How Do GM Wonder Bar Radios Work?

Introduced by Chevrolet (a GM brand) in 1957, the Wonder Bar radio was the first automatic "station-seeking" radio tuner of its kind. Drivers simply set the minimum desired signal strength. Once he pushed a button, the device would scan through the spectrum of AM/FM radio frequencies until it found the first sufficiently strong station.

While radio waves can travel hundreds of miles, the fidelity and strength of a radio signal decays as it gets further from its source. For a radio receiver and speakers, this decay is manifested as "static noise". Because different drivers have different tolerances for static noise, the inventors of the Wonder Bar radio designed it such that the driver could set the minimal signal strength for the automatic tuner by turning a knob. Thus, a driver who didn't mind some static could set the minimum lower than a driver who preferred only clear signals. The trade-off would be that the Wonder Bar tuner would "find" a greater number of total stations for the more tolerant driver.

Once the minimum signal strength is manually set, the driver pushes the "Wonder Bar" button into the radio and releases it. The button is attached to a push-to-make switch that briefly completes an electrical circuit, just long enough to trigger a separate scanner circuit. The scanner circuit consists of an electrical motor, power source, amplifier and a "heterodyne" (also known as a frequency mixer) tied into the antenna.

When the electromagnetic radio waves strike the antenna, they induce small pulses of electrical energy through it. These pulse patterns travel through the antenna and into the conductive metal wire attached to it. The wire leads to the amplifier, which increases the voltage of the pulses while preserving their pattern and frequency. The amplified pulses then travel to the heterodyne.

The heterodyne is basically an adjustable resonance device for electrical pulses. When triggered by the Wonder Bar switch, the motor in the scanner circuit gradually rotates the heterodyne, shifting its internal configuration through a progression of resonance frequencies.

If an incoming electrical pulse frequency doesn't match the resonance frequency configuration of the heterodyne, the heterodyne effectively "blocks" it. However, if the incoming frequency does match the heterodyne's resonance frequency, the heterodyne allows the incoming electrical signal to pass through to the rest of the circuit.

If the strength of this heterodyne-permitted signal meets or exceeds the minimum strength requirements originally set by the driver, the circuit shuts off the motor. By shutting off the motor, the heterodyne's resonance shape locks into place, allowing a steady stream of that frequency's radio signal. Finally, this signal stream is converted into sound by the speaker.

From the driver/listener's perspective, the turner moves, stops at the first intelligible station and keeps playing that station until it's turned.