Because 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. Each new current  flow creates new fields around the antenna that force the not- totally-collapsed fields from the previous AC cycle out into  space. These are the radio waves. The process is continuous  as long as AC is applied to the antenna. Thus, steady radio  waves of a frequency determined by the input AC frequency  propagate out into space. Radio waves are directional and propagate out into space at  186,000 miles per second. The distance they travel depends on  the frequency and the amplification of the signal AC sent to the  antenna. The electric field component and the electromagnetic  field component are oriented at 90° to each other, and at 90°  to the direction that the wave is traveling. [ Figure 11-77 ] Radio waves of different frequencies have unique  characteristics as they propagate through the atmosphere.  Very low frequency (VLF), LF, and medium frequency  (MF) waves have relatively long wavelengths and utilize  correspondingly long antennas. Radio waves produced at  these frequencies ranging from 3kHz to 3mHz are known  as ground waves or surface waves. This is because they  follow the curvature of the earth as they travel from the  broadcast antenna to the receiving antenna. Ground waves are  particularly useful for long distance transmissions. Automatic  direction finders (ADF) and LORAN navigational aids use  these frequencies.  [Figure 11-78] High frequency (HF) radio waves travel in a straight line  and do not curve to follow the earth’s surface. 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. HF radio waves bounce  off of the ionosphere layer of the atmosphere. This refraction  extends the range of HF signals beyond line-of-sight. As a  result, transoceanic aircraft often use HF radios for voice  communication. The frequency range is between 2 to 25  MHz. These kinds of radio waves are known as sky waves.  [Figure 11-78] Above HF transmissions, radio waves are known as space  waves. They are only capable of line-of-sight transmission  and do not refract off of the ionosphere.  [Figure 11-78] Most aviation communication and navigational aids operate  with space waves. This includes VHF (30-300MHz), UHF  (300MHz-3GHz), and super high frequency (SHF) (3Ghz- 30Ghz) radio waves.  To transmit radio waves, an AC generator is placed at the  midpoint of an antenna. As AC current builds and collapses in  the antenna, a magnetic field also builds and collapses around  it. An electric field also builds and subsides as the voltage  shifts from one end of the antenna to the other. Both fields,  the magnetic and the electric, fluctuate around the antenna at  the same time. The antenna is half the wavelength of the AC  signal received from the generator. At any one point along  the antenna, voltage and current vary inversely to each other. The frequency of an AC signal is the number of times the  AC cycles every second. AC applied to the center of a radio  antenna, a conductor half the wavelength of the AC frequency,  travels the length of the antenna, collapses, and travels the  length of the antenna in the opposite direction. The number  of times it does this every second is known as the radio wave  signal frequency or radio frequency as shown in  Figure 11-75 .  As the current flows through the antenna, corresponding  electromagnetic and electric fields build, collapse, build in  the opposite direction, and collapse again.  [Figure 11-76] 11-32 Types of Radio Waves Figure 11-75.  There is a wide range of radio frequencies. Only the  very low frequencies and the extremely high frequencies are not  used in aviation.  Loran C 100 KHz ADF 200 - 1600 KHz NDBs 190 - 535 KHz AM broadcast 550 - 1800 KHz HF comm 2 - 30 MHz Marker beacons 75 MHz FM broadcast 88 - 108 MHz VHF NAV (VOR) 108 - 118 MHz VHF comm 118 - 137 MHz Glideslope 328 - 336 MHz DME 960 - 1215 MHz Transponder 1030 & 1090 MHz GPS 1.6 GHz Radar altimeter 4.3 GHz Doppler NAV 8.8 GHz Weather radar 9.375 GHz Aviation Uses Radio Frequencies 300 GHz 30 GHz 3 GHz 300 MHz 30 MHz 3 MHz 300 KHz 30 KHz 3 KHz Very low  frequency Very low  frequency (LF) Medium  frequency (MF) High frequency (HF) Very high  frequency (VHF) Ultra high  frequency (UHF) Super high  frequency (SHF) Extremely high frequency (EHF)