Principles of Radio Observing

Radio observing is based on conceptually simple principles:

• Radio waves travel in straight lines.
  
  
• If the Earth's curvature places the horizon between you (the receiver) and the source (the transmitter), the transmission will be blocked.
  
  
• Meteors enter the Earth's atmosphere at very high speed. At an altitude of roughly 100 km the heat of friction is so high that electrons are stripped off atmospheric atoms, forming a temporary electrically charged column or layer of ions.
  
  
• With an efficiency of effect related to frequency and density of the charged mass, this accumulation of ions reflects or scatters radio energy. By this means a distant transmission normally prevented from reaching your receiver because of an obstruction (like the horizon) can briefly be heard at your location because the signal is bounced over the obstruction. 
  
  
• After a brief time (usually a few milliseconds but as long as several minutes on rare occasions), the plasma dissipates and the signal is lost.

The physics of the situation are such that radio observations of meteors are usually confined to the frequency range of about 40 MHz to 120 MHz (VHF band). At lower frequencies man-made noise, commercial/international broadcasting usually produce too much interference. Furthermore, the lower frequencies are readily deflected over the horizon by the ionosphere itself, especially when it is being irradiated by the sun. This is what makes long distance (especially "shortwave") reception possible at certain times of the day. The phenomenon tends to mask any meteor effects although as you will see in our demonstration images, many events can still be detected. On the other hand, the reflectivity of a plasma falls-of rapidly with rising frequency although useful work is still possible to about 400 MHz.

The ionosphere is a complex structure comprising several layers having a variety of characteristics. You will find a comprehensive treatment of the subject here:

What we are left with then, is an observing window of frequencies in the range indicated, where the ionosphere is generally (but not entirely) non-reflective while the more intense ionization produced by the passage of meteors is sufficient to result in readily detected returns.

There are other conditions, such as tropospheric ducting, and "sporadic E" effects promoting the transmission of radio waves over the horizon. These are sometimes, but not frequently, troublesome for meteor observers. In any event, they are in themselves interesting phenomena. The serious meteor astronomer might want to check regularly updated sites such as Space Weather  or the Tropospheric Ducting Forecast for information and additional links.  

Of course, we eventually want to do more than just detect scattered radio waves. With suitable equipment and computer software it becomes possible to count meteors, estimate their velocity,  observe upper atmospheric winds and deduce many other characteristics of both the meteors and their near-Earth environment.