HIGH_FREQUENCY_LOOP_ANTENNA

Published:2009/7/12 22:10:00 Author:May | From:SeekIC

Good performance in the 5- to 30-MHz range requires an amplifier with extremely high input im-pedance and low noise that can drive 75-Ω loads at high signal levels at frequencies over 30 MHz.Combining dual FET source followers and the new Maxim 436 wideband transconductance amplifier can produce just such an amplifier. A balanced configuration is used for the tuned loop to preserve the symmetry of the figure-eight polar antenna pattern. As a result of using FET source followers on the amplifier's front end, only the 1-MΩ gate resistors load the tuned circuit, so tuning is very sharp and resistance to off-frequency interference is very high. The FETs drive the differential inputs of the MAX436, which amplifies the balanced signal and converts it to a single-ended output. Voltage gain for this amplifier is switch selectable at either 8 dB or 20 dB into a 75-Ω load. Since the ampli-fier is designed to work into a 75-Ω load, the device can be connected to the receiver with a length of RG-59U cable. Maximum undistorted output is 1500 mV into 75 Ω, so overloading is unlikely even at the high-gain setting.



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