The Basics of Radio Wave Propagation
Edwin C. Jones, MD, PhD (AE4TM) Department of Physics and Astronomy, University of Tennessee, Knoxville, TN 37996-1200
Radio Wave Propagation
Aurora: A favorite propagation. When more than the usual levels of charged particles arrive at the earth (i.e., increased solar wind), as a result of a CME or coronal stream, many of these charged particles penetrate the weakest parts of the GMF near the polar regions. This is because the GMF field lines guide these charged particles into these regions; at these polar regions, extreme ionization can result at altitudes up to 1000km. Due to this increased ionization, a dynamic curtain shaped layer develops instead of the more typical horizontal shaped F2-layer. This auroral layer may reflect radio waves from the HF-band (3-30MHz) all the way up to and including the entire UHF-band (300-3000MHz). However, due to its very irregular shape and constant movement, heavy fading (QSB) is common in the reflected radio signals. This QSB can also result from multiple reflections within these auroral layers, causing rapid phase shifting. An auroral signal is easily recognized at 30MHz as a bubbling sounding modulation or "under-water-like" modulation. Finally, because of the extreme and sudden phase shifts, narrow band modes such as CW and digital are the most reliable modes for DX contacts.
Backscatter: A useful form of propagation which mostly occurs when the maximum usable frequency (MUF) rises above 30MHz. During these conditions, when radio waves reach the ionosphere (usually the F2-layer), they are reflected towards the earth's surface at a larger detectable continuum of angles than usual. In other words, a detectable fraction of a radio signal is now reflected at a very sharp angle back into region just surrounding the transmitting station but usually beyond the range of ground wave communications (i.e., blind zone). Therefore, backscatter signals are heard within a radius of 2000km from the transmitting station. Backscatter signals are generally weaker than the normal reflected radio waves and during periods of low solar flux, only radio stations using directional antennas can produce readable signals. However, during periods of very high solar flux, even small stations using 10 Watts and vertical ground plane antennas may produce readable signals. Backscatter signals are generally very stable and rarely influenced by QSB. Finally, backscatter signals are easily recognized as a "hollow" or "barrel-like" sound originating from the expected blind zones of a radio station.
Blind Zone: The blind zone is the area around a radio station which cannot normally be worked by either ground waves or normal ionospheric sky waves. Usually stations in the blind zone can only be worked via intermittent backscatter propagation. This zone is also called the "skip zone" by the US Military.
Es: A mode of propagation producing well known short skip radio contacts off the E-layer of the ionosphere. This propagation occurs most frequently during the summer months with a major node occurring during the summer, a minor node occurring during the winter, and "valleys" occurring around both equinoxes. During the summer, this mode is popular due to its high signal levels. Finally, the skip distances are generally around 1000 statute miles.
F2: The most common mode of propagation is sky waves reflected off the F2-layer of the ionosphere; these reflections are responsible for most DX contacts.
Gray-line: The area occurring along the sunset and sunrise zones (i.e. also called the terminator in astronomy) is known as the gray line and it has special significance to radio communications. Signals which travel along this gray line region often experience significant improvements in received signal strengths as compared to the direct shortest distance communications. This is because the radio wave absorbing D-layer disappears faster than the higher altitude radio wave propagating F2-layer around the time of sunset (and vise versa for sunrise). Because the F2-layer of the ionosphere remains strongly ionized along this gray line, HF signals often have less attenuation when they travel along the gray line as compared to the more direct shorter route.
LUF: Lowest usable frequency.
Meteor scatter: A remarkable type of propagation caused by the ionization by meteors (also known as "shooting stars") entering the earth's atmosphere. Meteors are small rocks orbiting in space and every year on certain dates, the earth passes through streams of these meteors. When the earth crosses an orbit of meteors, meteors hit the earth's atmosphere at a speeds of over 10.000km/h causing them they burn up at extremely high temperatures. The resulting high temperatures leave traces of ionized air behind them at 80-150km in altitude. Fortunately for radio operators, this trace of ionized air can reflect radio waves up to 500MHz and sometimes beyond. It can also reflect HF signals in the range of 30MHz. Each meteor entry results in a radio wave scatter that can be categorized into either a "ping" or "burst". Pings are short openings lasting a few seconds and bursts are openings lasting for minutes. During meteor storms (i.e., when meteors occur at high rates), both pings and bursts can occur so regularly that long QSO's are possible. The most famous meteor shower is called the Perseids and it occurs when the earth crosses the Perseid meteor orbit around August 12th of each year. This particular shower is known to have up to 120 meteors per hour. For instance, in 1994 the Perseids supported radio conversations having strong signal strengths for several hours and the skip distances ranged from 200 to 1800km. However, meteor scatter contacts are usually more brief; and a result, APRS and VHF packet radio is considered to be a good means of communication during meteor showers due to the mode's short packets of data containing useful information such as the transmitting station's callsign as well as location in each packet sent.
MUF: Maximum Usable Frequency.
TA: Trans-Atlantic. A mysterious and rare type of propagation named after the mysterious openings that occur between Europe and North America during the summer months, at a sunspot minimum, and well after sunset. In theory, openings such as these are unlikely, but there have been many occasions in 1995, 1996, and 1997 when such openings like these have occurred which allowed DX contacts across the Atlantic when DX seemed impossible. Even more mysterious is the fact that TV-amateurs received signals across the Atlantic well into the VHF-band during these openings. The mechanism of propagation is still unclear, but one proposed theory suggests that a gigantic Es-cloud forms above the entire Atlantic resulting in sky wave propagation.
TEP: Trans-Equatorial Propagation. This is another form of mysterious radio wave propagation which occurs during the spring and fall months during the sunspot minimum. This form of propagation allows two stations at nearly identical middle latitudes on opposite sides of the geomagnetic equator to communicate at frequencies up to 150 MHz. For example, communications can occur between Italy and South-Africa or between the West Indies and South America. Like Trans-Atlantic propagation, there is no widely accepted scientific explanation for this type of propagation.
Tropospheric scatter: The only form of propagation that is directly influenced by the surface weather of the earth. Our troposphere (0-10km altitude) is composed of layers of air having different temperatures and moisture contents. When a sharp transition, called an inversion, appears between a cold dry layer and a warm moist layer of air, this transition causes refraction of radio waves. This is analogous to the refraction caused by the transition between water and air. For instance, when you put a stick into the water, it looks like it is bent. This same type of refraction occurs when a radio wave travels through a climate inversion; if the inversion is strong enough, radio waves can be refracted back to the surface of the earth after traveling significant distances (up to several hundred kilometers on the 6m band). Finally, this propagation effect is seen most often in the VHF and UHF bands, especially the 6m band.
Ducting: On rare occasions, two or more inversions may appear at different altitudes. Sometimes certain radio waves can be transported between these two inversions. Therefore, this type of propagation is called "ducting" (or "tunnelling"). Records of over 2500km have been set due to such ducting on VHF and UHF. Unfortunately, the effect is usually confined to 2m, but it can occur as high as 1.2 GHz (usually along frontal systems), and it almost never occurs below frequencies of 50MHz. When ducting does occur on these frequencies, communication distances are typically in the range of ~400km. Inversions usually develop under the influence of high pressure weather systems when there is very little air movement. Also, low pressure systems may produce an inversion when a cold air mass collides with a warmer air mass (called a frontal system in meteorology). Inversions that occur along frontal systems support propagation along a line parallel to the weather front, and radio amateurs using frontal inversion often point their antennas parallel to the frontal system to take advantage of this form of propagation.
