Powerful Radio Signals add Free Soundtrack
It's one of life's little irritations---I answer my telephone, and the person on the other end sounds a lot like Elvis. Then I realize that a local a.m. radio station is broadcasting an Elvis song, which is somehow being picked up by my phone and competing with the caller for my ear.
How does my phone turn into a radio receiver? I spent four years in the Air Force working on radios, and I remember the receivers as rather large, complicated boxes, crammed with tiny electrical components and a web of wires. My phone doesn't seem that complicated.
According to Robert Hunsucker, a professor emeritus at the Geophysical Institute with the University of Alaska Fairbanks, my phone isn't that complicated, and neither is a receiver circuit. A receiver is so simple, Hunsucker said, that anything from a phone to a person's mouth can act as one.
At its most basic, a receiver circuit consists of only three elements: an antenna, which picks up an electromagnetic radio signal; a detector, which is an electrical component that converts the radio wave to an audio signal the human ear can pick up; and a transducer, which is anything that acts like a speaker.
When I asked him about my phone, Hunsucker correctly deduced that I live close to the transmitter antenna of a local a.m. radio station. Because I live less than a mile away from the station, Hunsucker said I'm in the near-field of the transmitter, an area where the strength of the signal emitted by the radio station is strongest. The near-field is the area within a few wavelengths of a transmitter antenna---usually within a mile or so depending on the characteristics of the transmitter.
Within the near-field, the radio-frequency signal emitted by the transmitter is so strong that things not designed to be antennas---like telephone wires---can act like them. Hunsucker said radio-frequency waves from the a.m. station probably are traveling over the phone line, into my house, and into my phone.
As the signal instantly reaches the phone, a detector pulls the audio signal off the radio wave. The component acting as a detector in my phone is probably a diode, Hunsucker said. A diode is a semiconductor device, which offers greater resistance to the flow of electrical current in one direction than in the opposite direction. This trait allows a diode (or, in the presence of a very strong radio signal, an overdriven component that acts as a diode) to convert the incoming radio frequency signal into a direct-current voltage.
The direct-current voltage produced by the diode has peaks and valleys that correspond to Elvis's voice and the sound of his backup band. This voltage is felt and expressed by a transducer, which, in the case of my phone, is the little speaker that functions as the ear piece, and I hear the King.
Hunsucker said the problem can sometimes be cured with a radio-frequency filter that can be attached to the phone line. He also said that if the radio signal is very strong, a filter might not be enough.
Such is the extremely rare case when a person's mouth acts as a receiver. The electrical conductivity of the human body can act as an antenna. A metallic filling in a tooth, reacting just so with saliva, can act as a semiconductor to detect the audio signal. The speaker in this case could be anything that vibrates within the mouth enough to produce noise, such as bridgework or maybe a loose filling.
In those cases of extremely strong radio-frequency waves, Hunsucker said the receiver effect can be eliminated by surrounding the bogus receiver with a grounded, copper-screened cage. Or you may choose to sit back and enjoy the music.
