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Chapter 3 – Radio Phenomena

• Propagation: How Signals Travel• Propagation On The HF Bands

– Ground-wave Propagation

– Sky-wave Propagation

– HF Scatter Propagation

• VHF/UHF Propagation Characteristics– Line-of-sight Propagation

– Tropospheric Bending and Ducting

– VHF/UHF Signals Through The Ionosphere

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Radio Wave Propagation

Radio waves travel to their destination in four ways:1. Line of Sight

Directly from one point to another.

2. Ground-Wave Along the ground, bending slightly to follow the Earth’s

curvature.

3. Tropospheric Bending and Ducting In the lower layer of the Earth’s atmosphere.

4. Sky-Wave Refracted or bent back to the Earth’s surface by ionized layers

in the ionosphere.

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Atmospheric Regions

Region Height Notes

Troposphere 7 miles Region where all weather occurs

Stratosphere 6 to 30 miles

Region where atmospheric gases “spread out” horizontally. The high speed jet stream travels in the stratosphere.

Ionosphere 30 to 400 miles

Region where solar radiation from the sun creates ions. Major influence on HF radio wave propagation.

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Atmospheric Regions

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HF Band Propagation

• Ground-Wave Propagation• Sky-wave Propagation• HF Scatter Mode

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Ground-Wave Propagation

• Results from a radio wave diffraction along the Earth’s surface.

• Primarily affects longer wavelength radio waves that have vertical polarization (electric field is oriented vertically).

• Most noticeable on AM broadcast band and the 160 meter and 80 meter amateur bands.

• Communication distances often extend to 120 miles or more.

• Most useful during the day at 1.8 MHz and 3.5 MHz when the D-Region absorption makes sky-wave propagation impossible.

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Ground-Wave Propagation

The curved surface of the Earth horizon can diffract long-wavelength(low frequency) radio waves. The waves can follow the curvature of theEarth for as much as several hundred miles.

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Sky-wave Propagation

• Ionization levels in the Earth’s ionosphere can refract (bend) radio waves to return to the surface.– Ions in the Earth’s upper atmosphere are formed when

ultraviolet (UV) radiation and other radiation from the sun knocks electrons from gas atoms.

– The ionization regions in the Earth’s ionosphere is affected the sunspots on the sun’s surface. The sunspots vary in number and size over a 11 year cycle.

• Sky-wave propagation is determined by radio wave frequency and level of ionization in the ionosphere.

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Sky-wave Propagation (Cont’d)• Communication distances of 2500 miles are

possible with one skip off the ionosphere.– Skip propagation has both minimum and maximum

ranges.

– The area between the maximum ground wave distance and the minimum skip distance is called the skip zone.

– World-wide communications is possible using several skips (or multi-hops)

• The highest frequency that a radio wave transmitted straight up is reflected back to Earth is called the critical frequency.

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Sky-wave Propagation (Cont’d)

• The maximum usable frequency (MUF) is the highest frequency at which the ionosphere bends radio waves back to a desired location on earth.– MUF is dependant on level of solar radiation strength

and time of day.

– The maximum usable frequency (MUF) tends to be higher during periods of high sunspots.

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Sky Wave Propagation

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Regions In The Ionosphere

The Earth’s ionosphere contains several regions of charged particles which affect radio signal propagation.

The ionization regions change from day to night periods.

Region Height Above Surface

D Region 30-60 miles

E Region 60-70 miles

F Region 100-310 miles

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Regions In The Ionosphere

• D Region– Height about 35 to 60 miles above Earth.

– Maximum ionization at or near noon.

– Ionization disappears by sunset.

– Absorbs energy from radio waves. Absorption on lower frequencies is greater than higher frequencies.

– Radio wave absorption is most pronounced at mid-day.

– Responsible for short daytime communication ranges on lower-frequency HF bands (160, 80 and 40 meters).

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Regions In The Ionosphere (Cont’d)

• E Region– Height about 50 to 70 miles above Earth.

– Ionization useful for bending radio waves when in sunlight.

– Reaches maximum ionization level around mid-day.

– Ionization reaches a minimum level just prior to sunrise.

– Radio wave propagation up to about 1250 miles in a single skip hop.

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Regions In The Ionosphere (Cont’d)

• F Region– Height ranges from 100 to 310 miles above Earth.

– Ionization reaches a maximum about noon and tapers off gradually toward sunset. Minimum ionization is reached just prior to sunrise.

– F region splits into two parts (F1 and F2) during the day and recombine at night.

• F1 region forms about 140 miles above Earth

• F2 region forms about 200 miles above Earth

– F2 region is responsible for long distance HF band communication with distances of about 2500 miles.

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HF Scatter Modes

• All electromagnetic wave propagation is subject to scattering influences from the Earth’s atmosphere, ionospheric regions and objects in radio path.

• Scattered signals may be received in sky-wave propagation skip zone.

• Scatter signals are generally weak and subject to echoes and distortion.

• Most common when operating near the MUF.• Under ideal conditions, scatter propagation is

possible over 3000 miles or more.

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The Scatter Modes

Back Scatter Propagation

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VHF/UHF Propagation

• Line Of Sight (LOS)• Tropospheric Bending• Tropospheric Ducting• VHF/UHF Signals Through The Ionosphere

– Sporadic “E”

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Line-Of-Sight Propagation

• Radio signals travel in a straight line from a transmitting antenna to the receiving antenna.

• Provides VHF/UHF communications within a 100 miles or so.

• Signals can be reflected by buildings, hills, airplanes, etc.

• Reflections vary the propagation path causing signal cancellation and reinforcement. This results in a rapid fluttering sound called picket fencing.

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Line-Of-Sight (LOS) Propagation

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Tropospheric Bending

• Slight bending of radio waves occur in the troposphere close to the Earth’s surface.– There is always a radio signal loss as radio waves travel

through the troposphere• Radio signal loss increases as the frequency increases

– The radio path horizon is generally 15 percent farther away than the visible horizon (typically 8 to 9 miles).

• Communication distances can be increased by increasing the antenna height above the terrain

– Tropospheric bending propagation is most useful at 144 Mhz and higher frequencies

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Radio Path Horizon

• The farthest point to which radio waves will travel directly.

• The structure of the atmosphere near the Earth’s surface causes the radio waves to bend in a curved path.

• The radio horizon exceeds the geometric horizon by approximately 15%.

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Radio Path Horizon

The distance D to the radio horizon is greater from a higherantenna. The maximum distance over which two stations maycommunicate by space wave is equal to the sum of theirdistances to the horizon.

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Radio Path Horizon

0.05.0

10.015.020.025.030.035.040.045.050.0

1.0 10.0 100.0 1000.0

Height(ft)

Distance(miles)

Chart shows theoretical communication distance (in miles) to the radio horizon for various transmitter antenna heights above terrain (in feet).

HxD 415.1

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Tropospheric Ducting

• Radio signals can also be trapped in the troposphere, traveling a long distance before returning to the Earth’s surface.

• Results when a “duct” is formed by a temperature inversion level (warm air over cold air) over land or water.– Adjacent tropospheric regions having different densities will bend

radio waves passing through the regions

• Most useful at VHF/UHF frequencies.

• Most frequent during spring, summer and fall.

• Can provide contacts of 950 miles or more over land and up to 2500 miles over ocean

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Tropospheric Ducting

When a cool air mass is overrun by a mass of warmer air, a “duct” may be formed, allowing VHF and UHF radio signals to travel great distances with little attenuation or signal loss.

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VHF/UHF Signals Through Ionosphere

• Sporadic E– A type of sky-wave propagation that allows long

distance communication on the VHF bands (6 meters, 2 meters and 220 Mhz) through the E region of the atmosphere.

– Occurs only sporadically during certain times of the year.

– Most common type of VHF atmospheric propagation.

– The 6 meter band is most likely experience sporadic-E propagation during the summer months ... even during periods of low sunspot activity.