May 26, 2023

Usefulness of Harmonics in Radio Direction Finding

Harmonics are undesired transmissions produced at multiples of the intended transmission frequency. 

Most radio frequency transmitters produce harmonics at relatively low power levels, as compared to the fundamental frequency (also called the first harmonic).  The second harmonic appears at double the frequency of the fundamental.  The third harmonic is at triple the frequency of the fundamental.  For example, if the fundamental transmit frequency is 146.550 MHz then the second harmonic would be at 146.550 * 2 = 293.1 MHz and the third harmonic would be at 146.550 * 3 = 439.65 MHz.  Harmonics are a potential source of radio frequency interference.

In general, in amateur radio transmitter harmonics are undesirable.  However, there is one situation where an amateur radio operating can benefit from harmonics - radio fox hunting.  

A wide-band receiver is useful for radio fox hunting

In a radio fox hunting, the objective is to locate a hidden transmitter.  Hunting with a wide-band receiver allows the radio fox hunter to tune to the hidden transmitted signal's 2nd and 3rd harmonic frequencies as a way of effectively reducing the received signal strength.  This can provide a competitive advantage since a weak received signal at a harmonic is much easier to locate directionally than a strong signal at the fundamental.

This technique works best when you are in close proximity to the fox transmitter.

Good DX and 73, NJ2X

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