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 NXP Semiconductors


  • Radio Direction Finder Basics

  • Created: Aug 28, 2014

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Description

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Summary

This designed circuit is a type of Radio Direction Finder(RDF) that is a common instrument to aircraft. It is a handheld RDF that is used to localize both AM and FM carrier-based signal sources with the use of a single connection to the antenna input of an FM receiver tuned to the frequency of interest in the range of 45 to 450MHz.


The circuit is comprised of HEF4047BT that is a gatable astable multivibrator with logic techniques incorporated to permit positive or negative edge-triggered monostable multivibrator action with retriggering and external counting options. The 1N4148 is a high-speed switching diode fabricated in planar technology that is used to manage the reception and transmission of signals. The signal direction is perpendicular to the plane of the antennas. There is 180-degree ambiguity, but this is not a problem in actual use because multiple "bearings" must be taken anyway in order to establish a "fix" on the location. Once close in, the operator works toward the general sense of the direction, and again the ambiguity is not a problem. The three-position switch is used for three purposes. The UP is a switched on and rotated function for a null in the audio tone which it adds to the audio coming out the receiver. The DOWN is used to stop the tone but still keeps the antenna activated for monitoring and standby purposes. The center position is used to shut off the unit. The capacitors and resistors are used as filters and stabilizers.


The design is suitable for low cost project but high end application. It is easy to build even to a beginner. It is an inexpensive way to implement the concept of direction finding, and provides something relatively compact to keep readily available for locating interferences.