01 Radio Physics v4.3

8/6/2019 01 Radio Physics v4.3

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Training materials for wireless trainers

Radio Physics


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Goals

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to introduce the fundamental concepts related toelectromagnetic waves (frequency, amplitude, speed,wavelength, polarization, phase)

to show where WiFi is placed, within the broaderrange of frequencies used in telecommunications

to give an understanding of behavior of radiowaves

(absorption, reflection, diffraction, refraction,interference)

to introduce the concepts of Fresnel zone


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What is a Wave?

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Electromagnetic Waves

Characteristic wavelength, frequency, and amplitude

No need for a carrier medium

Examples: light, X rays and radio waves

amplitude

amplitude

wavelength ( )

wavelength ( )

time: 1 second

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Quick review of unit prefixes

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Wavelength and Frequency

c = f * c = speed (meters / second)

f = frequency (cycles per second, or Hz)

= wavelength (meters)

If a wave on water travels at one meter per second,and it oscillates five times per second, then each wave

will be twenty centimeters long:

1 meter/second = 5 cycles/second * = 1 / 5 meters = 0.2 meters = 20 cm

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Wavelength and Frequency

Since the speed of light is approximately 3 x 108 m/s,we can calculate the wavelength for a given frequency.

Let us take the example of the frequency of 802.11b/gwireless networking, which is:

f = 2.4 GHz= 2,400,000,000 cycles / second

wavelength () = c / f

= 3 * 108 m/s / 2.4 * 109 s-1

= 1.25 * 10-1 m= 12.5 cm

Therefore, the wavelength of 802.11b/g WiFi is about12.5 cm.

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Electromagnetic propagation

An electromagnetic wave with a frequency of 2.4 GHzhas a wavelength of 12.5 cm

Light (or a radio signal) that travels at the speed oflight needs:

8 minutes to travel from the Sun to the Earth

1.3 seconds to travel from the Moon to Earth

333 microseconds to travel 100 km

3 nanoseconds to travel 1 meter

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Polarization

Electromagnetic waves have electrical and magneticcomponents.

The electrical and magnetic components oscillateperpendicular to each other and to the direction ofthe propagation.

direction of propagation

magnetic field

electric field

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radio

104 102 100 10-2 10-4 10-6 10-8 10-10 10-12 10-14 10-16

Approximate wavelength in meters

104 106 108 1010 1012 1014 1016 1018 1020 1022 1024

microwave

infrared

visible light

ultraviolet

X rays

gamma rays

Approximate frequency in Hz

Electromagnetic Spectrum

Approximate range for WiFi

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Perspective

FM radio

mobile phones satellite TV

telecom linksshortwaves

links withsubmarines

microwaves

radars

radiohams

AM radio

the radiospectrum

TV

GPSWiFi

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WiFi frequenciesand wavelengths

5 GHz2.4 GHz

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Behavior of radio waves

The longerthe wavelength, the further it goes

The longerthe wavelength, the better it travelsthrough and around things

The shorterthe wavelength, the more data it

can transport

There are a few simple rules of thumb that can proveextremely useful when making first plans for a wirelessnetwork:

All of these rules, simplified as they may be, are rathereasy to understand by example.

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Traveling radio waves

Absorption Reflection

Diffraction

Refraction

Interference

Radio waves do not move in a strictly straight line. Ontheir way from point A to point B, waves may besubject to:

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Absorption

Metal. Electrons can move freely in metals, and are

readily able to swing and thus absorb the energy of apassing wave.

Watermolecules jostle around in the presence ofradio waves, thus absorbing some energy.

Trees and wood absorb radio energy proportionallyto the amount of water contained in them.

Humans are mostly water: we absorb radio energyquite well!

When electromagnetic waves go through some material,they generally get weakened or dampened.

Materials that absorb energy include:

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Reflection

The rules for reflection are quite simple: the angle atwhich a wave hits a surface is the same angle at whichit gets deflected. Metal and water are excellentreflectors of radio waves.

!i

!r

!i =

!r

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Diffraction

Because of the effect of diffraction, waves will bendaround corners or through an opening in a barrier.

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The Huygens Principle

Potential spherical wavelets

Diffraction

Any point of a wavefront can be considered as the source ofa new spherical wavefront.

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Refraction

Refraction is the apparent bending of waves when they meeta material with different characteristics.

When a wave moves from one medium to another, it changesspeed and direction upon entering the new medium.

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Phase

The phase of a wave is thefraction of a cycle that the waveis offset from a reference point.It is a relative measurement thatcan be express in different ways

(radians, cycles, degrees,percentage).

Two waves that have the samefrequency and different phases

have a phase difference, andthe waves are said to be out ofphase with each other.

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Interference

When two waves of the same frequency, amplitude andphase meet, the result is constructive interference:the amplitude doubles.

When two waves of the same frequency and amplitude

and opposite phase meet, the result is destructiveinterference: the wave is annihilated.

+

+

=

=

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Line of Sight and Fresnel Zones

a free line-of-sight IS NOT EQUAL TO a free Fresnel Zone

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Fresnel zone radius

Partial obstruction

Line of sight


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Conclusions

Radio waves are very similar to light waves, but theyoccupy a volume in space, the Fresnel ellipsoid, whichshould be unobstructed for optimum reception.

Lower frequencies travel further, but at the expense ofthroughput.

The Huygens principle explains why waves can bend

around the corner.

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01 Radio Physics v4.3

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