Wave Propagation

Wave Propagation- means the medium by which the Radio waves travel from the Transmitter to the Receiver.

Radio Waves are a form of Electrical and Magnetic Radiation, they always travel at a constant rate of Light Waves, that is 162,000N.M/ 186,000 S.M/ 300,000 K.M per second.

In a Radio Wave the electrical and magnetic component is 90* to each other.

In a vertical polarized wave the electrical component is vertical and the associated magnetic component is horizontal.

Similarly In a horizontally polarized wave the electrical component is horizontal and the associated magnetic component is vertical.

Critical Frequency- is the highest frequency that can be returned by a Ionosphere layer, any frequency higher than Critical Frequency will cause Radio Wave to penetrate more deeply into the Ionosphere layer before returning to the Earth.

Critical Angle- is the lowest angle at which the sky wave returns to the Earth, any angle above this there will be a uninterrupted flow of Sky waves. Any angle below Critical Angle the Radio Wave will penetrate Ionosphere and will lost in the Space.

There are many types of Propagation-

Surface Wave- type of wave travels over ground, from the transmitter to the receiver. They follow the Earth’s curvature and are affected by the land properties.

Factors affecting Surface Wave-

  1. Vertical Polarization is used to get maximum range.
  2. Communication is reliable.
  3. They are absorbed by dry land and die out soon, therefore long distance coverage is not possible.
  4. No Atmospheric Disturbance.
  5. Ground Attenuation (Absorption) increases with increase in Frequency, therefore they are useful only at low Frequency.
  6. Ground features decides the Range of the waves

Surface Reflected wave- type of wave travels from the Transmitter to the receiver after being reflected from the Earth.

Factors affecting Surface Reflected Wave-

  1. Vertical Polarization is used to get maximum range
  2. Coverage is more reliable than ground wave.
  3. The effect of land properties is less than Surface waves.
  4. Marginal Atmospheric Disturbance.
  5. Ground Attenuation (Absorption) increases with increase in Frequency, therefore they are useful only at low Frequency.
  6. Range is higher as compared to Surface Wave alone.
  7. It suffers from Selective Fading in which the Received signal from the Surface Wave and from the Surface Reflected wave meets each other 180° out of phase at the receiver and hence cancel out each other, It can be corrected by Varying the height of Receiver or Transmitter.

Sky Wave (short wave)- – type of wave travels from the Transmitter to the receiver after being reflected from the Ionosphere layer (Ionosphere is a layer of free electrons consisting of positive and negative ionized particles, that surrounds the Earth, which absorbs ultra violet rays produced by Sun’s rays from reaching the Earth) above the Earth.

Factors affecting Sky Wave-

  1. Vertical Polarization is used to get maximum range
  2. The effect of land properties is negligible.
  3. It suffers from Atmospheric Disturbance.
  4. Long distance coverage is Possible.
  5. It suffers from Night effect/ Dusk & Dawn Error.

Night Effect/Dusk & Dawn Error- At night when Ionosphere layer changes due to the absence of Sun rays that causes a layer consisting of charged Ionized particles, caused by the atoms of high velocity and high energy Ultra violet rays in the Sun’s rays. It is the region of free electrons consisting of positive and negative ionized particles, which surrounds the Earth. The Earth is surrounded by few layers which absorbs these ultra violet rays from reaching the Earth. When sun is no longer over a particular area, it is called night, hence the direct ultra violet radiation in the night is absent and the layer that consisted charged ions in the day almost vanishes. For the Sky wave to be deflected from the atmosphere it has to travel much higher than in the day to be reflected back to the Earth. This causes the range of the Radio Aid to be increased in the night time.

  1. It suffers from Selective fading.

Selective Fading- It occurs when the received signal from the Surface Wave and from the Surface Reflected wave meets each other 180* out of phase at the receiver and hence cancel out each other, It can be corrected by Varying the height of Receiver or Transmitter.

  1. It suffers from Skip Zone and Skip Distance.

Skip Zone/Dead Space- The Ground distance between the point at which the maximum Surface wave goes and the point where the first Sky wave returns, No signal coverage is available in this area and it is also termed as Dead Space, The only method to receive signal in the Dead Space is to lower the frequency, or to increase the power of the Transmission, which is not very practical in flight.

  1. It suffers from Fading.

Fading- It occurs due to the fluctuations in the signal strength, It mainly occurs due to the continuous fluctuations in the Ionosphere. It also occurs when two Radio waves have traveled different routes and are in a phase difference. If they are In Phase they will strengthen each other giving even stronger reception. But if the waves are Out of Phase, they will eventually cancel out each other.

  1. Skip Zone The authenticity of the signal is not guaranteed.

Space Wave/Direct Wave/Line of Sight Wave- type of wave which travels almost in a straight line from the transmitter to the receiver. This type of waves do bend slightly due to Earth’s curvature, else  travel almost like a light. These types of waves are usually horizontally Polarized.

Factors affecting Space Wave-

  1. It has no Atmospheric Interference.
  2. It does not suffer from Night Effect, therefore one Frequency can be used round the clock.
  3. Range depends on the height of Transmitter and Receiver.
  4. It receives the Space Wave as well as Surface Reflected wave, and can cause fading if they are Out of Phase.
  5. These types of waves are found at VHF band and above

Super Refraction/ Duct Propagation- occurs due to Inversion layer very close to the Earth surface. A reflective layer of air forms very close to the Earth surface. It reflects the VHF band and above in the same way as Ionosphere and a Multi Hop phenomena takes place. It increases the range of Radio Waves to thousands of miles. As long as Duct remains they continue to travel in that duct.

It takes place in Tropical (Near Equator) or Sub Tropical Latitudes, because the land mass gets extremely hot during the day, followed by rapid cooling at night. It is a winter season phenomena and is more prevalent over Sea than Land.

The meteorological conditions required for Duct Propagation are-

  1. Warm Air
  2. Subsidence (air is descending down)
  3. Pronounced Radiation cooling of the Earth.