Heinrich Rudolf Hertz was a German physicist. He was born in 1857 and died in 1895. He confirmed Maxwell's electromagnetic theory of waves and discovered information about their behaviour. The measurement of the frequency of radio waves is named after him. Research Heinrich Hertz
Computer Screen Interception (CSI) is a popular method of reading confidential information from remote computers. The basic principle of Computer Screen Interception is that CRT video display units radiate radio waves, these can then be received and displayed on a modified televisionreceiver relatively quite simply, revealing a copy of everything being displayed on the originating computer screen. Research Computer Screen Interception
Infrared radiation (heat waves) consists of electromagnetic radiation of wavelengths shorter than those of the super-high-frequency radio waves and longer than 7600 angstroms - that is between about 0.75 and 1000 micro metres forming the radiation between the visible and microwave regions of the radiationspectrum. Research Infrared
Long Waves are radio waves having wavelengths between 1000 and 10,000 metres, corresponding to frequencies from 300 kHz down to 30 kHz. Research Long Waves
A nebula is a cloud of gas and dust in space. Before the invention of the telescope, the term nebula was applied to all celestial objects of a diffuse appearance. As a result, many objects now known to be star clusters or galaxies were originally called nebulas.
Nebulas exist within other galaxies as well as in our own Milky Waygalaxy. They are classified as planetary nebulas, supernova remnants, and diffuse nebulas, including reflecting, emission, and dark nebulas. Small, very bright nebulas called Herbig-Haro objects are found in dense interstellar clouds and are probably the products of gas jets expelled by new stars in the process of formation. Planetary nebulas, or planetaries, are so called because many of them superficially resemble planets through telescopes. They are actually shells of material that an old average star sheds during a late, red giant stage in its evolution, before becoming a white dwarf. The Ring nebula of the constellationLyra, a typical planetary, has a rotational period of 132, 900 years and a mass calculated to be about 14 times that of the earth's sun. Several thousand planetaries have been discovered in the Milky Way. More spectacular but fewer in number are nebulas that are the fragments of supernova explosions, perhaps the most famous of which is the Crab nebula in Taurus, now fading at the rate of about 0.4 percent per year. Nebulas of this kind are strong emitters of radio waves, as a result of the explosions that formed them and the probable pulsar remnants of the original star. Diffuse nebulas are extremely large structures, often many light-years wide, that have no definite outline and a tenuous, cloudlike appearance. They are either luminous or dark. The former shine as a result of the light of neighbouring stars. They include some of the most striking objects in the sky, such as the Great nebula in Orion. The tremendous streams of matter in the diffuse nebulas are intermingled in violent, chaotic currents. Many thousands of luminous nebulas are known. Spectral studies show that light emanating from them consists of reflected light from stars and also, in so-called emission nebulas, of stimulated radiation of ionised gases and dust from the nebulas themselves.
Dark, diffuse nebulas are observed as nonluminous clouds or faintly luminous, obscuring portions of the Milky Way and too distant from the stimulation of neighbouring stars to reflect or emit much light of their own. One of the most famous dark nebulas is the Horsehead nebula in Orion, so named for the silhouette of the dark mass in front of a more luminous nebular region. The longest dark rift observed on photographic plates of the star clouds of the Milky Way is a succession of dark nebulas. Both dark nebulas and luminous nebulas are considered likely sites for the processes of dust-cloud condensation and the formation of new stars. Research Nebula
Radio waves consist of magnetic and electrostatic fields at right angles to each other and also at right angles to the direction of travel. Radio communication is carried on by means of this energy that travels from the transmitter to the receiver. These waves move with the velocity of light and represent electrical energy that has escaped into free space. The chief characteristics of a radio wave are the frequency (or wave length) and the intensity. The frequency represents the number of complete cycles of oscillations that the transmitter sends out per second, and so is the frequency of the alternating current producing the wave. The wave length is the distance in space occupied by one complete cycle of oscillation. The wave length Wl in metres and the frequency f in cycles are related by the equation: Wl = 300,000,000 / f where the quantity 300,000,000 is the velocity of light in metres per second. The frequency is ordinarily expressed in kilohertz, abbreviated kHz, or in megahertz, abbreviated Mhz.
A low-frequency wave is seen from the equation to have a long wave length, while a high frequency corresponds to a short wave length. Radio waves differ from other electromagnetic waves, such as light, only in wave length (or frequency). The strength or intensity of a radio wave is measured in terms of the voltage stress produced by the electrostatic field of the wave, and is usually expressed in microvolts stress per meter. The strength in microvolts per metre is also exactly the same voltage that the magnetic flux of the wave induces in a conductor one metre long when sweeping across the conductor with the velocity of light. The minimum field strength required to give satisfactory reception of a radio wave varies with the amount of interference that is present. Under very favourable conditions waves having a strength of less than 1 uv per metre will produce intelligible Signals; Much greater field strengths are generally necessary, however, because of interfering waves generated by man-made and natural sources. Thus in rural areas experience has shown that it normally requires a field strength of the order of 100 uv per metre to give what the listener considers satisfactory service from a broadcast station, whereas in urban locations the man-made interference is so great that field strengths of 500 to 30, 000 uv per metre are ordinarily needed to insure good reception at all times. The strength of the radio wave reaching the receiver from a distant transmitter is affected by a number of factors. Most important of these are the spreading of the wave owing to distance, the absorption of energy by the earth, and the attenuation and refraction of the wave by the action of the ionised regions (ionosphere) of the upper atmosphere.
The effect of the earth and the ionised regions depends very greatly upon the frequency. Thus low-frequency radio waves such as 12 to 100 khz suffer very little attenuation other than that due to spreading, and the received signal strength does not differ greatly from day to night and season to season. In contrast with this, waves of medium wave broadcast frequencies (550 to 1500 khz) have such high attenuation in the daytime that only local stations can be heard, while at night the attenuation is frequently so low that very distant stations produce satisfactory signals.
High-frequency waves, such as those in the range 6 Mhz to 38 Mhz, behave in a still different manner. Here the ground very quickly absorbs the portion of the wave travelling along the earth's surface. Waves of such frequencies may, however, reach a distant point as a result of refraction of energy earthward by the ionised region in the upper atmosphere. Finally, at very high, ultra high and super high frequencies the ground absorbs the portion of the wave travelling along the earth's surface, while the ionised regions are not capable of bending the wave path appreciably. Communication at these frequencies is hence possible only over distances so short that the earth's curvature permits a substantially straight line path between transmitting and receiving points, or between the earth and satellites where the signal follows a straight line straight through the ionised regions of the atmosphere. Research Radio Waves
Solar radiation is radiation emanating from the Sun and consisting mainly of visible light, ultravioletradiation, and infraredradiation, although the whole spectrum of electromagnetic waves is present, from radio waves to X-rays. High-energy charged particles such as electrons are also emitted, especially from solar flares. When these reach the Earth, they cause magnetic storms (disruptions of the Earth's magnetic field), which interfere with radio communications. Research Solar Radiation
The Probert Encyclopaedia was designed, edited and programed by
Matt and Leela Probert
Abbreviations
Research Heinrich Hertz
