Project #1

The Radio Direction Finder - part 1

Never go to a gunfight armed with a flyswatter.  Good advice, but how many of us have participated in a foxhunt or set out to locate a stuck mike switch that has timed out the repeater, armed only with an S-meter and a gas gauge?  The purpose of this project is to enable you to show up derssed for the occasion, whether it be a foxhunt, tracking a stuck mike or in a real emergency, locating a lost ham or a downed aircraft.
Phase Error - not Doppler
  In operation, this rig alternately selects between two antennas at an audio rate.  If the transmitter is closer to one of the antennas, the signal will reach the antennas at different times.  This time difference will result in a phase difference or error between the two signals as they arrive at the receiver.  As we introduce this error at an audio rate (about 1000 hz.) the error modulates the existing signal.  The amplitude of the audio tone is greatest when one antenna is closer to

the transmitter, while the tone is almost non-existent or nulled when both antennas are equal distance from the transmitter.  In use, the rig, connected to a reciever (HT or scanner) is rotated until the phase error tone nulls to it's lowest level.  When this occurs, the transmitter should be located 90 degrees in front of or behind the plane of the antennas.

AM or FM?

The circuit was originally designed to operate in conjunction with an FM receiver and will locate an AM or FM transmitter equally well.  However, the aircraft band operates in AM mode.  This rig has also been tested successfully with an inexpensive AM-FM-AIR receiver for locating aircraft Emergency Locator Transmitters.

Printed circuit pattern.  Overall dimensions are 2" by 3".