williams@kirk.DEC (John Williams 223-3402) (07/19/85)
Yes, You can transmit audio electromagnetically, but there are two problems: 1) You need more amplitude to establish a certain field strength at low frequencies, which can be measured as Volts/meter along the length of the antenna. 2) At low frequencies, signals are absorbed by metal objects. A tremendous amount of energy would be dissipated by mechanical vibration. The second is the most important effect, and makes transmission impractical at low frequencies except within the confines of a transformer. John Williams
george@fear.UUCP (George White) (07/22/85)
The power grid makes a pretty good (and powerful) low frequency transmitter. A sizable fraction of what we pay for electricity is 'lost' on transmission. Just ask someone who lives close to a high voltage transmission line about the effects of this RF energy. Another demonstration is to put your finger on the input of an audio amplifier. Does anyone know how far away you can be from any known source of AC power and still be able to detect hum (middle of desert, south pole, middle of the ocean, the shuttle, a screen room, Gilroy, ...) ? I suspect that this energy is detectable quite far from the Earth as is a 10 HZ beat note between the two primary power grid frequencies. Should we try and detect such transmissions from other solar systems (Please, no flames for assuming intelligence spawns AC power distribution systems) ? George White
fowler@uw-beaver (Rob Fowler) (07/24/85)
How far do you have to go to get away from 60Hz hum? I used to do computerized cartography applied to doing near shore analytic tidal predictions. As such I had contact with people who actually go out measuring currents on the continental shelf in up to 300- 400 feet of water south of Long Island, N.Y. (there are others). They had to redesign their electro magnetic current sensors because of 60 Hz interference on the ocean floor. I won't get into the details of how they work, but they rely on using currents through moving salt water to generate detectable magnetic fields. It's necessary to use low freq. AC to do this in order to cancel out the earth's magnetic field, to cancel out chemical effects at the electrodes, and otherwise make the field detectable . If you use some random frequency and phase then your measurements contain an interference component generated by your signal beating against the background hum. The solution they used was to phase lock the oscillator that generates the AC with the background hum. Presto, all the problems go away. This apparently works anywhere on the continental shelf off the East Coast. Modulating the entire US power grid for communication purposes would be another story though. I have heard rumors that the Russians do have the potential to use part of their power grid as antennae for emergency submarine communication. An interesting experience I had related to background hum occured about 10 years ago. I went to a screening of a documentary that was done on a European tour of Julian Beck's Living Theater. At one point the performers started chanting and "OM"ing. It was really awful, unaesthetic, bad. They seemed to be out of tune with the universe. The reason? They were doing a European OM in resonance with the 50Hz grid and it sounded flat and unharmonious to an audience sitting in 60Hz Berkeley. -- Rob Fowler (uw-beaver!fowler or fowler@washington.arpa)