(尚未全寫 Not finished writing yet. 2019/10)
Here I'll list some techniques, for my quiet page, for finding a radio, left playing on some faraway hill, blasting all night to scare animals away from crops.
E.g., one day in October 2019 all night long from far away drifted the sound of a radio.
Because the sound wasn't too clear, I simply went looking. As a result made a big loop several kilometers with no results.
The next day I instead explored the usual suspects, finding nothing either.
Finally on the third day I decided to take to time to find the station first.
Always do this before running around! Not only will you know which of many radios you might find it probably is, you can then proceed to compute distance.
Comparing one by one from one end of the dial to the other (my radio: talk, their radio: music. Try next station. My radio: music, their radio: talk. Try next station. Both talk, or both music: listen longer to see it they are the same station. (Beware of different sub-station with same content. That could mess up our distance calculations...)
Note: in fact we should start by first just comparing the nearby popular stations, then do a total (FM) dial sweep, then even try again using the AM dial.
Then I used a see lightning / hear thunder app to compute the distance, using several samples 0800... 0800...
I computed the distance to between 1.0 and 1.1 KM. Even just with my ears and finger on the app, this turned out to be highly accurate.
A Mr. Liu was attempting to keep wild boars out of his orchard. He was delighted when I told him about the government's electric fence reimbursement program, and immediately turned down the volume. Problem solved!
So how could I have "walked right past it" on my previous two forays? Well it turns out it was on top of a little hill. Walking on the road in front or in back will miss it! Just like one cannot see what is on top of a flat roof of a building from the street.
圖：視野（綠） Image: viewshed: green. Made with Google Earth Pro (heywhatsthat viewsheds better for longer distances.)
But I even was on a mountain behind it, higher than it. How did I still not hear it? Well, it was on a front porch of a house. The house blocked the sound just enough to be hard to detect from behind.
To determine the source, one can tune into the same station with one's pocket radio, and use speed of sound, 3 seconds per kilometer, to estimate the distance of the delay. Several lightning distance calculator cell phone apps come in handy here. See also Sound ranging, Acoustic_location#Time-of-arrival_localization.
OK, we have tuned into the same stations as them. But guess what? Everybody also likes that same station. So if we go marching off in that direction and accost the the first person we see playing that station, the result will probably be we got an innocent person playing the station softly, and behind a hill. So even if they turn down the volume, when we return to our observation point, we find we didn't solve the problem one bit.
Therefore before we go marching off to find the culprit, we must make a distance estimate, using those lightning apps.
However in no way should we use our cellphone's built in FM Radio app, as it usually includes signal processor software etc. that could induce delays of a whole second (333 meters) or more. Same with any Internet radio, high-tech car radios, etc. or other fancy radios. The only radio we should use is the simplest pocket etc. dumb, raw, radios, that don't induce any delay.
Also note some programs broadcast via networks over several stations, each with their own internal delay, skewing the times! So, during different parts of a day, for the same station, we might calculate different distances... so we must double check if are we really tuned to the same station. Scan the dial for a 2nd or 3rd instance of the same program, with less delay. Tune in later when they are broadcasting a local program... we might find that it is an entirely different station than we thought.
And of course, not only us, but also the other person also might be using a fancy radio / amplifier, also inducing delays.
Any delays in our equipment mean the origin might be farther than we think. Any delays in their system mean the origin might be closer than we think.
Of course when you approach the person, don't make a big deal about *that* radio station, else later they might say "I now am listening to a different station. I wasn't sure if the new station also bothered you"!
OK, we've eliminated all the delays in our system. Let's consider all the delays in their system: amplifier, etc. Well, added all in it still means that the distance we compute is the maximum distance. If they have no delay it is the exact distance. If there is some delay then the point is actually closer to us.
Note throughout our story we never got the azimuth wrong. The angle where we heard the radio coming from turned out pretty much to be correct. It's just the distance that we have no initial proper idea of.
Now of course they could just be playing some song memory card. In this case there would be no characteristic song, song, song, advertisement, song, song. song, chit-chat, advertisement, etc. Just continual songs.
Or they could be playing from some website, via bluetooth, etc. So when adding up all the different delays, even if you did find the website, you probably couldn't pin down a good distance range.
Last modified: 2021-04-02 06:10:14 +0800