| 摘要 |
An antenna, which may be a search coil, connected to an active circuit which provides negative impedances, each of which is of the order of magnitude of the positive impedances which characterize this active antenna. In one embodiment, one coil terminal is connected to an amplifier which drives a voltage-controlled current source that, in turn, drives a feedback coil which is coupled to the original search coil. In another embodiment that additionally exhibits an advantageous signal-to-noise characteristic, both terminals of the search coil are connected to a differential amplifier that, in turn, provides the control voltage for a current source, which, as in the first embodiment, drives the feedback winding. The feedback coil is wound to provide positive feedback by additive superposition of both coil fields. The positive feedback provided by the feedback current lowers the antenna impedance which, in turn, increases the effective area of the antenna. This circuit configuration incorporates a differentiation inherent in the fundamental characteristic of a coil, which is sensitive to the rate-of-change of the magnetic field. The outstanding stability of this active antenna may be attributed to the inherent accuracy of this differentiation performed by the antenna coil, to the particular circuit configurations and to the particular form of feedback employed.
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