Propagation Of Radio Waves Or Otherwise Essay

, Research Paper

Propagation as defined by Webster Dictionary as “the phenomenon of radio frequency energy traveling through the earth’s atmosphere, as well as through the empty space above the atmosphere”.

Once radio-frequency energy has been emitted from an antenna it will travel away from the antenna, where it is generated from, at the speed of light. Once it is sent the wave exists and propagates independently of the system that produces it. This means that changes, which occur at the transmitter (antenna), will have no effect on the energy that has already been radiated. The front of the wave is round or spherical in nature. The direction in which the wave will travel is dependent on the characteristics of the antenna from which the wave is generated.

For the purpose of analyzing and predicting propagation, it will take place in two manners or modes: ground wave propagation mode and sky wave propagation mode. These two modes are very different and distinct, as described below.

Ground – Wave Propagation Mode:

Ground – Wave propagation mode refers to the propagation effects, which are most directly determined by the earth’s presence. Ground – Wave propagation include several different components (See Figure 1). The direct wave component will include the radiation that travels through the atmosphere on a direct line from the sending antenna to the receiving antenna. The direct wave case is sometimes referred to as line of sight propagation since it is limited by the curvatures of the earth’s surface. (See Figure 2). However in actual fact radio waves travelling near the earth’s surface bend and follow the surface to a small degree. This bending is refraction caused by the atmosphere. As a result of the refraction, the “radio horizon” is slightly beyond the visual horizon. Direct – wave propagation is especially important when the frequency of radiation is above approximately 30 MHz. At such frequencies the direct wave is the major component of ground-wave propagation. It is also important to remember to know that line of sight transmission means that signals which are propagated mainly as direct waves do not go around or through objects or any other obstructions of large size.

Therefore, a ground wave includes these components; surface wave, direct wave, tropospheric wave, and ground reflected waves. A surface wave will travel near the earth’s surface and is significantly affected by that surface. This wave is works best with vertical polarization and relatively low frequencies, approximately 2MHz. Absorption by the earth’s surface limits the range of the wave. Direct wave propagation is utilized for transmissions of frequencies higher than those for which the surface wave is used. Polarization may be vertical or horizontal – both are used. Vertical polarization is used commonly for two-way radio communication; horizontal polarization is used for commercial broadcast FM and television transmissions. The range of direct wave propagation is limited to line of sight, which, because an effect called atmospheric refraction, is approximately 15% greater than visual line of sight. The tropospheric wave is a direct wave that has received significant refraction by the troposphere, the lower atmosphere. Refraction occurs when weather conditions, such as temperature inversion, produce layers of differing refractive indexes in the atmosphere. The boundaries of such layers produce refraction. Tropospheric wave propagation’s achieve remarkable ranges at times, but may also be unreliable. Ground reflected wave might enhance a direct wave but often cause undesirable effects such as television ghost images.

Sky – Wave Propagation Mode:

The second major type of wave propagation includes what is called a sky wave. The sky wave is the product of radiation whose direction initially is above the horizontal, toward the sky. It is a relatively complex event; there are several factors that determine the ultimate nature of this mode of propagation (See Figure 3). This diagram depicts the several different paths of transmission associated with sky waves. The most important factor that effects the propagation of sky waves is the ionosphere.

The Ionosphere:

The meaning of ionosphere means literally means the sphere of ions. It is a sphere of ionized gases that encompass the earth. An ion is an atom or molecule (group of atoms) which has a net electrical charge. The sphere of ions has four particular layers; they are termed the D, E, F1 and F2 layers. The layers are different heights above the earth’s surface, the order of the heights is the same as that shown in (See Figure 4), with the D layer nearest to the earth’s surface.

The molecules of the gases at the upper ends of the earth’s atmosphere are ionized by radiation from the sun. Ionization is maintained for long periods because the low density of the gases in the upper atmosphere reduces significantly, this will produce what is called de-ionization, the recombination of electron and atom. Most of the ionizing energy is produce from the sun, there are also other sources of ionizing energy, and these are a direct result of cosmic energies.

The amount of ionization, is a directly depends on the intensity of the ionizing energy. Since the majority of that energy is ultraviolet radiation from the sun, therefore the effect on radio wave propagation is pr