Basic Radio Principles A radio wave is an electrom agnetic (EM) wave with  frequency characteristics that  make it useful. The wave  travels long distances throug h space (in or out of the  atmosphere) without losing too  much strength. An antenna  is used to convert electric current into a radio wave so it can  travel through space to the receivi ng antenna, which converts  it back into an electric current for use by a receiver. All matter has a varying degree  of conductivity or resistance  to radio waves. The Earth itself  acts as the greatest resistor  to radio waves. Radiated energy  that travels near the ground  induces a voltage in the ground th at subtracts energy from the  wave, decreasing the strength of t he wave as the distance from  the antenna becomes greater.  Trees, buildings, and mineral  deposits affect the strength to  varying degrees. Radiated  energy in the upper atmosphere  is likewise affected as the  energy of radiation is absorbed  by molecules of air, water,  and dust. The characteristics of  radio wave propagation vary  according to the signal frequency and the design, use, and  limitations of the equipment. A ground wave travels across the surface of the Earth. You  can best imagine a ground  wave’s path as being in a tunnel  or alley bounded by the surfac e of the Earth and by the  ionosphere, which keeps the  ground wave from going out  into space. Generally, the lower the frequency, the farther  the signal travels. Ground waves are usable for navigation purposes because  they travel reliably and predictably along the same route  day after day and are not influenced by too many outside  factors. The ground wave frequency range is generally from  the lowest frequencies in the radio range (perhaps as low as  100 Hz) up to approximately 1,000 kHz (1 MHz). Although  there is a ground wave component to frequencies above this,  up to 30 MHz, the ground wave at these higher frequencies  loses strength over very short distances. The sky wave, at frequencies of 1 to 30 MHz, is good for  long distances because these frequencies are refracted or  “bent” by the ionosphere, causing the signal to be sent back  to Earth from high in the sky and received great distances  away.  [Figure 9-1]  Used by high frequency (HF) radios in  aircraft, messages can be sent across oceans using only 50  to 100 watts of power. Frequencies that produce a sky wave  are not used for navigation because the pathway of the signal  from transmitter to receiver is highly variable. The wave is  Space waves have another characteristic of concern to users.  Space waves reflect off hard objects and may be blocked if  the object is between the transmitter and the receiver. Site  and terrain error, as well as propeller/rotor modulation error  in very high omnidirectional range (VOR) systems, is caused  by this bounce. Instrument landing system (ILS) course  distortion is also the result of this phenomenon, which led  to the need for establishment of ILS critical areas. When able to pass through the ionosphere, radio waves  of 15 MHz and above (all the way up to many GHz), are  considered space waves. Most navigation systems operate  with signals propagating as space waves. Frequencies above  100 MHz have nearly no ground or sky wave components.  They are space waves, but (except for global positioning  system (GPS)) the navigation signal is used before it reaches  the ionosphere so the effect of the ionosphere, which can  cause some propagation errors, is minimal. GPS errors  caused by passage through the ionosphere are significant  and are corrected for by the GPS receiver system. For aeronautical communication purposes, the sky wave  (HF) is about 80 to 90 percent reliable. HF is being gradually  replaced by more reliable satellite communication. “bounced” off of the ionosphere, which is always changing  due to the varying amount of the sun’s radiation reaching it  (night/day and seasonal variations, sunspot activity, etc.). The  sky wave is, therefore, unreliable for navigation purposes. How Radio Waves Propagate Ground Wave Sky Wave Space Wave Not refracted  Ionosphere  Ground wave  Space wave  Sky wave  Figure 9-1.  Ground, space, and sky wave propogation. 9-2