Direction of propagation Electric field Magnetic field Ionosphere Radio station Repeater Receiver Line-of-sight Line-of-sight (VHF-UHF) Space waves Sky wave (HF) G r o u n d $_{ }$w a v e ( V L F t o Receiver 11-34 Figure 11-78.  Radio waves behave differently in the atmosphere depending in their frequency.  Figure 11-77. The electric field and the magnetic field of a radio wave are perpendicular to each other and to the direction of propagation  of the wave.  VHF communication radios are the primary communication  radios used in aviation. They operate in the frequency range  from 118.0 MHz to 136.975MHz. Seven hundred and twenty  separate and distinct channels have been designated in this  range with 25 kilohertz spacing between each channel. Further  division of the bandwidth is possible, such as in Europe  where 8.33 kilohertz separate each VHF communication  channel. VHF radios are used for communications between  aircraft and air traffic control (ATC), as well as air-to-air  communication between aircraft. When using VHF, each  party transmits and receives on the same channel. Only one  party can transmit at any one time.  The production and broadcast of radio waves does not convey  any significant information. The basic radio wave discussed  above is known as a carrier wave. To transmit and receive  useful information, this wave is altered or modulated by  an information signal. The information signal contains the  unique voice or data information desired to be conveyed. The  Loading Information onto a Radio Wave