A utility radio is a broadcast radio receiver used during World War II that used four vacuum tubes to produce audio that was clear enough for critical public information purposes. It was a very important communication radio equipment because there was no other type of broadcast receiver during the 1940s. The modern utility radio, however, is used mostly for safety purposes and is a must-have for any kind of weather and other natural disturbances. Many voice public safety announcements are transmitted over the air by broadcast stations and received by the utility radio.
Amplitude modulation (AM) is used for the utility radio in the standard AM broadcast band and in shortwave, which are radio frequencies allocated at frequencies higher than the AM band. Numerous worldwide audio broadcasts are available on the shortwave band. Many government-sponsored shortwave broadcasts reach across continents to provide information and entertainment to people in most places.
There are different types of radios in radio electronics. The simplest is a direct-conversion receiver type that amplifies the incoming radio frequency once or several times without translating the frequency. After the radio frequency amplification, it is fed to a detector that extracts the original message.
The incoming radio frequency is translated into an intermediate frequency in a single-conversion receiver, which is then fed to a detector as in the direct-conversion receiver. In a double-conversion receiver, the first intermediate frequency is fed into another mixer to generate a second intermediate frequency. The double-conversion receiver is best suited for sub-bands with lots of co-channel users and interference such as in a two-way radio.
The single-conversion receiver uses an internal oscillator called the local oscillator to produce a fixed amplitude signal. This can be mixed with the incoming radio frequency to produce the difference frequency, which is the intermediate frequency. If the incoming radio frequency needs to be increased in amplitude, or amplified 5,000 times, most of the amplification is done by a series of intermediate frequency amplifiers.
The great news is that intermediate frequency amplifiers are tuned once in the factory, while only the front-end amplifier and the local oscillator need to be tuned to incoming signal and corresponding local oscillator frequency. Given the characteristics of a superheterodyne or superhet radio receiver, the utility radio will contain a final-stage radio frequency amplifier that is tuned to the intermediate frequency. This is different from the receiver frequency. For instance, a single-conversion AM broadcast receiver tuned to 1,000 kilohertz (kHz) may have a local oscillator frequency of 1,455 kHz so that the difference or beat frequency is 455 kHz or the intermediate frequency.