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Current Source for Magnetic Sensor


OBJECTIVE: To design, develop and build a prototype RF current source or RF power amplifier that drives the type of low impedance magnetic current loop for the magnetic sensor described below. DESCRIPTION: Operation of the sensor is dependent on the magnetic field that is projected and that in turn is directly related to the current in the magnetic current loop. Conventional 50 ohm amplifiers require considerable matching resulting in a narrow bandwidth and excessive operational sensitivity. Current bandwidths are less than 0.5 % and it is expected that an RF current source will provide a bandwidth of greater than 10%. This will allow for much greater flexibility of mounting configurations. The only option today is to use a 50 ohm power amplifier. Using a conventional 50 ohm power amplifier requires a matching network to transform from 50 ohms to approx. 1.005-j3.09 ohms. Thus, the conventional approach would be to start with a voltage controlled current source to a 50 ohm power amplifier which is then impedance matched to 1.005-j3.09 ohms. This process provides for a maximum bandwidth of only 1% and results in poor efficiency. Developing a RF current source or RF power amplifier that drives a low impedance magnetic sensor will result in a minimum of a 10x increase in bandwidth and approximately a 5-10x reduction in required power with the sensor operating at approx. 1.005-j3.09 ohms. This type of magnetic sensor can easily penetrate the ground to detect deeply buried threats, such as landmines, etc., while its design will reduce unwanted electromagnetic (EM) interference. The commercial and military applications include the development of greatly improved metal/anomaly sensors. The concept behind transmitting a large magnetic field while minimizing the generation of a propagating EM wave, is to use a current loop in which the current around the loop has a constant magnitude and a constant phase [1]. Usually in a current loop sensor the current changes phase around the loop and this phase change generates a propagating EM signal. By keeping both the magnitude and phase constant, little EM signal is projected but a strong magnetic signal is produced that extends normal to the plane of the loop creating a large magnetic field in the near field. This field will penetrate conducting dielectrics such as ground which have little effect on the magnetic field but substantially terminate the electric field and thus, a propagating EM wave. In [1] the in-phase current loop is created using multiple small loops. In [2] an in-phase current loop design is presented in which reactive compensation is used. Periodic series capacitors placed around the loop compensate for the"time-of-flight"phase change along a segment of the loop. Thus a magnetic current loop was developed for use in a magnetic-current-loop-based communication system. This design divided the loop into small segments and reactive compensation is added to each segment. Adding reactive compensation to each segment of the loop cancels the series reactance of each segment of the loop and provides for current magnitude and phase uniformity along the loop at any given instant in time [2]. We have built and modeled such a magnetic sensor and the impedance at 13.56 MHz is around 1.005-j3.09 ohms [3]. PHASE I: The contractor shall conduct a feasibility study to develop a current source which can greatly improve the bandwidth and reduce the required power needed to drive a low impedance magnetic sensor. The contractor shall submit a report which shall detail the results of the feasibility study of the sensor to be used to perform this mission. The report should contain a description of the sensor, as well as technical details of how the sensor will perform the required task(s) and expected performance. A brief high level plan for phase II work should be included in this report in the event of a phase II selection. PHASE II: The contractor shall develop a robust prototype sensor based on the results of the Phase I effort. The prototype sensor will be able to be drive a low impedance magnetic sensor and demonstrate an ability to penetrate the ground to detect deeply buried threats. A demonstration of the sensor will be done at a location determined by the government. PHASE III: Based upon Phase II results the sensor will be improved upon and optimized for commercialization. Multiple military programs and commercial applications can benefit from this sensor including: R & D laboratories and both military and commercial metal/anomaly sensor developers/manufactures. The most likely path for transition to operational capability is development of a superior metal/anomaly sensor than the sensors presently available.
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