Tuesday, November 24, 2009
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Under the action of solar radiation and the hail of meteorites, an ionized layer is formed in the upper part of the Earth's atmosphere. In this layer, the neutral air molecules are decomposed into ions and electrons and the whole layer presents a chaos of charged particles.Short wave radio signals are reflected from this layer just as light rays are reflected from the surface of a mirror, or sound from a barrier.Likewise, this layer can be compared with the edge of billiard table: if the ball does not go straight into the pocket, it can be sent on rebound. In a situation, a radio receiver set located at a distance of 200 kilometres (say) away from the wireless transmitting station can not receive signals from the transmitting station. This is because the ground waves are stopped by the Earth's curvature and the sky wave will not reach the receiver, because it bounces again more than 200 kilometres way. So some 'blind zones' are formed and if the receiver is located in that blind zone it will receive no signal or very weak signal. In such a situation, another station can relay the message to the target station. The distance of the intended receiver from the transmitter is then termed as 'skip distance'. So it is not always necessary that a receiving station located nearer (than a station located further away from the transmitting station) to the transmitting station will be able to receive its signal.
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The First proclaimed Radio Amateur (HAM), Guglielmo Marconi was not using VHF or UHF for his famous Trans-Atlantic radio communication experiment nor was he employing any artificial relay mechanism. Yet his radio signal traveled half the world. Marconi's wireless transmitter powered by 2,000 volts from a generator driven by a 32 horsepower petrol engine pumped out 25,000 watts (25 KW) of power at a carrier frequency of about 328 kHz (Kilo Hertz). Alternately. the wavelength of the radio frequency used was approximately 915 metres. The formula to calculate wavelength of an electromagnetic wave is Wavelength in metres = 300/frequency In Mega Hertz. The dial of a radio receiver also marks either wavelength or frequencies or both.
On frequencies below 30 MHz, long distance radio communication is the result of refraction (bending) of the wave in the ionosphere. Oliver Heaviside in England and A.E. Kennally in America,in 1902, suggested that there must be some kind of reflecting medium in the upper atmosphere that caused the radio waves to be returned to Earth at considerable distances from the transmitter. Under the action of solar radiation and the hail of meteorites, an ionised layer is formed in the upper part of the Earth's atmosphere. In this layer, the neutral air molecules are decomposed into ions and electrons and the whole layer presents a chaos of charged particles.
Short wave or High Frequencies (HF) in the range of 3-30 MHz propagates through this invisible layer which consists of charged particles located at altitudes of between 250 and 400 km in the atmosphere surrounding the Earth. This layer of charged air particles called F2 layer of the ionosphere plays a vital role in HF propagation by reflecting or refracting the HF signals back to Earth.
The ionosphere has got different sub-layers. The lowest is D-layer at altitudes ranging from 50 to 90 km. High frequencies (3-30 MHz) penetrate this layer, while low frequency (LF: 30-300 kHz) or medium waves are absorbed by this layer. To some extent LF and Very Low Frequency (VLF: 3 to 30 kHz) are reflected during daytime. It slightly scatter and absorbs HF. This layer subsists only during daytime.
The E-layer extends from an altitude of 100 km. Though sunlight is an important factor for its existence, after sunset also it exists for some time. This layer is responsible for evening and early night time propagation of medium waves (low frequency) upto a distance of about 250 km. Propagation of lower short wave frequencies, e.g. 2 MHz , up to distance of 2000 km at daylight time is due to this layer. It has little effect at night.
F1 layer exists at an altitude of 200 km during daytime and its characteristics are very similar to E-layer which emerges into F2 layer at night. F2 layer is the most important layer which exists at altitudes ranging from 250 to 400 km and HF long distance propagation round the clock is due to this layer. The behaviour of this layer is influenced by the time of the day, by season and by sunspot activity. F2 layer was formerly known as Appleton layer. This layer has a high ionization gradient. This layer exists both in the daytime and nighttime. Since at such an altitude air density is extremely low, the free ions and electrons (due to the action of ultraviolet radiation from the Sun) can not recombine readily and so can store energy received from the Sun for many hours; that is the reason the refractive property of this layer changes only to a negligible extent during day and night. The path which the short wave signal follows through the F2 layer is in reality a curved one. Degree of the curve depends on the angle of incidence of the wave, ionization gradient of the layer and frequency of the signal.
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The radio frequencies above 30 MHz has the tendency to penetrate the ionosphere making them unsuitable for long distance propagation. So, the range of frequencies from 30 to 300 MHz (also 300 MHz and above), which are placed under the Very High Frequency (VHF) category are mainly used for line-of-sight communication. The most common example of line-of-sight communication is the TV Telecast. A TV transmission tower is made as tall as possible so that its signals can have a wide area of coverage. To receive a TV telecast, we have to turn our TV antenna (known as a Yagi antenna) towards the TV transmission tower. In areas where the TV transmission tower is located at a far away place from a viewer, the viewer has to increase the height of his TV receiving antenna. This means that both the transmitting and receiving antenna should literally see each other to make the communication effective. Otherwise there should be some means to redirect the signal back to the receiver. Artificial Satellites in space (which houses active electronic relaying device), terrestrial relay station and passive reflectors (the metallic plates we see above the hills) are employed to extend the VHF coverage. Line-of-sight communication is considered reliable within a short distance.
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