Radio Navigation (062)

A radio wave is a changing electromagnetic wave which moves away from an aerial/ antenna at the speed of light.

The speed of light is constant (in a vacuum) at 300,000,000 m/s (3 x 10⁸ m/s).

Electromagnetic waves are referred to in terms of:

1. Wavelength;
2. Frequency;
3. Amplitude;

The wavelength is defined as the length of one complete cycle of a wave, and is represented by the letter λ (measured in units of length – meters).

The frequency is defined as the number of cycles the wave makes in a second, and is measured in the unit hertz (Hz).

The relationship between wavelength and frequency is expressed by the formula:

λ = speed of light (c) / frequency (Hz)

Positions on a radio wave are described in degrees from 0° to 360°, called the phase angle. By convention, the phase is 0° where the amplitude is zero and rising.

If two transmissions with the same wavelength (and thus frequency) start at the same point, the waves will be synchronized and in phase. If one starts (slightly) after another, they will be out of phase by an amount measured in degrees. Or in other words there is a phase difference / shift between the two waves.

Modulation is the process of adding and transporting information by a carrier radio wave (see 062.01.01.05.03) and uses a process called heterodyning to add (impose) the intelligence on the carrier wave.

It creates two new frequencies alongside the original carrier wave, the carrier wave plus the intelligence and the carrier wave minus the intelligence.

These extra frequencies are called sidebands, the upper sideband at the higher frequency and the lower sideband at the lower frequency.

In example: when an Amplitude Modulated signal is created from a carrier wave of 400 kHz and intelligence (perhaps a voice signal) of 1 to 3 kHz, then the result is:

• The original carrier wave of 400 kHz;
• The upper sideband between 401 kHz and 403 kHz (400 + 1 to 400 +3);
• The lower sideband between 397 kHz and 399 kHz (400 – 3 to 400 – 1);

Question alarm: based on this example remember “modulation is the addition of a low frequency signal (tone, voice) to a high frequency carrier wave“. Not the other way around.

As heterodyning produces two sidebands including the intelligence, it is possible to reduce the amount of bandwidth taken by the transmission and subsequently reduce the required power of the transmitter, by deliberately not transmitting the carrier wave and one of the sidebands.

This is called a single sideband (SSB) transmission.

In order to optimize the range, this technique is particularly used in HF long range communications.

Radio transmissions are classified by a three digit ITU code. The symbols indicate:

1. the type of modulation of the main carrier;
2. the nature or type of the signal modulating the main carrier;
3. the type of information carried;

The designators that could come from Non-Directional Beacons (NDBs) are part of the Learning Objectives (062.02.02.01.17) and classified as:

N0N: carrier without modulation;
A1A: carrier with keyed Morse code modulation;
A2A: carrier with amplitude modulated Morse code;

Any information to be transmitted must be laid on top of the carrier wave before transmission and decoded on reception. This process is know as modulation and demodulation.

Modulation is the process of adding (impressing) and transporting information by a carrier radio wave.

Modulation can be achieved by changing the amplitude, the frequency or the phase of the carrier wave. Each method has its advantages and disadvantages.

The simplest form of modulation is to vary the amplitude of the carrier wave to transmit information.

The ionosphere is the ionised component of the Earth’s atmosphere, stretching from approximately 60 to 400 km above the surface and is divided in three layers: D, E and F.

In the Ionosphere the (few) gas molecules that are present are being bombarded with solar and cosmic radiation, which split the molecules into positively and negatively charged ions. The amount of ions / density of ions increases with the intensity of radiation.

The density is higher in the upper ionosphere (F), as these molecules are subject to a higher intensity of radiation, than those in the lower (D / E) ionosphere. Furthermore, as solar radiation is absent at night, the density of the ionosphere is less at night in comparison to daytime.

As such the density of the layers vary with time of day, season and solar activity.

Radio waves will be refracted at the boundary where the ion density changes (E & F layer). At low frequencies and at the correct angle of incidence the radio wave will be refracted enough to return back to Earth, these radio waves are called Sky waves.

Explain how the different layers of the ionosphere influence wave propagation (062.01.03.01.04).

Surface waves or ground waves are radio waves which tend to follow the curvature of the Earth due to diffraction ( see 062.01.03.06.05 ).

As diffraction is greatest at low frequencies, surface waves form at the low frequency end of the radio spectrum.

The distance a surface wave can travel is limited by surface attenuation, which is as well lowest at lower frequencies.

These two factors mean that surface waves are longest at the lowest frequencies.

Space waves are line of sight waves.

Naming is not intuitive. A reverse definition between sky and space waves would have been more applicable. So don’t mix them up!

Space waves are the only propagation path for frequencies in the range from higher VHF to EHF.

Max theoretical range. = 1.23 x √H₁ + 1.23 x √H₂
(H₁ height of the transmitter, H₂ height of the receiver (in feet above mean sea level))

Radio waves at VHF, UHF, SHF and EHF are certainly subject to refraction as they pass through the ionosphere, but are not bent (refracted) enough to come all the way back to Earth. So, radio waves with frequencies at VHF and higher normally don’t tend to propagate as sky waves.

In addition, at frequencies at VHF and higher the radio waves are not subject to diffraction, therefore not propagating as ground waves either.

Radio waves in VHF, UHF, SHF and EHF only propagate as space waves.

Radio waves can lose their energy and be bent or reflected to change their path through the atmosphere.

Bending

Bending can be caused by either refraction or diffraction.

Refraction

Refraction is the change of direction of a radio wave caused by a change of speed due to the difference in optical density between the surface it passes over or the medium it passes through.

The amount of refraction depends on the change in speed of the wave, its frequency and the angle at which it hits the new medium.

In the ionosphere refraction is greater at low frequencies.

Diffraction

Diffraction is the change in direction of a radio wave when it moves over a sharp object or through a hole.

In the later, the hole will act like a point source and effectively retransmits the wave.

Diffraction is greater at low frequencies.

Reflection

Reflection is a radio wave bouncing off a surface (like a mirror reflecting light).

How the radio waves are reflected depends on the angle of attack, the density of the material and how smooth the reflecting surface is; rough surfaces scatter energy in all directions, smooth surfaces act like a mirror.

Whether or not a surface acts like a mirror depends on the wavelength of the radio wave. A surface might appear smooth for a wavelength in the order of a meter, but may not be smooth for a wavelength of a millimeter.

Attenuation

Attenuation is the loss of signal strength (power) of a wave.

Atmospheric attenuation

Atmospheric attenuation is the loss of signal strength due to electrons in dust and water droplets absorbing some of the energy of the radio wave.

Atmospheric attenuation increases as frequency increases.

Surface attenuation

Surface attenuation is when radio waves passing over the Earth’s surface and lose their energy and slow down.

Surface attenuation is greatest over the ice caps, then desert areas, then over land and least over water.

Surface attenuation increases as frequency increases.

Ionospheric attenuation

Ionospheric attenuation occurs when waves are passing through the layer of electrically charged atoms at the edges of space – the ionosphere.

Ionospheric attenuation increases as frequency decreases.

1. refraction
2. attenuation / absorption
3. reflection