There are a limited number of frequencies within the usable frequency ranges (i.e., LF, HF, and VHF). But the amplitude of the oscillator output varies in relation to the fluctuating current input. The oscillator frequency output is consistent because it is built into the oscillator circuit. The oscillator that creates the carrier wave does so with this varying current. As the varying DC information signal is amplified, the amplifier output current varies proportionally. A DC signal, for example from a microphone, is amplified and then superimposed over the AC carrier wave signal. Two common methods of modulating carrier waves are amplitude modulation and frequency modulation.Īmplitude Modulation (AM)A radio wave can be altered to carry useful information by modulating the amplitude of the wave. The modulated carrier wave then carries the information from the transmitting radio to the receiving radio via their respective antennas. The information signal contains the unique voice or data information desired to be conveyed. To transmit and receive useful information, this wave is altered or modulated by an information signal. The basic radio wave discussed above is known as a carrier wave. Loading Information onto a Radio WaveThe production and broadcast of radio waves does not convey any significant information. Only one party can transmit at any one time. When using VHF, each party transmits and receives on the same channel. VHF radios are used for communications between aircraft and air traffic control (ATC), as well as air-to-air communication between aircraft. Further division of the bandwidth is possible, such as in Europe where 8.33 kilohertz separate each VHF communication channel. Seven hundred and twenty separate and distinct channels have been designated in this range with 25 kilohertz spacing between each channel. They operate in the frequency range from 118.0 MHz to 136.975MHz. VHF communication radios are the primary communication radios used in aviation. This includes VHF (30-300MHz), UHF (300MHz-3GHz), and super high frequency (SHF) (3Ghz-30Ghz) radio waves. Most aviation communication and navigational aids operate with space waves. They are only capable of line-of-sight transmission and do not refract off of the ionosphere. Ībove HF transmissions, radio waves are known as space waves. These kinds of radio waves are known as sky waves. The frequency range is between 2 to 25 MHz. As a result, transoceanic aircraft often use HF radios for voice communication. This refraction extends the range of HF signals beyond line-of-sight. HF radio waves bounce off of the ionosphere layer of the atmosphere. This would limit transmissions from the broadcast antenna to receiving antennas only in the line-of-sight of the broadcast antenna except for a unique characteristic. High frequency (HF) radio waves travel in a straight line and do not curve to follow the earth’s surface. Thus, steady radio waves of a frequency determined by the input AC frequency propagate out into space. The process is continuous as long as AC is applied to the antenna. Each new current flow creates new fields around the antenna that force the nottotally-collapsed fields from the previous AC cycle out into space. At any one point along the antenna, voltage and current vary inversely to each other.īecause of the speed of the AC, the electromagnetic fields and electric fields created around the antenna do not have time to completely collapse as the AC cycles. The antenna is half the wavelength of the AC signal received from the generator. Both fields, the magnetic and the electric, fluctuate around the antenna at the same time. An electric field also builds and subsides as the voltage shifts from one end of the antenna to the other. As AC current builds and collapses in the antenna, a magnetic field also builds and collapses around it. To transmit radio waves, an AC generator is placed at the midpoint of an antenna.
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