why is it that when i’m listening to the radio and turn on something electric (such as my keyboard… the instrument; not computer… or the sewing machine), it makes the radio get alot of static?
Lots of electronic and electrical emit broadband RF and EMI for various reasons. Digital electronics use square waves which have (theoretically) an infinite number of harmonics; such devices, if not properly shielded can generate quite a bit of electrical noise. Devices which use AC motors can also generate a lot of noise, due to arcing in the brushes.
what do you mean by “properly shielded”?
Generally EMI shielding involves enclosing the electronics in a metal case, or in a plastic case lined either with metal foil, or coated with a conductive spray-on material, all of which must be grounded. Any wires or cables which may carry high-frequency RF will often have ferrite beads near where they exit the enclosure, both to reduce EMI output and to reduce received interferrence from outside sources. Many electronic devices must meet minimum FCC requirements for EMI emissions and shielding.
Simple answer: any extremely sudden change in an electrical current creates radio waves. For example, turn the lights in a room on or off and you’ll hear a little “click” in your radio loudspeaker. Most small motors have “brushes” or little switches which turn their electromagnet coils on and off at high speed as the motor spins. A radio can pick up the “silent whirr” made by an electric motor.
If you want to be a cyborg with electromagnetic senses, just wear a small AM radio with headphones. Tune the receiver to a spot between stations. Then wear the thing all day. You’ll hear all sorts of interesting sounds. Here in Seattle the electric busses make weird musical notes, and the radio-wave sound can be heard whenever you’re near an overhead bus cable.
Hair blow-dryers and electric mixers are really bad for AM radios since they do lots of work, which means they create a larger electric current in the power line, which makes a much louder radio wave as the brushes turn the current on and off.
Hey, wasn’t it your AM radio that picks up the garbage? FM radios ignore normal radio waves (they can only detect CHANGES in continuous radio waves.)
FM receivers can pick up EMI too. Run your vacuum cleaner next to your TV (in the NTSC sytem, TV audio is FM, the video is AM) or FM radio some time. You’ll be able to hear the noise.
The noise of the vacuum cleaner? 
Well, yes. Unless you accidentally vacuum your SO. Then you’ll hear the screaming first. 
how likely is it for radio to pick up conversation on a cordless phone? don’t tell me this is impossible, because i’ve heard my sister’s phone conversation in the other roon thru the radio before (FM).
Depends on the phone and the radio. The phone has to be an analog model, no digital or spread spectrum, and operating on the old 47/49 MHz cordless phone band. If the phone’s circuitry is cheap, it might be outputting on one or more harmonics, which are multiples of the fundamental frequency. 47 MHZ x 2 is 94 MHz or 49 MHz x 2 = 98 MHz, which are squarely in the middle of the 88 to 108 MHZ broadcast FM band. These phones transmitted to the base on one of ten channels (later expanded to 25) in the 47 MHz band, and received from the base in the 49 MHz band.
Oh, and the range will be very limitied, because the harmonics are seaker than the fundamental, and the even harmonics are weaker than the odd harmonics.
Eh. Not really.
The company I work for has done some piddly experiments with digital transmission of audio, and I can tell you for sure that you can understand a none encrypted digital signal with nothing more than an AM radio. The lowpass filters work together with the digital signal to create a kind of pulse width modulation digital to analog converter. It sounds pretty shitty, but it can be done.
Also, most FM receivers will receive AM to some extent. If you transmit an AM signal on a frequency an FM receiver can get, then you will hear audio. I used to retune car radios (FM band 88-108 MHZ) to pick up aircraft band (AM, 120 MHZ.) It sounded crappy and the receivers weren’t very sensitive, but if you were close to an airport it did the job.
You won’t pick up spread spectrum (or really well encrypted digital) with an ordinary radio, though. Not unless something is really wrong.
Spread spectrum can be picked up and listened to without a proper receiver, however. You need a special “near field” receiver and a lot of signal strength (receiver close to the transmitter) but it can be done. I have done it to a system that used to be commercially available. No ordinary scanner could even detect the radios in operation (2-way spread spectrum,) but placing a near field receiver close to the repeater would let me listen to both sides of a conversation. That system isn’t sold anymore, but more for commercial reasons than because it was insecure.
Mort,
That is interesting about digital though a low pas filter. Do you know how they were encoding bits because I don’t see how what you are saying could possibly work unless the encoding was done specifically to be put though a low pass filter like you guys did.
It’s called pulse-width modulation. The transmitter modulates the length of each pulse to correspond to the amplitude of the original analog signal. The different “on” times of the pulses charge the caps in the filters to different, proportional voltage levels, thus creating a very crude AM signal in the receiver.
I should add that this will not work for any digital encoding scheme other than PWM.
It wasn’t PWM being transmitted. It was a PSK system transmitting the actual binary values at 64KHertz and 8 bits resolution. You actually got audio out of it, but distorted as all hell. Depending on the 8 bit number being transmitted, you get bit patterns that charge the capacitors differently. “11111111” generates something like “10101010” in PSK, which will generate a low voltage in the receiver. “10101010” as the original value will translate as “11111111” in PSK, which generates a different voltage. It isn’t a proper PWM modulator by any stretch of imagination, but it works well enough to be understood. Encryption pretty well randomizes the bits and causes you to get what amounts to white noise in the receiver.