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RF Basics: Radio Propagation

Radio wave propagation.

It is fitting in a book about wireless communication to look at the phenomena that lets us transfer information from one point to another without any physical medium—the propagation of radio waves. If you want to design an efficient radio communication system, even for operation over relatively short distances, you should understand the behavior of the wireless channel in the various surroundings where this communication is to take place. While the use of “brute force” increasing transmission power could overcome inordinate path losses, limitations imposed on design by required battery life, or by regulatory authorities, make it imperative to develop and deploy short-range radio systems using solutions that a knowledge of radio propagation can give.

The overall behavior of radio waves is described by Maxwell’s equations. In 1873, the British physicist James Clerk Maxwell published his Treatise on Electricity and Magnetism in which he presented a set of equations that describe the nature of electromagnetic fields in terms of space and time. Heinrich Rudolph Hertz performed experiments to confirm Maxwell’s theory, which led to the development of wireless telegraph and radio. Maxwell’s equations form the basis for describing the propagation of radio waves in space, as well as the nature of varying electric and magnetic fields in conducting and insulating materials, and the flow of waves in waveguides.

From them, you can derive the skin effect equation and the electric and magnetic field relationships very close to antennas of all kinds. A number of computer programs on the market, based on the solution of Maxwell’s equations, help in the design of antennas, anticipate electromagnetic radiation problems from circuit board layouts , calculate the effectiveness of shielding, and perform accurate simulation of ultra-high-frequency and microwave circuits. While you don’t have to be an expert in Maxwell’s equations to use these programs (you do in order to write them!), having some familiarity with the equations may take the mystery out of the operation of the software and give an appreciation for its range of application and limitations.

The Four Types of Radio Wave Propagation

What are the four types of radio wave propagation? Radio waves can propagate from transmitter to receiver in four ways: through ground waves, sky waves, free space waves, and open field waves. Ground waves exist only for vertical polarization, produced by vertical antennas, when the transmitting and receiving antennas are close to the surface of the earth. The transmitted radiation induces currents in the earth, and the waves travel over the earth’s surface, being attenuated according to the energy absorbed by the conducting earth. The reason that horizontal antennas are not effective for ground wave propagation is that the horizontal electric field that they create is short circuited by the earth.

Ground wave propagation is dominant only at relatively low frequencies, up to a few MHz, so it needn’t concern us here. Sky wave propagation is dependent on reflection from the ionosphere, a region of rarified air high above the earth’s surface that is ionized by sunlight (primarily ultraviolet radiation).

The ionosphere is responsible for long-distance communication in the high-frequency bands between 3 and 30 MHz. It is very dependent on time of day, season, longitude on the earth, and the multiyear cyclic production of sunspots on the sun. It makes possible long-range communication using very low power transmitters. Most short-range communication applications that we deal with in this chapter use VHF, UHF, and microwave bands, generally above 40 MHz. There are times when ionospheric reflection occurs at the low end of this range, and then sky wave propagation can be responsible for interference from signals originating hundreds of kilometers away. However, in general, sky wave propagation does not affect the short-range radio applications that we are interested in.

The most important propagation mechanism for short-range communication on the VHF and UHF bands is that which occurs in an open field, where the received signal is a vector sum of a direct line-of-sight signal and a signal from the same source that is reflected off the earth. Later we discuss the relationship between signal strength and range in line-of-sight and open field topographies.

The range of line-of-sight signals, when there are no reflections from the earth or ionosphere, is a function of the dispersion of the waves from the transmitter antenna. In this free-space case the signal strength decreases in inverse proportion to the distance away from the transmitter antenna. When the radiated power is known, the field strength is given by equation (5.1):

Range can be calculated on this basis at high UHF and microwave frequencies when high-gain antennas are used, located many wavelengths above the ground. Signal strength between the earth and a satellite, and between satellites, also follows the inverse distance law, but this case isn’t in the category of short-range communication! At microwave frequencies, signal strength is also reduced by atmospheric absorption caused by water vapor and other gases that constitute the air.

Open Field Propagation

Although the formulas in the previous section are useful in some circumstances, the actual range of a VHF or UHF signal is affected by reflections from the ground and surrounding objects. The path lengths of the reflected signals differ from that of the line-of-sight signal, so the receiver sees a combined signal with components having different amplitudes and phases.

The reflection causes a phase reversal. A reflected signal having a path length exceeding the line-of-sight distance by exactly the signal wavelength or a multiple of it will almost cancel completely the desired signal (“almost” because its amplitude will be slightly less than the direct signal amplitude). On the other hand, if the path length of the reflected signal differs exactly by an odd multiple of half the wavelength, the total signal will be strengthened by “almost” two times the free space direct signal.

In an open field with fl at terrain there will be no reflections except the unavoidable one from the ground. It is instructive and useful to examine in depth the field strength versus distance in this case. The mathematical details are given in the Mathcad worksheet “Open Field Range.”

In Figure 5.1 we see transmitter and receiver antennas separated by distance d and situated at heights h 1 and h 2 . Using trigonometry, we can find the line of sight and reflected signal path lengths d 1 and d 2 . Just as in optics, the angle of incidence equals the angle of reflection θ. We get the relative strength of the direct signal and reflected signal using the inverse path length relationship. If the ground were a perfect mirror, the relative reflected signal strength would exactly equal the inverse of d 2 . In this case, the reflected signal phase would shift 180 degrees at the point of reflection. However, the ground is not a perfect reflector. Its characteristics as a reflector depend on its conductivity, permittivity, the polarization of the signal, and its angle of incidence.

5.1. Open field signal paths In the Mathcad worksheet we have accounted for polarization, angle of incidence, and permittivity to find the reflection coefficient, which approaches -1 as the distance from the transmitter increases. The signals reaching the receiver are represented as complex numbers, since they have both phase and amplitude. The phase is found by subtracting the largest interval of whole wavelength multiples from the total path length and multiplying the remaining fraction of a wavelength by 2π radians, or 360 degrees.

Figure 5.2 gives a plot of relative open field signal strength versus distance using the following parameters : Polarity: horizontal Frequency: 300 MHz Antenna heights: both 3 meters Relative ground permittivity: 15

5.2. Field strength versus range at 300 MHz Also shown is a plot of free space field strength versus distance (dotted line). In both plots, signal strength is referenced to the free space field strength at a range of 3 meters. Notice in Figure 5.2 that, up to a range of around 50 meters, there are several sharp depressions of field strength, but the signal strength is mostly higher than it would be in free space. Beyond 100 meters, signal strength decreases more rapidly than for the free space model. Whereas there is an inverse distance law for free space, in the open field beyond 100 meters (for these parameters) the signal strength follows an inverse square law. Increasing the antenna heights extends the distance at which the inverse square law starts to take effect. This distance, dm, can be approximated by

Part 2 will cover Multipath Phenomena Part 3 will cover Diversity Techniques

References Gibson, J. D. (ed.), The Mobile Communications Handbook, CRC Press, Inc., 1996. Rappaport, T. S., Wireless Communications, Principles and Practice, Prentice Hall, Upper Saddle River, NJ, 1996. Spix, G. J., “Maxwell’s Electromagnetic Field Equations,” unpublished tutorial, copyright 1995 (http://www.connectos.com/spix/rd/gj/nme/maxwell.htm).

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Copyright: Printed with permission from Newnes, a division of Elsevier. Copyright 2008. “RF & Wireless Technologies” by Bruce A. Fette. For more information about this title and other similar books, please visit www.newnespress.com.

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RF Basics: Radio Propagation

It is fitting in a book about wireless communication to look at the phenomena that lets us transfer information from one point to another without any physical medium—the propagation of radio waves. If you want to design an efficient radio communication system, even for operation over relatively short distances, you should understand the behavior of the  wireless  channel in the various surroundings where this communication is to take place. While the use of “brute force” —increasing transmission power—could overcome inordinate path losses, limitations imposed on design by required battery life, or by regulatory authorities, make it imperative to develop and deploy short-range radio systems using solutions that a knowledge of radio propagation can give.

The overall behavior of radio waves is described by Maxwell’s equations. In 1873, the British physicist James Clerk Maxwell published his  Treatise on Electricity and Magnetism  in which he presented a set of equations that describe the nature of electromagnetic fields in terms of space and time. Heinrich Rudolph Hertz performed experiments to confirm Maxwell’s theory, which led to the development of wireless telegraph and radio. Maxwell’s equations form the basis for describing the propagation of radio waves in space, as well as the nature of varying electric and magnetic fields in conducting and insulating materials, and the flow of waves in waveguides.

From them, you can derive the skin effect equation and the electric and magnetic field relationships very close to antennas of all kinds. A number of computer programs on the market, based on the solution of Maxwell’s equations, help in the design of antennas, anticipate electromagnetic radiation problems from  circuit  board layouts, calculate the effectiveness of shielding, and perform accurate simulation of ultra-high-frequency and microwave circuits. While you don’t have to be an expert in Maxwell’s equations to use these programs (you do in order to write them!), having some familiarity with the equations may take the mystery out of the operation of the software and give an appreciation for its range of application and limitations.

Mechanisms of Radio Wave Propagation Radio waves can propagate from transmitter to receiver in four ways: through ground waves, sky waves, free space waves, and open field waves. Ground waves exist only for vertical polarization, produced by vertical antennas, when the transmitting and receiving antennas are close to the surface of the earth. The transmitted radiation induces currents in the earth, and the waves travel over the earth’s surface, being attenuated according to the energy absorbed by the conducting earth. The reason that horizontal antennas are not effective for ground wave propagation is that the horizontal electric field that they create is short circuited by the earth.

Ground wave propagation is dominant only at relatively low frequencies, up to a few MHz, so it needn’t concern us here. Sky wave propagation is dependent on reflection from the ionosphere, a region of rarified air high above the earth’s surface that is ionized by sunlight (primarily ultraviolet radiation).

The ionosphere is responsible for long-distance communication in the high-frequency bands between 3 and 30 MHz. It is very dependent on time of day, season, longitude on the earth, and the multiyear cyclic production of sunspots on the sun. It makes possible long-range communication using very low power transmitters. Most short-range communication applications that we deal with in this chapter use VHF, UHF, and microwave bands, generally above 40 MHz. There are times when ionospheric reflection occurs at the low end of this range, and then sky wave propagation can be responsible for interference from signals originating hundreds of kilometers away. However, in general, sky wave propagation does not affect the short-range radio applications that we are interested in.

The most important propagation mechanism for short-range communication on the VHF and UHF bands is that which occurs in an open field, where the received signal is a vector sum of a direct line-of-sight signal and a signal from the same source that is reflected off the earth. Later we discuss the relationship between signal strength and range in line-of-sight and open field topographies.

The range of line-of-sight signals, when there are no reflections from the earth or ionosphere, is a function of the dispersion of the waves from the transmitter antenna. In this free-space case the signal strength decreases in inverse proportion to the distance away from the transmitter antenna. When the radiated power is known, the field strength is given by equation (5.1):

where P t  is the transmitted power, G t  is the antenna gain, and d is the distance. When P t  is in watts and  d  is in meters,  E  is volts/meter. To find the power at the receiver (P r ) when the power into the transmitter antenna is known, use (5.2):

G t  and G r  are the transmitter and receiver antenna gains, and λ is the wavelength.

Range can be calculated on this basis at high UHF and microwave frequencies when high-gain antennas are used, located many wavelengths above the ground. Signal strength between the earth and a satellite, and between satellites, also follows the inverse distance law, but this case isn’t in the category of short-range communication! At microwave frequencies, signal strength is also reduced by atmospheric absorption caused by water vapor and other gases that constitute the air.

Open Field Propagation Although the formulas in the previous section are useful in some circumstances, the actual range of a VHF or UHF signal is affected by reflections from the ground and surrounding objects. The path lengths of the reflected signals differ from that of the line-of-sight signal, so the receiver sees a combined signal with components having different amplitudes and phases.

The reflection causes a phase reversal. A reflected signal having a path length exceeding the line-of-sight distance by exactly the signal wavelength or a multiple of it will almost cancel completely the desired signal (“almost” because its amplitude will be slightly less than the direct signal amplitude). On the other hand, if the path length of the reflected signal differs exactly by an odd multiple of half the wavelength, the total signal will be strengthened by “almost” two times the free space direct signal.

In an open field with fl at terrain there will be no reflections except the unavoidable one from the ground. It is instructive and useful to examine in depth the field strength versus distance in this case. The mathematical details are given in the Mathcad worksheet “Open Field Range.”

In  Figure 5.1  we see transmitter and receiver antennas separated by distance  d  and situated at heights h 1  and h 2 . Using trigonometry, we can find the line of sight and reflected signal path lengths d 1  and d 2 . Just as in optics, the angle of incidence equals the angle of reflection θ. We get the relative strength of the direct signal and reflected signal using the inverse path length relationship. If the ground were a perfect mirror, the relative reflected signal strength would exactly equal the inverse of d 2 . In this case, the reflected signal phase would shift 180 degrees at the point of reflection. However, the ground is not a perfect reflector. Its characteristics as a reflector depend on its conductivity, permittivity, the polarization of the signal, and its angle of incidence.

5.1. Open field signal paths

In the Mathcad worksheet we have accounted for polarization, angle of incidence, and permittivity to find the reflection coefficient, which approaches -1 as the distance from the transmitter increases. The signals reaching the receiver are represented as complex numbers, since they have both phase and amplitude. The phase is found by subtracting the largest interval of whole wavelength multiples from the total path length and multiplying the remaining fraction of a wavelength by 2π radians, or 360 degrees.

Figure 5.2  gives a plot of relative open field signal strength versus distance using the following parameters : Polarity: horizontal Frequency: 300 MHz Antenna heights: both 3 meters Relative ground permittivity: 15

5.2. Field strength versus range at 300 MHz

Also shown is a plot of free space field strength versus distance (dotted line). In both plots, signal strength is referenced to the free space field strength at a range of 3 meters. Notice in Figure 5.2 that, up to a range of around 50 meters, there are several sharp depressions of field strength, but the signal strength is mostly higher than it would be in free space. Beyond 100 meters, signal strength decreases more rapidly than for the free space model. Whereas there is an inverse distance law for free space, in the open field beyond 100 meters (for these parameters) the signal strength follows an inverse square law. Increasing the antenna heights extends the distance at which the inverse square law starts to take effect. This distance, dm, can be approximated by

where h 1  and h 2  are the transmitting and receiving antenna heights above ground and λ is the wavelength, all in the same units as the distance d m . In plotting Figure 5.2, we assumed horizontal polarization. Both antenna heights, h 1  and h 2 , are 3 meters. When vertical polarization is used, the extreme local variations of signal strengths up to around 50 meters are reduced because the ground reflection coefficient is less at larger reflection angles. However, for both polarizations, the inverse square law comes into effect at approximately the same distance. This distance in Figure 5.2 where λ is 1 meter is, from equation (5.3): d m  _ (12 _ 3 _ 3)/ λ _ 108 meters. In Figure 5.2 we see that this is approximately the distance where the open-field field strength falls below the free-space field strength.

Part 2  will cover Multipath Phenomena Part 3 will cover Diversity Techniques

References Gibson, J. D. (ed.), The Mobile  Communications  Handbook, CRC Press, Inc., 1996. Rappaport, T. S., Wireless Communications, Principles and Practice, Prentice Hall, Upper Saddle River, NJ, 1996. Spix, G. J., “Maxwell’s Electromagnetic Field Equations,” unpublished tutorial, copyright 1995 (http://www.connectos.com/spix/rd/gj/nme/maxwell.htm).

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Radio Waves

WHAT ARE RADIO WAVES?

Radio waves have the longest wavelengths in the electromagnetic spectrum. They range from the length of a football to larger than our planet. Heinrich Hertz proved the existence of radio waves in the late 1880s. He used a spark gap attached to an induction coil and a separate spark gap on a receiving antenna. When waves created by the sparks of the coil transmitter were picked up by the receiving antenna, sparks would jump its gap as well. Hertz showed in his experiments that these signals possessed all the properties of electromagnetic waves.

You can tune a radio to a specific wavelength—or frequency—and listen to your favorite music. The radio "receives" these electromagnetic radio waves and converts them to mechanical vibrations in the speaker to create the sound waves you can hear.

RADIO EMISSIONS IN THE SOLAR SYSTEM

Astronomical objects that have a changing magnetic field can produce radio waves. The radio astronomy instrument called WAVES on the WIND spacecraft recorded a day of bursts of radio waves from the Sun's corona and planets in our solar system.

Data pictured below show emissions from a variety of sources including radio bursts from the Sun, the Earth, and even from Jupiter's ionosphere whose wavelengths measure about fifteen meters in length. The far right of this graph shows radio bursts from the Sun caused by electrons that have been ejected into space during solar flares moving at 20% of the speed of light.

RADIO TELESCOPES

Radio telescopes look toward the heavens to view planets, comets, giant clouds of gas and dust, stars, and galaxies. By studying the radio waves originating from these sources, astronomers can learn about their composition, structure, and motion. Radio astronomy has the advantage that sunlight, clouds, and rain do not affect observations.

Since radio waves are longer than optical waves, radio telescopes are made differently than the telescopes used for visible light. Radio telescopes must be physically larger than an optical telescopes in order to make images of comparable resolution. But they can be made lighter with millions of small holes cut through the dish since the long radio waves are too big to "see" them. The Parkes radio telescope, which has a dish 64 meters wide, cannot yield an image any clearer than a small backyard optical telescope!

A VERY LARGE TELESCOPE

In order to make a clearer, or higher resolution, radio image, radio astronomers often combine several smaller telescopes, or receiving dishes, into an array. Together, these dishes can act as one large telescope whose resolution is set by the maximum size of the area. The National Radio Astronomy Observatory's Very Large Array (VLA) radio telescope in New Mexico is one of the world's premier astronomical radio observatories. The VLA consists of 27 antennas arranged in a huge "Y" pattern up to 36 km across (roughly one-and-one-half times the size of Washington, DC).

The techniques used in radio astronomy at long wavelengths can sometimes be applied at the shorter end of the radio spectrum—the microwave portion. The VLA image below captured 21-centimeter energy emissions around a black hole in the lower right and magnetic field lines pulling gas around in the upper left.

THE RADIO SKY

If we were to look at the sky with a radio telescope tuned to 408 MHz, the sky would appear radically different from what we see in visible light. Instead of seeing point-like stars, we would see distant pulsars, star-forming regions, and supernova remnants would dominate the night sky.

Radio telescopes can also detect quasars. The term quasar is short for quasi-stellar radio source. The name comes from the fact that the first quasars identified emit mostly radio energy and look much like stars. Quasars are very energetic, with some emitting 1,000 times as much energy as the entire Milky Way. However, most quasars are blocked from view in visible light by dust in their surrounding galaxies.

Astronomers identified the quasars with the help of radio data from the VLA radio telescope because many galaxies with quasars appear bright when viewed with radio telescopes. In the false-color image below, infrared data from the Spitzer space telescope is colored both blue and green, and radio data from the VLA telescope is shown in red. The quasar-bearing galaxy stands out in yellow because it emits both infrared and radio light.

Next: Microwaves

National Aeronautics and Space Administration, Science Mission Directorate. (2010). Radio Waves. Retrieved [insert date - e.g. August 10, 2016] , from NASA Science website: http://science.nasa.gov/ems/05_radiowaves

Science Mission Directorate. "Radio Waves" NASA Science . 2010. National Aeronautics and Space Administration. [insert date - e.g. 10 Aug. 2016] http://science.nasa.gov/ems/05_radiowaves

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Radio Waves

Radio waves are a type of electromagnetic radiation. A radio wave has a much longer wavelength than visible light. Humans use radio waves extensively for communications.

This radio tower has both rectangular and circular antennas to transmit and receive radio frequency energy.

Photo by Christina Telep on Unsplash

The wavelengths of radio waves range from a few millimeters (tenths of inches) to hundreds of kilometers (hundreds of miles). Visible light, for comparison, has wavelengths in the 400 to 700 nanometer range, about 5,000 times shorter than the shortest wavelength radio waves. Radio waves oscillate at frequencies between a few kilohertz (kHz or thousands of hertz) and a few terahertz (THz or 1012 hertz). "Far infrared" radiation borders radio waves along the electromagnetic spectrum  and has slightly higher energy and shorter wavelengths than radio waves.

Microwaves are short wavelength radio waves which we use for cooking and for communication. Microwaves have wavelengths between a few millimeters and tens of centimeters (tenths of inches to tens of inches).

Various frequencies of radio waves are used for television and FM and AM radio broadcasts, military communications, mobile phones, ham radio, wireless computer networks, and numerous other communications applications.

Most radio waves pass freely through Earth's atmosphere. However, some frequencies can be reflected or absorbed by the charged particles in the ionosphere .

© 2018 UCAR with portions adapted from Windows to the Universe (© 2005 NESTA)

• Electromagnetic (EM) Spectrum

2 minute read

Radio Waves

Propagation of radio waves.

Radio waves travel by three different routes from their point of propagation to their point of detection. These three routes are through the troposphere, through the ground, and by reflection off the ionosphere. The first of these routes is the most direct. A radio wave generated and transmitted from point A may travel in a relatively straight line through the lower atmosphere to a second point, B, where its presence can be detected by a receiver. This "line of sight" propagation is similar to the transmission of a beam of light from one point to another on Earth's surface. And, as with light, this form of radio wave propagation is limited by the curvature of Earth's surface.

This description is, however, overly simplified. Radio waves are deflected in a number of ways as they move through the troposphere. For example, they may be reflected, refracted, or diffracted by air molecules through which they pass. As a consequence, radio waves can actually pass beyond Earth's optical horizon and, to an extent, follow Earth's curvature.

Line-of-sight transmission has taken on a new dimension with the invention of communications satellites. Today a radio wave can be aimed at an orbiting satellite traveling in the upper part of the atmosphere. That satellite can then retransmit the signal back to Earth's surface, where it can be picked up by a number of receiving stations. Communications satellites can be of two types. One, a passive satellite, simply provides a surface off which the radio wave can be reflected. The other type, an active satellite, picks up the signal received from Earth's surface, amplifies it, and then retransmits it to ground-based receiving stations.

Since radio waves are propagated in all directions from a transmitting antenna , some may reflect off the ground to the receiving antenna, where they can be detected. Such waves can also be transmitted along Earth's surface in a form known as surface waves. Radio waves whose transmission takes place in connection with Earth's surface may be modified because of changing ground conditions, such as irregularities in the surface or the amount of moisture in the ground.

Finally, radio waves can be transmitted by reflection from the ionosphere. When waves of frequencies up to about 25 megahertz (sometimes higher) are projected into the sky, they bounce off a region of the ionosphere known as the E layer. The E layer is a region of high electron density located about 50 mi (80 km) above Earth's surface. Some reflection occurs off the F layer of the ionosphere also, located about 120 mi (200 km) above Earth's surface. Radio waves reflected by the ionosphere are also known as sky waves.

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Catch a Wave: Radio Waves and How They Work

About the Author: Andrew Schnickel

In Fall 2000, Andrew Schnickel was a Junior majoring in CECS who enjoyed running barefoot in the rain.

Introduction

You are driving down the open road, not a care in the world. It’s a sunny summer day and you have the car windows down. Everything seems almost perfect, yet something is missing. You turn on the radio, tune it to your favorite station (see Fig. 1), and instantly the car fills with sound. Suddenly, everything feels right. Radio is something most of us take for granted. Have you ever stopped to consider how a radio works? How does a radio know which station to play? What is AM, what is FM, and how are the two different? Why does FM sound better, but AM can be heard farther away? You have probably experienced a time when finding a particular radio station has been difficult. There are many factors involved in finding and receiving stations, factors such as modulation, broadcasting power, time of day, and geographical location. The key to getting the most out of your radio is to understand how radio works and how engineering has played a part in the development of a device which most of us use everyday.

Radio Basics

Am and fm differences.

Two characteristics, amplitude and frequency (see Fig. 2), mark the difference between AM and FM radio. AM stands for amplitude modulation, which means the amplitude of the radio signal is used to encode information. FM denotes frequency modulation, which uses a change in frequency to encode information. From this you can see that both AM and FM radio use modulation to encode information. “Modulation is the variation of some property of the radio carrier in a manner that conveys information” [1].

Modulation Explained

Amplitude modulation (am), frequency modulation (fm), sound quality and performance, signal strength, frequency range, interference, broadcast range.

• [1] J.J. Carr. “Elements of Electronic Communications.” Reston, Virginia: Reston Publishing Company, Inc., 1978.
• [2] R.S. Carson. “Radio Communications Concepts: Analog.” New York: John Wiley & Sons, 1990.
• [3] Editors and Engineers  The “Radio” Handbook.  Los Angeles: Editors and Engineers, 1942.
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What radio waves can travel through and what they cannot.

When Guglielmo Marconi began research into long-range radio in the late 19th century, the scientific community was skeptical. That he succeeded in transmitting radio signals more than 2000km met with much astonishment. What he had unwittingly done was discover the ionosphere, a layer in the atmosphere that is ionized to a degree by the sun’s ultraviolet rays. 

This affects radio waves and reflects them to earth, which bounces them upwards again. This repeated sequence enables us to transmit radio waves around the world. So, if radio waves can’t penetrate the ionosphere, what else will hinder them? Let’s look at what radio waves consist of, and then we’ll talk about their ability or otherwise to pass through certain materials.

Radio Waves Explained 

Developments in radio technology have brought us a long way in the past 100 years and more, but how do radio waves work? Before Marconi’s discovery, scientists believed that radio signals could only travel a straight line, as they were known to do in free space. Hence, there was a necessity for tall towers from which to transmit signals. A 100m high tower, for example, could send a signal up to around 30km, the limit set by the curvature of the earth. Marconi showed that radio waves could be bent and bounced by the ionosphere.

Radio waves form a small part of what is known as the electromagnetic spectrum (EMS). This consists not just of radio waves but also other forms of electromagnetic radiation such as gamma rays, microwaves, visible light, and more. Radio waves have among the longest wavelengths on the EMS. Imagine a rope held between two people. Move the rope up and down at one end, and a waveform moves across the rope.

This is what a radio wave would look like if we could see it. The wavelength is the distance between two ‘peaks’ in the wave. We also have to consider the frequency, which the number of waves that pass a given point in a set time. This is vital to understand so that different transmissions do not interfere with each other. 

Television signals are radio waves, but they are broadcast in a different frequency range to the radio.

What has this to do with the materials that radio waves can and cannot pass through? Let’s look more closely at this. 

What Can’t Radio Waves Pass-Through?

We’ve already seen that radio waves do not penetrate the ionosphere. We should explain that this refers to radio waves of up to 40MHz, the highest frequency bounced off the ionosphere. Why do radio waves bounce back?

As we mentioned, the ionosphere is partly ionized. 

This is because the UV light from the sun causes electrons to shake free from atoms, leaving a large proportion of free electrons hanging about in the ionosphere. When a radio wave hits the ionosphere, the energy created by the free electrons, as a result, causes the radio waves to be repelled or reflected. 

So, we know that radio waves up to 40MHz won’t pass through an ionized layer in the atmosphere. Remember that we explained the perceived limitations of radio transmissions before Marconi’s discovery. A line of sight was necessary, without obstructions, for successful transmission.

In some radio transmissions, interference can be caused by the waves being blocked by materials that they cannot penetrate. While radio waves can move through wood, bricks, and concrete to a certain extent, these materials can still cause interference. This is because radio waves meeting such objects can be reflected from buildings and other large structures, just like the ionosphere. This explains why the reception of radios and other radio frequency devices such as smartphones vary between locations.

For the record, as it covers a large bandwidth necessarily, television signals need to be above 40MHz so they cannot be bounced off the ionosphere, hence the tall towers in high locations used for TV transmissions. 

Radio waves will also be adversely affected by metal and water, which they cannot pass through. That is because water and metal are both electrical conductors. Like the ionosphere, metal and water contain many free electrons, which will vibrate when a radio wave hits the surface, and the wave will bounce back. 

To summarize, we now know that radio waves are affected by free electrons in electrical conductors such as metals and water and cannot pass through these materials. 

We have discovered that they can pass through non-conducting materials quite well but are losing propagation power with reflecting surfaces such as water, metal, and other materials. Furthermore, worldwide radio transmission is made possible by the radio waves being reflected by the ionosphere and the earth in a continuous sequence, allowing for transmissions to be made across great distances. 

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August 17, 2024

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Scientists discover phenomenon impacting Earth's radiation belts

by Rod Boyce, University of Alaska Fairbanks

Two University of Alaska Fairbanks scientists have discovered a new type of "whistler," an electromagnetic wave that carries a substantial amount of lightning energy to the Earth's magnetosphere.

The research is published today in Science Advances .

Vikas Sonwalkar, a professor emeritus, and Amani Reddy, an assistant professor, discovered the new type of wave. The wave carries lightning energy, which enters the ionosphere at low latitudes, to the magnetosphere . The energy is reflected upward by the ionosphere's lower boundary, at about 55 miles altitude, in the opposite hemisphere.

It was previously believed, the authors write, that lightning energy entering the ionosphere at low latitudes remained trapped in the ionosphere and therefore was not reaching the radiation belts . The belts are two layers of charged particles surrounding the planet and held in place by Earth's magnetic field.

"We as a society are dependent on space technology ," Sonwalkar said. "Modern communication and navigation systems , satellites, and spacecraft with astronauts aboard encounter harmful energetic particles of the radiation belts, which can damage electronics and cause cancer.

"Having a better understanding of radiation belts and the variety of electromagnetic waves, including those originating in terrestrial lightning, that impact them is vital for human operations in space," he said.

Sonwalkar and Reddy's discovery is a type of whistler wave they call a "specularly reflected whistler." Whistlers produce a whistling sound when played through a speaker.

Lightning energy entering the ionosphere at higher latitudes reaches the magnetosphere as a different type of whistler called a magnetospherically reflected whistler, which undergoes one or more reflections within the magnetosphere.

The ionosphere is a layer of Earth's upper atmosphere characterized by a high concentration of ions and free electrons. It is ionized by solar radiation and cosmic rays , making it conductive and crucial for radio communication because it reflects and modifies radio waves.

Earth's magnetosphere is a region of space surrounding the planet and created by Earth's magnetic field. It provides a protective barrier that prevents most of the solar wind's particles from reaching the atmosphere and harming life and technology.

Sonwalkar and Reddy's research shows that both types of whistlers—specularly reflected whistlers and magnetospherically reflected whistlers—coexist in the magnetosphere.

In their research, the authors used plasma wave data from NASA's Van Allen Probes, which launched in 2012 and operated until 2019, and lightning data from the World Wide Lightning Detection Network.

They developed a wave propagation model that, when considering specularly reflected whistlers, showed the doubling of lightning energy reaching the magnetosphere.

Review of plasma wave data from the Van Allen Probes showed that specularly reflected whistlers are a common magnetospheric phenomenon.

The majority of lightning occurs at the low latitudes, which are tropical and subtropical regions prone to thunderstorm development.

"This implies that specularly reflected whistlers probably carry a greater part of lightning energy to the magnetosphere relative to that carried by magnetospherically reflected whistlers," Sonwalkar said.

The impact of lightning -generated whistler waves on radiation belt physics and their use in remote sensing of magnetospheric plasma have been researched since the 1950s.

Sonwalkar and Reddy are with the Department of Electrical and Computer Engineering in the UAF College of Engineering and Mines. Reddy is also affiliated with the UAF Geophysical Institute.

Sonwalkar and Reddy's research is supported by grants from the National Science Foundation and NASA EPSCoR, the Established Program to Stimulate Competitive Research.

Journal information: Science Advances

Provided by University of Alaska Fairbanks

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Turns out you can transmit sound in a vacuum, just not very far

For the first time, researchers were able to transmit, or "tunnel," sound waves across extremely small distances between two crystals in a vacuum.

For the first time, scientists have shown that sound can travel through the emptiness of a vacuum. However, the rule-breaking trick requires specific circumstances and can only be carried out over extremely small distances.

The iconic tagline of the 1979 sci-fi film "Alien" tells us that "in space no one can hear you scream." This was based on the fact that space is a vacuum, a region devoid of any particles. Sound waves travel by vibrating through the particles of a medium, such as air or water, from a source to a receiver. So in a vacuum, there is no travel medium. (Outer space is not actually a total vacuum because it does contain small amounts of gas, plasma and other particles. But this matter is surrounded by vast swathes of emptiness.)

But in a new study, published July 14 in the journal Communications Physics , researchers showed that sound can move through a vacuum. Unfortunately for space explorers being hunted by aliens, this does not extend to human screams.

In the new experiment, researchers transmitted, or "tunneled," sound waves across a vacuum between two zinc oxide crystals by transforming the vibrating waves into ripples within an electric field between the objects.

Related: Eerie sounds triggered by plasma waves hitting Earth's magnetic field captured in new NASA sound clip

A zinc oxide crystal is a piezoelectric material, which means that when force or heat is applied to it, the material produces an electrical charge. Therefore, when sound is applied to one of these crystals, it creates an electrical charge that disrupts nearby electric fields. If the crystal shares an electric field with another crystal, then the magnetic disruption can travel from one to the other across a vacuum. The disruptions mirror the frequency of the sound waves, so the receiving crystal can turn the disruption back into a sound on the other side of the vacuum.

However, the disruptions cannot travel a distance greater than the wavelength of a single sound wave. In theory, this works with any sound no matter how small the wavelength of that sound is, as long as the gap between the crystals is small enough. 

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The method is not always reliable. In a large percentage of the experiments, the sound was not perfectly transmitted between the two crystals: parts of the wave were warped, or reflected, as it passed through the electric field, the researchers found. However, occasionally the piezoelectric crystals perfectly transmitted the entire sound wave.

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"In most cases the effect [sound transmitted] is small, but we also found situations, where the full energy of the wave jumps across the vacuum with 100 % efficiency, without any reflections," study co-author Ilari Maasilta , a material physicist at the University of Jyväskylä in Finland, said in a statement .

The finding could one day help develop microelectromechanical components, like those found in smartphones and other technology, the researchers said. 

Harry is a U.K.-based senior staff writer at Live Science. He studied marine biology at the University of Exeter before training to become a journalist. He covers a wide range of topics including space exploration, planetary science, space weather, climate change, animal behavior, evolution and paleontology. His feature on the upcoming solar maximum was shortlisted in the "top scoop" category at the National Council for the Training of Journalists (NCTJ) Awards for Excellence in 2023. 

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Scientists discover phenomenon impacting Earth's radiation belts

Two University of Alaska Fairbanks scientists have discovered a new type of "whistler," an electromagnetic wave that carries a substantial amount of lightning energy to the Earth's magnetosphere.

The research is published in Science Advances.

Vikas Sonwalkar, a professor emeritus, and Amani Reddy, an assistant professor, discovered the new type of wave. The wave carries lightning energy, which enters the ionosphere at low latitudes, to the magnetosphere. The energy is reflected upward by the ionosphere's lower boundary, at about 55 miles altitude, in the opposite hemisphere.

It was previously believed, the authors write, that lightning energy entering the ionosphere at low latitudes remained trapped in the ionosphere and therefore was not reaching the radiation belts. The belts are two layers of charged particles surrounding the planet and held in place by Earth's magnetic field.

"We as a society are dependent on space technology," Sonwalkar said. "Modern communication and navigation systems, satellites, and spacecraft with astronauts aboard encounter harmful energetic particles of the radiation belts, which can damage electronics and cause cancer.

"Having a better understanding of radiation belts and the variety of electromagnetic waves, including those originating in terrestrial lightning, that impact them is vital for human operations in space," he said.

Sonwalkar and Reddy's discovery is a type of whistler wave they call a "specularly reflected whistler." Whistlers produc e a whistling sound when played through a speaker.

Lightning energy entering the ionosphere at higher latitudes reaches the magnetosphere as a different type of whistler called a magnetospherically reflected whistler, which undergoes one or more reflections within the magnetosphere.

The ionosphere is a layer of Earth's upper atmosphere characterized by a high concentration of ions and free electrons. It is ionized by solar radiation and cosmic rays, making it conductive and crucial for radio communication because it reflects and modifies radio waves.

Earth's magnetosphere is a region of space surrounding the planet and created by Earth's magnetic field. It provides a protective barrier that prevents most of the solar wind's particles from reaching the atmosphere and harming life and technology.

Sonwalkar and Reddy's research shows that both types of whistlers -- specularly reflected whistlers and magnetospherically reflected whistlers -- coexist in the magnetosphere.

In their research, the authors used plasma wave data from NASA's Van Allen Probes, which launched in 2012 and operated until 2019, and lightning data from the World Wide Lightning Detection Network.

They developed a wave propagation model that, when considering specularly reflected whistlers, showed the doubling of lightning energy reaching the magnetosphere.

Review of plasma wave data from the Van Allen Probes showed that specularly reflected whistlers are a common magnetospheric phenomenon.

A majority of lightning occurs at the low latitudes, which are tropical and subtropical regions prone to thunderstorm development.

"This implies that specularly reflected whistlers probably carry a greater part of lightning energy to the magnetosphere relative to that carried by magnetospherically reflected whistlers," Sonwalkar said.

The impact of lightning-generated whistler waves on radiation belt physics and their use in remote sensing of magnetospheric plasma have been researched since the 1950s.

• Solar Energy
• Energy Technology
• Geomagnetic Storms
• Severe Weather
• Radiant energy
• Ionizing radiation
• Solar radiation
• Seismic wave
• Breaking wave
• Introduction to quantum mechanics
• Photoelectric effect

Story Source:

Materials provided by University of Alaska Fairbanks . Original written by Rod Boyce. Note: Content may be edited for style and length.

Journal Reference :

• Vikas S. Sonwalkar, Amani Reddy. Specularly reflected whistler: A low-latitude channel to couple lightning energy to the magnetosphere . Science Advances , 2024; 10 (33) DOI: 10.1126/sciadv.ado2657

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