There has been significant interest in retrodirective antennas, especially considering the wealth of applications that could be significantly enhanced, or created, by the use of such technology. There is enormous potential for retrodirective antennas where complicated automatic tracking systems would benefit from being replaced by much simpler systems. Retrodirective array technology offers one solution pathway since it can offer extremely fast tracking with relatively simple circuitry. Retrodirective or self‐steering arrays are suited for low radio frequency (RF) power mobile terminal use particularly on or between un‐stabilised vehicles. In this type of operational scenario, high degrees of relative movement are expected, and power consumption and weight of the antenna must be kept to a minimum. In this study, the authors give a brief historical review of basic retrodirective technology and elaborate on some recent developments at Queens University of Belfast associated with retrodirective antenna technology in relation to, two‐way communications, ultrafast RADAR, microwave imaging, spatial power transmission, mitigation of multipath effects and spatial encryption.

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This letter demonstrates an extremely simple, low cost, chipless RFID reader. The reader consists of only a single Voltage Controlled Oscillator (VCO) as the detector front end. The information from the chipless RFID tag appears as a low frequency variation on the VCO DC bias supply which is readily detected and demodulated. Experimental results are shown for successful detection of a 10 bit RFID tag, operating over the frequency range 2.4-3.4 GHz. The results when compared to those taken from a vector network analyser (VNA) prove that the VCO detector can provide a similar level of performance to the more sophisticated VNA arrangement. The proposed solution provides a significant simplification of the chipless RFID reader.Introduction: Research into Chipless RFID is already producing small credit card sized tags that are completely passive and require no active (Chip) devices. This leads to the possibility of ultra-low cost tags that could be inkjet printable, in a similar way to optical bar codes. Despite the simplicity of such tags, the hardware that is required to read them needs to be high performance, complicated and therefore expensive. This is because, in order to yield the information stored within them, chipless RFID tags usually produce small changes in resonance. These small changes in resonance are difficult to detect since the reflected signal from the tag is very low due to low amounts of backscatter from such small size tags, and the fact that these tags have no on-board active components to amplify the signals returned back to the reader.Most experimental results reported for chipless RFID tags are measured using a vector network analyser (VNA), e.g. [1]. Here the differences in resonance that are required to be detected often result in a change in the S parameters of as little as 0.5 dB. Such small changes suggest that a high performance instrument, such as a VNA is needed for reliable detection. There has been work ongoing to produce stand alone chipless RFID reader hardware, that does not need the use of a VNA [2,3]. These readers still require reasonable performance and sophistication at the receiver front end. Consequently they contain most of the components that would be required for a high performance superheterodyne receiver.In this letter, we present, for the first time, a high performance chipless RFID reader which uses only a single active component, a voltage controlled oscillator (VCO), as the receiver front end. The resonance information from the tag appears at the reader as a low frequency signal superimposed on the bias current of the VCO. This proposed solution has the potential for significant cost reduction of chipless RFID readers, which could allow them to deployed, in volume, on platforms such as hand held mobile devices.

A new type of broadband retrodirective array, which has been constructed using a microstrip Rotman lens, is presented. Automatic tracking of targets is obtained by exploiting the conjugate phase response of the beamforming network which is exhibited when the input ports are terminated with either open or short circuits. In addition, the true time-delay property of the Rotman lens gives broadband operation of the self-tracking array when used in conjunction with Vivaldi antennas. The simulated and measured bistatic and monostatic radar cross-section (RCS) patterns of a structure consisting of 13 beamports and 12 array ports are presented at frequencies in the range 8-12 GHz. Significantly enhanced RCS within the scan coverage 40 is demonstrated by comparing the retrodirective behavior of a 12-element Vivaldi array terminated with and without the Rotman lens.

Abstract-We present an in depth look at the challenges involved in using analogue retrodirective arrays for satellite communications. The main technical issues surrounding the development of a retrodirective (self-steering) Satellite Communications (SATCOM) system are given and techniques for mitigating these issues provided. Detailed results are given for a prototype high performance circularly polarized retrodirective array architecture suitable for mounting on an un-stabilized mobile platform. The paper concludes with practical retrodirective L-band array results with the array used to acquire actual broadband satellite data signals from a commercial L-band satellite system. Received satellite signals as low as -130dBm at the antenna elements are tracked. Accurate self-tracking occurs over the azimuth range of up to ± ± ± ±40°°°°.

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