Radio-wave propagation has been a fascinating science ever since Marconi and Tesla discovered, around the year 1900, that EM fields can travel over long distances without any supporting infrastructure whatsoever. Let’s look at a few of the factors that affect wireless communications at radio frequencies.
The orientation of the E field lines of flux is defined as the polarization of an EM wave. If the E field flux lines are parallel to the earth’s surface, you have horizontal polarization. If the E field flux lines are perpendicular to the surface, you have vertical polarization. Polarization can also be slanted, of course. In some situations, the E flux lines rotate as the wave travels through space. This is circular polarization if the E field intensity remains constant. If the E field intensity is more intense in some planes than in others, the polarization is elliptical. Rotating polarization can be clockwise or counterclockwise, viewed as the wavefronts approach. The rotational direction is called the sense of polarization.
The Line-of-Sight Wave
Electromagnetic waves follow straight lines unless something makes them bend. Line-of-sight propagation can take place even when the receiving antenna cannot be seen from the transmitting antenna. To some extent, radio waves penetrate nonconducting objects such as trees and frame houses. The line-of-sight wave consists of two components: the direct wave and the reflected wave. The direct wave: The longest wavelengths are least affected by obstructions. At very low, low, and medium frequencies, direct waves can diffract around things. As the frequency rises, especially above about 3 MHz, obstructions have a greater and greater blocking effect.
The reflected wave: Electromagnetic waves reflect from the earth’s surface and from conducting objects like wires and steel beams. The reflected wave always travels farther than the direct wave. The two waves are usually not in phase at the receiving antenna. If they’re equally strong but 180° out of phase, a dead spot occurs. This phenomenon is most noticeable at the highest frequencies. At VHF and UHF, an improvement in reception can sometimes result from moving the transmitting or receiving antenna just a few inches. In mobile operation, when the transmitter and/or receiver are moving, dead spots produce rapid, repeated interruptions in the received signal. This is called picket fencing.
The Surface Wave
At frequencies below about 10 MHz, the earth’s surface conducts ac quite well. Because of this, vertically polarized EM waves follow the surface for hundreds or even thousands of miles, with the earth actually helping to transmit the signals. The lower the frequency, the lower the ground loss, and the farther the waves can travel by surface-wave propagation. Horizontally polarized waves do not travel well in this mode, because horizontal E field flux is shorted out by the earth. Above about 10 MHz, the earth becomes lossy, and surface-wave propagation is not useful for more than a few miles.
Sky-Wave EM Propagation
Ionization in the upper atmosphere, caused by solar radiation, can return EM waves to the earth at certain frequencies. The ionization takes place at three or four distinct layers. The lowest ionized region is called the D layer. It exists at an altitude of about 30 mi (50 km), and is ordinarily present only on the daylight side of the planet. This layer absorbs radio waves at some frequencies, impeding long-distance ionospheric propagation.
The E layer, about 50 mi (80 km) above the surface, also exists mainly during the day, although nighttime ionization is sometimes observed. The E layer can provide medium-range radio communication at certain frequencies.
The uppermost layers are called the F1 layer and the F2 layer. The F1 layer, normally present only on the daylight side of the earth, forms at about 125 mi (200 km) altitude; the F2 layer exists at about 180 mi (300 km) over most, or all, of the earth. Sometimes the distinction between the F1 and F2 layers is ignored, and they are spoken of together as the F layer. Communication by means of F-layer propagation can usually be accomplished between any two points on the earth at some frequencies between 5 MHz and 30 MHz.
At frequencies above about 30 MHz, the lower atmosphere bends radio waves toward the surface. Tropospheric banding occurs because the index of refraction of air, with respect to EM waves, decreases with altitude. The effect is similar to the way sound waves sometimes travel long distances over the surface of a calm lake in the early morning or early evening, letting you hear a conversation more than a mile away. Tropospheric propagation makes it possible to communicate for hundreds of miles when the ionosphere will not return waves to the earth.
Another type of tropospheric propagation is called ducting. It takes place when EM waves are trapped in a layer of cool, dense air sandwiched between two layers of warmer air. Like bending, ducting occurs almost entirely at frequencies above 30 MHz.
Still another tropospheric-propagation mode is known as troposcatter. This takes place because air molecules, dust grains, and water droplets scatter some of the EM field. This effect is commonly seen at VHF and UHF.
Tropospheric propagation in general, without mention of the specific mode, is sometimes called tropo.
In the presence of unusual solar activity, the aurora (northern lights or southern lights) can return radio waves to the earth. This is known as auroral propagation. The aurora occur at altitudes of about 40 to 250 mi (65 to 400 km). Theoretically, auroral propagation is possible, when the aurora are active, between any two points on the surface from which the same part of the aurora lie on a line of sight. Auroral propagation seldom occurs when one end of the circuit is at a latitude less than 35° north or south of the equator.
Auroral propagation is characterized by rapid and deep fading. This almost always renders analog voice and video signals unintelligible. Digital modes are most effective for communication via auroral propagation, but the carrier is often spread out over several hundred hertz as a result of phase modulation induced by auroral motion. This severely limits the maximum data transfer rate. Auroral propagation is often accompanied by deterioration in ionospheric propagation.
Meteors produce ionized trails that persist for approximately 0.5 s up to several seconds, depending on the size of a particular meteor, its speed, and the angle at which it enters the atmosphere. This is not enough time for the transmission of much data, but during a meteor shower, multiple trails can result in almost continuous ionization for a period of hours. Such ionized regions reflect radio waves at certain frequencies. This is meteor scatter propagation. It can take place at frequencies considerably
above 30 MHz, and occurs over distances ranging from just beyond the horizon up to about 1500 mi (2400 km), depending on the altitude of the ionized trail and the relative positions of the trail, the transmitting station, and the receiving station.
The moon, like the earth, reflects EM fields. This makes it possible to communicate by means of earth-moon-earth (EME), also called moonbounce. High powered transmitters, sophisticated antenna systems, and sensitive receivers are needed for EME. Some moonbounce communication is done by radio amateurs at frequencies from 50 MHz to over 2 GHz.