Radio waves and the EM spectrum

 

Introduction

Since the discovery of radio waves and the electromagnetic spectrum in later part of the 19th century, we now depend on it for many applications from industrial use all the way down to daily use in our homes and personal lives.

This article introduces the basic make up of electromagnetic waves and its parameters.

What is an electromagnetic wave?

In 1867, Maxwell predicted that oscillating magnetic and electric fields coupled together could form a wave that propagates through space. It was not until 1887 that the first radio waves were produced in the laboratory by Heinrich Hertz. The below image shows the structure of the electromagnetic wave.

There are two oscillating fields - one magnetic (blue) and the other electric (red) with both being at right angles to one another. Both components make up the electromagnetic wave (EM wave). One main distinction between EM waves and ordinary sound waves is that sound waves require a medium such as air or wood to propagate, but EM wave can propagate in a vacuum. In addition all EM waves propagate through a vacuum at the speed of light, c.

EM wave parameters

There are various parameters that make up the EM wave.

Amplitude is the maximum field strength of the electric and magnetic fields (for simplicity we show the just the electric field). The stronger the field strength, the higher the peaks.

Wavelength (m) in the image above is the horizontal distance from one peak point to the next peak point. Usually this is given the symbol λ (pronounced "lambda").

Frequency, f [Hz] is the reciprocal of wave period, T. Since the electric and magnetic fields cycle between minimum and maximum values, one cycle will have a duration of T seconds.

So frequency in Hz is given by:

Phase velocity is the speed at which the wave propagates through the medium. Since the phase velocity of all EM waves travel at speed of light, c in a vacuum and by definition varies on the time taken for a point on the wave to travel one wavelength, we can write it as:

$$c = \frac{λ}{T} = {λ}f$$

where c = 300 000 000 m/s. Since this is the fixed value for waves travelling in a vacuum, the relationship between wavelength and frequency become directly related. For example:

For 100 kHz (100 000 Hz), wavelength = 3 km

For 145 MHz (145 000 000 Hz), wavelength = 2 m

For 430 MHz (430 000 000 Hz), wavelength = 70 cm

For 868 MHz (868 000 000 Hz), wavelength = 35 cm

For 2.4 GHz (2400 000 000 Hz), wavelength = 12.5 cm

Note that the value "c" is the speed of light in a vacuum. Under different mediums e.g. coaxial cable, this value can vary slightly.

 

Polarisation is which orientation or plane the wave is travelling in. Conventionally, the electric field orientation is used to determine the polarisation. Polarisation is important as the orientation of the receiving antenna must match the wave polarisation. Usually denoted as "H" (Horizontal) or "V" (Vertical) . Another less common type of polarisation is the circular type used in some satellite communications, but we will limit this topic to just horizontal and vertical for now.

The EM spectrum

Waves below 3 kHz	Radio waves	Infrared	Visible	Ultraviolet	X-rays	γ-rays
3 kHz and below	3 kHz to 3 THz	3 THz to 380 THz	380 THz to 790 THz	790 THz to 100 000 THz	100 000 THz to 10 000 000 THz	10 000 000 THz and above

The EM spectrum is a continuous spectrum from low frequency waves to high frequency waves. It is possible to divide the spectrum into wave types according to how they interact with nature.

Below 3 kHz: Waves at these low frequencies are found naturally as part of physical processes in the earth and space.

3 kHz to 3 THz: Used as the designation for RF "radio frequency" waves.

3 THz to 380 THz: This is the infrared. It lies just outside the visible range and is therefore invisible to the naked eye, but can be sensed as heat.

380 THz to 790 THz: This is the portion of the EM spectrum visible as ordinary light to humans.

790 THz to 100 000 THz is the ultraviolet (UV) just outside the visible range.

100 000 THz to 10 000 000 THz: X-rays discovered by German scientist Wilhelm Röntgen.

Over 1 000 THz: Gamma rays.

RF applications

For the RF portion of the EM spectrum, we can sub-divide this range as follows:

RF range	Frequency	Wavelength	Main applications
VLF (Very Low Frequency)	3 kHz to 30 kHz	100 km to 10 km
LF (Low Frequency)	30 kHz to 300 kHz	10 km to 1 km	Ship / aircraft beacons
MF (Medium Frequency)	300 kHz to 3 MHz	1 km to 100 m	AM radio, ship communication, amateur radio
HF (High Frequency)	3 MHz to 30 MHz	100 m to 10 m	SW broadcast, ship/aircraft, amateur radio
VHF (Very High Frequency)	30 MHz to 300 MHz	10 m to 1 m	Low power radio, FM radio, Fire crew/police communications, disaster prevention
UHF (Ultra High Frequency)	300 MHz to 3 GHz	1 m to 10 cm	Low power radio, Wireless LAN, Bluetooth, Mobile phones, TV broadcasting, MCA, amateur radio
SHF (Super High Frequency)	3 GHz to 30 GHz	10 cm to 1 cm	Satellite communications, radar
EHF (Extremely High Frequency)	30 GHz to 300 GHz	1 cm to 1 mm	Satellite, radio astronomy, radar
Sub-millimeter	300 GHz to 3 THz	1 mm to 0.1 mm

Low power radio

This is the main focus area for Circuit Design radio modules which use frequencies generally in the range 150 / 300 / 400 / 800 / 900 / 1200 / 2400 MHz (Data communication) and 800 MHz (audio and speech communication). The RF power level of Circuit Design radio modules generally do not exceed 10 mW.