Electromagnetic Radiation and Radio Waves
(Natural and Man-Made Miracles)
Electromagnetic radiation is a wonderful thing. It brings us heat and lights up our day, it brings us radio and television and carries our telephone conversations. It brings us the Sun's energy which is needed by all plants for photosynthesis and growth. It brings warmth to the inhabitants of the Earth's animal kingdom and to some of them to tan their bodies. We also use it to see through solid bodies, to find our way around the planet and to cook our food.
In a tremendous intellectual leap, in 1873 James Clerk Maxwell suggested the existence of electromagnetic waves and worked out mathematically what their properties might be before anybody had ever observed, or even thought of, such a phenomenon. Since then, communications engineers have performed miracles harnessing this radiation for a myriad of uses.
Electromagnetic radiation has the following interesting properties
- It can be found in nature or be man-made.
- It does not require a medium for propagation.
- It travels with the speed of light.
- It carries energy as it propagates. The higher the frequency, the higher the energy associated with the wave.
- It can transfer its energy to the matter on which it impinges.
- Its transferred energy may be sufficient to break chemical bonds, ionising the matter on which it impinges.
- Its propagation obeys the inverse square law.
- It can be used to carry information.
- It can be broadcast outwards to reach many locations or it can be formed into beams to reach a particular spot.
- It can be be reflected or refracted.
- It can be split and recombined to form diffraction patterns.
- It can travel great distances. The radiation resulting from a simple100 volt, 1 MHz sine wave fed into a suitable antenna can be detected as far away as the next planet.
- It travels in straight lines.
- It can be bent around the Earth's circumference by reflection from the ionosphere.
- It can pass through walls.
- It can be captured by placing a metal rod, a loop, parabolic metal dish or horn in its path and it can be launched into the atmosphere with the same tools.
Radio waves are a specific example of electromagnetic radiation. Despite all the communications benefits "electromagnetic radiation" makes possible, the name has a sinister connotation. The alternative name, "radio waves", does not seem nearly so threatening. But too much of a good thing, even water, can be dangerous if present in excessive quantities at the wrong place or time. So it is with electromagnetic radiation.
We are in fact swimming in an ocean of radio waves of various strengths. At home we have high frequency radiation coming from
- Hundreds of long wave, medium wave. short wave and UHF radio broadcasts
- Dozens of terrestrial television signals
- Television signals at microwave frequencies beamed down by satellites
- UHF signals from hundreds of mobile phones and their local base stations
- Private mobile radio signals used by the emergency services and private networks
- Television remote controls
- Remote control toys (cars and planes)
- Microwave GPS satellite navigation location signals whether we use them or not.
- Wi-Fi networks for communications between computers and computer networks
- Bluetooth connections between electronic appliances
- Laser light in CD players
- Infra red television remote controls
- Wireless security sensors
- Garage door openers
- Car door remote locking keys
- Infra red radiation from cookers and domestic heaters
- Leakage from microwave ovens
- Continuous unwanted radio frequency interference (RFI) generated by computer circuit boards and oscillators in radio reception and transmission equipment
- Random RFI due to local electrostatic discharges from motor commutators on household equipment and power tools as well as automobile ignition systems (sparking plugs).
- Random RFI due to distant electrostatic discharges from lightning strikes anywhere between the signal source and the home
And at the other end of the spectrum we have
- Very low frequency radiation from power cables, electric motors, domestic appliances, transformers and battery chargers
Depending on where we live we may also be near enough to experience signals from other sources even though we may not have the equipment to capture them
- Air traffic control systems
- Aircraft instrument landing systems
- Radar surveillance
- Microwave repeater systems used for broadband communications links
- Speed cameras
- Very low frequency radiation from electric fields radiating from high voltage electricity grid transmission lines, transformers and power cables
Closer to home we submit ourselves to high levels of radiation from medical equipment
- X Ray machines
- X Rays from CAT scanners
- Electromagnetic fields from MRI scanners
But curiously many hospitals ban the use of mobile phones because their tiny transmitters might interfere with sensitive medical equipment
Then we are all bathed in more general background sources of radiation most of which we can not avoid and some we can.
- High frequency radiation from the Sun and other artificial light sources at optical frequencies
- Infra red radiation (heat) from the Sun
- Man made heat and light sources
High energy, short wavelength electromagnetic radiation such as ultra-violet rays, X rays and gamma rays can cause ionisation of other materials when present at high enough energies and this can cause serious and permanent damage to human tissue. This radiation may be found in nuclear installations and may also be used in controlled medical treatments. Such radiation may be found in the domestic environment but fortunately not at dangerous levels
The first man-made radio waves were created in 1888 by Heinrich Hertz, three years after the world's first practical automobile was launched by Karl Benz. Before that, apart from light waves and the odd lightning discharge, there were almost no radio waves in the atmosphere. The growth of radio waves in the atmosphere in the last one and a half centuries has followed the growth of industrial development, just like the concentration of carbon dioxide in the atmosphere, but at least radio waves have not been blamed for global warming. (Not yet anyway!)
Communications and Engineering Miracles
- With all these radio signals vying for our attention, amongst a background of unwanted radiation sources, all whizzing by with a speed of 186,000 miles per second, thanks to communications engineers you can poke your mobile phone or radio antenna into the air and select just the signal that was intended just for you.
- The very limited bandwidth available within the electromagnetic spectrum, which is suitable for radio communications, accommodates millions of communications links with a collective bandwidth of many times the available bandwidth by simultaneously using the same frequencies without interfering with eachother. Another set of challenges answered by communications engineers.
- We might also expect that all the radio signals in the atmosphere would be completely scrambled with each other. Fortunately by some natural miracle the signals retain their integrity. They may be superimposed on eachother or swamp eachother and they may pick up electrical noise during their travels but they do not mix to form sum and difference frequencies as they would in a non linear device and so no miraculous engineering solution is needed to decode or operate upon the new frequency components to reconstruct the original signal. They only need to be separated from each other.
The Electromagnetic Wave
Maxwell's equations describe how electromagnetic radiation is propagated. He showed that a varying magnetic field induces an associated varying electric field perpendicular to the magnetic field and this varying electric field in turn induces an associated varying magnetic field in the plane of the initial magnetic field. Together these two varying fields form an electromagnetic wave propagating at the speed of light in a direction perpendicular to both the electric and magnetic fields as shown in the diagram below.
Source - G.R. Delpierre and B.T. Sewell, University of Capetown (Modified)
Radiation Wavelength and Frequency
The frequency f (Hertz) of the wave is inversely proportional to the wavelength λ (metres) and is given by the relationship
f = c / λ
where c is the speed of light (m/sec).
Radiation and the Inverse Square Law
The rate at which energy emanating from a fixed, constant source of electromagnetic radiation passes through a surface at a distance d from the source is proportional to 1/d2. This is known as the Inverse Square Law. It arises simply because the surface enclosing the source is a sphere, centred on the source, through which all the energy must pass and the surface area of this sphere increases as the square of the distance d from the source. Thus the energy flow (measured in Watts per square metre (W/m2)) falls off rapidly as the distance from the source increases.
Radiation and Polarisation
The individual electric and magnetic fields in an electromagnetic wave are orthogonal (at right angles) to eachother with the plane of oscillation of the fields determined by the orientation of the radiating element such as an antenna. By convention the polarisation refers to the plane of oscillation of the electric field.. In the diagram above the polarisation is vertical as represented by the direction of the electrical field E and is said to be linear.
Electromagnetic waves may also be circularly polarised, in which case, the tip of the electric field vector E, describes a helix along the direction of propagation. Such waves may be generated from two crossed dipoles fed with a 90° degree time-phase difference (phase quadrature) or by a helical antenna radiating in the direction of its axis.
The Electromagnetic Wave Spectrum
Electromagnetic waves can typically be described by any of the following three physical properties: the frequency f, wavelength λ, or photon energy E. The diagram below shows all possible frequencies of electromagnetic radiation and the corresponding photon energies and some of the applications for which they are used. The spectrum covers an enormous range with wavelengths ranging from the size of an atom to almost the size of the universe, (Over 26 orders of magnitude). The corresponding photon energies occupy a similar range, from the unmeasurable to the highly dangerous.
Wave - Particle Duality
Quantum mechanics wave - particle duality theory showed that paradoxically, electromagnetic radiation and particles of matter could exhibit both wave-like and particle-like properties but not at the same time. In practice this means that some properties of radiation can best be explained by wave theory while others can better be explained by particle theory which describes electromagnetic radiation as an energy flow carried by particles called photons, each with a characteristic energy which depends on the frequency of the radiation.
The photon energy E of a single photon associated with the electromagnetic wave increases with frequency and is given by the relationship
E = h x f (Joules) or h x c / λ
where h is Planck's constant (6.63 X 10-34 Joule seconds or 4.14 X 10−15 eV seconds) and f is the frequency of the wave and c is the velocity of light (299.8 x 106 m/sec) .
- The spectrum above shows that the individual photons in visible light have energies of a few electron Volts while the particles in cosmic rays with an equivalent frequency of around 1025 Hertz have relatively enormous energies of over 10 billion electron Volts (1.6 nanoJoules). Though a nanoJoule is very small, the total energy flow associated with the radiation is many, many times greater due to the very high number of photons making up the overall photon flux (See below).
- Below a frequency of around 100 GHz, which includes most of the spectrum used for radio communications, the energy of individual photons is almost negligible at less than 10−4 eV or 10−24 Joules.
The photon flux Φ of a radiated wave, defined as the number n of photons per second per unit area of the wave is given by
Φ = n/m2/sec
The energy E associated with the photons is given by
E = n x h x f(Joules)
The radiation intensity P or power density (radiated power per unit area) associated the photon flux is given by
P = Φ x E = n x h x f / sec / m2 (Joules / sec / m2 or Watts / m2)
The number of photons n in E Joules of energy at any frequency or wavelength is given by
n = E x h / f = E x h x λ / c
The number of photons per Joule (setting E = 1Joule) for light is given by
n = h x λ / c
Note that a the radiation intensity depends on BOTH the photon flux AND the frequency of the radiation.
- Common Light Sources
- For visible green light with a wavelength λ = 500 nm (500 x 10-9 metres)
The photon energy E = h x c / λ = (6.63 X 10-34) x (299.8 x 106) / (5 x 10-7) = 3.98 x 10-19 Joules or 2.48 eV
The number of photons per Joule of radiated energy is = 1 / ( 3.98 x 10-19) = 2.513 x 1018 (a very large number!)
- Making some gross assumptions we can calculate the rate at which photons are emitted by a 100 Watt incandescent light bulb.
- The rate energy is supplied to the lamp = 100 Watts = 100 Joules per second.
- But only about 2.25% of this energy is converted to visible light. (See Energy Efficiency) Thus the lamp emits 2.25 Joules of radiant light energy per second.
- The lamp actually emits a wide spectrum of radiation, most of which is infra red radiation but we are only considering the visible energy here which amounts to about 10% of the total radiated energy. The visible energy is emitted over the spectrum from red to violet (wavelength 7.5 X 10-7m to 3.5 X 10-7m) with varying intensity, but for the purposes of this calculation we can assume that the average wavelength of the radiation is 5.0 X 10-7m which is the wavelength of green light near the middle of the visible spectrum. See the graph of Solar Radiation which has a similar spectrum and also the Electromagnetic Wave Spectrum above.
- From the above, the rate at which visible light photons are radiated from a 100 Watt incandescent light is 2.25 x 2.517 x 1018 = 5.66 x 1018 photons per second.
- The total number of photons emitted per second over the full radiation spectrum of the light source (heat and light) will depend on the temperature of the source and will be about 10 times the number of photons contained in the visible light.
- Cosmic Radiation
Cosmic radiation is not strictly electromagnetic radiation. Cosmic rays are in fact streams of high energy particles originating from outside the earth's atmosphere. They are not homogenious and may have different constituent particles. Typically they consist mainly of protons, (positively charged Hydrogen nuclei) which make up around 89% to 90% of the stream, alpha particles (Helium nuclei) which make up around 9% of the stream, the nuclei of other heavier elements which account for about 1% of the particles and beta particles (electrons) make up the remaining 1%. Similarly the cosmic ray particles may have different energy levels with particles originating from the sun, the so called "solar wind" having relatively low energy levels of around 106 eV, while particles emanating from outside the solar system typically have energy levels ranging from about 108 to1012 eV, though energy levels of up to 1021 eV have been recorded. This is many orders of magnitude greater than the 1013 eV which the best terrestrial particle accelerator, CERN's Large Hadron Collider (LHC) can produce.
Before the invention of particle accelerators such as the cyclotron and the synchrotron, nuclear physics experimentors often used cosmic rays as the source of high energy particles in their experiments.
Being composed of sub-atomic particles, cosmic rays do not propagate with the speed of light, but at some speed close to it. Their particle energy levels are close to the photon energies in the higher frequency electromagnetic waves and simply for convenience, cosmic rays are often included in graphical representations of the electromagnetic spectrum with an equivalent wavelength or frequency for their energy levels (as in the diagram above).
Similar to nuclear radiation, the high energy cosmic ray particles can cause ionisation of materials on which they impinge and as such can have dangerous physiological effects. See Physiological Effects of Electromagnetic Radiation (below) and Physiological Effects of Nuclear Radiation.
Fortunately the earth's magnetic field deflects much of the cosmic radiation away from the earth and some of what get's through is absorbed by the earth's atmosphere. Nevertheless, cosmic radiation accounts for about 13% of all background radiation at the earths surface. The radiation dose at the earth's surface attributable to cosmic radiation amounts to about 3.6 milliSieverts (mSv) whereas the dosage from all sources of background radiation (including the nuclear decay of the earth's elements) is around 3.0 mSv in the USA and 2.0 mSv in the UK. The cosmic energy dosage however increases with altitude which can be a health hazard for airline crews and frequent fliers and is positively dangerous for astronauts. It is estmated that cosmic rays contribute to 100,000 cancer deaths per year.
- For cosmic radiation with an equivalent wavelength of 10-16 metres:
The photon energy is (6.63 X 10-34) x (299.8 x 106) / (10-16) = 1.99 x 10-9 Joules or 1.24 x 109 eV
The number of photons per Joule is = 1 / (1.24 x 109) = 5.03 x 1010
Note that as a consequence of the shorter wavelength, each photon of cosmic radiation contains 5 x 109 times as much energy as the green light photons and can consequently be much more damaging. (See following section)
By the same token, green light radiation needs correspondingly 5 x 109 more photons to make up one Joule of radiated energy than cosmic radiation because of the lower energy level of the photons emitted by green light.
Ionisation Effects of Electromagnetic Radiation
Ionisation is the breaking of chemical bonds holding matter together, releasing ions or electrons from the molecules or atoms, leaving two charged particles or ions: molecules with a net positive charge, and the free electrons with a negative charge. This can occur naturally by dissociation when salts are dissolved in aqueous solutions causing their constituent elements to separate into ions.
In the case of electromagnetic radiation ionisation occurs in a more forcible manner when matter is bombarded with high energy photons. If the photon energy is high enough it can knock electrons out of molecules or atoms leaving positively charged ions and negatively charged electrons.
The electromagnetic radiation spectrum diagram above shows how the photon energy increases with frequency and that at frequencies above the visible light spectrum, the photon energy of the radiation is sufficient to cause ionisation of the matter on which it impinges. Below the frequency of visible light, and this includes the emissions from microwave ovens and all the frequencies used for radio communications, the radiation is non-ionising since the photon energy of the radiation is so small that ionisation is not normally possible unless the intensity is exceptionally high.
Long distance radio communications depend on ionisation of the upper layers of the earth's atmosphere by cosmic rays. The resulting free ions form a conductive blanket, known as the ionosphere, which reflects radio waves enabling radio signals to reach beyond the horizon by bending around the curvature of the earth.
Physiological Effects of Electromagnetic Radiation
Ionising radiation is particularly hazardous to living organisms because its effects are painless, cumulative and latent : you can't sense that radiation damage is happening and symptoms may take up to several weeks to develop.
At frequencies above the upper end of the visible light spectrum, starting with ultra violet (UV) radiation, the photon energy becomes sufficient to cause ionisation damage to human body tissue. Overexposure can cause burns due to the heating effect of the radiation but prolonged exposure can result in chemical changes to the skin tissue. Ionisation can cause DNA mutation leading to tissue damage and the possible formation of cancerous tumours. At progressively higher frequencies, such as X-rays and above, the greater photon energy of the radiation not only causes increased damage but it penetrates deeper into the body with even more serious consequences.
Higher energy (gamma) radiation is still more dangerous. Its properies together with those of other ionising radiation are outlined in the section on Nuclear Radiation
See also Conducting Gas Plasmas
The physiological effect on the body of non-ionising radiation, (frequencies below the visible light spectrum) is the heating of the exposed tissue, often referred to as its "thermal" effect. For short exposures this is not dangerous but damage can be caused by prolonged exposure to high levels of radiation.
It is important to distinguish between electromagnetic radiation and nuclear radiation.
- Electromagnetic radiation is the propagation of energy by means of electromagnetic waves (interlinked, varying electric and magnetic fields) such as heat, light, radio waves, X rays and gamma rays, all travelling with the speed of light. It is relatively harmless below the frequency of X rays, but at X ray frequencies and above, the electomagnetic wave carries sufficient energy to cause ionisation of the materials on which it impinges and hence can be hazardous to humans and other life.
- Nuclear radiation is the flow of diiscrete, high energy sub-atomic matter particles, not waves, resulting from the natural decay of nuclear materials or from nuclear reactions such as fission and fusion. The velocity of the particles may approach, but can never reach, the speed of light. The ever present background radiation on earth is due to the decay of earthly nuclear materials found in the earth's crust but also due to debris from the extra-terrestrial fission and fusion reactions taking place on the sun and the stars in the cosmos which result in the constant bombarding of the earth by cosmic rays. Fortunately the level of background radiation is so low that the human race is able to live with it. Evolution has not however equipped us to live with high levels of nuclear radiation which could possibly occur from man made nuclear reactions here on earth. Every attempt is made to contain the radiation produced in controlled nuclear reactions employed in the electrical power industry, but very rarely things may go wrong. On the other hand, nuclear weapons depend on unfettered, runaway nuclear reactions which spread nuclear radiation indiscriminately.
The Eye - A Biological Miracle
The eye is essentially a very sensitive radio receiver and image sensor.
- It has a wide band tuner, the retina, with a bandwidth of 390 THz (TeraHertz = 1012Hz) which can detect electromagnetic radiation in the frequency range from 400 to 790 THz, (200,000 times higher than microwaves).
In more detail:
- It has an automatic gain control system, the iris, which protects against signal overload.
- It has a broadband, narrow beamwidth, directional, variable focus antenna, the lens, which captures the radiation.
- It has an automatic focusing system, accommodation by cilary muscles, which optimises the reception for different distances, from close-ups to infinity, by controlling the shape of the lens.
- It has a rangefinder function, as well as 3D vision, provided by the eyes taken in pairs, parallax between the images.
- It has an image scanning system, the rods and cones, with a resolution of 150,000 pixels/ sq. mm. which enables the relative spatial position of the sources to be identified.
- It has signal amplitude sensors, the rods, which measure pixel luminance (brightness) with a dynamic range of more than 10 million to 1
- It can detect amazingly low photon fluxes of 5 to 9 photons per millisecond. (See Photon Energy above)
- It has signal frequency sensors, the cones, which identify the pixel chrominance (colour) with a frequency range of 390 TeraHertz..
- It has a spectrum analyser display mechanism, colour. The received radiation itself has no colour. Colour is the way the eye perceives and represents the frequency of the radiation.
- It has a self cleaning and protection mechanism, the eyelid.
- It has an expected lifetime of 70 years or more.
There is no electronic equipment which comes anywhere near to this level of performance.
We could also consider that some people think it's a biological miracle that we don't all die from exposure to all the electromagnetic radiation in the atmosphere.
Radio Frequency Safety Limits
Specific Absorption Rate (SAR)
The magnitude of the effect of radio frequency radiation on the body depends on the intensity and duration of the radiation. The specific absorption rate (SAR) is commonly used to measure the power absorbed by the body from microwave ovens, mobile phones and MRI scans. It is a measure of the potential thermal effects on the patient's tissue due to exposure of the body to electromagnetic radiation and is defined as the power absorbed per mass of tissue in Watts per kilogram. It is not the power emitted by the source. The actual energy absorbed by the body depends on its distance from the source as well as the shapes of the source and the body and their relative exposure and orientation towards each other
The tolerance of the body to radio frequency radiation depends on which part is involved, vital organs being much more susceptible to damage than the body's extremities. The SAR may be averaged either over the whole body, or over a small sample volume weighing a few grams.
- For mobile phones, for which absorption of RF energy by the body is an unwanted consequence, the safe SAR limit is specified by the FCC in the USA as 1.6 W/kg (averaged over 1 gram of tissue) whereas in Europe the IEC specifies 2.0 W/kg (averaged over 10 grams)
- For MRI scans, whose function depends on the absorption of electromagnetic energy by the body, the US, FDA limits are:
- 4 W/kg averaged over the whole body for any 15-minute period
- 3 W/kg averaged over the head for any 10-minute period; or
- 8 W/kg in any gram of tissue in the extremities for any period of 5 minutes.
For reference an SAR of 2 W/kg would take 2 days to melt a kilogram of ice. (Since the latent heat of fusion of water is 334 kJ/Kg, it will require 334,000 Watt seconds of energy to melt. With a 2 Watt source it will take 167,000 seconds)
See also Nuclear Radiation Effects and Safety Limits
A Word About Microwave Ovens
Microwave ovens operate in the same 2.4 GHz frequency band as Wi-Fi, Bluetooth and ZigBee wireless communications systems but at a much higher power. (See power level comparisons below). Since the frequencies and the associated quantum energies used by all of these applications, including microwave ovens, are a million times lower than those of x-rays (see the radiation spectrum above), they cannot produce the damaging ionisation associated with high frequency electromagnetic radiation.
The microwave energy used in the oven does not actually transform or oxidise the organic compounds which make up the ingredients in the food as in normal cooking. It merely excites dipole molecules, mainly water and fats, contained in the food increasing their kinetic energy. Dipole molecules are those with a positive charge at one end and a negative charge at the other. The alternating electric field of the microwaves causes the molecules to rotate with each cycle as they try to align themselves with the field. As the oscillating molecules become involved in collisions with other molecules, putting them also into motion, the agitation causes the molecules to heat up. This heat is passed on by conduction to everything in contact with the dipole molecules so that the heat spreads through the food finally heating up the container or plate holding the food.
At 2,450 MHz, the frequency of the microwave radiation is in the non ionising region of the electromagnetic spectrum and hence the radiation does not have the energy to cause tissue damage by ionising and breaking down the molecules or atoms in the food. Though human tissue also contains dipole molecules, a short exposure to the microwave radiation produced by a microwave oven is likely to be much less damaging than momentarily putting your hand on a hot stove. To make doubly sure of safety, microwave ovens have safety interlocks which switch off the magnetron completely if the oven door is open and in addition they incorporate shielding to ensure that the maximum leakage of radiation from the oven when the door is closed is limited to agreed national standards. The United States FDA requirement states that new ovens may not leak microwave radiation in excess of 1 mW/cm2 at 5 cm (2 inches) from the oven surface and that, once placed into service, the maximum permissible microwave radiation is 5 mW/cm2 at 5 cm from the oven surface.
Some Facts to Put the Power Levels of Received RF Radiation into Context.
See also Solar Power