THE CREATION AND BEHAVIOR OF RADIO WAVES

[Music] to understand the fundamental principles of the antenna on either fixed or aircraft installations it is first necessary to conception of how radio waves are generated and how they behave if we hold a compass near a wire for which a direct current of electricity is passed will be deflected a magnetic field exists around the wire as long as the cook continues to flow lines of force are created around the conductor whenever the flow of current stops the direction of these lines of course making up the magnetic field changes whenever the direction of current flow is changed if we charge a glass rod by friction it will attract objects from a distance this is static electricity around the charged object lines of force extend out at right angles from the surface in this case the field is not magnetic it is a dielectric field this field remains around the object as long as it remains charred thus we see that electricity flowing as a direct current generates a magnetic field while around the charged object in which there is no flow of current there is a dielectric field sometimes called an electrostatic field in a hookup consisting of a battery switch and condenser maximum current flows the instant the switch is closed resulting in the creation of a mag at the instant of closing the circuit this magnetic field remains around the conductor and condenser flows when the plates of the condenser are fully charged current ceases to flow the magnetic field around the condenser and conductors collapses as current diminishes the condenser is then a charged object and the dielectric field is built up around it as the charge accumulates if the battery is reversed the dielectric field collapses the energy returns to the circuit and then builds up in the opposite direction rapid changes in polarity in the circuit result in rapidly changing dielectric and magnetic fields if the condenser plates in such a circuit are moved apart the dielectric and magnetic fields move out the grid system when the polarity of the battery is reversed the field collapses and is built up in the opposite direction both fields are projected out and returned at the speed of light one hundred eighty six thousand three hundred miles per second very rapid changes in polarity are possible if a high frequency generator is used when the polarity of the system is suddenly changed the field which has been built up starts to return to the circuit however the reversal in polarity starts another electromagnetic field moving outward this new field or wave of energy meets the first one before it has had time to return to its source the returning energy in each case meeting the oncoming new field is forced out as an electromagnetic wave when each new field starts to collapse it meets another surge of energy and is driven out as another wave with a radio tube as oscillator a rapid continuous series of waves results these are radio waves radio waves move in all directions from the radiating source or antenna traveling at the speed of light all radio waves move at the same velocity low frequency transmission results in long wavelengths the more rapidly the waves follow each other the higher the frequency the higher the frequency the shorter the wavelength radio waves tend to travel in a straight line much as does light they may be reflected or refracted an example of reflection is seen near an antenna where the earth is the reflecting medium the refracting media are layers of ionized gas in a region from 50 to 300 miles above the earth in this region known as the ionosphere the layers of ionized atmosphere so bend radio waves that they act as if they had been reflected an upper angle or sky wave may thus be returned to the earth at a great distance where it may be heard with good volume the skyway returning from the ionosphere may be reflected by the earth going into the upper atmosphere again where it is refracted back toward the earth a second time this refraction and reflection may be repeated several times so that a message may often be heard thousands of miles from the transmitting station the higher frequencies are refracted the some extremely short waves failing to return to the earth at all this low refractive index of the medium-high frequencies makes them desirable for long-distance communication antenna installations are often adapted to the particular characteristics of these high frequencies that portion of the antennas radiation traveling near the surface of the earth is called the ground wave the polarization of this ground wave changes as it travels tending to lean forward the ground wave loses energy accordingly it cannot be used for long distance communication the ground wave of the lower frequencies travels farther than due to higher frequencies extremely high frequencies act almost as does light failing to follow the earth's curvature or to go around obstacles in many cases a silent strip exists between the point where the sky wave returns to earth and where the ground wave has become or too weak to be detected this is known as the skip distance or zone of silence there may be more than one skipped distance when the wave is reflected into the sky from where it is returned and reflected again for example a powerful station in cincinnati may not be heard in st louis yet beyond a zone of silence it comes in clearly at kansas city beyond the second zone of silence the station may again be heard in denver fading may be due to changes in the ionosphere level this shifts the position of the returned skyways the signal is heard when it is returned to the receiving station and fades out when it misses the station fading is often due to interference effects between the ground wave and the returned skyways the two waves strengthen each other when in phase when out of phase they weaken or destroy the signal altogether distortion as well as fading may be due to this interference effect between waves both of which are from the same station space about is filled with unseen waves sent from many antennas out of the maze an efficient receiver and antenna will select but one translating it into an intelligible message [Music] you