DIRECTION FINDING SYSTEM


Posted on Feb 5, 2014

If you have the time and money then make this plate bigger, if possible. 5-meters would be ideal for a home base. Similar to the system used in modern aviation where an antenna array modulates the phase of an incoming signal. The commercial system is known as Commutated Aerial Direction Finding (CADF) where a ring of 18 identical antennas are each switched on in


DIRECTION FINDING SYSTEM
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sequence, just like winding a rotary switch round and round so that one antenna is always connected to the receiver. Consider: If the receiver is connected to the take-off point in the centre of the above diagram, then any received signal will become phase modulated, the phase delay being dependant upon the direction of the received signal. Consider for one moment a signal comes from due North of the above array then Antenna-1 will receive signals that are part of one cycle in advance of all the other antennas. As the switch (commutator) is rotated then the signal fed to the receiver will gradually lag in phase more and more until Antenna -5 is selected. Continuing the switch rotation round, the signal will again begin to lead. The overall effect will give us a sine (ish) audio signal out of the receiver. If we now triggered an oscilloscope so that the NORTH antenna made the trace start (the trailing edge of Antenna-9 switching pulse), then the waveform received from the FM receiver would look something like this (after filtering): If the station received now came from due East then as the antennas are switched the received phase would advance until Antenna-3 was selected then continue to lag again. The greatest lag would occur when Antenna-7 was selected then the signal would again begin to lead again. If we still triggered the oscilloscope so that the NORTH antenna made the trace start, then the waveform received from...




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