How does radio work?

[vexed]

But if that is how the song is transported, then I am still wondering how the song itself is encoded. How are all the tones that happen simultaneously encoded in a binary datastream that is sent/received/read/decoded sequentially? Or am I all wrong and is this not how it works?

Oh, and book suggestions are always welcome of course.

I googled for you: http://www.explainthatstuff.com/radio.html

Feel free to do your own research next time.

 

[ddt]

My school was in an academic town where new products and concepts were tested.

Difficult to google this one, but I'm learning they were called "induction loops" and may have been mainly targeted for hearing impaired students.

Oh, it seems they are still commercially available

A bit of a derail... Induction loops indeed are still widely in use. I've never heard of their use for ordinary hearing people, BTW. On the receiving side the technology is the simplest: most hearing aids and cochlear implants are fitted with a so-called "T coil" which picks up the EM radiation from the induction loop. And the signal is an "open standard", so to say. When you have a meeting in a room with various deaf/hard-of-hearing people, it's easiest to install an induction loop - mike, amplifier, and a wire running along the edge of the room - so all people can use it. The downside, of course, is that the signal can be picked up by anyone, also someone eavesdropping from, e.g., the floor below, so don't discuss confidential information.

The competing systems, FM and IR (infrared) are more targeted for one-to-one or one-to-few situations (indeed, a classroom with one or two deaf students). They're also proprietary technology, so an IR system of manufacturer X cannot communicate directly with a hearing aid of manufacturer Y. Commonly, the transmission of the signal then takes place via an induction loop: the transmitter part of the IR system hangs from the teacher's neck, it picks up his voice and converts it into an IR signal; the IR signal is picked up by the receiver, which hangs from the student's neck; it then creates an induction loop in the neck band of the receiver, and that EM signal is then picked up by the T-coil in the hearing aid of the student, which recreates sound in the student's ear.

(I have the impression that IR systems are more popular than FM systems over here).

 

[daxi]

As a radio amateur I am trained to train others to gain their licences. The Science of how Radio works depends upon what you want to know..Do you want to know how the receiver works? Do you want to know how the transmitter works?. Do you want to understand how the antenna works?. Would you want to know about how the signal propagates, that is what happens once it leaves the transmitting antenna, but before it reaches the receiving antenna?.
The words simplest sound receiver is two components. A Germanium diode ( 1N34A ) And a high impedance ear phone. You simply strip the plug off the ear phone & connect the bare wires one to either side of the diode, which then picks up strong radio signals if you have any...This can then be expanded to a third part & made more sensitive by unsoldering one of the ear phone wires from the diode & inserting a small coil of stiff wire between one leg of the diode & the ear phone..This is a small antenna & can be made to receive specific frequencies. You can then get a piece of tin foil, a book & a couple of lengths of wire with small clips at one end & make a simple capacitor which tunes the frequency.
Radio Frequency is transferred across the air waves as a sine wave. The frequency is measured in both MHz (And fractions such as KHz) And by the length of the sine wave (Wave length.)..The wave length is produced by a radio wave travelling at 300,000 kilometres per second & oscillating at a know speed the MHz or KHz figure, This effects the length of the sine wave. (The closeness of the peaks & troughs to each other) So in simple figures 300 speed of a radio wave divided by MHz the oscillation gives a wavelength & this wave length effects the way the signal reacts to it's surroundings & with in the earth's atmosphere & the size/design of the antenna.
I use a simple torch bulb & a couple of known lengths of wire to demonstrate the effect of wave length & polarity on reception. I use a amateur radio 433 MHz hand held radio, this frequency has a wave length of 70Cm which is small enough to be easy to work with.
I connect two lengths of stiff wire, one each side of the bulb & hold this a few inches from my hand held radio parallel to the antenna & key the mike the small torch bulb, only needs a couple of Volts to light it & Electro Magnetic Fields (as thus Radio Frequency) is measured in Volts per Square Meter, so a small field is all thats needed to work. I explain that what I have made is a dipole, two elements each a quarter of a wave length one top & one bottom, one in a real antenna would be the radiating/receiving element & connected to the feed line line centre core & the other would be the counterpoise (balance) & would be the outer core & connected to chassis (earth.) It is a balanced antenna. Each leg having the same impedance, in this case we will call it 50 ohms & go no further as then things start to get really complicated. I turn the antenna to 90 degrees to the transmitting antenna & the bulb goes out, this is caused by lack of polarity between the transmitting antenna & receiving antenna. I then swap the two wires either side of the bulb for wires that make a poor match for the used frequency & the bulb will not light. This is caused as the antenna is not resonant at the used frequency.

 

[daxi]

Induction loops indeed are still widely in use. I've never heard of their use for ordinary hearing people.

Induction loop systems are very widely used in commercial applications. Both for security/stock control application as well as for broadcasting within a defined perimeter.
Ever seen those tabs stuck to clothes etc in shops that are used to stop (slow up) theft & wondered why they have no battery? And why shop lifters use a bag lined with tin foil to beat them? It's because an induction loop system gives a protection against anyone leaving the defined perimeter..Shop doors. with receivers at them. In a factory or commercial premises it can be used to track both stock & staff/visitors. An induction loop system can identify movement without most people being aware that it is happening, unlike needing to use a swipe card for example.

 

[Mr. Scott]

As always, light is a mysterious thing.

Yes. How radio works is answerable, but the true nature of things like light waves and radio waves, photons and wave partical duality, are not inuitively understandable. Our only way to get a grip on it is with mathematics, but it's a subject for another thread.

 

[Mr. Scott]

Before I forget, sometimes people with metal fillings or braces in their teeth can hear a radio broadcast if they are very close to a strong AM transmitter. Most likely metal in the tooth generates an electrical current induced from the transmitted waves, and if certain layers of materials are present, the current is rectified into the audio signal which may vibrate the tooth and skull and sound will be picked up by the inner ear. There's disagreement about whether or not this is urban myth. Mythbusters tested it but I don't think they tested it properly.

 

[davefoc]

Before I forget, sometimes people with metal fillings or braces in their teeth can hear a radio broadcast if they are very close to a strong AM transmitter. Most likely metal in the tooth generates an electrical current induced from the transmitted waves, and if certain layers of materials are present, the current is rectified into the audio signal which may vibrate the tooth and skull and sound will be picked up by the inner ear. There's disagreement about whether or not this is urban myth. Mythbusters tested it but I don't think they tested it properly.

I looked into this often reported story once and left thinking that it probably wasn't true. As I recall the story originated with Lucille Ball and it was on shaky grounds from the beginning. I was an electrical engineer but I am not knowledgeable enough on radio wave detection to know that it couldn't be true though, so for me it still falls into the plausible but probably not true range. At least three things would need to be true for this to work I think:
1. Enough energy from the radio source is coupled into part of the human body acting as an antenna to produce a sound wave or vibration that is large enough to be perceived by the human.
2. Some kind of detection/rectification device exists in the human body such that the energy from the radio wave could be converted to a signal that could excite some kind of transducer to produce the sound or vibration that the human perceived.
3. Some kind of transducer that could convert the electrical energy to mechanical movement exists.

Of the requirements only one seems likely to be true. The second one requires some kind of solid state diode/detection device (a naturally occurring vacuum tube is a non-starter I think) would need to exist by happenstance. This seems right at the barely plausible limit to me. And finally the human would need some method to perceive the electrical signal. Could a human perceive audio information from an audio frequency electrical signal connected to it? Is there anything in the body could conceivably convert the electrical energy to a mechanical vibration? I'm leaning to no on this also.

 

[Mr. Scott]

I looked into this often reported story once and left thinking that it probably wasn't true. As I recall the story originated with Lucille Ball and it was on shaky grounds from the beginning. I was an electrical engineer but I am not knowledgeable enough on radio wave detection to know that it couldn't be true though, so for me it still falls into the plausible but probably not true range. At least three things would need to be true for this to work I think:
1. Enough energy from the radio source is coupled into part of the human body acting as an antenna to produce a sound wave or vibration that is large enough to be perceived by the human.
2. Some kind of detection/rectification device exists in the human body such that the energy from the radio wave could be converted to a signal that could excite some kind of transducer to produce the sound or vibration that the human perceived.
3. Some kind of transducer that could convert the electrical energy to mechanical movement exists.

Of the requirements only one seems likely to be true. The second one requires some kind of solid state diode/detection device (a naturally occurring vacuum tube is a non-starter I think) would need to exist by happenstance. This seems right at the barely plausible limit to me. And finally the human would need some method to perceive the electrical signal. Could a human perceive audio information from an audio frequency electrical signal connected to it? Is there anything in the body could conceivably convert the electrical energy to a mechanical vibration? I'm leaning to no on this also.

If you read the link you'd see there is more to it than the Lucille Ball story. It's hard to imagine an actress/comedienne would invent something so implausible to a layman, that tooth fillings would pick up radio transmissions. I bet it happened which is how she heard about it, just not to her.

The original radio detectors used naturally occurring galena crystals as rectifiers, not vacuum tubes. AM transmitters sometimes radiated enormous amounts of power (up to a million watts and more), and ears are extremely sensitive. What mythbuster should have done was advertise for people who could hear stations, and test them with lab equipment. A diode forming of mouth metal would require chance accumulations of just the right layers of materials. That would require lots of trial and error to test in a laboratory. Today's fillings and braces are not the same as those in the great AM radio era, so I remain open minded.

The point, nevertheless, is that radio reception does not need a very complicated device.

At the turn of the 20th century, an American scientist, Greenleaf Whittier Pickard, found that a number of naturally occurring crystalline minerals could be used to detect radio signals.

 

[Dikken]

I was just about to mention my galena cat's whisker radio that I made in the 1950's. I clipped it onto the hot-water radiator system for an antenna and could not only listen to local stations but distant ones just by moving the wire. There was no amplifier, so I needed an ear piece.

I spent many nights listing to music types I had never heard before.

 

[Mr. Scott]

I was just about to mention my galena cat's whisker radio that I made in the 1950's. I clipped it onto the hot-water radiator system for an antenna and could not only listen to local stations but distant ones just by moving the wire. There was no amplifier, so I needed an ear piece.

I spent many nights listing to music types I had never heard before.

Like!

I also as a kid made a working radio receiver with a wire outside my window, a germanium diode (probably a 1N34), and an earpiece like this one. Literally, only those 3 parts. I couldn't tune mine -- yours must have been very good, but mine proved how simple a receiver can be.

 

[davefoc]

If you read the link you'd see there is more to it than the Lucille Ball story. It's hard to imagine an actress/comedienne would invent something so implausible to a layman, that tooth fillings would pick up radio transmissions. I bet it happened which is how she heard about it, just not to her.

The link seems to be to a Mythbuster's forum where a variety of people, not all of which seemed to be knowledgeable on the subject, are commenting on the plausibility of this or the test methods that were employed in the segment. I didn't notice one of the posters that particularly stood out because of his expertise on this matter. Perhaps you could suggest one that had something particularly interesting to say?

The original radio detectors used naturally occurring galena crystals as rectifiers, not vacuum tubes. AM transmitters sometimes radiated enormous amounts of power (up to a million watts and more), and ears are extremely sensitive.

The vacuum tube comment was a bit of a joke, perhaps I should have employed a smily? Here's one to make up for it: Note I am often the best audience for my humor and the fact that I'm the only one laughing sometimes is a source of yet more humor for myself.

I know powerful AM transmitters exist and have existed, although 50KW in the US has been the limit for a very long time. There is no question that enough energy can be coupled into a passive receiver with a small antenna to create a sound that people can hear from even modest transmitters. I am not sure that enough energy can be captured from the metal in a person's dental work to power something that could create something that a human could hear. But I thought that it was possible.

What mythbuster should have done was advertise for people who could hear stations, and test them with lab equipment.

That sounds like a really good idea to me. I don't think you'd need to test them with lab equipment though. I think you could just ask them what was playing on the radio before you tried to analyze the mechanism that allowed it to happen.

A diode forming of mouth metal would require chance accumulations of just the right layers of materials. That would require lots of trial and error to test in a laboratory. Today's fillings and braces are not the same as those in the great AM radio era, so I remain open minded.

I'm open minded also in the sense that I don't have personal knowledge that would make it so I can rule out the possibility. I might be more skeptical than you are about whether it is a real effect or not though.

The point, nevertheless, is that radio reception does not need a very complicated device.

I thought daxi did a pretty good job of making this point. I would like to have seen some pictures of his gadgets.

 

[jj]

I've also made a crystal set, in fact a tuned one, using a coil, a variable capacitor, a 1U4a running as a grid diode (with a filament battery), and a 2K headset. Using the 1N34 worked, but the better high-pereviance diode you get from a 1U4 running as a diode using the control grid is even better.

I also used a 2N404 to make a simple gain stage, and I used the 1U4 also as a gain device, but it had way the (bleep) too much gain.

Worked rather well, too.

 

[daxi]

The stories about metal plates in heads receiving radio transmissions are certainly true. I am not sure that a tooth filling under normal circumstances would act as a receiver, unless the signal was VERY strong, but it is certainly within the laws of physics for it to happen.
I know of Microwave ovens, un-connected speakers, hair driers, electric organs & even kids toys & light bulbs that have picked up AM RF. The audio is sometimes difficult to understand, whilst in other devices it may be perfectly understandable.
The term crystal radio is to a certain extent misleading as no crystal is actually needed. During WW2, many American service man, as an example, had what was called a "Trench" or "Foxhole" radios & these were often made using a razor blade as the "Crystal" detector, rather than a crystal. And the radio used by British prisoners in Colditz & described by Pat Ried used a small pea sized lump of Coke.
These radio are simple to build & use nothing more than a odd piece of wood as a base, some drawing pins instead of solder to join wires, a safety pin with some graphite tied to it's point as the contact with the crystal detector that can be tuned slightly. A coil of wire which could simply be the wire wrapped around the base, or around a cardboard former. An earphone which can be easily home made & a length of wire for the earth & a longer length as an antenna.
A receiver for AM transmissions can be very simple indeed & if you want to receive (resolve) Single Side Band transmissions then tune two radios into the same signal & then tune one to beat against the other until it's readable & you have just made a BTO (Beat Frequency Oscillator.)
If someone wants to make a diode receiver from scratch. Cut about an inch off one of the clear plastic biro's Insert a wire & seal one end with blue tack so the wire extends a small way into the tube & at least half an inch out from it. Pour sufficient Copper Oxide into the tube to cover the exposed wire. Fill the rest of the tube with Copper filings or cut some fine copper wire into very small bits. Place a second wire in the open end of the tube, so that it is at least touching or preferably slightly into the copper bits & again seal with blue tack...One working diode is now yours.

 

[Mr. Scott]

This link provides some believable testimony to hearing radio on braces or fillings, and says the episode about this on mythbusters was poorly done.

This was a terrible segment [on mythbusters]. While the particular Lucy story is no doubt bunk, the physics is really simple. They displayed great ignorance in how a crystal radio works. You don't need a battery to pick up radio folks. I listened to radio stations 2 thousand miles away on a crystal radio as a kid.

And ... I once got AM in my right ear after biting down hard on a peppercorn and jammed a filling.

It so happens that lead and ceramic can be parts of radio detector diodes (that translate radio carrier waves to sound current) and some fillings contain lead and parts of teeth are enamel, and loose fillings can vibrate and produce sound. There's testimony that, after a loose filling was replaced, the effect vanished.

The idea is plausible, the testimony I'm finding is convincing, mythbusters made technical and historical errors in their study of it, but I don't want to debate it forever.

 

[ddt]

Induction loop systems are very widely used in commercial applications. <snip>

Thanks for that, I didn't know - never thought about the technology used. My comment was a bit sloppy, I only discussed application of induction loops for transmitting sound/voice.

BTW, welcome to the forum!

 

[davefoc]

I looked around the web a bit more on the dental radio story and didn't find anything convincing. The idea that it is plausible but unconfirmed comes up a lot. And then as has been noted there are a few anecdotal reports by people claiming to have experienced it. Right now I don't think there is much to debate about the topic without either some quantitative experiments that evaluated the potential for a radio receiver to occur through happenstance inside a human or some experimental data where individuals were tested to see that they could report the content of a radio transmission without a purpose built radio receiver.

I have my own anecdotal story that is somewhat related though. A few years before the development of wi-fi we had modified parts of a hand held transceiver to transmit data as part of a data collection terminal product we developed. We were walking around our facility testing coverage back to a spectrum analyzer in the lab. The transmitter didn't have a microphone attached to it. The spectrum analyzer had a speaker that could be used to hear the audio in a signal if any existed. Much to our amazement something in the transmitter modulated the output with the sound of our voices so that the people in the lab could hear us as we walked around the facility. I hadn't thought about this in years but our theory, I think, was that a component in the radio was vibrating enough that the change in its electrical characteristics was enough to modulate the output frequency enough that detectable audio information was transmitted. This was not a good thing for us because the audio information was essentially noise that was laid on top of the digital data that we were trying to transmit. As to the product, it was more or less a failure. We sold a few of the devices but our company in a different division developed a spread spectrum data radio that we integrated into our newer generation terminals that was very successful.

 

[jj]

The dental filling thing has a few gaps. First, electrical stimulation of the cochlea will not induce the same filtering effects that acoustic excitation will. The filtering effects are, in fact, how we create signals on the auditory nerve that we can make sense of.

Now, it is possible, perhaps, to create a "sensation of pitch" via time-domain firing over a broad spectrum, but only maybe, kind of, perhaps, and unproven.

But accurate hearing? No.

This is why cochlear implants are so hard to make. They have to mimic the filtering present in the ear to start with, and then apply the signals, in the right places, to the basilar membrane. Just exciting the whole basilar membrane (what radio waves rectified by a filling are likely to do) is not going to create any normal kind of sensation.

Sorry, but no.

 

[Mr. Scott]

The dental filling thing has a few gaps. First, electrical stimulation of the cochlea will not induce the same filtering effects that acoustic excitation will. The filtering effects are, in fact, how we create signals on the auditory nerve that we can make sense of.

Now, it is possible, perhaps, to create a "sensation of pitch" via time-domain firing over a broad spectrum, but only maybe, kind of, perhaps, and unproven.

But accurate hearing? No.

This is why cochlear implants are so hard to make. They have to mimic the filtering present in the ear to start with, and then apply the signals, in the right places, to the basilar membrane. Just exciting the whole basilar membrane (what radio waves rectified by a filling are likely to do) is not going to create any normal kind of sensation.

Sorry, but no.

Few are claiming radio reception by dental work happens by direct electrical stimulation of the nerves in the ear. The best hypothesis is that loose fillings vibrate mechanically, because of the huge power of nearby transmitter towers, and that mechanical vibration is transmitted to the ear through the bones.

 

[psionl0]

The dental filling thing has a few gaps.

Detection of AM signals is relatively easy - all you need is a "non-linear" device in an electrical circuit (if it were any more complicated than this then radio might never have taken off).

Any junction of two dissimilar compounds (eg fillings and teeth) can act as such a non-linear device and provide part of the apparatus that is necessary to receive AM signals.

I have heard of washing machines and similar appliances acting like an AM receiver but I don't have any first hand information about dental fillings - although it is definitely theoretically possible.

 

[gnome]

Electricity goes in, right wing talk shows come out. You can't explain that!