For part II of the ARRL examination study, we will study propagation of radio waves.
Radio waves spread out when transmitted from an antenna in straight lines unless they are reflected or refracted by some object. Due to this spreading and scattering, the waves become weaker as they propagate farther into the air. This limits the “range” a radio transmission can communicate over. The curvature of the Earth creates a “radio horizon” that limits the range of radio propagation. “Line of sight” propagation is when radio waves are transmitted within direct sight of the receiver. This is commonly done in VHF frequencies and higher. Lower frequencies travel as “ground waves”.
Radio waves are partially reflected when the medium through which the wave propagates changes due to a change in intrinsic impedance (a property defined by permittivity and permeability). Radio waves can even be reflected by change in weather patterns. The figure below shows the concept of diffraction (bending past an obstruction) of radio waves. Diffraction can also refer to spreading when a wave travels through a narrow medium into an open area.
Light waves also bend by “refraction” which is exactly how radio waves travel around the earth. The earth is curved and therefore the waves need to bend to propagate past “line of sight” distances. The shorter the wavelength (and hence higher frequency), the easier the wave can travel in and out of buildings by penetration of openings in solid objects.
It interesting to note that different waves received by an antenna can interfere if they are out of phase (destructive interference). This is called “multipath” which is when antennas receive waves from different paths. Moving an antenna a few feet can counterract this. Multipath propagation results in irregular fading. VHF and UHF signals propagating with multipath propagation experience fluttering or “picket-fencing” which comes from rapid variation of the signal strength. Tropospheric propagation or “tropo” is propagation of VHF or higher frequencies assisted by atmospheric phenomena such as weather fronts or temperature inversions. It is not uncommon for Tropo signals to propagate over 300 miles. Reflections can also be caused by conductors such as airplanes. Satellites reflect waves with conductive plating.
Thirty to 260 miles above the earth, the ionospheric layer resides. Atoms of nitrogen and oxygen are ionized by UV rays from the sun and become positively charged. The separation of the electrons and the creation of positive ions creates a weakly conductive region. The ionosphere is composed of many different regions. The E, F1 and F2 layers tend to reflect radio waves and the D and E regions tend to absorb waves.
“Skip” or sky wave propagation is when HF waves are completely bent back towards the earth. The conductive surface of the earth reflects the wave back and the process repeats. These “hops” or reflections allow the waves to be received at farther distances. Lower frequencies are bent more than higher frequencies. For this reason, UHF signals are rarely heard beyond the radio horizon. The MUF (maximum usable frequency) and LUF (lowest usable frequency) are the highest and lowest frequencies that can be reflected by the ionosphere without absorption. When sunspot activity increases, the makes the ionosphere more conductive and increases the MUF.
Sporadic or “E-Skip” propagation is when patches of the ionosphere become ionized enough to reflect frequencies as high as VHF and UHF. This is most common during early summer and mid winter months.