Radio Wave Propagation and Frequency Allocation

• Radio wave propagation occurs through different modes, each with a unique mechanism.
• The modes of radio wave propagation are classified as ground (surface) waves, space (tropospheric) waves, and sky (ionospheric) waves.

Allocation of Frequencies for Broadcasting

• Long Wave Band is not used in India.
• Medium Wave (MW) Band ranges from MF 300 - 3000 kHz.
• MW Band frequencies are between 531 kHz to 1602 kHz with a channel spacing of 9 kHz.
• Short Wave (SW) Band ranges from HF 3-30 MHz.
• VHF ranges from 30-300 MHz.
• UHF ranges from 300-3000 MHz.
• SHF ranges from 3-30 GHz.

Earth's Layers

• The layers of Earth are the troposphere, stratosphere, mesosphere, and ionosphere.
• The troposphere extends to 18 km from Earth's surface.
• The stratosphere extends to 50 km from Earth's surface.
• The mesosphere extends to 90 km from Earth's surface.
• The ionosphere extends to 350 km from Earth's surface.

Ground (Surface) Waves

• Medium wave (MW) propagates along Earth's surface.
• MW induces current in the ground and loses energy through absorption.
• Range depends on frequency, transmitter power, ground conditions (salinity, conductivity), and water vapor content.
• Received signal strength (V) is determined by the formula: V = (120π * ht * hr * I) / (λ * d)
• 120π represents the characteristic impedance of free space.
• ht is the effective height of the transmitting antenna.
• hr is the effective height of the receiving antenna.
• I is the antenna current.
• d is the distance from the transmitting antenna.

Fading of Signal

• Fading occurs when both ground and sky wave signals are received with comparable strength, creating a 'fading zone'.
• The fading zone should be as far from the transmitter as possible.
• An antenna height of 0.55λ, where λ is the wavelength of the operating frequency, achieves this objective.

Space (Tropospheric) Waves

• They travel in straight lines, limited by the earth's curvature due to line-of-sight conditions.
• Space waves consist of direct and reflected waves from the earth's surface.
• Direct waves are steady and strong.

Line of Sight (LOS)

• Radio waves propagate in a curved path due to refraction in the troposphere.
• Receiving antenna height is as important as the transmitting antenna height.
• LOS = √(2a) * (√ht + √hr) m
• 'a' is the radius of the Earth, 6370 km or 6.37 x 10^6 m.
• ht is the transmitting antenna height in meters.
• hr is the receiving antenna height in meters.

Fresnel Zone

• Propagation occurs through a volume around the line of sight, known as the "First Fresnel Zone."
• This volume should be free of surfaces or buildings to avoid reflections.
• Line of sight alone is insufficient; the First Fresnel Zone must be clear.

Environment Effects

• Buildings have little effect on low frequencies (few MHz).
• Above 30 MHz, obstruction loss and shadow loss become important.
• Attenuation by walls can be 2-5 dB at 30 MHz, increasing to 10-40 dB at 3000 MHz.
• Thick vegetation absorbs RF energy, more significantly with vertical polarization.
• Clutter losses relate to natural and man-made obstructions, statistically evaluated and considered in field strength calculations, dependent on frequency and the area around the transmitter.

Effective Radiated and Isotropic Power

• Effective Radiated Power (ERP) is the product of intrinsic transmitter power and transmitting antenna gain over a dipole.
• ERP = Transmitter power (kW) x antenna gain (In kW) or Transmitter power (dBm) + antenna gain (dBm).
• Effective Isotropic Radiated Power (EIRP) is similar to ERP, but the gain is expressed relative to an isotropic antenna.
• Isotropic antenna gain = 1.64 times or 2.15 dB of a dipole.
• EIRP = ERP (dBW) + 2.15 dB (In dBW) or EIRP = 1.64 x ERP.
• An isotropic antenna is a theoretical point antenna radiating uniformly in all directions.

Field Strength

• Minimum signal strength for satisfactory reception with a 10 m receiving antenna:
  • Band 1*: 48 dBμV/m
  • Band 3: 55 dBμV/m
  • Band 4: 65 dBμV/m
  • Band 5: 70 dBμV/m
• Doordarshan adopted 40 dBμV/m.
• Field strength is calculated as: 134.8 + 10 log P - 20 log d - F dBμV/m.
  • P = EIRP in Watts.
  • d = distance of receiving point in meters.
  • F = Loss experienced in propagation.

Protection Ratio (PR)

• Ratio of wanted to unwanted field strength at a point, indicating signal quality.
• PR = Xw/Yuw, where Xw is the wanted transmitter's field strength and Yuw is the unwanted transmitter's field strength.
• PR (dB) = 10log(Xw/Yuw).

Duct Propagation

• Troposphere refractivity 'n' decreases with height (-40 to -80 units/km under normal conditions).
• Ducting mode exists when refractivity is -157 N units/km or less.
• VHF/UHF waves refract (bend) rapidly during ducting, reflecting off the ground repeatedly.

Types of Interference

• Co-Channel Interference: No interference if the desired TV signal exceeds the interfering signal by 55 dB or more; Venetian blind interference (horizontal black and white bars) occur when the desired signal is weaker.
• Adjacent Channel Interference: Results from beats between carriers with a 1.5 MHz difference, producing a coarse beat pattern.
• Ghost Interference: Caused by reflections from tall obstructions, reducing resolution; mitigated by shifting the receiving antenna.

Ionosphere Sky Waves

• Short wave (SW) propagates as sky waves.
• Ionization of the Earth's upper atmosphere plays a role in propagation.
• Solar energy splits atmospheric molecules into ions which remain for some time.
• Ionosphere extends from 50 to 400 km and contains ionized particles.
• Passing sunrays create imaginary but distinct layers such as D, E, F1, and F2 due to different densities.

Ionospheric Layers

• The D layer is the lowest at 70 km with a 10 km thickness.
  • Ionization depends on the sun's altitude, disappears at night.
  • Absorbs MF and HF waves but reflects some VLF and LF waves.
• The E layer is above the D-layer at 100 km with a 25 km thickness.
  • Disappears at night as ions recombine due to lack of solar radiation.
  • Aids MF surface wave propagation and reflects some HF waves in daytime.
• The Es-layer is a sporadic E-layer with high density and sometimes appears with the E-layer.
  • May persist at night, aiding long-distance propagation and allows unexpected good reception.
  • It's causes are not well known.
• Sporadic E is irregular and scatters radio frequencies up to 150 MHz seasonally within the E region.
  • Common daytime occurrence over equatorial regions and in temperate latitudes.
  • Supports reflections for distances up to 2,400 km.
• The F1 layer is at a height of 180 km in daytime that combines with the F2 layer at nighttime.
  • In day time is has a approximate thickness of 200 km.
  • Most radio waves passes through it and are reflected by the F2 layer.
  • Main effect is absorption for HF waves.
  • Absorption increases as frequency increases.
• The F2 layer is the most important for reflecting HF waves.
  • Can be up to 200 km thick. Its height ranges from 290 to 400 km.
  • At night, falls to ≈300 km.
  • Height and ionization density vary with time of day, ambient temperature, and sunspot cycle.

Virtual Height & Critical Frequency

• Electromagnetic waves refract (bend) gradually.
• Below the ionized layer, the path of incident and refracted rays are same as if from a surface at a greater height, known as the virtual height.
• Once the virtual height is known, the angle of incidence required for the return of the wave can be calculated.
• Critical Frequency (fc) is obtained when sending a signal pulse directly upwards.
• The reflected return time is measured to give an indication of the layer height.
• Above this frequency, it signal passes right through the layer, and onto the next layer/space.

Maximum Usable Frequency

• Highest frequency for sky wave communication between two points.
• MUF = Critical Frequency / cosθ = fc sec θ