Radio Waves in Aviation

What determines which frequency to use?

The ionized part of the Earth’s atmosphere is known as the ionosphere. Ultraviolet light from the sun collides with atoms in this region knocking electrons loose. This creates ions, or atoms with missing electrons. This is what gives the Ionosphere its name and it is the free electrons that cause the reflection and absorption of radio waves.

https://sciencestruck.com/ionosphere-facts

Sun’s UV light collides with atoms knocking electrons loose:

• Ions are atoms with missing electrons

Free electrons cause reflection and absorption of radio waves.

Interesting facts about the Ionosphere:

• One of the most noted and interesting facts would be the formation of aurora. The aurora is an illusive natural phenomenon, which usually occurs near the Northern (Aurora Borealis) and Southern (Aurora Australis) Hemisphere.
• It is associated with a magnificent display of chromatic lights swirling in the northern and southern skies. They were considered to be indications of an approaching adversity or catastrophe in ancient Greek and Roman cultures.
• They occur in the ionosphere due to the exchange of photons from unstable nitrogen and oxygen atoms, destabilized by collision of the solar wind particles.
• It is also considered that auroras are the result of emission of atmospheric gases.
• What causes lightning could be a question which might intrigue us. Lightning is said to take place in the ionosphere.
• When liquid and ice particles clash with each other, they form massive electrical fields, dissipating an electric spark, which we refer to as lightning.
• The temperature inside a lightning bolt reaches around 50,000º C, which is even greater than the surface temperature of the sun!

Think about visualizing water waves, then consider how we know light behaves too – to help understand these behaviours.

Ground Wave Propagation:

• Refraction – Follows the contour of the earth
• E.g. Radio below 2MHz frequency

The strength of the groundwave is inversely proportional to frequency, being strongest at low frequencies. As frequency is increased, the distance covered by the groundwave decreases.

A ground wave is considered to have 2 components: a surface wave (travels entirely along the earth’s surface) and a space wave.

Sky Wave Propagation:

The Ionosphere reflects the electromagnetic waves, this zigzag pattern will vary depending on the atmosphere.

Reflects off the Ionosphere, e.g. HF Radio.

The Ionosphere reflects the electromagnetic waves, this zigzag pattern will vary depending on the atmosphere.

For HF radios note the reception created by where the wave is meeting the ground. There are areas of no reception for HF radios, yet at greater distances from the source, the reception improves again. This is normal for HF radios.

Space Wave (Line of Sight) Propagation:

E.g. VHF Radio & Mobile Phones.

While the distance between towers is not so relevant, it is more relevant to consider how far from the VHF transmitter we can operate.

D and R calculate the distance between towers for effective communication

To work out the exact distance a VHF signal will travel, you can use the following equation:

So, with an aircraft at 36,000 feet and the ATC radio tower at 100 feet, communication will be possible up to 250 nautical miles away. However, this is the theoretical optimum and will be much less in practice due to transmitter power and receiver inefficiencies.

Ionosphere affects HF Radio:

During the day, energy from the sun causes the D, E and F layers to become heavily ionized, making the layer more active. At night, with less energy from the sun, only the E and F layers are active. As a result, lower frequencies are better quality during the night and higher frequencies are better during the day.

However, what HF gains in distance, it loses in quality. Quite often, the quality of the signal is so poor that it’s impossible for either station to hear each other.

Terrain – VHF Radio:

Just like your mobile phone if there is terrain between you and the transmitting tower, you will not be able to send and receive messages via your VHF radio.

Sunspot activity – HF Radio:

The level of sunspot activity has an enormous effect on the ionosphere and hence on HF radio propagation conditions.

These sunspots are areas where there is intense magnetic activity. 

Around the sunspot there is an area called a plage. This is slightly brighter than the surrounding area and it is a large radiator of cosmic rays, ultra-violet light and X-rays. In fact it results in the overall level of radiation coming from the sun to increase. In turn this increased radiation level from around the sunspots causes the ionosphere to become ionised to a greater extent. This means that higher frequencies can be reflected from the ionosphere.

The higher the levels of radiation received from the Sun, the greater the levels of ionisation in the ionosphere and in general this brings better propagation conditions for HF radio communications.

It is found that over a period of approximately eleven years over which the sunspots vary. At the peak of this cycle conditions on the bands at the top of the short wave spectrum are very good. Low power stations can be heard over remarkably long distances. At the bottom of the cycle bands around 30 MHz will not usually support normal propagation via the ionosphere.

Thunderstorms and Atmospheric Static – HF & VHF:

What is thunderstorms and atmospheric static, and what it means for radio:

• Discharge of static electricity in the Atmosphere
  • Lightning from Thunderstorms is the most visual source
    • Lightning scatters radio waves for a fraction of a second
    • Brief noise burst makes frequency unusable
  • Non-Lightning discharges also occur regularly
• HF – Noise is louder on lower frequency bands
  • Can be heard several hundred miles from the source
• VHF – Noise also occurs on lower frequencies when storms close to the receiving antenna 

Atmospheric noise, caused by lightning discharges in thunderstorms, is normally the major contributor to radio noise in the HF band:

• Thunderstorm occurrence is greatest at low latitudes and varies seasonally.
• Atmospheric noise can be impulsive when storms are proximate to the receiving antenna, or display a broad continuum when storms are remote.
• This noise may display some directionality depending on the location and spread of storms with respect to the receiving antenna.
• Atmospheric noise effects are greater at lower HF frequencies so are likely to affect signal quality during the night and solar minimum when lower frequencies are in use.

Lightning scattering has sometimes been observed on VHF and UHF over distances of about 500 km. The hot lightning channel scatters radio-waves for a fraction of a second. The RF noise burst from the lightning makes the initial part of the open channel unusable and the ionization disappears quickly because of recombination at low altitude and high atmospheric pressure. Although the hot lightning channel is briefly observable with microwave radar, no practical use for this mode has been found in communications.

Static/noise also occurs from other Atmospheric Static sources:

• Rain Static
• Snow Static

Installation of Static Dischargers or Wicks:

Static dischargers, or wicks, are installed on aircraft to reduce radio receiver interference.

Static dischargers are normally mounted on the trailing edges of the control surfaces, wing tips and the vertical stabilizer. They discharge precipitation static at points a critical distance away from avionics antennas where there is little or no coupling of the static to cause interference or noise.

Interference from electrical equipment – HF & VHF:

Just like your car radio receives interference from electronic devices, so does your Avionics Radio – HF & lower frequencies most affected.

Any other device creating an electromagnetic wave could create interference:

• Electrical motors
• Power lines
• Mobile Phones

Attenuation is the reduction of signal strength.

This can be caused by:

• Distance from the transmitting station.
• Atmospheric conditions:
  • As a radio wave passes through the atmosphere the electrons in dust and water droplets absorb some of the energy causing atmospheric attenuation.
  • In a vacuum there is no attenuation as the radio wave has nothing to give up its energy to.
• Physical Blockages by obstruction, creating shadowing:
  • Buildings
  • Trees
  • Terrain