Description
Object description
An American technical instructional film showing how radio waves are generated, the different types of wave, direct, sky, and reflected. A simple battery/capacitor/oscillator circuit demonstrates how an AC wave may be generated from a DC wave. The film proceeds to introduce the antennae and how the radiation from them is effected by local conditions, aided by clear, animated diagrams.
Content description
The film shows the large variety of antennae installed both on the ground and aircraft required to maintain communications. Radio signals are electromagnetic waves directed through space by an antenna and can travel great distances. To understand the principals of the antenna it is first necessary to learn how radio waves are generated.
Content description
A direct current passing through a wire generates a magnetic field and if the current is reversed so does that of the magnetic field. A dielectric field is demonstrated by a charged (static electricity) glass rod attracting small pieces of paper. The lines of force extend at right angles to the glass surface and are also known as an electrostatic field. This field is maintained as long as it is charged. A capacitor is connected to a battery, current flows and an electromagnetic field emanates form the conducting wire. When the capacitor is fully charged the current ceases to flow and so does the electromagnetic field, but a dielectric field emanates from the capacitor. Reversing the battery repeats the whole cycle. The generation of a rapid change in polarity of these fields is possible when the oscillator valve replaces the capacitor and the waves thus generated push each other outwards, collapse and are regenerated. These are radio waves which move in all directions from the radiating source at a velocity of 186,000 miles/second, the speed of light. All radio waves move at the same velocity and their frequency is determined by their wavelength: the higher the frequency the shorter the wavelength.
Content description
"Radio waves in space": Radio waves travel in a straight line much as does light and may be reflected or refracted by the earth's surface and the ionosphere, layers of ionised gas in the region 50 to 300 miles above the earth acting as a reflecting medium. Such waves undergoing repeated reflections are known as skywaves, and signals may be received thousands of miles away from the transmitter. The high frequencies are refracted the least, some failing to return to the earth. The refractive index of high to medium frequencies make them suitable for medium/long distance communications, in conjunction with dipole antennae adapted for these frequencies. Film cuts to examples of dipole antennae. The portion of the transmitted signal that travels near the earth's surface is called the ground wave, whose strength falls off rapidly and is not suited for long distance communication. Lower frequencies travel further than high frequencies, extremely high frequencies travel almost as light, not following the curvature of the earth or passing around obstacles on the ground.
Content description
Signal loss and fading may occur at receivers located in the Skip zone, an area between the end of the ground wave and the start of the first reflected wave at the surface of the earth. There may be more than one skip distance. Fading can be due to changes in the ionosphere levels, or due to interference between the ground and sky waves. If in phase they strengthen the signal, if out of phase can destroy the signal.
Content description
An efficient receiver and suitable antenna will receive but one out of many filling the sky. Film closes with officer in the control tower talking to the pilot of a B29 bomber as he takes off.