Radio propagation

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Radio propagation is a term used to explain how radio waves behave when they are transmitted, or are propagated from one point on the Earth to another.

In free space, all electromagnetic waves (radio, light, X-rays, etc) obey the inverse-square law which states that an electromagnetic wave's strength is proportional to:

  • Signal Strength \propto \frac {1}{r^2} where r is the distance (range) from the source.

Doubling the distance from a transmitter means the signal strength is reduced to nearly one quarter.

Propagation on Earth is not only affected by the inverse-square model, but by a number of other factors determined by its path from point to point. This path can be a direct line of sight path or an over-the-horizon (see also radio horizon) path aided by refraction in the ionosphere.

Lower frequencies (between 30 and 3000 kHz) have the property of following the curvature of the earth via groundwave propagation in the majority of occurrences. The interaction of radio waves with the ionized regions of the atmosphere makes radio propagation more complex to predict and analyze than in free space. Ionospheric radio propagation has a strong connection to space weather.

Since radio propagation is somewhat unpredictable, such services as emergency locator transmitters, in-flight communication with ocean-crossing aircraft, and some television broadcasting have been moved to satellite transmitters. A satellite link, though expensive, can offer highly predictable and stable line of sight coverage of a given area.

A sudden ionospheric disturbance is often the result of large solar flares directed at Earth. These solar flares can disrupt HF radio propagation and affect GPS accuracy.

Contents

[edit] Propagation modes

Radio waves at different frequencies propagate in different ways.

Radio Frequencies and their Primary mode of Propagation
Radio Band Frequency Propagation Via
VLF Very Low Frequency 3 - 30 kHz - Guided between the earth and the ionosphere
LF Low Frequency 30 - 300 kHz - Guided between the earth and the ionosphere

- Ground Waves
MF Medium Frequency 300 - 3000 kHz - Ground waves

- E layer ionospheric refraction at night, when D layer absorption disappears
HF High Frequency (Short Wave) 3 - 30 MHz - E layer ionospheric refraction

- F layer ionospheric refraction
VHF Very High Frequency 30 - 300 MHz

- Line-of-sight
UHF Ultra High Frequency 300 - 3000 MHz

- Line-of-sight
SHF Super High Frequency 3 - 30 GHz - Line-of-sight

[edit] The antenna

The beginning and end of a communication circuit is the antenna. The antenna can provide gain and directivity on both transmit and receive. The take-off angle of the antenna is based on the type of antenna, the height of the antenna above ground, and the type of ground below and in front of the antenna. The take-off angle will determine the angle of incidence on the ionosphere, which will affect where the signal will be refracted by the ionosphere.

[edit] Ground wave

Main article: Surface wave

Ground waves are radio waves that follow the curvature of the earth. These waves may be vertically polarized to allievate short circuiting the electric field through the conductivity of the ground. Since the ground is not a perfect electrical conductor, ground waves are attenuated as they follow the earth’s surface. At low frequencies, ground losses are low and become lower at lower frequencies. The VLF and LF frequencies are mostly used for military communications, especially with ships and submarines.

Early commercial and professional radio services relied exclusively on long wave, low frequencies and ground-wave propagation. To prevent interference with these services, amateur and experimental transmitters were restricted to the higher (HF) frequencies, felt to be useless since their ground-wave range was limited. Upon discovery of the other propagation modes possible at medium wave and short wave frequencies, the advantages of HF for commercial and military purposes became apparent. Amateur experimentation was then confined only to authorized frequency segments in the range.

[edit] Line-of-sight propagation

Line-of-sight is the direct propagation of radio waves between antennas that are visible to each other. This is probably the most common of the radio propagation modes. Because radio signals can travel through many non-metallic objects, radio can be picked up through walls even though you cannot see through them. This is still line-of-sight propagation. Examples would include propagation between a satellite and a ground antenna or reception of television signals from a local TV transmitter.

Ground plane reflection effects are an important factor in VHF line of sight propagation. The interference between the direct beam line-of-sight and the ground reflected beam often leads to an effective inverse-fourth-power law for ground-plane limited radiation.

[edit] Skywave propagation

Skywave propagation, also referred to as skip, is any of the modes that rely on refraction of radio waves in the ionosphere, which is made up of one or more ionized layers in the upper atmosphere. F2-layer is the most important ionospheric layer for HF propagation, though F1, E, and D-layers also play some role. These layers are directly affected by the sun on a daily cycle, and the 11-year (sunspot cycle) determines the utility of these modes. During solar cycle minimas frequencies only up to about 15 MHz are usable. During solar maxima whole HF range up to 30 MHz can be used and F2 propagation up to 50 MHz are observed frequently depending up on daily solar flux values.

Forecasting of skywave modes is of considerable interest to amateur radio operators and commercial marine and aircraft communications, and also to shortwave broadcasters.

[edit] Tropospheric scattering

At VHF and higher frequencies, small variation (turbulence) in the density of the atmosphere at a height of around 6 miles (10 kilometres) can scatter some of the normally line-of-sight beam of radio frequency energy back toward the ground, allowing over-the-horizon communication between stations as far as 500 miles (800 km) apart.

[edit] Tropospheric ducting and enhancement or refraction via inversion layer

Sudden change in atmosphere's vertical moisture content and temprature profiles can on random occations make microwave and UHF & VHF signals propagate hundreds of km up to about 2000 km and for Ducting mode even farther, beyond normal radio-horizon. The inversion layer is mostly observed over high pressure regions, but there are several tropospheric weather conditions which create these randomly occurring propagation modes. Inversion layer's altitude for non-Ducting is typically found between 100 m to about 1 km and for Ducting about 500 m to 3 km and the duration of the events are typically from several hours up to several days. Higher frequencies experience most dramatic increase of signal strengths, while on low-VHF and HF the effect is neglibigle. Propagation path attenuation may be below free-space loss. Some of the lesser inversion types related to warm ground and cooler air's moisture content occur regularly at certain times of the year and time of day.

[edit] Meteor scattering

Meteor scattering relies on reflecting radio waves off the intensely ionized columns generated by meteors. While this mode is very short-duration, often only from a fraction of second to couple of seconds per event, digital Meteor burst communications allows remote stations to communicate to a base that may be hundreds of miles up to over 1000 miles (1600 km) away, without the expense and power input required for a satellite link.

[edit] Auroral reflection

Intense colums of Auroral ionisation at 100 km altitudes within the auroral oval reflect radio waves, perhaps most notably on HF and VHF. The reflection is angle-sensitive - incident ray vs. magnetic field line of the column must be very close to right-angle. Random motions of electrons spiraling around the field lines create a doppler-spread that broadens the spectra of the emission to more or less noise-like - depending on how high radio frequency is used. The radio-aurora is observed mostly at high latitudes and rarely extend down to middle latitudes. The occurrences of radio-auroras depends on solar activity (flares, coronal holes, CMEs) and annually the events are more numerous during solar cycle maximas. Radio aurora includes the so-called afternoon radio aurora which produces stronger but more distorted signals and after the Harang-minima, the late-night radio aurora (sub-storming phase) returns with variable signal strength and lesser doppler spread. The propagation range for this predominantly back-scatter mode extends up to about 2000 km in east-west plane, but strongest signals are observed most frequently from north at nearby sites on same latitudes.

Rarely, a strong radio-aurora is followed by Auroral-E, which resembles both propagation types in some ways.

[edit] Sporadic-E propagation

Sporadic-E (Es) propagation can be observed on HF and VHF bands. It must not be confused with ordinary HF E-layer propagation. Sporadic-E at mid-latitudes occurs mostly during summer season, from May to August. There is no single cause for this mysterious propagation mode. The reflection takes place in thin sheet of ionisation around 90 km height. The ionisation patches drift westwards at speeds of few hundred km per hour. There is a weak periodicity noted during the season and typically Es is observed on 1..3 successive days and remains absent for a few days to reoccur again. Es do not occur during small hours, the events usually begin at dawn, there is a peak in the afternoon and a second peak in the evening. Es propagation is usually gone by local midnight.

Maximum observed frequency (MOF) for Es is found to be lurking around 30 MHz on most days during the summer season, but sometimes MOF may shoot up to 100 MHz or even more in ten minutes to decline slowly during the next few hours. The peak-phase includes oscillation of MOF with periodicity of approximately 5...10 minutes, possibly related to gravity waves. The propagation range for Es single-hop is typically 1000...2000 km, but with multihop, double range is observed. The signals are very strong but also with slow deep fading.

[edit] Rain scattering

Rain scattering is purely a microwave propagation mode and is best oberved around 10 GHz, but extends down to few GHz - the limit being the size of the scattering particle size vs. wavelength. This mode scatters signals mostly forwards and backwards when using horizontal polarisation and sidescattering with vertical polarisation. Forward-scattering typically yields propagation ranges of 800 km. Scattering from snowflakes and ice pellets also occurs, but scattering from ice without watery surface is less effective. The most common application for this phenomenon is microwave rain radar, but rain scatter propagation can be a nuisance causing unwanted signals to intermittently propagate where they are not anticipated or desired. Similar reflections may also occur from insects though at lower altitudes and shorter range. Rain also causes attenuation of point-to-point and satellite microwave links. Attenuation values up to 30 dB has been observed on 30 GHz during heavy tropical rain.

[edit] Aeroplane scattering

Aeroplane scattering (or most often reflection) is observed on VHF through microwaves and besides back-scattering, yields momentary propagation up to 500 km even in a mountain-type terrain. The most common back-scatter application is air-traffic radar and bistatic forward-scatter quided-missile and aeroplane detecting trip-wire radar and the US space radar.

[edit] Lightning scattering

Lightning scattering has sometimes been observed on VHF and UHF over distance of about 500 km. The hot lightning channel scatters radiowaves for a fraction of a second. The RF noise burst from the lightning makes the initial part of the open channel unusable and the ionisation disappears soon because of combination at low latitude high atmospheric pressure. This mode has no practical use.

[edit] Diffraction

Knife-Edge diffraction is the propagation mode where radio waves are bent around sharp edges. For example, this mode is used to send radio signals over a mountain range when a line-of-sight path is not available. However, the angle cannot be too sharp or the signal will not diffract. The diffraction mode requires increased signal strength, so higher power or better antennas will be needed than for an equivalent line-of-sight path.

Diffraction phenomena by small obstacles are also important at high frequencies. Signals for urban cellular telephony tend to be dominated by ground-plane effects as they travel over the rooftops of the urban environment. They then diffract over roof edges into the street, where multipath propagation, absorption and diffraction phenomena dominate.

[edit] Absorption

Low-frequency radio waves travel easily through brick and stone and VLF even penetrates sea-water. As the frequency rises, absorption effects become more important. At microwave or higher frequencies, absorption by molecular resonance in the atmosphere (mostly water, H2O and oxygen, O2) is a major factor in radio propagation. For example, in the 58 - 60 GHz band, there is a major absorption peak which makes this band useless for long-distance use. This phenomenon was first discovered during radar research during World War II. Beyond around 400 GHz, the Earth's atmosphere blocks some segments of spectra while still passes some - this is true up to UV light, which is blocked by ozone, but visible light and some of the NIR is transmitted.

Heavy rain and snow also affect microwave reception.

[edit] See also

[edit] References

  • Larry D. Wolfgang et. al, (ed), The ARRL Handbook for Radio Amateurs, Sixty-Eighth Edition , (1991), ARRL, Newington CT USA ISBN 0-87259-168-9

[edit] Further reading

  • Boithais, Lucien, "Radio Wave Propagation". McGraw-Hill Book Company, New York. 1987. ISBN 0-07-006433-4

[edit] External links

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