a phase shift of 0, 90, 180, or 270 degrees. The trace  40  shows the average of these four traces. 
         [0030]    It can be seen that each one of the four random configurations dramatically reduces the main lobe from −4 dB to around −40 dB. Thus, switching the phased array from transmit/receive modes using matched impedance elements to an inactive mode using random impedance elements reduces the peak backscattering power by around 35 dB. The grating lobe pattern is disrupted by the random configurations. The reduced peak backscattering power comes at the cost of a higher average backscatter power across the spatial range, although the backscattering power is fairly consistent from 0 degrees to 90 degrees. 
         [0031]    Further embodiments falling within the scope of the appended claims will also be apparent to those skilled in the art.