Technician License Class

Technician License Class Chapter 4 Propagation, Antennas, and Feed Lines

Propagation • Radio waves travel outward from an antenna in a straight line • 3 phenomena possible after that: • Reflection • Bouncing wave off reflective surface • Refraction • Gradual bending of wave while traveling through atmosphere • Diffraction • Redirection of wave around edge of solid object

Propagation • Line-of-sight (LOS) • Direct from transmitting antenna to receiving antenna • Radio horizon • Point at which radio signals are blocked by curvature of the earth • Slightly greater than visual horizon • Refraction increases radio horizon distance by about 15%

Propagation • Diffraction: Redirection of wave around edge of solid object • Knife edge diffraction

Propagation • Multi-path • Transmitted radio waves reflected off various objects will arrive at receive antenna at different times • Result: “Picket fencing”

Propagation • Radio waves can pass through openings in solid objects • Longest dimension of opening at least ½ wavelength • Because of shorter wavelength, UHF signals can pass through building openings better than VHF signals

Propagation • Tropospheric Ducting • Radio waves can travel for long distances along vertical boundaries of different temperature air layers • Propagation of 300 miles or more possible on VHF or UHF

Propagation • Ionosphere • The upper layers of the atmosphere are ionized by UV radiation from the sun • 30 to 260 miles above the Earth’s surface

Propagation • Ionosphere • The ionosphere is divided into layers or regions • Each layer has unique characteristics

Propagation • Ionosphere • Transmissions in some radio frequency bands (e.g., HF & lower VHF) will be refracted off the ionosphere and returned to earth • Called “skip” • Skip distances are 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 http://www.spaceweather.com/ • Ionosphere • The higher the amount of ionization, the better radio waves are refracted by 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 and size of sunspots varies over an 11-year cycle • Currently in declining portion of Cycle 24

Propagation Ionosphere

Propagation • Ionosphere • Skip is refraction (bending), not reflection (bouncing) • The shorter the wavelength (higher frequency), the less the signal is refracted (bent). • At some given frequency, the wave is no longer bent enough to return to earth • Known as the “critical frequency” • Skip normally occurs in the F-layer (F1 & F2) • Can occur in the E-layer

Propagation • 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 over any 24-hour period depending on the amount of ionization in the ionosphere

Propagation Ionosphere

Propagation • Ionosphere • E-Layer Propagation • Sporadic-E • Can occur any time during the solar cycle • Highest probability: Early summer and mid-winter • Bands: 10 meters, 6 meters, and 2 meters • Aurora (Northern latitudes) • Rapid signal strength changes • Sounds fluttery or distorted • Primarily on 6 meters • Meteor scatter • Primarily 6 meters

Propagation • 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 • Little to no communications possible on lower frequency bands during the day when D and E layer are present

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? A. Change the batteries in your radio to a different type B. Turn on the CTCSS tone C. Ask the other operator to adjust his squelch control D. Try moving a few feet or changing the direction of your antenna if possible, as reflections may be causing multi-path distortion

T3A02 -- Why might the range of VHF and UHF signals be greater in the winter? new A. Less ionospheric absorption B. Less absorption by vegetation C. Less solar activityD. Less tropospheric absorption

T3A06 -- What term is commonly used to describe the rapid fluttering sound sometimes heard from mobile stations that are moving while transmitting? A. Flip-flopping B. Picket fencing C. Frequency shifting D. Pulsing

T3A08 -- Which of the following is a likely cause of irregular fading of signals received by ionospheric reflection? A. Frequency shift due to Faraday rotation B. Interference from thunderstorms C. Random combining of signals arriving via different paths D. Intermodulation distortion

T3A10 – What may occur if data signals arrive via multiple paths? new A. Transmission rates can be increased by a factor equal to the number of separate paths observed B. Transmission rates must be decreased by a factor equal to the number of separate paths observed C. No significant changes will occur if the signals are transmitted using FM D. Error rates are likely to increase

T3A12 – How might fog and light rain affect radio range on 10 meters and 6 meters? new A. Fog and rain absorb these wavelength bands B. Fog and light rain will have little effect on these bands C. Fog and rain will deflect these signals D. For and rain will increase radio range

T3A13 – What weather condition would decrease range at microwave frequencies? new A. High winds B. Low barometric pressure C. Precipitation D. Colder temperatures

T3C05 -- Which of the following effects might cause radio signals to be heard despite obstructions between the transmitting and receiving stations? A. Knife-edge diffraction B. Faraday rotation C. Quantum tunneling D. 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? A. Tropospheric scatter B. D layer refraction C. F2 layer refraction D. Faraday rotation

T3C08 -- What causes tropospheric ducting? A. Discharges of lightning during electrical storms B. Sunspots and solar flares C. Updrafts from hurricanes and tornadoes D. Temperature inversions in the atmosphere

T3C11 -- Why do VHF and UHF radio signals usually travel somewhat farther than the visual line of sight distance between two stations? A. Radio signals move somewhat faster than the speed of light B. Radio waves are not blocked by dust particles C. The Earth seems less curved to radio waves than to light D. Radio waves are blocked by dust particles

T3A11 -- Which part of the atmosphere enables the propagation of radio signals around the world? A. The stratosphere B. The troposphere C. The ionosphere D. The magnetosphere

T3C01 -- Why are direct (not via a repeater) UHF signals rarely heard from stations outside your local coverage area? A. They are too weak to go very far B. FCC regulations prohibit them from going more than 50 miles C. UHF signals are usually not reflected by the ionosphere D. UHF signals are absorbed by the ionospheric D layer

T3C02 -- Which of the following is an advantage of HF vs VHF and higher frequencies? new A. HF antennas are generally smaller B. HF accommodates wider bandwidth signals C. Long distance ionospheric propagation is far more common on HF D. There is less atmospheric interference (static) on HF

T3C03 -- What is a characteristic of VHF signals received via auroral reflection? A. Signals from distances of 10,000 or more miles are common B. The signals exhibit rapid fluctuations of strength and often sound distorted C. These types of signals occur only during winter nighttime hours D. 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? A. 10 meters B. 6 meters C. 2 meters D. 70 centimeters

T3C09 -- What is generally the best time for long-distance 10 meter band propagation via the F layer? A. From dawn to shortly after sunset during periods of high sunspot activity B. From shortly after sunset to dawn during periods of high sunspot activity C. From dawn to shortly after sunset during periods of low sunspot activity D. From shortly after sunset to dawn during periods of low sunspot activity

T3C10 -- Which of the following bands may provide long distance communications during the peak of the sunspot cycle? new A. Six or ten meters B. 23 centimeters C. 70 centimeters or 1.25 meters D. 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 and 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 the antenna • Feed Point Impedance: Ratio of RF voltage to RF current at the feed point • If impedance is purely resistive (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 • Reflector • Director

Antenna Fundamentals • Polarization • An electromagnetic wave consists of an electric wave and magnetic wave that propagate at right angles to each other • Polarization is the orientation of the electric wave with respect to the earth

Antenna Fundamentals • Polarization • If the 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 orientation of the antenna element • Loop antennas and circular polarization are exceptions • At VHF and UHF frequencies, if polarizations of transmit and receive antenna do not match, reduced received signal strength results • Polarization of refracted sky wave signals is random and continuously changing • Elliptically polarized • An antenna of any orientation may be used

Antenna Fundamentals • Decibels • The difference in strength between 2 signals is often expressed in decibels (dB) • Ratio between 2 values… a comparison • Based on logarithmic scale

Antenna Fundamentals • Decibels • Commonly used to: • Specify gain of an amplifier • Specify gain of an antenna • Specify loss in a feed line • Comparing output vs input level

Antenna Fundamentals • Decibels • Ratio between 2 values • One value is often a standard reference value, e.g., 1 watt or 1 milliwatt • Logarithmic scale • Power ratio: dB = 10 log10(P2/P1) • Voltage ratio: dB = 20 log10(E2/E1) • Where P2 is output and P1 is input • If dB is positive => gain • If dB is negative => loss

Antenna Fundamentals Loss Gain

Antenna Fundamentals • Antenna Gain • Omni-directional antennas radiate equally in all directions • Directional antennas focus radiation in one or more specific directions, e.g., beam antennas • Gain is the apparent increase in power in a particular direction because energy is focused in that direction • Measured in decibels (dB)

Antenna Fundamentals • Radiation Patterns • A way of visualizing antenna performance • The further the line is away from the center of the graph, the stronger the signal in that direction (gain)

Antenna Fundamentals Radiation Patterns

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