Two Homing type Direction-Finding

  
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The antenna is simply a pair of vertical dipoles. The diodes in each dipole allows the circuit to select which antenna is active at any given time. When a positive voltage is sent up the coax feedline, diode D1 will conduct and D2 will block, making the left antenna active and disabling the right antenna. A negative voltage will cause D2 to conduc
Two Homing type Direction-Finding - schematic

t and D1 to block, reversing the situation - hence the need for the diodes to be installed "backwards" with respect to each other. Either version of the homing unit will switch back and forth between the two antennas at an audio rate. If the antenna plane is oriented broadside to the incoming signal, there will be little change as the switching is done and the tone will disappear. However, if the antenna plane is oriented in any other way, the two antennas will be at different distances from the source: This will cause a change in phase of the received signal at each switch, or, in other words, phase modulation and a resulting tone. Since phase changes also cause a momentary frequency shift, these switches will appear in the audio of an FM receiver as positive and negative pulses (e. g. the tone. ) If one antenna is closer to the signal source than the other (even if this means an inch or two - even though the signal you are receiving may be miles away) this will cause a tone to appear in the aural unit, allowing the unit to determine which antenna is closer to the source. The only active device in the aural circuit is an NE565 phase-locked loop (PLL) IC. The circuit does not use it as a PLL, but rather uses its two components: an oscillator and a phase detector. The oscillator is used to send the switching signal to the antennas through R1 and its associated 10-uF capacitor. The phase comparator portion of the IC is used...



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