Propagation Modes & Atmospheric Interactions

Comms 4 Modes of Propagation

Terrestrial Propagation

Propagation of radio waves within the Earth's atmosphere between two or more points on the Earth's surface.

Modes of Wave Propagation

1. Ground/Surface Wave, 2. Sky/Ionospheric Wave, 3. Space/LOS/Tropospheric Wave.

ELF (Extremely Low Frequency)

3-30 Hz.

SLF (Super Low Frequency)

30-300 Hz.

ULF (Ultra Low Frequency)

300-3000 Hz.

VLF (Very Low Frequency)

3-30 kHz; uses Ground Wave propagation.

LF (Low Frequency)

30-300 kHz; uses Ground Wave propagation.

MF (Medium Frequency)

300-3000 kHz; uses Ground Wave propagation; used for AM Broadcasting.

HF (High Frequency)

3-30 MHz; uses Sky Wave propagation.

VHF (Very High Frequency)

30-300 MHz; uses Space Wave propagation.

UHF (Ultra High Frequency)

300-3000 MHz; uses Space Wave propagation.

SHF (Super High Frequency)

3-30 GHz.

EHF (Extremely High Frequency)

30-300 GHz.

Ground Wave Propagation

An Earth-guided electromagnetic wave that travels over the surface of the Earth, following its contour due to diffraction.

Ground Wave Frequency Range

Effective on VLF, LF, and MF bands (frequencies up to 2 MHz).

Ground Wave Polarization

Must be vertically polarized to prevent short-circuiting the electric field component by the ground.

Surface Conductivity for Ground Waves

Sea water is 'Good'; Flat, loamy soil is 'Fair'; Jungle is 'Unusable'.

Ground Wave Application

Primarily used for AM radio broadcasting.

Ground Wave Disadvantages

Requires relatively high transmission power, large antennas, and ground losses vary with surface materials.

Ground Wave Field Strength Formula

E = (120 * pi * I * ht) / (lambda * d).

Induced Antenna Voltage Formula

Va = E * hr (Electric field * height of receive antenna).

Sky Wave Propagation (Ionospheric Propagation)

Radio waves strike the ionosphere and are refracted back to the ground; used primarily in the HF band.

Cause of Ionization

Ultraviolet radiation from the sun is the primary cause of ionization in the atmosphere.

Troposphere

The lowest layer of the atmosphere (8-10 miles high) where weather disturbances take place.

Stratosphere

The isothermal region of the atmosphere located above the troposphere with constant temperature.

Ionosphere

The region of the atmosphere containing ionized layers and low-density gas.

D Layer

The lowest ionospheric layer (30-55 miles); exists only during the day; absorbs MF and attenuates HF.

E Layer (Kennelly-Heaviside Layer)

Exists 55-90 miles high; refracts HF waves during the day; disappears by midnight; can refract signals up to 20 MHz.

F Layer

Exists 90-240 miles high; splits into F1 and F2 during the day; responsible for HF long-distance transmission.

Daily Variations

Variations in the ionosphere resulting from the 24-hour rotation of the Earth.

Seasonal Variations

Variations resulting from the Earth revolving around the sun and the relative position of the sun.

11-Year Sunspot Cycle

A cycle of minimum and maximum sunspot activity affecting ionization levels every 11 years.

27-Day Sunspot Cycle

Variations caused by the rotation of the sun on its own axis, making sunspots visible at 27-day intervals.

Sporadic E

Irregular cloud-like patches of unusually high ionization forming near the normal E layer.

Sudden Ionospheric Disturbances (SID)

Disturbances caused by gigantic emissions of hydrogen (solar flares) from the sun; can last minutes to hours.

Ionospheric Storms

Disturbances caused by particle radiation from the sun, which moves slower than UV light.

Index of Refraction in Ionosphere (n)

Formula: n = square root of (1 - (81 * N) / f^2); where N is electron density.

Critical Frequency (fc)

The highest frequency that will be returned to earth when beamed vertically upward.

Critical Frequency Formula

fc = square root of (81 * Nmax).

Maximum Usable Frequency (MUF)

The highest frequency returned to earth at a given distance when beamed at a specific angle (Secant Law).

MUF Formula

MUF = fc / cos(theta) = fc * sec(theta).

Optimum Working Frequency (OWF/FOT)

The frequency that gives the most consistent communication; chosen to be 15% lower than MUF.

OWF Formula

OWF = 0.85 * MUF.

Virtual Height

The apparent height of the ionized layer measured by sending a wave vertically and timing its return.

Skip Zone

The zone of silence between the point where the ground wave becomes too weak and the point where the sky wave first returns.

Skip Distance

The distance from the transmitter to the point where the sky wave is first returned to Earth.

Space Wave Propagation (LOS)

Line-of-Sight or Tropospheric propagation; composed of a direct wave and a reflected wave.

Radio Horizon

The boundary beyond which direct rays cannot be propagated; it is approximately 4/3 times the optical horizon.

Radio Horizon Formula (Metric)

D(km) = sqrt(17 * htmeters) + sqrt(17 * hrmeters).

Radio Horizon Formula (English)

D(miles) = sqrt(2 * htfeet) + sqrt(2 * hrfeet).