Technician License Class Propagation, Antennas and Feed Lines
Propagation • Radio waves travel in straight lines. • Except: • Reflection. • Bouncing off reflective surface. • Refraction. • Gradual bending while traveling through atmosphere. • Diffraction. • Bending around edge of solid object.
Propagation • Radio waves travel in straight lines. • Line-of-sight. • Radio horizon. • Distance at which radio signals are blocked by curvature of the earth. • Slightly greater than optical horizon. • Refraction increases radio horizon by about 15%.
Propagation • Diffraction.
Propagation • Multi-path. • Radio waves reflected off of many objects arrive at receive antenna at different times. • Picket fencing.
Propagation • Radio waves can pass through openings in solid objects. • Longest dimension of opening at least 1/2λ. • Because of shorter wavelength, UHF signals can pass through buildings better than VHF signals.
Propagation • Tropospheric Ducting • Radio waves can travel for long distances along boundaries of different temperature air layers. • Propagation of 300 miles or more on VHF or UHF.
T3A01 -- What should you do if another operator reports that your station’s 2 meter signals were strong just a moment ago, but now they are weak or distorted? • Change the batteries in your radio to a different type • Turn on the CTCSS tone • Ask the other operator to adjust his squelch control • Try moving a few feet or changing the direction of your antenna if possible, as reflections may be causing multi-path distortion
T3A02 -- Why are UHF signals often more effective from inside buildings than VHF signals? • VHF signals lose power faster over distance • The shorter wavelength allows them to more easily penetrate the structure of buildings • This is incorrect; VHF works better than UHF inside buildings • UHF antennas are more efficient than VHF antennas
T3A06 -- What term is commonly used to describe the rapid fluttering sound sometimes heard from mobile stations that are moving while transmitting? • Flip-flopping • Picket fencing • Frequency shifting • Pulsing
T3A08 -- Which of the following is a likely cause of irregular fading of signals received by ionospheric reflection? • Frequency shift due to Faraday rotation • Interference from thunderstorms • Random combining of signals arriving via different paths • Intermodulation distortion
T3C05 -- Which of the following effects might cause radio signals to be heard despite obstructions between the transmitting and receiving stations? • Knife-edge diffraction • Faraday rotation • Quantum tunneling • Doppler shift
T3C06 -- What mode is responsible for allowing over-the-horizon VHF and UHF communications to ranges of approximately 300 miles on a regular basis? • Tropospheric scatter • D layer refraction • F2 layer refraction • Faraday rotation
T3C08 -- What causes tropospheric ducting? • Discharges of lightning during electrical storms • Sunspots and solar flares • Updrafts from hurricanes and tornadoes • Temperature inversions in the atmosphere
T3C10 -- What is the radio horizon? • The distance over which two stations can communicate by direct path • The distance from the ground to a horizontally mounted antenna • The farthest point you can see when standing at the base of your antenna tower • The shortest distance between two points on the Earth's surface
T3C11 -- Why do VHF and UHF radio signals usually travel somewhat farther than the visual line of sight distance between two stations? • Radio signals move somewhat faster than the speed of light • Radio waves are not blocked by dust particles • The Earth seems less curved to radio waves than to light • Radio waves are blocked by dust particles
Propagation • The Ionosphere. • The upper layers of the atmosphere are ionized by UV radiation from the sun. • 30 to 260 miles above the surface.
Propagation • The Ionosphere. • The ionosphere is divided into layers or regions. • Each layer has unique characteristics.
Propagation • The Ionosphere. • Some radio frequency ranges (HF & lower VHF frequencies) will be reflected off of the ionosphere & return to earth. • Called “skip”. • Distances well beyond the range of line-of-sight. • Several hundred to several thousand miles. • Maximum of about 2500 miles for a single hop. • Can have multiple hops.
Propagation • The Ionosphere. • The higher the amount of ionization, the better radio waves are reflected off of the ionosphere. • Amount of ionization varies with time of day. • Sunrise to sunset higher ionization level. • Amount of ionization varies with sunspot activity. • More sunspots higher ionization level. • Larger sunspots higher ionization level. • Number & size of sunspots varies over an 11-year cycle. • Currently in declining portion of Cycle 24.
Propagation • The Ionosphere.
Propagation • The Ionosphere. • Skip is not really reflection (bouncing) but rather refraction (bending). • The shorter the wavelength (higher frequency), the less the signal is refracted (bent). • At some frequency, the wave is no longer bent enough to return to earth. • Critical frequency. • Skip normally occurs in the F-layer (F1 & F2). • Can occur in the E-layer.
Propagation • The Ionosphere. • The highest frequency that can be used to communicate between 2 points is called the Maximum Useable Frequency (MUF). • The lowest frequency that can be used to communicate between 2 points is called the Lowest Useable Frequency (LUF). • MUF & LUF vary depending on amount of ionization of the ionosphere.
Propagation • The Ionosphere.
Propagation • The Ionosphere. • E-Layer Propagation. • Sporadic-E. • Any time during solar cycle. • Early summer & mid-winter • 10m, 6m, & 2m. • Aurora. • Rapid signal strength changes. • Sounds fluttery or distorted. • Primarily 6m. • Meteor scatter. • Primarily 6m.
Propagation • The Ionosphere. • The lowers regions of the ionosphere absorb radio waves. • Primarily D-layer. • Some absorption in E-layer. • The longer the wavelength (lower frequency), the more absorption.
T3A11 -- Which part of the atmosphere enables the propagation of radio signals around the world? • The stratosphere • The troposphere • The ionosphere • The magnetosphere
T3C01 -- Why are direct (not via a repeater) UHF signals rarely heard from stations outside your local coverage area? • They are too weak to go very far • FCC regulations prohibit them from going more than 50 miles • UHF signals are usually not reflected by the ionosphere • They collide with trees and shrubbery and fade out
T3C02 -- Which of the following might be happening when VHF signals are being received from long distances? • Signals are being reflected from outer space • Signals are arriving by sub-surface ducting • Signals are being reflected by lightning storms in your area • Signals are being refracted from a sporadic E layer
T3C03 (B)-- What is a characteristic of VHF signals received via auroral reflection? • Signals from distances of 10,000 or more miles are common • The signals exhibit rapid fluctuations of strength and often sound distorted • These types of signals occur only during winter nighttime hours • These types of signals are generally strongest when your antenna is aimed west
T3C04 -- Which of the following propagation types is most commonly associated with occasional strong over-the-horizon signals on the 10, 6, and 2 meter bands? • Backscatter • Sporadic E • D layer absorption • Gray-line propagation
T3C07 -- What band is best suited for communicating via meteor scatter? • 10 meters • 6 meters • 2 meters • 70 cm
T3C09 -- What is generally the best time for long-distance 10 meter band propagation via the F layer? • From dawn to shortly after sunset during periods of high sunspot activity • From shortly after sunset to dawn during periods of high sunspot activity • From dawn to shortly after sunset during periods of low sunspot activity • From shortly after sunset to dawn during periods of low sunspot activity
T3C12 -- Which of the following bands may provide long distance communications during the peak of the sunspot cycle? • Six or ten meters • 23 centimeters • 70 centimeters or 1.25 meters • All of these choices are correct
Antenna Fundamentals • Definitions: • Antenna: Converts an RF electrical signal into an electromagnetic wave (radio wave) or vice versa. • Any electrical conductor can act as an antenna. • Some sizes & configurations work better than others. • Feed-line: Conducts the RF electrical signal to/from the antenna. • a.k.a. – Transmission Line.
Antenna Fundamentals • Definitions: • Feed Point: Place where the feed-line is connected to antenna. • Feed Point Impedance: Ratio of RF voltage to RF current at the feed point. • If impedance is pure resistance (no reactance) then antenna is said to be “resonant”
Antenna Fundamentals • Definitions: • Antenna Elements: Conductive parts of an antenna. • Driven Element: Element that feed-line is connected to. • Driven Array: More than one driven element. • Parasitic Element(s): Element(s) not directly connected to feed-line.
Antenna Fundamentals • Polarization. • An electromagnetic wave consists of an electric wave & a magnetic wave at right angles to each other. • Polarization is the orientation of the electric wave with respect to the earth.
Antenna Fundamentals • Polarization. • If electric wave is horizontal (parallel to the ground), then wave is said to be horizontally polarized. • If electric wave is vertical (perpendicular to the ground), then wave is said to be vertically polarized.
Antenna Fundamentals • Polarization.
Antenna Fundamentals • Polarization. • The direction of the electric field is the same as the direction of the antenna element. • Loop antennas & circular polarization are exceptions. • If polarizations are not matched, reduced signal strength results. • Polarization of sky wave signals (skip) is random & continuously changing. • Elliptically polarized. • Any polarization antenna may be used.
T3A04 -- What can happen if the antennas at opposite ends of a VHF or UHF line of sight radio link are not using the same polarization? • The modulation sidebands might become inverted • Signals could be significantly weaker • Signals have an echo effect on voices • Nothing significant will happen
T3A07 -- What type of wave carries radio signals between transmitting and receiving stations? • Electromagnetic • Electrostatic • Surface acoustic • Magnetostrictive
T3A09 -- Which of the following results from the fact that skip signals refracted from the ionosphere are elliptically polarized? • Digital modes are unusable • Either vertically or horizontally polarized antennas may be used for transmission or reception • FM voice is unusable • Both the transmitting and receiving antennas must be of the same polarization
T3B02 -- What property of a radio wave is used to describe its polarization? • The orientation of the electric field • The orientation of the magnetic field • The ratio of the energy in the magnetic field to the energy in the electric field • The ratio of the velocity to the wavelength
T3B03 -- What are the two components of a radio wave? • AC and DC • Voltage and current • Electric and magnetic fields • Ionizing and non-ionizing radiation
T5C07 -- What is a usual name for electromagnetic waves that travel through space? • Gravity waves • Sound waves • Radio waves • Pressure waves
T9A02 -- Which of the following is true regarding vertical antennas? • The magnetic field is perpendicular to the Earth • The electric field is perpendicular to the Earth • The phase is inverted • The phase is reversed
Antenna Fundamentals • Decibels. • The difference in strength between 2 signals is often expressed in decibels (dB). • Ratio between 2 values • Logarithmic scale.
Antenna Fundamentals • Decibels. • Commonly used to: • Specify gain of an amplifier. • Specify gain of an antenna. • Specify loss in a feed line.