Amateur Extra License Class Chapter 10 Topics in Radio Propagation
HF Propagation • In nearly all cases, HF waves travel along the surface of the earth or they are returned to earth after encountering the upper layers of the ionosphere.
HF Propagation • All types of waves can change direction due to two different phenomena: • Diffraction. • Encountering a reflecting surface’s edge or corner. • Refraction. • Change in velocity due to change in properties of medium wave is traveling through.
HF Propagation • Ground Wave • Special type of diffraction. • Lower edge of wave (closest to the earth) loses energy due to induced ground currents. • Lower edge slows, tilting wave front forward. • Primarily effects vertically-polarized waves. • Most noticeable on longer wavelengths. • AM broadcast, 160m, & 80m. • Over distance, ground wave signal is absorbed, decreasing strength. • More pronounced at shorter wavelengths. • Most useful during daylight on 160m & 80m.
E3C12 -- How does the maximum distance of ground-wave propagation change when the signal frequency is increased? It stays the same It increases It decreases It peaks at roughly 14 MHz
E3C13 -- What type of polarization is best for ground-wave propagation? Vertical Horizontal Circular Elliptical
HF Propagation • Sky Wave • Radio waves refracted in the E & F layers of the ionosphere. • Maximum one-hop skip distance about 2500 miles.
HF Propagation • Sky Wave • Pedersen Ray. • High angle wave. • Provides propagation beyond normal maximum skip distance.
HF Propagation • Sky Wave • Absorption. • D layer. • Ionized only during sunlight. • Absorbs RF energy. • The longer the wavelength, the more absorption. • Kills sky wave propagation on 160m & 80m during daylight hours.
E3C08 -- What is the name of the high-angle wave in HF propagation that travels for some distance within the F2 region? Oblique-angle ray Pedersen ray Ordinary ray Heaviside ray
HF Propagation • Long Path and Gray Line • Long path. • Radio waves travel a great-circle path between 2 stations. • The path is shorter in one direction & longer in the other. • The normal path is the shorter. • The long path is 180° from the short path.
HF Propagation • Long Path and Gray Line • Long path. • A slight echo on the received may indicate that long-path propagation is occurring. • With long path propagation, the received signal may be stronger if antenna is pointed 180° away from the station. • Long path propagation can occur on all MF & HF bands. • 160m through 10m. • Most often on 20m.
HF Propagation • Long Path and Gray Line • Gray line propagation. • At sunset, D layer collapses rapidly, reducing adsorption. • F layer collapses more slowly. • Similar effect occurs at sunrise. • Net result is that long distance communications are possible during twilight hours on the lower frequency bands. • 8,000 to 10,000 miles. • 160m, 80m, 40m, & possibly 30m.
HF Propagation • Long Path and Gray Line • Gray line propagation.
E3B04 -- What type of propagation is probably occurring if an HF beam antenna must be pointed in a direction 180 degrees away from a station to receive the strongest signals? Long-path Sporadic-E Transequatorial Auroral
E3B05 -- Which amateur bands typically support long-path propagation? 160 to 40 meters 30 to 10 meters 160 to 10 meters 6 meters to 2 meters
E3B06 -- Which of the following amateur bands most frequently provides long-path propagation? 80 meters 20 meters 10 meters 6 meters
E3B07 -- Which of the following could account for hearing an echo on the received signal of a distant station? High D layer absorption Meteor scatter Transmit frequency is higher than the MUF Receipt of a signal by more than one path
E3B08 -- What type of HF propagation is probably occurring if radio signals travel along the terminator between daylight and darkness? Transequatorial Sporadic-E Long-path Gray-line
E3B09 -- At what time of day is gray-line propagation most likely to occur? At sunrise and sunset When the Sun is directly above the location of the transmitting station When the Sun is directly overhead at the middle of the communications path between the two stations When the Sun is directly above the location of the receiving station
E3B10 -- What is the cause of gray-line propagation? At midday, the Sun being directly overhead superheats the ionosphere causing increased refraction of radio waves At twilight, D-layer absorption drops while E-layer and F-layer propagation remain strong In darkness, solar absorption drops greatly while atmospheric ionization remains steady At mid afternoon, the Sun heats the ionosphere decreasing radio wave refraction and the MUF
E3B11 -- Which of the following describes gray-line propagation? Backscatter contacts on the 10 meter band Over the horizon propagation on the 6 and 2 meter bands Long distance communications at twilight on frequencies less than 15 MHz Tropospheric propagation on the 2 meter and 70 centimeter bands
HF Propagation • Fading • Variations in strength of received signals. • Changes in height of ionized layers. • Changes in amount of absorption. • Random polarization shifts. • Multi-path reflections.
HF Propagation • Fading • Selective fading. • Fading can have a different effect signals that are only a few hundred Hertz apart. • Can cause loss of mark or space signal of RTTY transmission. • Most severely affects wide-bandwidth signals such as AM or FM. • Can cause moderate to severe distortion of received signal.
E3C05 -- Which of the following describes selective fading? Variability of signal strength with beam heading Partial cancellation of some frequencies within the received pass band Sideband inversion within the ionosphere Degradation of signal strength due to backscatter
VHF/UHF/Microwave Propagation • Above 30 MHz, radio waves are rarely refracted back to earth by the ionosphere. • Must use other techniques for long-distance communications. • Low-angle of radiation from the antenna is more important than on HF. • It is more important for polarization of transmitting & receiving antennas to match than on HF.
VHF/UHF/Microwave Propagation • Radio Horizon • Radio horizon not the same as visual horizon. • Refraction in the atmosphere bends radio waves & increases “line-of-sight” distance by about 15%. Visual Horizon (miles) ≈ 1.32 Hft Radio Horizon (miles) ≈ 1.415 Hft
VHF/UHF/Microwave Propagation • Multipath • Radio waves reflected off of many objects arrive at receive antenna at different times. • Waves reinforce or cancel each other depending on phase relationship. • Picket fencing.
E3C06 -- By how much does the VHF/UHF radio-path horizon distance exceed the geometric horizon? By approximately 15% of the distance By approximately twice the distance By approximately one-half the distance By approximately four times the distance
E3C14 -- Why does the radio-path horizon distance exceed the geometric horizon? E-region skip D-region skip Downward bending due to aurora refraction Downward bending due to density variations in the atmosphere
VHF/UHF/Microwave Propagation • Tropospheric Propagation • VHF/UHF propagation normally limited to 500 miles. • Certain atmospheric conditions can create a “duct” where radio waves can travel for hundreds or thousands of miles. • Bands: • 6m – Rare. • 2m – Fairly common. • 70cm – Common.
VHF/UHF/Microwave Propagation • Tropospheric Propagation
E3C09 -- Which of the following is usually responsible for causing VHF signals to propagate for hundreds of miles? D-region absorption Faraday rotation Tropospheric ducting Ground wave
VHF/UHF/Microwave Propagation • Transequatorial Propagation • Communications between stations located an equal distance north & south of the magnetic equator.
VHF/UHF/Microwave Propagation • Transequatorial Propagation • Most prevalent around the spring & autumn equinoxes. • Maximum effect during afternoon & early evening. • Allows contacts up to about 5,000 miles. • Useable up to 2m & somewhat on 70cm. • As frequency increases, paths more restricted to exactly equidistant from and perpendicular to the magnetic equator.
E3B01 -- What is transequatorial propagation? Propagation between two mid-latitude points at approximately the same distance north and south of the magnetic equator Propagation between any two points located on the magnetic equator Propagation between two continents by way of ducts along the magnetic equator Propagation between two stations at the same latitude
E3B02 -- What is the approximate maximum range for signals using transequatorial propagation? 1000 miles 2500 miles 5000 miles 7500 miles
E3B03 -- What is the best time of day for transequatorial propagation? Morning Noon Afternoon or early evening Late at night
VHF/UHF/Microwave Propagation • Auroral Propagation
VHF/UHF/Microwave Propagation • Auroral Propagation • Charged particles from the sun (solar wind) are concentrated over the magnetic poles by the earth’s magnetic field & ionize the E-layer. • VHF & UHF propagation up to about 1,400 miles.
VHF/UHF/Microwave Propagation • Auroral Propagation • Reflections change rapidly. • All signals sound fluttery. • SSB signals sound raspy. • CW signals sound like they are modulated with white noise. • CW most effective mode. • Point antenna toward aurora, NOT towards station. • In US, point antenna north.
E3C01 -- Which of the following effects does Aurora activity have on radio communications? SSB signals are raspy Signals propagating through the Aurora are fluttery CW signals appear to be modulated by white noise All of these choices are correct
E3C02 -- What is the cause of Aurora activity? The interaction between the solar wind and the Van Allen belt A low sunspot level combined with tropospheric ducting The interaction of charged particles from the Sun with the Earth’s magnetic field and the ionosphere Meteor showers concentrated in the northern latitudes
E3C03 -- Where in the ionosphere does Aurora activity occur? In the F1-region In the F2-region In the D-region In the E-region
E3C04 -- Which emission mode is best for Aurora propagation? CW SSB FM RTTY
E3C11 -- From the contiguous 48 states, in which approximate direction should an antenna be pointed to take maximum advantage of aurora propagation? South North East West
VHF/UHF/Microwave Propagation • Meteor Scatter Communications • Meteors passing through the ionosphere collide with air molecules & strip off electrons. • Ionization occurs at or near the E-region. • Best propagation 28 MHz to 148 MHz. • 20 MHz to 432 MHz possible.
VHF/UHF/Microwave Propagation • Meteor Scatter Communications • Major meteor showers. • Quadrantids January 3-5. • Lyrids – April 19-23. • Arietids – June 8. • Aquarids – July 26-31. • Perseids – July 27 to August 14. • Orionids – October 18-234. • Taurids – October 26 to November 16. • Leonids – November 14-16. • Geminids – December 10-14. • Ursids – December 22.
VHF/UHF/Microwave Propagation • Meteor Scatter Communications • Operating techniques. • Keep transmissions SHORT. • Divide each minute into four 15-second segments. • Stations at west end of path transmit during 1st & 3rd segments. • Stations at east end of path transmit during 2nd & 4th segments.