Radio 
    Wave Propagation

  









  
As you use Citizens 
  Band radio and become a more skilled radio operator you will probably promote 
  your own interest into the mysterious world of radio frequency and radio 
  waves. The following information is designed to provide you with a basic 
  working knowledge of the Radio Frequency Spectrum and its components. 
  

  
 





  
RADIO WAVES

  Radio waves surround you! Signals are transmitted and received by millions of 
  stations. AM and FM radio stations, television, mobile business radios, fire 
  and police radio systems, amateur radio stations, cordless and cellular 
  telephones, pagers, nursery room monitors, garage door openers, and of course 
  your personal Citizens Band Radio Station! Radio waves travel at the speed of 
  light. The invisible radio wave consists of an electrostatic field and a 
  magnetic field traveling at right angles in the direction of their 
  transmission.. 




 




  
RADIO FREQUENCIES

  To help you to visualize the behavior of invisible radio waves imagine the 
  effect on the surface of the water when you toss a small stone into the air 
  and it lands in the water. Tiny ripples radiate in all directions. If you 
  increase the size of the stone the size of the ripples will also increase and 
  the distance between the peak of each ripple will increase also.

  If you could count the number of times the peak of each wave went past a 
  specific point over one second, you would know the waves' frequency and 
  wavelength. In radio waves, we measure frequency by the number of wave cycles 
  per second. The more waves that pass a specific point in one second, the 
  higher the frequency. And the distance that the wave travels in one cycle is 
  the wavelength.. 





 




  
RADIO SPECTRUM

  The unit for measuring frequency is hertz (Hz). It is named after Heinrich 
  Hertz, a German physicist who discovered electromagnetic waves. The 
  frequencies that human can hear are audio frequencies and range from about 20 
  Hz to about 18,000 Hz or 18 kHz. A dog can hear up to about 20 kHz or possibly 
  even higher. The radio frequency spectrum starts at around 10 kHz and ranges 
  up to 300,000 Megahertz (MHz). A frequency of 10 kHz means that the wave 
  cycles 10,000 times per second. Similarly, a frequency of 300 million hertz 
  (300 MHz) means that the wave cycles 300 million times per second. 

  
In mobile radio 
  communications we use the ranges of frequencies in the medium frequency (MF), 
  high frequency (HF), and very high frequency (VHF) range..

  
 





  
CB RADIO

  Guess where the FCC assigned the short-range citizens band (CB) service. They 
  allocated 23 channels - ultimately expanded to 40 channels - at 27 MHz. Is 
  that below the magic 30 MHz dividing line? Sure is, and the planned 
  short-range unlicensed CB radio service instantly turned into an undisciplined 
  sky-wave free-for-all! Although intended to fulfill the need for personal 
  short-range communications, regular high-frequency sky-wave skip signals 
  blasted in from the ionosphere, causing short-range channel chaos. 

  
During periods of reduced ionosphere- 
  activity, the short-range capabilities of CB can be realized. But when the 
  skip is active, a CB radio transmitter with only a few watts of output power 
  can put strong signals into receivers thousands of miles away. This is 
  commonly referred to as ‘DXing’. DXing has taken on a very active part in the 
  CB world and continues to promote the use of Citizens Band use and operation.
  

  
Many well-established CB radio operators are 
  strongly dedicated to the ‘art’ of DXing. Although skip is mainly 
  unpredictable, DXers spend many long hours studying and charting the activity 
  of skip signals. Some have even come very close to an established pattern that 
  reveals some level of reliability. However, the pattern of skip is solely 
  chance. Most DXers are satisfied to make distant contacts wherever and 
  whenever the ionospheric skip permits.





 




  
MOBILE RADIO WAVE 
  RANGE

  Different radio frequencies travel over the surface of the earth and into 
  space in many different ways. Sometimes the bounce, sometimes they are 
  refracted (bent) by the ionosphere. Some radio frequencies easily penetrate 
  the thickness of a forest, yet other radio frequencies can be blocked by a 
  single leaf. 

  
 





  
GROUND WAVES
  

  Every radio transmission travels out from the antenna over two paths: the 
  ground wave and the sky-wave. Ground waves are the real work horse for vehicle 
  mobile communications up to 100 miles. Ground waves normally are vertically 
  polarized signals that travel out in all directions from an omni-directional 
  antenna and are usually strong enough to be detected up to 50 to 100 miles 
  away.










Refraction




REFRACTION



  




  
In rural areas, land mobile radio systems 
  may use frequencies in the VHF and UHF spectrum to maximize the potential of 
  ground wave propagation. VHF signals propagate best in areas of rolling hills 
  and over water. UHF signals become quite reflective, and are best propagated 
  in the downtown area of cities with huge skyscrapers. When VHF and UHF signals 
  require a range of more than 100 miles, repeater relay stations and satellites 
  may have to be used to get these normally "line-of-sight" signals to go a lot 
  further. 

  
 

  





  
SKY WAVES

  The second component of a transmitted radio signal is the sky-wave. In VHF and 
  UHF mobile radio bands, sky-waves normally travel out into space and keep 
  right on going. UHF and microwave sky-waves have traveled millions of miles to 
  earth from space probes sent out into deep space.

  
Sky-waves on medium frequencies and high 
  frequencies take a remarkable skip off of the ionosphere that circles our 
  planet at altitudes of 50 to 250 miles. 'When a medium or high frequency radio 
  wave enters the ionosphere, the radio wave is refracted back to earth (see 
  drawing). Just as a prism refracts sunlight into different color bands over a 
  range from blue too red, radio waves refracted by the ionosphere may skip back 
  to earth over a range of 200 to 3000 miles away from the transmitter. If the 
  hop back to earth is strong enough and lands in the very conductive ocean, 
  that medium frequency or high frequency wave takes a double hop - sometimes a 
  triple hop - and literally skips around the world. 

  
Ham radio operators regularly take advantage 
  of this sky-wave excitement on their specific frequencies in the medium and 
  high frequency spectrum. Same thing with ocean voyaging mariners - they chose 
  their bands carefully to bounce their signal back to shore to stay in touch 
  with their mobile marine station hundreds and thousands of miles away. On a 
  hot summer afternoon, 30 MHz is the usual dividing line between sky-wave 
  communications that come back to earth and sky-waves that simply keep on going 
  into outer space. For frequencies below 30 MHz, the sky waves bounce back to 
  earth. Above 30 MHz, skip usually is not a problem..










 




  
HOW IMPORTANT IS 
  POWER OUTPUT?

  The output power of a mobile transmitter, which might be as low as 1 watt or 
  as high as 1000 watts, has very little to do with the maximum distance that 
  the signal may travel. Although signal strength varies inversely with the 
  distance, a major increase in power output will have little or no effect in 
  establishing communications to a distant point that couldn't hear the ground 
  wave or sky-wave at the original power setting. 

  
Out on the oceans, mariners use 25-watt 
  radios for short-range communications on the VHF band. Up to about 100 miles, 
  the sea water has little effect on VHF waves, but on the worldwide lower 
  frequencies, the high conductivity of sea water causes the signal strength of 
  the ground wave to vary inversely with the distance. When operating on these 
  frequencies, mariners require more power and rely almost entirely on sky-wave 
  propagation to get their signals received hundreds and thousands of miles 
  away.













  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  








Collin's 
Home Page    Colin's Amateur Radio Page   
Colin's Snooker Page   
Colin's Guest Book   
Contact Me   
About Me 




This Web page was last updated 
on 
Friday September 21, 2001 




© 2001 Designed by Colin McCord