Education.com
Try
Brainzy
Try
Plus

Rf Waves Help (page 3)

By — McGraw-Hill Professional
Updated on Sep 12, 2011

Auroral And Meteor-scatter Propagation

In the presence of unusual solar activity, the aurora often reflect radio waves at some frequencies. This is called auroral propagation . The aurorae occur in the ionosphere at altitudes of 25 km (40 mi) to 400 km (250 mi) above the surface. Theoretically, auroral propagation is possible, when the aurorae are active, between any two points on the Earth’s surface from which the same part of the aurora lies on a line of sight. Auroral propagation seldom occurs when either the transmitter or the receiver is at a latitude less than 35 degrees north or south of the equator. Auroral propagation can take place at frequencies well above 30 MHz and often is accompanied by deterioration in ionospheric propagation via the E and F layers.

When a meteor from space enters the upper part of the atmosphere, an ionized trail is produced because of the heat of friction. Such an ionized region reflects EM energy at certain wavelengths. This phenomenon, known as meteor-scatter propagation , can result in over-the-horizon radio communication or reception.

A meteor produces a trail that persists for a few tenths of a second up to several seconds depending on the size of the meteor, its speed, and the angle at which it enters the atmosphere. This amount of time is not sufficient for the transmission of very much information, but during a meteor shower , ionization can be almost continuous. Meteor-scatter propagation has been observed at frequencies considerably above 30 MHz and occurs over distances ranging from just beyond the horizon up to about 2,000 km (1,200 mi) depending on the altitude of the ionized trail and on the relative positions of the trail, the transmitting station, and the receiving station.

Tropospheric Bending

The lowest 13 to 20 km (8 to 12 mi) of the Earth’s atmosphere comprise the troposphere . This region has an effect on radio-wave propagation at certain frequencies. At wavelengths shorter than about 15 m (frequencies above 20 MHz), refraction and reflection can take place within and between air masses of different density. The air also produces some scattering of EM energy at wavelengths shorter than about 3 m (frequencies above 100 MHz). All these effects generally are known as tropospheric propagation , which can result in communication over distances of hundreds of kilometers.

A common type of tropospheric propagation takes place when radio waves are refracted in the lower atmosphere. This is most dramatic near weather fronts, where warm, relatively light air lies above cool, more dense air. The cooler air has a higher index of refraction than the warm air, causing EM fields to be bent downward at a considerable distance from the transmitter. This is tropospheric bending . It is often responsible for anomalies in reception of FM and TV broadcast signals.

Troposcatter

At frequencies above about 100 MHz, the atmosphere has a scattering effect on radio waves. The scattering allows over-the-horizon communication at VHF, UHF, and microwave frequencies. This is called tropospheric scatter , or troposcatter . Dust and clouds in the air increase the scattering effect, but some troposcatter occurs regardless of the weather. Troposcatter takes place mostly at low altitudes where the air is the most dense. Some effects occur at altitudes up to about 16 km (10 mi). Troposcatter can provide reliable communication over distances of several hundred kilometers when the appropriate equipment is used.

Figure 18-4 shows tropospheric scatter and bending. The transmitting station is at the lower left. There is a temperature inversion in this example; it exaggerates the bending. If the boundary between the cool air near the surface and the warm air above is well defined enough, reflection can occur in addition to the bending. If the inversion covers a large geographic area, signals can bounce repeatedly between the inversion boundary and the surface, providing exceptional long-range communication, especially if the surface is salt water.

 

Forms of Radiation Rf Waves Duct Effect

Fig. 18-4 . The troposphere can bend and scatter radio waves at some frequencies.

View Full Article
Add your own comment