The Electromagnetic Spectrum by Year 12 Physics Students

Radio waves

By Jemin Jayapal 12O

https://www.youtube.com/watch?v=al7sFP4C2TY

Radio waves are a type of electromagnetic wave and are part of the electromagnetic spectrum. Like all EM waves they travel at a speed of 3 x E8 m/s in a vacuum but their wavelength range is between 1 millimetre and 100 kilometres (frequency range of 300 GHz to 3 kHz). (1)

(1) https://www.boundless.com/physics/textbooks/boundless-physics-textbook/electromagnetic-waves-23/the-electromagnetic-spectrum-165/radio-waves-592-11171/

Radio waves are the lowest-energy, lowest-frequency and longest-wavelength of all the electromagnetic waves. There are naturally occurring radio waves made by lightning or by astronomical objects and some are artificially produced when an alternating current flows in an aerial.

Different frequencies of radio waves have different propagation characteristics in the Earth’s atmosphere; long waves may cover a part of the Earth very consistently, shorter waves can reflect off the ionosphere and travel around the world, and much shorter wavelengths bend or reflect very little and travel on a line of sight.

http://en.wikipedia.org/wiki/Radio_wave

image

The above left image shows a rough plot of Earth’s atmospheric transmittance (or opacity) to radio waves and the above right image shows the effect of the ionosphere on radio waves (6)

(6) http://www.bbc.co.uk/schools/gcsebitesize/science/aqa_pre_2011/radiation/the_electromagnetic_spectrumrev6.shtml

The longer the wavelength the better the radio waves can spread around large objects by diffraction. (2)

(2) http://revisionworld.com/gcse-revision/physics/electromagnetic-radiation/radio-waves-microwaves

A radio programme receiver does not need to be directly in view of the transmitter to receive programme signals. For low frequency radio waves diffraction can allow them to be received behind hills, although repeater stations are often used to improve the quality of the signals.

The lowest frequency radio waves are also reflected from an electrically charged layer of the upper atmosphere, called the Ionosphere. This means that they can reach receivers that are not in the line of sight because of the curvature of the Earth’s surface.

Radio waves possess certain properties:

They are reflected by metal surfaces;

They can pass through glass, plastics and brick;

Some pass through the atmosphere;

Transmission is affected by wave effects such as reflection, refraction, diffraction and interference.” (3)

(3) http://revisionworld.com/gcse-revision/physics/electromagnetic-radiation/radio-waves-microwaves

History

James Clerk Maxwell first predicted and theorized radio waves in 1865 through the use of some complex mathematical calculations. Unfortunately his predictions had no solid evidence of their existence at the time. (4) He had observed the properties of light and proposed equations that interlinked them and radio waves that travelled in space.

(4) Harman, Peter Michael (1998). The natural philosophy of James Clerk Maxwell. Cambridge, England: Cambridge University Press. p. 6. ISBN 0-521-00585-X.

In 1887 Heinrich Hertz generated radio waves, which proved the reality of Maxwell’s electromagnetic waves.

For his radio wave transmitter he used a high voltage induction coil, a condenser (capacitor, Leyden jar) and a spark gap – whose poles on either side are formed by spheres of 2 cm radius – to cause a spark discharge between the spark gap’s poles oscillating at a frequency determined by the values of the capacitor and the induction coil.

image

Conceptual Schematic of Hertz’s Experiment

To prove there really was radiation emitted, it had to be detected. Hertz used a piece of copper wire, 1 mm thick, bent into a circle of a diameter of 7.5 cm, with a small brass sphere on one end, and the other end of the wire was pointed, with the point near the sphere. He added a screw mechanism so that the point could be moved very close to the sphere in a controlled fashion. This “receiver” was designed so that current oscillating back and forth in the wire would have a natural period close to that of the “transmitter” described above. The presence of oscillating charge in the receiver would be signaled by sparks across the (tiny) gap between the point and the sphere (typically, this gap was hundredths of a millimeter).

So in this experiment Hertz confirmed Maxwell’s theories about the existence of electromagnetic radiation.

In more advanced experiments, Hertz measured the velocity of electromagnetic radiation and found it to be the same as the light’s velocity. He also showed that the nature of radio waves’ reflection and refraction was the same as those of light and established beyond any doubt that light is a form of electromagnetic radiation obeying the Maxwell equations.

Summing up Hertz’s importance: his experiments would soon trigger the invention of the wireless telegraph and radio by Marconi and others and TV.

In recognition of his work, the unit of frequency – one cycle per second – is named the “hertz”, in honor of Heinrich Hertz.

http://www.juliantrubin.com/bigten/hertzexperiment.html

How are radio waves produced?

To produce a Radio wave you need to apply a direct electrical current to a wire. This produces a magnetic field which is sent outwards as a wave from the wire.

When the current is removed, the field collapses which again sends a wave.

If the current is applied and removed over and over for a period of time, a series of waves is propagated at a discrete frequency. An easier way of doing this is to use an alternating current.

http://revisionworld.com/gcse-revision/physics/electromagnetic-radiation/radio-waves-microwaves#XBGrMeefFlPyfPp9.99

If the current changes polarity or direction repeatedly, that could make waves, too. This phenomenon is the basis of electromagnetivity and basically describes how radio waves are created within transmitters.

Other kinds of electromagnetic radiation, including radio waves, are made by natural processes such as the nuclear reactions in a star.

http://www.qrg.northwestern.edu/projects/vss/docs/communications/3-how-do-you-make-a-radio-wave.html

Uses of Radio Waves

There are a variety of applications of Radio waves. Some are household and some industrial. The primary and most common use of Radio waves is to transmits information from one place to another, using transmitters and receivers. This can be seen in Radio, Television etc. Radio Waves are also used in Radar, as they are sent and the radio waves are able to reflect off the objects from in their path. ‘For navigation of ships and aircraft the radio range, radio compass (or direction finder), and radio time signals are widely used. Radio signals sent from global positioning satellites can also be used by special receivers for a precise indication of position’. Various remote-control devices, including rocket and artificial satellite operations systems and automatic valves in pipelines, are activated by radio signals’ (5)

https://www.boundless.com/physics/textbooks/boundless-physics-textbook/electromagnetic-waves-23/the-electromagnetic-spectrum-165/radio-waves-592-11171/

(5) http://www.infoplease.com/encyclopedia/science/radio-uses-radio-waves.html

image

http://pixgood.com/what-uses-radio-waves.html

Benefits of Radio Waves

The transmitters that are transmitting radio waves do not have to be directly in line or in view of each other. Due to one of the properties of radio waves, they can be reflected by the ionosphere, therefore enabled radio waves to travel across the curvature of the earth. (6)

Radio waves are also able to diffract around objects (low frequency radio waves). So this means that although there is an object blocking the path of microwave, therefore it is able to diffract (spread out) around the object. Although this cause deterioration of signal and could reduce the quality of the signal. (6)

The other benefits of radio waves links it to the uses such as communication, navigation and accuracy (linking to long distance transmission)

Negatives on Radio Waves

Interference is an issue with radio waves. This can mean interference between radio waves but also it could mean interference due to bad weather conditions etc. This can affect the quality of the signal received thus the signal would be distorted.

Diffraction of Radio waves although this is an advantage of radio waves it can also be a disadvantage as it can reduce the quality of the waves as it is spreading out.

Finally, radio communication relies on a very limited spectrum of bandwidth frequencies. This is why commercial radio stations sometimes seem to overlap or blend together (7)

(7) http://www.ehow.co.uk/list_6181295_disadvantages-radio-communication.html

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