Electromagnetic Wave Propagation

Cycle

One Complete rotation through all of the values of magnitude and direction of oscillation

Frequency

The number of cycles per second. One cycle per second is called 1 Hertz

Wavelength

The physical distance between one part of a cycle and the identical part of the following cycle.

Speed of the wavelength

300,000,000 m/s

Wavelength calculations

Speed of the wave / Frequency

Frequency calculations

Speed of the wave / Wavelength

Amplitude

Maximum displacement of the oscillation away from the mean value during a cycle. (Strength of oscillation)

Phase

Amplitude of wave corresponding to an angular displacement

Phase Difference

Relationship between 2 cycles of the same frequency ("Out of Phase" = Phase difference)

Attenuation

Reduction in signal strength as distance from transmitter is increased

Types of Surfaces and there effects

Due to varying surface conductivities, a wave will travel further over sea than over dry land

Waves tend to bend towards least favourable area (bend [slow down] towards land when crossing a coast)

Modulation

The changing of frequency or amplitude by superimposing an audio frequency on a carrier frequency

Amplitude Modulation

- Frequency kept constant

- Amplitudes added to reflect modulation

Disadvantages of Amplitude Modulation

- More power required to produce AM

- For a given transmitter power, un-modulated signals will travel further than a modulated signal

- AM signals are more vulnerable to atmospheric and similar interference (static)

Frequency Modulation

- Amplitude remains constant

- Frequency is altered by superimposing modulating AND carrier waves

Advantages of Fm over Am

- Less power required

- Due to constant amplitude of transmissions, static is eliminated

Disadvantages of FM

- Radio receiver is more complex

- 'Extra' frequencies must be available to accommodate frequency modulation

Phase Modulation

: Uses the phase difference of 2 signals on the same frequency to carry intelligence.

Phase Modulation is a modulation in which the phase of the carrier wave changes according to the instantaneous amplitude of the modulating signal keeping amplitude and frequency constant

Pulse Modulation

The transmission of energy in concentrated pulses instead of a continuous wave.

Single Sideband

- Sidebands are additional frequencies

- Occur when a carrier wave is modulated by a frequency lower and higher than itself (audio frequencies)

- Carrier wave modulation consists of:

Carrier Frequency

Carrier Frequency + Audio Frequency

Carrier Frequency - Audio Frequency

- SSB = removing either 2 or 3 to reduce bandwidth usage.

3 Types of Radio Wave Propagation

- Surface Wave - Sky Wave - Direct Wave

Surface / Ground waves

- Follow earth's curvature due to:

- Diffraction and Scattering

- Caused due to obstacles redirecting radio waves downward

- Influences lower frequency waves more

Wave Tilting

- Wave contacts earth and slows down

- Responds by tilting down but energy lost = increased attenuation.

Surface/Ground Wave Restrictions, Advantages and Dis-Advantages

Restricted to VLF, LF and MF frequency ranges

Disadvantages therefore:

- Low-efficiency aerials - long aerials required

- Higher static levels

- High power demand

- High installation costs

Advantage: Long ranges of transmission possible

Sky Waves and Factors affecting Sky Waves

Caused due to refraction in ionosphere

Factors affecting Sky Waves

Frequency:

Low frequency bands become trapped or absorbed in the ionosphere

Medium/High Frequency bands are most suitable

Frequency bands >HF penetrate ionosphere and continue into space

Critical angle

The smallest angle to the vertical which is capable of producing a return wave from the ionosphere

Skip Distance

- Earth distance between the transmitter and the first returning sky wave.

- Influenced by condition of ionosphere

- Skip distance during day may be 50 miles

- As ionosphere rises during night skip distance may increase to 60 miles or more (perhaps twice)

- Signal strengths therefore vary for a given location

Dead Space

Spaces between returning Sky Waves and space from end of the Surface Wave to the next returning Sky Wave

Sky Wave Propagation within the Ionosphere

Ionised gases respond to solar radiation changes by changing density.

During the Night -

Solar radiation ceases

Ionic layers contract + density reduces

Bottom layer disappears

Ionosphere rises and influences the distance from transmitter at which a sky wave contacts Earth's surface

Factors Affecting Propagation

- Frequency in Use

- Power output of transmitter

- Obstacles in path of transmitter

Other factors affecting HF Band & GPS

- Diurnal activity/Night effect

- Seasonal activity (Annual Effects)

- 11-year sun-spot cycle

- Density of ionosphere layer

- Penetration Depth

- Critical Angle

- F layer