What are the most effective audio tone frequency(s) and pattern(s) used in direction finding in the world of amateur radio? Also: Is there a specification for the tones that amateur radio fox hunt transmitters use, and if so, is it designed with effectiveness in mind?
I am designing a Fox-hunt transmitter for the 70-cm band. I'm developing it to produce a range of tones in sequence, such as 440 Hz for 500 ms, 740 for 300 ms, 2200 Hz for 300 ms, etc. These are square-wave tones generated on a microcontroller and fed to a FSK transceiver module which is configured for direct modulation. The transceiver is configured for 5 kHz deviation. Thus, when an amateur radio HT is tuned to the FM center frequency, the tones can be heard.
The prime design goal is for this to be heard as far away as possible, which means making the signal as unique and identifiable as possible for a human listener. This basically means finding the best signal-to-noise ratio in the audible spectrum, the signal being the tone(s) and the noise being the background static present at the given tone frequency. According to this document, the human ear is most attuned to tones around 3 kHz at the threshold of hearing. Thus, it would make sense to use 3 kHz tones exclusively. However, as I have experimented with white noise mixed with various tones (using Audacity), it seems that the range from 300 Hz to 2 kHz is more conducive for identifying the tone from the background noise. Thus I'm somewhat stumped. I've found a few videos of Fox-hunt transmitters in action, but these offer no consensus on tones or pattern.
There is one precedent I am aware of which does define a set of parameters for radio direction finding: Aircraft Emergency Location Transmitters. According to Industry Canada RS-287 Section 5.3(a), 121.5 MHz aircraft radio distress beacons sweep down from between 1600 Hz and 300 Hz, covering no less than 700 Hz of this range. The document does not reference any research or reason for the range chosen.
I am also familiar with the Emergency Alert System's dual-tone alert, which is the combination of 853 Hz and 960 Hz sine waves. This irritating tone gets your attention, but I'm not sure if it is relevant to radio, and as it is a composite tone, generating it with my microcontroller will add more complexity to the system.