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Question:

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?

Supplemental Discussion:

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.

References:

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  • $\begingroup$ Would you really need to modulate an audio tone on top of FM in order for it to be detected? You may just focus on sending out the strongest possible signal and then at the receiver end do whatever you need to pick it up. No squelch and amplify the signal from the noise as much as you can. THEN add some tone by the receiver if you so wish. $\endgroup$ – captcha Sep 10 '15 at 4:26
  • $\begingroup$ The transceiver can do 100 mW: I assume that is the EIRP for all cases, whether information is being modulated with a wide band (say, 25 kHz deviation), or just an unmodulated carrier. So the real question is what the most robust signal a typical HT can receive? I'm using a UV-5R, so unless there is a feature I have not discovered yet, I'm fairly sure it only receives FM with a deviation up about 5 kHz, and does not do any fancy "amplify the signal .. add some tone by the receiver" processing. This is a Fox hunt transmitter, thus audio is a given? $\endgroup$ – Steve Sep 14 '15 at 4:24
  • $\begingroup$ Two things conflict in your statement when you say that your "prime design goal is for this to be heard as far away as possible" and "I'm using a UV-5R". If you're going to be on the edge of reception the last thing you want is to use a UV-5R for reception. If you want maximum transmitter range use more power. Collinear perhaps. $\endgroup$ – captcha Sep 14 '15 at 5:14
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I would make something discrete, as sudden changes are easier to detect than large ones. Stick into the middle of the hearing range (Many Hams are older, and tend to lose the extreme hearing range with time). I would choose a random sweep of tones in the 1000-1500 Hz range, changing the tone 2-5 times per second (Random time intervals might also work). This should give a pretty good indicator of what the signal is, and make it easy to find.

Don't forget to identify the station every 10 minutes as required.

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  • $\begingroup$ "..as sudden changes are easier to detect than large ones". Could you elaborate? I'm not seeing the difference between "sudden" and "large". Also, examples of real-world foxhunt transmitter metrics would be helpful. And shouldn't a signal be consistent, why have random sweep of frequencies and time intervals? $\endgroup$ – Steve Aug 13 '15 at 16:42
  • $\begingroup$ The changing tone makes it easier to find. A single tone is harder to detect, unless you turn the signal on/off (CW). The different frequencies makes it easier to detect. Signals turning on/off are harder for a fox hunt, the continuous wave makes it easier. $\endgroup$ – PearsonArtPhoto Aug 13 '15 at 19:09
  • $\begingroup$ @Steve: I just looked at this again, and realized the right word would be tone, not frequency. I've corrected this mistep. $\endgroup$ – PearsonArtPhoto Sep 9 '15 at 14:02
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It depends on how you define "Most effective". Are you looking to help or hinder the hunters?

I have used audio tones that were very close to the commutation rate of a doppler DF unit, specifically intending to confuse it.

If you are looking to help people identify the fox, then you could transmit morse as bursts of tone, covering the ID requirement and the ability to distinguish the fox from other transmissions. Something like "DE K9FOX"

If you can find it online somewhere, the instructions for the Agrelo fox hunt transmitter might give you some clues. I wrote the firmware for that unit many years ago. I had a lot of options for audio tones and CW messages. If you can't find it, email me at KC6ETE at gmail, and I'll send you info.

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