Radio Wave Propagation - Report Example

Radio Wave Propagation Introduction: Even though they can't be seen, the universe is filled with waves of all sizes, ranging from seismic waves with lengths in miles to x-rays. Some waves are useful for communication because information can coded into waves in a variety of ways: for example amplitude can code for volume, and television signals are coded by tiny ripples in the wave. They can also be generated at a fixed frequency, and receivers can be set to accept that frequency. Radio waves are transmitted from a point source in straight lines filling a sphere. Wave propagation occurs by the inverse square law: signal strength is proportional to 1/x2 where x = distance from the source. So waves lose strength rapidly. [1] Radio waves travel very fast but only in a straight line. Thus the curvature of the earth should limit the distance between transmitter and receiver, a distance of about 60 miles 100 km). However, the earth's atmosphere has properties that allow enhanced propagation. The atmosphere is layered, and these layers have important effects on waves that are propagated on the earth's surface. The layer called the troposphere is a heavy, oxygen filled layer that extends from the surface to about 30 miles (50 km) altitude. From 30m (50 km) to 260m (416 km) is a highly charged layer called the ionosphere. The ionosphere affects radio signals in different ways depending on their frequencies. [2] The Propagation of Waves The frequencies used for radio propagation range from 30 kHz to 3 GHz. Frequencies are continuous but are defined in bands according to their properties and uses. Higher frequencies have shorter wave lengths and more energy: Band Frequency Range Type of Use LF Low Frequency 30 - 300 kHz Radio navigation MF Medium Frequency. 300 - 3000 kHz AM broadcasting, maritime communications HF High Frequency (Short Wave) 3 - 30 MHz International short wave, ham radio VHF Very High Frequency 30 - 300 MHz Television 2-13, FM Broadcasting UHF Ultra High Frequency 300 - 3000 MHz UHF Television, cellular phones Table after [3] LF waves are propagated as ground waves which, as the name implies, travel along the ground. Because the ground and its terrain interfere with ground waves, transmission requires lots of power. LF waves are used mainly in maritime communications over the sea and the navigational system called LORAN. [4] Sometimes ground waves suffer from a reflecting wave off the ionosphere that can return to earth out of phase near the receiver and interfere with the direct reception. MF wave frequencies are used for AM radio broadcasting. They are also ground waves and during the daylight hours are limited to a range of about 60 miles (100 km). But radio waves can be bent or refracted by changes in the earth's atmosphere particularly by weather patterns and by the ionosphere. The Ionosphere consists of the "D" (30-60 miles [50-100 km]), "E" (60-100 miles [100-160 km]), F1 (90-160 miles [144-256 km]), and the F2 (160-250 miles [256-400 km]) layers. At night, the "E" layer disappears and the F1 and F2 layers combine to form the "F" layer (somewhere between 90 and 250 miles [144-400 km]). Since the "E" layer disappears at night, the lower frequency Sky Waves (MF) travel further up into the atmosphere, where they are REFRACTED by the "F" layer[ up to 300m]. That's why at night, your radio often picks up many more AM broadcast stations! [4] HF waves have enough energy to reach the ionosphere during the day and are refracted by its various layers. Thus short-wave can travelled much further than the curvature of the earth would normally allow. This Sky Wave propagation depends strongly on the ionosphere which is in constant flux. The ionosphere is affected by many astronomical events like meteor showers and solar flares; it is also affected by seasons. Because the ionosphere changes so much, short-wave is considered unreliable for important commercial use. However, some of the effects on radio transmission are positive. These so-called anomalous propagations are of considerable interest to short-wave broadcasters and enthusiasts. Sky-wave propagation also leads to an area between the transmitter and the refracted signal where no reception is possible. "It's like shining a searchlight up and having it reflected back to illuminate the ground many miles from the light. You can only talk to those people in the patch of earth that your antenna is 'illuminating' with it's signal. The place that the signal skipped over is called the skip zone." [5] Under some circumstances, the F2 layer of the ionosphere reflects signals greater than 30 MHz in all directions including back towards the transmitter. This backscatter radiation produces signals in the blind zone that have a characteristic hollow sound. [6] Both VHF and UHF signals have enough energy to pass right through the ionosphere. Thus they are ground waves; transmitter and receiver must be in line-of-sight. In areas that don't have cable television, home TV antennae are placed on the top of the building to improve line-of-sight. However, these waves can be reflected. Billboard-like reflective surfaces are set on the tops of hills for these kinds of waves, a way to pass them along. Another form of passive reflection called Moonbounce or earth-moon-earth (EME) uses the surface of the moon as a passive reflector. It was first used in 1945 by the US military for radar waves.[7] Unfortunately, these reflective surfaces absorb much of the energy from these waves, making them inefficient. These systems require expensive booster or repeater stations. Instead, the earth's surface has been covered by the transmission beams of satellites which run in geosynchronous orbit around the earth, orbiting at exactly the same rate that the earth rotates so the relative position between them is always the same. Since VHF/UHF pass through the ionosphere, the waves can easily reach the satellites where the signal is boosted and broadcast back to receivers on earth. [7] Powerful transmitters are needed to reach the geosynchronous satellites, and signals require a noticeable amount of time to reach for the round trip. This lag time is called latency and is noticeable in conversations. So this system is best for one-way communications. [7] Indeed, television channels are found in both VHF and UHF bands [2]. Anomalous Propagation: From the beginning of radio wave communications, researchers noticed that signals sometimes appeared in unexpected places. Signals can sometimes be received at hundreds or even thousands of miles outside their expected coverage area. For instance, Marconi who is often credited with inventing radio was surprised when he was able to communicate more than 1,000 miles across the ocean. [8] It turns out that waves can skip around the surface of the earth by a variety of mechanisms. Tropospheric ducting occurs during stable, high pressure weather systems when the temperature differential of two bodies of the atmospheric air create a refraction. The denser air closer to the ground causes the wave to bend. Sometimes the interface between these air masses can extend 1,000 miles or more and remain for months. This is particularly true over the sea along coastal regions where the land helps maintain the cold air mass. Ducting is known to occur over the Mediterranean Sea, the Persian Gulf, between California and Hawaii, and between Brazil and Africa. Signals greater than 90MHz are particularly well propagated. [9] Sporadic E2 propagation produces "well known short skip radio contacts off the E-layer of the ionosphere. This propagation occurs most frequently during the summer months with a major node occurring during the summer, a minor node occurring during the winter, and "valleys" occurring around both equinoxes. During the summer, this mode is popular due to its high signal levels. Finally, the skip distances are generally around 1000 statute miles." [6] Signals can sometimes skip more than one time, and multi-hop e-skip can range up to 4,500 miles. E-skip usually affects the lower VHF range. [9] Trans-Atlantic propagation (TA) occurs sporadically in the summer months after sunset. VHF bands are particularly well served. No one knows why this type occurs. Another mysterious propagation is trans-equatorial propagation (TEP) that happens in spring and fall when sunspot activity is at a minimum. TEP allows communication at frequencies up to 150 MHz between two stations of middle latitudes on equal sides of the equator like between the West Indies and South America or Italy and Africa. [6] Meteors are rocks that orbit the sun ad some planets. Occasionally, earth passes through a cloud of meteors that heat up in the earth's atmosphere and burn at very high temperatures (also called shooting stars). Ionized air is left behind, and it can reflect waves in the range 30 to 500 MHz. Some meteor showers are predictable like the Perseids on 12 August, so radio operators can take advantage of these anomalous propagations, sending signals up to 1,000 miles. [6] Auroral propagation occurs during times of increased solar activity when heavy streams of charged particles reach the earth. Planetary lines of magnetic flux guide these particles to the poles of the earth where they can cause intense ionization. This auroral layer may relect waves from 3 MHz all the way up to 3,000 MHz. The layer is very dynamic, however, and signals are subject to phase shifting a rapid modulation. [6] Conclusion Radio communication have come a long way in the last 100 years. Besides expanding the frequency range and establishing geosynchronous satellite systems, a whole community of radio aficionados has grown enormously, studying the different forms of anomalous propagation. With the addition of computer technology, radio experts have now linked computers with transceivers and have programs that anticipate periods, sites, and frequencies of enhanced propagation. For example on 12 August, hundreds of ham operators around the world will try to take advantage of the meteor scatter from the Perseids even though the reflection will only take place for a total of a few minutes. Up until recently, the problems of radio wave propagation have entailed transmission from a fixed location to another fixed location. Cellular phone technology which utilizes bands at 900 MHz and 2.4 GHz presents new challenges. Mobile propagation and reception requires a high density of repeater stations. There is also the complicated question of propagation indoors and in other areas of high interference. Scatter, reflection, motion, and diffraction interfere with signal propagation. [1] These problems will be focused on because men will forever want to improve communications with one another. Works Cited [1] A Tutorial on Indoor Radio Propagation. 23 February 2005. Spread Spectrum Scene On-line Magazine. Pegasus Technologies. 14 April 2006. [2] Norm Cohen and Kenneth Davis. Radio Wave Propagation. 1994. Space Environment Laboratory. 14 April 2006. < www.sec.noaa.gov/info/Radio.pdf > [3] Radio Frequency Use. 25 October 2004. Tampa Bay Interactive. 14 April 2006. [4] Radio Communications. 25 October 2004. Tampa Bay Interactive. 14 April 2006. [5] Terry R. Dettmann. Types of Propagation. 15 November 1995. NASA Glenn Research Centre. 14 April 2006. [6] Edwin C. Jones. The Basics of Radio Wave Propagation. Department of Physics and Astronomy, University of Tennessee, Knoxville, TN 37996-1200. 14 April 2006. [7] Sandeep Baruah. FAQ Related to Radio Wave Propagation. Ham Radio for the Novice. 14 April 2006. [8] Larry D. Wolfgang et. al, (ed), The ARRL Handbook for Radio Amateurs, Sixty-Eighth Edition , (1991), ARRL, Newington CT USA 26 March 2006. Wikimedia Foundation, Inc. 14 April 2006. [9] TV DX and FM DX. Astronomy Encyclopedia. 14 April 2006 Read More