Transmitter Hunt

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Benjo

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Hi there. Might be a bit of an odd one, but I'm trying to research the plausibility of movie story line. During the 70s there used to be clunky old FM system hearing aids (Phonic Ear was a brand) that kids would strap the receiver to their chests and the teacher would wear the transmitter/microphone. Would it be plausible to track down the transmitter along the lines of transmitter hunting? Any help greatly appreciated!
 

vagrant

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Would it be plausible to track down the transmitter along the lines of transmitter hunting?
Maybe. Much will depend on the transmitters output power, antenna, and location. Something like that would probably not put out much power so that it would not require a license to transmit. I do not know how much power output those things had, but if it was similar to those FM microphones people can use with their home stereo's, then the signal would not reach too far, but "far" is relative.

I have hunted down 15mW transmitters that were miles away, but their geographic location and other things were a factor. Sometimes that same transmitter could not be heard 100' away. That transmitter was also using amateur radio frequencies around 146 MHz. If that Phonic ear was using an FM broadcast frequency it would not be worth the effort due to the interference from regular broadcast stations.

Another potential issue with tracking down the transmitter is attenuation. A signal can be attenuated if the transmitter is inside a building or a metal enclosure, versus outdoors in an open area.

I would say your premise may be plausible if that transmitter was 100 to 200 feet away in a building or built-up area. Another factor is the equipment used to do the hunting, such as directional antennas and the receiving device itself the hunter would use.
 

mastr

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...research the plausibility of movie story line. During the 70s there used to be clunky old FM system hearing aids...teacher would wear the transmitter/microphone. Would it be plausible to track down the transmitter along the lines of transmitter hunting?...
If your movie is set in the 70's - plausible, or close enough to plausible for a movie anyway. The required equipment to "track down" such a transmitter circa 1975 (within a reasonable time) would have cost many thousands of dollars and might fit in a large van with conspicuous antennas.

If your movie is set in the last 5 to 10 years - plausible, or close enough to plausible for a movie anyway. The required equipment to "track down" such a transmitter (within a reasonable time) still costs many thousands of dollars, but is less conspicuous and uses the now ubiquitous wireless data network to send location data from fixed, mobile or aeronautical infrastructure to a smart phone or tablet.
 

vagrant

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Why would it cost thousands of dollars? I used a CB radio back in the day to track down friends.

A person could hold a radio with a built-in antenna near their body and simply turn 360, listening to the signal to hear which direction it is strongest and walk that direction and do the 360 again. That technique is called "body fade". Using the Phonic Ear device the student would wear on their chest would facilitate that technique.

A signal strength meter on the radio would be nice, but somewhere along the way one may need to introduce some attenuation. This can be done by progressively tuning off frequency slightly as the signal strength increases, or clarity of the audio if only by ear. Another way would be to place one's hand or a book over the receiver to block the antenna. Basically, if the signal is strong enough to receive (overloading) and the receiver is off frequency and or the signal attenuated, you may be near the transmitter. Remember, body fade and directivity works by facing toward and away from the transmitter. As the receiver gets closer to the transmitter, facing your back toward the transmitter and turning side to side will give you directivity via attenuation. One does not always need to face forward toward the transmitter to get directivity. Thus, I have found it crucial to always turn in a circle again and again.

The frequency that Phonic ear used in the 70's is obviously important as well as the power output of the transmitter. If it was using a broadcast FM frequency, then a handheld transistor radio could be used. Still, the concern there would be the other FM broadcast stations, but typically the transmitter would have been tuned to a frequency away from strong stations.

Ultimately, the plausibility would be when you introduce distance between the transmitter and receiver. At that point a directional antenna would be prudent or may not be enough. If I knew more about the 1970's Phonic Ear frequency and power output, it would help.

The premise is plausible to a degree. If only using the Phonic Ear equipment, it would seem suitable for hide and seek within a limited area.
 

Benjo

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Thanks so much for the responses! Can't find any technical details of these types of FM system hearing aids used back then unfortunately.
Anyway, here's a bit more info about the story line; A deaf kid leaves the transmitter part of his hearing aid in the villain's van before the villain drives away. When the protagonist thinks all is lost and he'll never find the villain, the deaf kid realises what must have happened to the transmitter. I was wondering whether the protagonist could locate the transmitter either with a van load of tracking equipment or perhaps through his CB in his truck? Boosting the signal of the hearing aid receiver through his CB antenna?
So the villain would be a few miles away rather than localised. Any plausibility there or just flights of fancy? Oh, by the way, it is set in 1981.
 

slicerwizard

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It really doesn't matter how likely detection of the signal is (and why is the kid's gear sending a signal in the first place - isn't that what the instructor's half does?), your audience won't know anything about RF blocking or inverse cube law, so you're good to go.
 

mastr

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Why would it cost thousands of dollars?...
My experience has been that the cost is inversely proportional to the amount of time allotted to find the transmitter. I presumed that the OP would want time in his movie for some story line devices other than just some guy looking for the transmitter.

Circa (about) 1981, Doppler Systems had some pretty good stuff that might adapt to use in a movie. One model used 4 antennas on the vehicle roof and a ring of LEDs that indicated direction. I still have a couple around here some place.
 

vagrant

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Thank you Benjo for explaining. Let me give you something that would be plausible.

- If the child is fully deaf, full stop -

If not...

Now, I'm figuring the child is not totally deaf, as they are using the Phonic Ear in school.

The child could have a 49MHz VOX headset radio back in the 1980's. He could crank the volume to full or someone could have wired an earphone for it, due to his needs. I believe those things were $50 for a pair, so someone else has the other like another kid he would talk with. Those radios had VOX (voice activated transmission ability) so no need to press a button.

That VOX will help in two ways:
1. It only transmits when the audio level is loud enough to activate it.
2. Since it is not always transmitting it will save on battery life. (Those things used 9v batteries)

There are several things you are going to need to stretch here on the plausibility:
1. How long the battery lasts. If the child is able to turn it on just before leaving the van, that would be good but not mandatory. Still, having it on for several days would be a stretch, even if only occasionally transmitting.

2. I presume that radio would be in a bag, so figure out a way to get the bag near the window of the van. Alternatively, can your antagonist live in a two story building? Have them take the bag with other stuff from the van and put it near a window. If it needs to stay in the van then leave it, but if it could be on the dash or near a window it would help.

At this point we have a device that was around back in the 80's and occasionally transmitting on 49 MHz.

3. Your protagonist may need to be a radio buff, or their uncle is or someone. This is where the frequency hunting gear will come in. Back in the 70's there were portable radios $50 that could receive AM and FM broadcast bands as well as amateur radio, and bands used by police, fire, etc. Have your protagonist turn on the other VOX headset radio and then use that wide band portable to dial in on the exact frequency. That is critical.

They could then use this wide band radio, which is portable, and do the body fade once they heard a signal. Have them go to the top of a building or hill in order to get an initial bearing. Also have them bring along the other VOX headset. (They could attempt to use the VOX headset only, but the antennas on those things sucked. You could probably get away with using only that, but that would be a stretch and you don't have to do that because of the wide band radio.)

If your villain is in another town, this whole premise will not work. Those little radios do not transmit that far. If your protagonist is an amateur radio buff, which would make sense as why the child has the headset in the first place, you really can make this plausible. Or perhaps the child's uncle if the protagonist and child are unrelated. The amateur radio buff will be able to put together a dipole using Hamsticks in the 6 meter band. The six meter frequency ranges from 50 - 54 MHz, that makes it an antenna that is close to 49MHz and provides improved receive and directivity, to some degree. The coaxial cable going from the dipole antenna would plug into the wide band radio. An adapter would be needed but you can gloss over that.

A dipole antenna has nulls. If your protagonist is standing holding a pole that the two 6m Hamsticks are attached to (it would look like the letter T) and he is looking north, the nulls will be toward the east and west directions where the arms of the antennas are stretching out in both directions. A null is the direction or directions where the antenna does not receive signals that well. Again, if he is looking north, he would hear better in that direction and equally south, depending on what terrain is in the way. (Or you could simply write that they turned the antenna to find the direction. hahahaha that would be much easier than killing the reader with too many details) ahaha my long explanation is to make it plausible in my head.

Anyways, you pretty much have it. The protagonist may need to drive and wait and drive and wait until the villain makes noise, or turns on music or a TV that triggers the VOX. Once they get close, the protagonist could use the other VOX headset and listen to the villain as he approaches the location. Perhaps the villain is snoring or something or still listening to music.
 
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Benjo

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Thanks again for the feedback. This is really useful stuff. I like the Vox idea Vagrant. I'll let you know how I get on - hope you don't mind if I pick your brains again if I get stuck?
 

vagrant

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No problem. I just subscribed to this thread so if you make a post here I will see it, or send me a message through the forum.
 

Benjo

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Hi Vagrant. Another quick question if you don't mind? Is it vaguely plausible to connect a dipole antenna directly to the hearing aid receiver to hunt down the transmitter? Or would it be more the case of tuning into the frequency of the transmitter with a wide band radio? I've had to ditch any notion of the VOX headset idea as it too incongruous to the story...
 

K2RNI

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Not even remotely a maybe, that's a definite yes it can be tracked down and very easily with really cheap equipment. Remember people doing that on 2m with modified weather cubes. Not only did people do it all the time back then with basic cb radios and omni antennas it was also done with home brew little receivers as well. Sad how out of touch these users are lately. IDK what's the matter people these days.

I still DF 49 MHz ISM devices just using the rubber duck on a handheld scanner. You just gotta know what you're doing.
 
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vagrant

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Hi Vagrant. Another quick question if you don't mind? Is it vaguely plausible to connect a dipole antenna directly to the hearing aid receiver to hunt down the transmitter? Or would it be more the case of tuning into the frequency of the transmitter with a wide band radio? I've had to ditch any notion of the VOX headset idea as it too incongruous to the story...
It is your story. You can take whatever liberties you want. Plausibility is the factor if the transmitter is some distance away. You see, people either forget or do not realize that cellular phones are radios, but they are aware of signals dropping due to distance/cellular coverage. Will your readers believe it, especially if the very low power transmitter is miles away and not out in the open? While they may will probably not be familiar with that device, they will relate it to what they know. I can safely say that most people do not analyze things as much as I do, so you're probably fine.

One can make or repurpose something to create the antenna/dipole to be used to track down the transmitter. Could one take a telescoping aerial from the back of a TV set and rig it to the hearing aid receiver? Sure. Could one adjust the length to more closely match the frequency...yeah sure maybe, it depends. Keeping it simple may be best to keep the flow of the story and not lose your readers. Just pop an antenna on the hearing aid receiver and keep the story rolling. If you're going to do that, then lose the signal at one point, at least, and make the "hunting party" go back to where they last heard the signal and get a new bearing.

Yes, the wide band radio would be more apt to work, as they often had an input port where an external antenna could be connected.
 
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