Long distance propagation of radio waves depends on an invisible layer of charged particles, which envelops the Earth. This layer of charged particles known as the ionosphere has been in existence for millions of years. For those, who pioneered the long distance radio communication during the early part of the twentieth century, the ionosphere came as a boon. During the formative days of radio communication, radio scientists could not come to a definite conclusion about how radio waves propagated round the world. Both Radio and Television utilise radio wave, a form of electromagnetic wave, that travels at a velocity of 3,00000 km per second in vacuum. Its velocity gets changed very negligibly in a different medium, which is insignificant, because the earth is a very small place with a radius of only 6000-km. Communication between any two points on the earth is thus almost instantaneous. But electromagnetic waves travel in straight lines until they are deflected by something. The father of radio, Guglielmo Marconi himself was at a loss to explain how, on 12th December, 1901, he established the first real long distance wireless communication between St. Johns, New Foundland, USA and Poldhu in the Southern Tip of England, a distance of more than 3,000 kms across the Atlantic ocean. At that time, it was known that except for very short distances, the radio waves did not follow the natural curvature of the earth. Earth's curvature is a direct block to line-of-sight communication. When enough distance separates the two radio stations so that their antennas fall behind the curvature, the Earth itself blocks the transmitted signals from the receiver.
    There are certain radio frequencies, which can travel only in the line-of-sight. This means that higher the antenna of the radio transmitter, greater the distance covered by its transmission. That explains why television transmission towers are made as high as possible. Radio frequencies in the range of 30 to 300 MHz (known as Very High Frequency) normally propagate only in the line-of-sight. The frequencies in the range of 300 MHz (Mega Hertz) to 3000 MHz (known as Ultra High Frequency) also propagate in line-of-sight. To receive radio signals in these ranges at a far away place from their place of origin, we need some kind of a reflector in between. You might have noticed big metallic plates on the mountain tops (or on top of other tal structures, which have a similarity to the roadside signboards. These are passive reflectors, which reflect VHF and UHF signals to far away places. A passive reflector is an object, which is not equipped with any kind of electronic circuitry to relay the radio signal. The moon, which had been in orbit for some 5 million years or more, was used as a natural passive reflector by the U.S. Army Signal Corps for the first time on 11 January, 1946 by bouncing radar (Radio Detection And Ranging} signals off the moon during Project Diana. On 29 November, 1959, voice transmissions were relayed from Holmdel, New Jersey to Goldstone via this same natural satellite.
    The Moon was also used as a reflector of radio waves by the U.S. Army in the 1950s, when the existing channels between the US mainland and Hawaii failed because of atmospheric disturbances. This type of radio communication is known as Moonbounce radio communication. Ham radio operators refer to it as E.M.E., i.e. Earth-Moon-Earth and this describes exactly what happens; the radio signal leaves the earth, is reflected back off the moon, and comes back to earth. The reflected signal spreads out, and can be received at any place on earth where the moon is above the horizon. In case of using passive reflector to reflect VHF and UHF radio signals, large signal loss takes place between the transmitter and the reflector and an equal loss between the reflector and receiving station. It requires significant amount of power to assure a strong enough signal back on earth after the reflection process has taken effect. So, radio communication via artificial communication satellites, equipped with active electronic circuitry, which can re-transmit the received signal with an amplified power, was another proposition put forward by Arthur C Clarke during the 1945s. The artificial communication satellites overcome the problems involved in radio signal to span oceans and continents with sub-marine cables, landlines and terrestrial relay stations, for long-distance transmission of radio, telephone and television signals. VHF or UHF(also called microwave) travel in line-of-sight. So, the relay stations to receive, amplify and retransmit signals must be spaced approximately 35 miles or so because of the curvature of the earth. The cost of providing enough relay stations to encircle the world completely would still be prohibitive. The visionary idea put forward by famous science fiction writer Arthur C. Clarke, that three artificial satellites spaced at 1200 intervals about the equator at a distance of 35,200 km above the earth in a Geo-synchronous Earth Orbit (GEO) would effectively cover the entire globe, soon became a reality.
    A GEO satellite revolves round the Earth at the same speed as the Earth rotates around its axis and thus the satellite's position remains static relative to an area on the Earth. India's INSAT series of satellites are GEO satellites. Because of their high altitude, they require high transmission power for communication as a result of which the land based GEO transmission units are heavy and consequently unsuitable for mobile users. Another drawback resulting from the high altitude is called latency. As the distance between the satellite and the land-based transmitter-receiver is large, radio signals take time to travel from Earth to satellite and back, resulting in a time lag of 0.24 seconds, a time lag unacceptable in interactive communication. That is why GEO satellites are used in non-interactive radio communication such as the Television. Nowadays, a constellation of 66 orbiting communication satellites (known as IRIDIUM) exists just 780 km above the surface of the Earth classified as Low Earth Orbiting (LEO) satellites. Because of their lower altitude, latency (time lag) is almost negligible and user-devices do not have to be very powerful or bulky. Cellular & mobile phones also utilise frequencies in the UHF range and a city with cellular facility is divided into some small cells-each cell having its own relay station.
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. Under normal atmospheric circumstances, frequencies above 30 MHz do not propagate beyond the line sight. These signals penetrate the ionosphere and are lost into space.
    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 radio signals (radio signals which fall in the range of 3 to 30 MHz) are reflected from this layer just as light rays are reflected from the surface of a mirror, or sound wave from a barrier. likewise, this layer can be compared to the edge of a billiard table. Communication specialists use this layer like the edge of a billiard table: if the ball does not go straight into the pocket, it can be directed on the rebound. In the same way, the short wave signals radiated by distant radio stations get to our receiver on the rebound. They can continue traveling to several places round the world, for the Earth is also like the edge of a billiard-table.
    Unlike the short wave or high frequencies (HF), the frequencies ranging from 300 kHz to 3000 kHz, are known as medium frequencies and the band is known as Medium Wave band. There is very little daytime reflection of medium wave radio signal from the ionosphere resulting in a coverage of about 100 kms only.The ionosphere is located above the troposphere, starting at an altitude of 30 miles above the surface of the earth and extending upto an altitude of 260 miles. The troposphere is the region of the earth's atmosphere immediately adjacent to the earth's surface and extending upward for some tens of kilometres. Radio waves are refracted or bent slightly, when traveling from one medium to another. Refraction is caused by a change in the velocity of a wave when it crosses the boundary between one propagating medium and another. If this transition is made at an angle, one portion of the wave-front slows down or speeds up before the other, thus bending the wave slightly. Radio waves are commonly refracted when they travel through different layers of the atmosphere, whether it is highly charged ionospheric layers 100 km and higher, or weather-sensitive area near the Earth surface. When the ratio of the refractive indices of two media is great enough, radio waves can be reflected, just like light waves striking a mirror.
    The role of ionosphere in radio wave propagation can be discussed only in terms of the different radio frequencies available for communication and in the light of the existence of different ionospheric layers. Although the various methods used confirmed the theories of Heaviside and Kennally, there were differences between the results obtained by Professor Appleton and other investigators. It was discovered that there was not one, but two reflecting layers. The first trials with pulse waves in 1925 by Breit and Tuve in America were successful in that the method proved to be much more practicable. Since radio waves take 1 millisecond to travel 300 km, the height of the layer established from the first echo in this case was found to be 300 km. The ionised layers were designated with letters of the alphabet by E. V. Appleton, the lowest layer known at a height of about 60 to 90 km being called the D region because this is not strictly a layer but a relatively dense part of the atmosphere where atoms are broken up into ions by sunlight that recombine very quickly. The amount of ionisation therefore depends on the amount of sunlight and the region has the effect of absorbing the energy from a radio wave, particularly at frequencies in the band of 3 to 4 MHz and frequently as high as 7 MHz.
    Most of the long distance communication results from ionisation of the F layer, the most applicable radio bands in the High Frequency being 3.5 MHz, 7 MHz, 14 MHz and 21 MHz. The layer height may vary from a little over 200 km to as high as 400 to 500 km depending on the time of the year, latitude and time of the day and particularly the amount of sun-spot activity. During the peak period of the 11 year maximum sun spot activity cycle, propagation via the F layer extends up to around 30 MHz.
    The problem of variable propagation conditions can be partially overcome by using frequency diversity, in which an allotted wireless communication network is provided with several frequency assignments spanning the high frequency (short wave) band of frequencies. The radio operator can thus choose the channel that gives the best results at any given time. The 1800 kHz (1.8 MHz or 160 metre band) band suffers from extreme daytime D-layer absorption. Even at high radiation angles, virtually no signal can pass through the F layer and daytime communication is limited to ground-wave coverage. At night, the D layer quickly disappears and world- wide 160m communication becomes possible via F2-layer skip. Atmospheric and man-made noise limit propagation of this band. Tropical and mid-latitude thunderstorms cause high levels of static in summer, making winter evenings the best time to work long distance at 1.8 MHz. A proper choice of receiving antenna can often significantly reduce the amount of received noise while enhancing desired signals.
    The 3500 kHz (3.5 MHz or 80 metre band) is the lowest HF band, which is similar to 160 m in many respects. Daytime absorption is significant, but not quite as extreme as at 1.8 MHz. High-angle signals may penetrate to the E and F layers. Daytime communication range is typically limited to 400 km, primarily via ground-wave propagation. At night, signals are often propagated halfway around the world. As at 1.8 MHz, atmospheric noise is a nuisance, making winter the most attractive season for the 80 m. The 7000 kHz (7 MHz or 40 metre) band is useful for daytime communication up to a distance of 800 km via E and F layers. Long distance world-wide communication takes place in this band through F2 layer. The 10 MHz or 30 metre band is unique because it shares characteristics of both daytime and night-time bands. Communication up to 3000 km is typical during daytime, and this extends halfway around the world. The band is generally open via F2 on a 24-hour basis.
    Despite the fact that the introduction of artificial communication satellites for long distance radio communication made communication more reliable and there is very little role left to be played by the ionosphere in the professional telecommunication networks, it still draws the attention of communication enthusiasts, arm forces, spies and ham radio operators. Ionosphere is a gift of nature. Unlike the costly artificial satellites, we need not subscribe to anybody to get access to a facility, which can transfer our radio messages to distant parts of the world. It is worthwhile for a radio user to learn more about the ionosphere.