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Propagation by the ionosphere (space wave)

For this type of propagation, electromagnetic waves are reflected by a well defined layer of the atmosphere, called the ionosphere. Ionospheric waves are used for long distance communications in the decimeter-band. The ionospheric waves, after reflection by an ionospheric layer, are once again reflected by the earth surface. Those waves follow a kind of zigzag pattern between ionospheric layer and earth surface and are able to travel over a very long distance.

During daylight, 4 different layers exist, which are reduced to two at night. Each layer has the characteristics to attenuate the radio waves and given certain circumstances, to reflect them. This is the principle used by worldwide communications.

Antennas

Every radio technical system must employ an antenna to radiate and receive electromagnetic energy.

Antenna is a specialized transducer that converts incoming electromagnetic fields into alternating electric currents having the same frequencies (receiving antenna), or converts an alternating current at a specific frequency into an outgoing electromagnetic field at the same frequency (transmitting antenna). An antenna can be a simple wire or rod, or a complicated structure. Thousands of geometries and specifications are possible.

Antenna parameters

•antenna gain;

•antenna beam width;

•radiation pattern;

•aperture;

•side lobes

Antenna gain

Independent of the use of a given antenna for transmitting or receiving, an important characteristic of this antenna is the gain. The ratio between the amount of energy propagated in these directions compared to the energy that would be propagated if the antenna were not directional (isotropic radiation) is known as its gain.

Some antenna sources radiate energy equally in all directions. Radiation of this type is known as isotropic radiation.

Antenna aperture (area)

The effective aperture of an antenna is the area presented to the radiated or received signal. It is a key parameter, which governs the performance of the antenna. The effective antenna aperture is measured in m2. It depends on the physical area of the antenna, the overall efficiency of the antenna (generally ranging from 50% to 80%) and the wavelength.

Radiation pattern

The radiation pattern of an antenna is a graphical depiction of the gain of the antenna (usually expressed in dB) versus angle. This will be a two-dimensional pattern, a function of both the azimuth and elevation angles.

Radiation pattern

Beam width

The angular range of the antenna pattern in which at least half of the maximum power is still emitted is described as a „beam width”. The angle θ can be determined in the horizontal plane (with notation θAZ) as well as in the vertical plane (with notation θEL).