FM Radio Headsets, Antennas, & Tuners
Many crystal set cabinets provide storage space for FM radio headsets. To designate the headphone terminals on panels of crystal sets, either "T" or "P" is used most often, but occasionally "R", for receivers, appears on some of the earlier FM radio headsets. In recent years, the term earphone has often been restricted to the small phone that fits into the external ear canal. FM radio headsets consist of a headband and two phones within metal or molded watchcase-type shells; many of the earliest FM radio headsets have only the phone.
Phones for use in wireless sets are minor modifications of the early telephone operators listening apparatus: a headband with a single phone. Indeed, most early FM radio headsets were made by telephone companies or their production sources but more than 100 manufacturers of FM radio headsets have been identified. The words aerial and antenna are generally used interchangeably to indicate a system of wires or rods used either to radiate or collect radio waves from space. Some experts however prefer the term antenna when the purpose of the device is reception and aerial when it serves for transmission. Early antennas consisted of insulators and enough wire to suspend 100 to 150 feet of it as well as enough for the lead-in to the receiver and connection to the ground. Effective reception of radio signals is greatly determined by the antenna system. Several types of tuning devices were available to adjust early crystal sets to receive the specific radio frequency waves of a particular broadcasting station. In general, early wireless broadcasts used a broad range of RF and few competing stations were in operation until the later 1920s. Simple devices were often adequate such as single or two slide tuners in which varying the slider position on the coil changes the number of coils in the antenna circuit to produce inductance tuning. This same sort of variable selection of the number of inductance coil windings in the antenna circuit was accomplished by a tap switch with the coil tapped at appropriate intervals. Tuning the circuit is accomplished by varying either it inductance or capacitance or both. A tuning coil is a coil of wire constructed each turn electrically insulated from the adjacent ones and with a device permitting contact with the desired numbers of turns in the circuit so that inductance is varied and thus tuned to different radio wave lengths. A loose coupler consists of two closely placed coils; one, the primary is stationary and the other the secondary slides on rods in and out of the interior of the primary coil. Withdrawing the secondary coil loosens the coupling and decreases transfer of energy from the primary circuit to the secondary circuit. |
