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    Radio Waves Propagation
    Experiments, Labs, Studies and Background Information







    Experiments and Studies

    • Heinrich Hertz: The Discovery of Radio Waves (Hertz’s Experiments that Confirmed the Existence of Electromagnetic Radiation [View Experiment]
    • Ionospheric Physics of Radio Wave Propagation [View Experiment]
    • Measure the radiation pattern and radio wave propagation of a half-wave dipole antenna [View Experiment]
    • An Experiment Study of the Propagation of Radio Waves in a Scaled Model of Long-Wall Coal Mining Tunnels [View Experiment]
    • Lab: Interference of Light Waves [View Experiment]
    • Measurement of Antenna Radiation Patterns: Laboratory Manual [View Experiment]
    • Lab: Wave Propagation: High Frequency Slotted Line and Reflectometer Measurements [View Experiment]
    • Tesla’s Contribution to Radio Wave Propagation [View Experiment]
    • Thesis: The propagation of VHF and UHF radio waves over sea paths [View Experiment]
    Radio Waves Propagation

    Definition

    Radio waves 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.

    Radio Waves Propagation

    See also Solar Cycle
    See also Multipath & Fading

    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 the power density of an electromagnetic wave is proportional to the inverse of the square of "r" (where "r" is the distance [radius] from the source) or:

    \rho_P \propto \frac{1}{r^2}

    Doubling the distance from a transmitter means that the power density of the radiated wave at that new location is reduced to one-quarter of its previous value.

    The far-field magnitudes of the electric and magnetic field components of electromagnetic radiation are equal, and their field strengths are inversely proportional to distance. The power density per surface unit is proportional to the product of the two field strengths, which are expressed in linear units. Thus, doubling the propagation path distance from the transmitter reduces their received field strengths by one-half.

    Electromagnetic wave propagation is also affected by several 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 path aided by refraction in the ionosphere.

    Lower frequencies (between 30 and 3,000 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.

    Radio waves at different frequencies propagate in different ways.

    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 terrain 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.


    Radio frequencies and their primary mode of propagation
    Band Frequency Wavelength Propagation via
    VLF Very Low Frequency 3 – 30 kHz 100 – 10 km Guided between the earth and the ionosphere.
    LF Low Frequency 30 – 300 kHz 10 – 1 km Guided between the earth and the D layer of the ionosphere.

    Surface waves.

    MF Medium Frequency 300 – 3000 kHz 1000 – 100 m Surface waves.

    E, F layer ionospheric refraction at night, when D layer absorption weakens.

    HF High Frequency (Short Wave) 3 – 30 MHz 100 – 10 m E layer ionospheric refraction.

    F1, F2 layer ionospheric refraction.

    VHF Very High Frequency 30 – 300 MHz 10 – 1 m Infrequent E ionospheric refraction. Extremely rare F1,F2 layer ionospheric refraction during high sunspot activity up to 80 MHz. Generally direct wave. Sometimes tropospheric ducting.
    UHF Ultra High Frequency 300 – 3000 MHz 100 – 10 cm Direct wave. Sometimes tropospheric ducting.
    SHF Super High Frequency 3 – 30 GHz 10 – 1 cm Direct wave.
    EHF Extremely High Frequency 30 – 300 GHz 10 – 1 mm Direct wave limited by absorption.


    Radio spectrum
    ELF SLF ULF VLF LF MF HF VHF UHF SHF EHF
    3 Hz 30 Hz 300 Hz 3 kHz 30 kHz 300 kHz 3 MHz 30 MHz 300 MHz 3 GHz 30 GHz
    30 Hz 300 Hz 3 kHz 30 kHz 300 kHz 3 MHz 30 MHz 300 MHz 3 GHz 30 GHz 300 GHz


    The above table listed as radio spectrum is incorrect in its identification of frequency bands for ULF, ELF and AF (audio frequency) ULF = 3 to 30 Hz, ELF = 30 to 300 Hz, Audio Frequency = 300 to 3000 Hz, VLF = 3000 to 30000 Hz, LF = 30 kHz to 300 kHz, MF = 300 kHz to 3 MHz, HF = 3 to 30 MHz, VLF = 30 to 300 MHz, UHF = 300 to 3000 MHz SHF = 3 to 30 GHz EHF = 30 to 300 HGz

    Modes

    Surface Modes

    The mode is commonly called the "Ground wave". This can cause confusion since the Direct mode is also sometimes called the "Ground wave".

    In this mode the radio wave propagates by interacting with the semi-conductive surface of the earth. The wave "clings" to the surface and thus follows the curvature of the earth. Vertical polarization is used to alleviate short circuiting the electric field through the conductivity of the ground. Since the ground is not a perfect electrical conductor, ground waves are attenuated rapidly as they follow the earth’s surface. Attenuation is proportional to the frequency making this mode mainly useful for LF and VLF frequencies.

    Today LF and VLF are mostly used for time signals, and 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.

    Direct Modes (Line-of-sight)

    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 at VHF and higher frequencies. Because radio signals can travel through many non-metallic objects, radio can be picked up through walls. 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.

    Ionospheric Modes (Sky-Wave)

    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, the seasons and the 11-year sunspot cycle determines the utility of these modes. During solar maxima, the whole HF range up to 30 MHz can be used and F2 propagation up to 50 MHz are observed frequently depending upon daily solar flux values. During solar minima, propagation of higher frequencies is generally worse.

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

    Meteor scattering

    Meteor scattering relies on reflecting radio waves off the intensely ionized columns of air 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 station that may be hundreds of miles up to over 1,000 miles (1,600 km) away, without the expense required for a satellite link. This mode is most generally useful on VHF frequencies between 30 and 250 MHz.

    Auroral reflection

    Intense columns of Auroral ionization 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.

    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 in the northern hemisphere and from November to February in the southern hemisphere. There is no single cause for this mysterious propagation mode. The reflection takes place in a 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 to 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 multi-hop, double range is observed. The signals are very strong but also with slow deep fading.

    Thomas F. Giella, a noted retired Meteorologist, Space Plasma Physicist and Amateur Radio Operator, KN4LF cites the following from his professional research.

    Just as the E layer is the main refraction medium for medium frequency (300-3000 kc) signal propagation within approximately 5000 km/3100 mi, so is a Sporadic-E (Es) cloud. Sporadic-E (Es) clouds occur at approximately 100 km/60 miles in altitude and generally move from ESE to WNW. Like Stratosphere level warming and Troposphere level temperature and moisture discontinuities, Sporadic-E (Es) clouds can depending on the circumstances absorb, block or refract medium, high and very high frequency RF signals in an unpredictable manner.

    The main source for "high latitude" Sporadic E (Es) clouds is geomagnetic storming induced radio aurora activity.

    The main source for "mid latitude" Sporadic-E (Es) clouds is wind shear produced by internal buoyancy/gravity waves (IBGW's), that create traveling ionosphere disturbances (TID's), most of which are produced by severe thunderstorm cell complexes with overshooting tops that penetrate into the Stratosphere. Another tie in between Sporadic-E (Es) and a severe thunderstorm is the Elve.

    The main sources for "low latitude" Sporadic-E (Es) clouds is wind shear produced by internal buoyancy/gravity waves (IBGW's), that create traveling ionosphere disturbances, most of which are produced by severe thunderstorm cell complexes tied to tropical cyclones. High electron content in the Equatorial Ring Current also plays a role.

    The forecasting of Sporadic-E (Es) clouds has long been considered to be impossible. However it is possible to identify certain troposphere level meteorological conditions that can lead to the formation of Sporadic E (Es) clouds. One is as mentioned above the severe thunderstorm cell complex.

    Sporadic-E (Es) clouds have been observed to initially occur within approximately 150 km/90 mi to the right of a severe thunderstorm cell complex in the northern hemisphere, with the opposite being observed in the southern hemisphere. To complicate matters is the fact that Sporadic-E (Es) clouds that initially form to the right of a severe thunderstorm complex in the northern hemisphere, then move from ESE-WNW and end up to the left of the severe thunderstorm complex in the northern hemisphere. So one has to look for Sporadic-E (Es) clouds on either side of a severe thunderstorm cell complex. Things get even more complicated when two severe thunderstorm cell complexes exist approximately 1000- 2000 miles apart.

    Not all thunderstorm cell complexes reach severe levels and not all severe thunderstorm cell complexes produce Sporadic-E (Es). This is where knowledge in tropospheric physics and weather analyses/forecasting is necessary.

    Some of the key elements in identifying which severe thunderstorm cell complexes have the potential to produce Sporadic-E (Es) via wind shear, from internal buoyancy/gravity waves, that produce traveling ionosphere disturbances include:

    1.) Negative tilted mid and upper level long wave troughs.

    2.) Approximate 150 knot/170 mph jet stream jet maxes that produce divergence and therefore create a sucking vacuum effect above thunderstorm cells, that assist thunderstorm cells in reaching and penetrating the Tropopause into the Stratosphere.

    3.) 500 mb temperatures of -20 deg. C or colder, which produce numerous positive and negative lightning bolts and inter-related Sprites and Elves.

    4.) Approximate 150-175 knot/172-200 mph updrafts within thunderstorm cells complexes that create overshooting tops that penetrate the Tropopause into the Stratosphere, launching upwardly propagating internal buoyancy/gravity waves, which create traveling ionosphere disturbances and then wind shear.

    Tropospheric modes

    Tropospheric scattering

    At VHF and higher frequencies, small variation (turbulence) in the density of the atmosphere at a height of around 6 miles (10 km) 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.

    Tropospheric ducting and enhancement or refraction via inversion layer

    Sudden changes in the atmosphere's vertical moisture content and temperature profiles can on random occasions make microwave and UHF & VHF signals propagate hundreds of kilometers up to about 2,000 kilometers (1,300 mi)—and for ducting mode even farther—beyond the 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 meters (300 ft) to about 1 kilometer (3,000 ft) and for ducting about 500 meters (1,600 ft) to 3 kilometers (2 mi), and the duration of the events are typically from several hours up to several days. Higher frequencies experience the most dramatic increase of signal strengths, while on low-VHF and HF the effect is negligible. Propagation path attenuation may be below free-space loss. Some of the lesser inversion types related to warm ground and cooler air moisture content occur regularly at certain times of the year and time of day.

    Rain scattering

    Rain scattering is purely a microwave propagation mode and is best observed 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 polarization and side-scattering with vertical polarization. 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 have been observed on 30 GHz during heavy tropical rain.

    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 guided-missile and aeroplane detecting trip-wire radar and the US space radar.

    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.


    Other effects

    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 depends on the relationship between the wavelength and the size of the obstacle. In other words, the size of the obstacle in wavelengths. Lower frequencies diffract around large smooth obstacles such as hills more easily. For example, in many cases where VHF (or higher frequency) communication is not possible due to shadowing by a hill, one finds that it is still possible to communicate using the upper part of the HF band where the surface wave is of little use.

    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.

    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.

    Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License and Creative Commons Attribution-ShareAlike License.)

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