RADIO TRANSMITTER [Main Title]

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  • Title: RADIO TRANSMITTER [Main Title]
  • Film Number: AMY 449
  • Other titles:
  • Summary: An American training film about the basic principles of transmitter/receiver systems. Each element is presented in a logical sequence which combine to form a complete transmitter or receiver. The explanations are lucid and not too detailed, accompanied by clear, animated diagrams, and film clips of a variety of operational radio equipment.
  • Description: “Principles and typical circuits”: Radio technicians are seen at work receiving and transmitting signals from antennae mounted on a tall tower. Film cuts to a technician operating a morse key and the commentator observes how radio has developed from the simple crystal set to the sophisticated radios of today, which find wide application in all types of military services. Cut to ground controllers speaking to pilots of three Airacobra aircraft in flight. The various elements that comprise a transmitter/receiver unit are explained using schematic diagrams. The antennae may be thought of as a capacitor which transmits electrical vibrations, coupled inductively on the ground to a triode vacuum tube: the grid tank circuit. This forms an oscillator capable of radiating waves of radio frequencies. Place the morse key in the circuit and we have a simple transmitter. A diagram shows how the oscillator functions, using the grid bias battery versus the plate potential of the vacuum tube. The frequency of a transmitter is controlled by a tuned circuit incorporating a variable capacitor: this is also known as a tank circuit. A tuned circuit is the most common frequency determining element in radio apparatus. Film cuts to large open to air coils (for cooling) of a high power transmitter supplying inductance to the circuit, beside an open vane variable capacitor supplying capacitance to the circuit. “Action of the oscillator”: The oscillator is the basic element in all transmitter installations. The basic vacuum tube incorporates grid, filament and plate elements incorporated into the tuned circuit, an animated diagram shows the resultant changing flow of the electrons. The Hartley circuit has feed back from the plate coil as part of the tuned circuit, and thus differs from all other oscillators. Any oscillator coupled to the antenna will emit a continuous radio wave. A morse key inserted in the circuit will interrupt the continuous wave and a message may be sent: this is known as modulation. “The amplifier”: The amplifier is inserted between the oscillator and the antennae resulting in greater oscillator stability and stronger signals. Film cuts to technician turning a single dial to change frequency of the oscillation. “Back waves” occur if the amplifier itself oscillates, and is overcome by variable capacitors in the Hartley oscillator circuit. “The microphone”: The radio-telephone requires a microphone and the most common type incorporates a diaphragm in contact with a pile of carbon granules, voice pressure cause the pile resistance to vary which coupled to an oscillator will modulate the transmitted frequency. “The crystal oscillator”: Certain crystals when struck will vibrate for a very long time, Quartz is an example. Such crystals have almost frictionless physical resonance. Another property is the generation of alternating charges of electricity on opposing faces of the crystal. This property makes it possible to generate accurate control of frequencies and is barely affected by temperature. So accurate is a crystal oscillator it may be substituted for the tuned grid circuit of an oscillator. The frequency of a crystal is determined by the angle of its cut and the thickness of the slab of crystal, the thicker the crystal the lower the frequency. The simple fundamentals are basic to all transmitters. The film ends with a series of clips depicting a variety of receivers/transmitters in use by the army: a large static radio station, a small hand held radio-telephone, assorted mobile communication systems, and the different aviation systems deployed as three Dakotas take to the air.
  • Alternative Title:
  • Colour: B&W
  • Digitised:
  • Object_Number: AMY 449
  • Sound: Sound
  • Access Conditions:
  • Featured Period:
  • Production Date: 1942
  • Production Country: United States of America
  • Production Details:
  • Personalities, Units and Organisations:
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  • Physical Characteristics: Colour format: B&W Sound format: Sound
  • Technical Details: Format: 35mm Number of items/reels/tapes: 2 Footage: 1608ft; Running time:
  • HD Media:
  • Link to IWM Collections page:

    AMY 449

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