SCARS Technician / General License Course Week 4

Transcription

1 SCARS Technician / General License Course Week 4

2 Radio Wave Propagation: Getting from Point A to Point B Radio waves propagatein many ways depending on Frequency of the wave Characteristics of the environment We will discuss three basic ways: Line of sight Ground wave Sky wave

3 Line-of-Sight Radio energy can travel in a straight line from a transmitting antenna to a receiving antenna called the direct path There is some attenuation of the signal as the radio wave travels due to spreading out This is the primary propagation mode for VHF and UHF signals.

4 Ground Wave At lower HF frequencies radio waves can follow the Earth s surface as they travel. These waves will travel beyond the range of lineof-sight. Range of a few hundred miles on bands used by amateurs.

5 Reflect, Refract, Diffract Diffraction occurs when a wave encounters a sharp edge (knife-edge propagation) or corner

6 VHF and UHF Propagation Range is slightly better than visual line of sight due to gradual refraction (bending), creating the radio horizon. UHF signals penetrate buildings better than HF/VHF because of the shorter wavelength. Buildings may block line of sight, but reflected and diffracted waves can get around obstructions.

7 VHF and UHF Propagation Multi-path results from reflected signals arriving at the receiver by different paths and interfering with each other. Picket-fencing is the rapid fluttering sound of multi-path from a moving transmitter

8 Tropo -Tropospheric Propagation The troposphere is the lower levels of the atmosphere to about 30 miles high Radio waves can be reflected or scattered by clouds, rain, and density variations in the troposphere range up to about 300 miles Temperature inversions and weather fronts can form ductsthat trap and conduct VHF and UHF radio waves for hundreds of miles

9 A region from 30 to 260 miles above the surface of the Earth Atmosphere thin enough for atoms to be ionized by solar ultraviolet radiation Ions are electrically conductive The Ionosphere

10 Higher ionization refracts or bends radio waves more strongly Ionospheric Levels

11 Sunspot Cycle The level of ionization depends on the intensity of radiation from the Sun. Radiation from the Sun varies with the number of sunspots on the Sun s surface. High number of sunspots results in high levels of ionizing radiation emitted from the Sun. Sunspot activity follows an 11-year cycle.

12 The Ionosphere An RF Mirror Reflection depends on frequency and angle of incidence. Too high a frequency or angle and the waves are lost to space.

13 The Ionosphere An RF Mirror Sky-wave or skip propagation is responsible for most over-the-horizon propagation on HF and low VHF (10 and 6 meters) during peaks of the sunspot cycle. Skip is very rare on the 144 MHz and higher UHF bands. Each ground-to-sky-to-ground trip is called a hop.

14 The Ionosphere An RF Mirror Signals can take many paths through the ionosphere. Randomly combining at the receiving antenna, signals can partially cancel, creating irregular fading as the ionosphere changes. The resulting echo and flutter distort speech and CW. Fading causes data errors for digital signals.

15 Sporadic E (Es) and Aurora Highly ionized patches of the E layer can reflect HF and VHF signals best on 10, 6, and 2 meters. Aurora near the north and south poles can also reflect VHF and UHF waves with a distinctive distorted sound.

16 Meteor Scatter Thousands of meteors enter the Earth s atmosphere every day most quite small. Meteors leave trails of highly ionized gas that last for several seconds. Trails can reflect radio waves called meteor scatter.the best band for this is 6 meters. Mostly in the E layer, meteor scatter and sporadic E supports contacts up to about 1500 miles.

17 The Antenna System Antenna: Transforms current into radio waves (transmit) and vice versa (receive). Feed line: Connects your station to the antenna. Test and matching equipment: Allows you to monitor and optimize antenna system performance.

18 The Antenna (Some Vocabulary) Element: The conducting part or parts of an antenna designed to radiate or receive radio waves. Driven element: The element supplied directly with power from the transmitter. Array: An antenna with more than one element.

19 The Antenna (Some Vocabulary) Parasitic element: Elements not connected directly to a feed line. Resonant: An antenna is resonant when its feed point impedance has zero reactance. Feed point: Where the transmitted energy enters the antenna. Radiation: NOTradioactivity! An antenna emitting electromagnetic waves.

20 Electromagnetic Waves Radio waves are electromagnetic waves Electric and magnetic fields at right angles to each other, oscillating at the wave s frequency Spread out into space from the antenna Created by ac current Wave and current have the same frequency

21 Wave Polarization Orientation of the wave s electric field component with respect to the surface of the Earth Vertical or horizontal determined by elements Can be circular if the orientation twists as the wave spreads through space Combinations of polarization are called elliptical polarization

22 Cross-Polarization Antenna and wave polarization must match for maximum reception. Cross-polarized: antenna elements and the wave s electric field at right angles Can reduce reception by a factor of 100 For elliptically polarized waves (such as HF skywave) any antenna will respond at least partially.

23 The Decibel (db) A ratio expressed as an power of 10 to make large numbers easier to work with. db = 10 log (power ratio) db = 20 log (voltage ratio) Positive values in db indicate ratios > 1 and negative values of db are for ratios < 1. Antenna gain is discussed in terms of db.

24 Antenna Radiation Patterns Radiation patterns are a way of visualizing antenna performance. The further the line is from the center of the graph, the stronger the signal at that point. Graph calibrated in db.

25 Radiation Pattern Vocabulary Nulls: Directions of minimum gain Lobes: Regions between nulls Main lobe: Lobe with highest gain Side lobe: Any lobe other than the main lobe Forward gain: Gain in the direction assigned as forward

26 Radiation Pattern Vocabulary Azimuth pattern: Radiation pattern showing gain in all horizontal directions around the antenna. Elevation pattern: Radiation pattern showing gain at all vertical angles from the antenna. Often restricted to angles above horizontal

27 Azimuth Pattern Elevation Pattern

28 Radiation Pattern Vocabulary Front-to-back ratio: Ratio of forward gain to gain in the opposite direction. Front-to-side ratio: Ratio of forward gain to gain at right angles to the forward direction.

29 Feed Lines The purpose of the feed line is to get RF power from your station to the antenna. Basic feed line types Coaxial cable (coax) Open-wire line (OWL) also called ladder line or window line Power lost as heat in the feed line is called lossand it increases with frequency.

30 Feed Line Vocabulary Center conductor: Central wire Dielectric: Insulation surrounding center conductor Shield: Braid or foil surrounding dielectric Jacket: Protective outer plastic coating Forward (reflected) power: RF power traveling toward (away from) a load such as an antenna

31 Coaxial Cable Most common feed line Easy to use Not affected by nearby materials Has higher loss than open-wire line at most frequencies Air-insulated hard line has lowest loss

32 Open-Wire Line Lighter and less expensive than coax Has lower loss than coax at most frequencies More difficult to use since it is affected by nearby materials Requires impedance matching equipment to use with most transceivers

33 Characteristic Impedance The impedance presented to a wave traveling through a feed line Given in ohms (Ω), symbolized as Z 0 Depends on how the feed line is constructed and what materials are used Coax: 50 and 75 Ω OWL: 300, 450, and 600 Ω

34 Standing Wave Ratio (SWR) If the antenna feed point and feed line impedances are not identical, some RF power is reflected back toward the transmitter. Called a mismatch Forward and reflected waves create a pattern of standing waves of voltage and current in the line SWR is the ratio of standing wave max to min Measured with an SWR meteror SWR bridge

35 Standing Wave Ratio (SWR) Reflected power is re-reflected at the transmitter and bounces back and forth. Some RF power is lost as heat on each trip back and forth through the feed line All RF power is eventually lost as heat or transferred to the antenna or load High SWR means more reflections and more loss of RF power (less transferred to the antenna or load).

36 Nothing Is Perfect SWR equals the ratio of feed point (or load) and feed line impedance, whichever is greater than 1 (SWR always greater than 1:1). What is an acceptable SWR? 1:1 SWR is perfect no power reflected Up to 2:1 SWR is normal Modern radios lower transmitter output power for protection when SWR is above 2:1

37 Nothing Is Perfect SWR above 3:1 is considered high in most cases. Erratic SWR readings may indicate a faulty feed line, faulty feed line connectors, or a faulty antenna. High SWR can be corrected by Tuning or adjusting the antenna With impedance matching equipment at the transmitter Called an antenna tuner or transmatch Does not change SWR in the feed line

38 Adjusting SWR An SWR meter is inserted in the feed line and indicates the mismatch at that point. Either adjust the antenna to minimize the reflected power or adjust the antenna tuner for minimum SWR at the transceiver.

39 Dummy Loads A dummy load is a resistor and a heat sink Used to replace an antenna or other piece of equipment during testing. Dummy loads dissipate signals in the feed line as heat Allows transmitter testing without sending a signal over the air Helpful in troubleshooting an antenna system

40 Most basic antenna The Dipole Total length is ½ wavelength (½ l) Usual construction: Two equal halves of wire, rod, or tubing Feed line connected in the middle Length (in feet) usually estimated 468 / frequency (in MHz) often too short

41 The Dipole Radiates strongest broadside to the dipole, weakest off the ends If oriented horizontally, the radiated waves are horizontally polarized 3D radiation pattern looks like a donut or bagel This is a free-spacepicture

42 The Ground-Plane

43 The Ground-Plane One-half of a dipole (1/4-wavelength long) oriented perpendicularly to a ground plane that acts as an electrical mirror Replaces the dipole s missing half Any conducting surface can act as the groundplane, including the ground! Car roof or trunk, or other metal surface Radial wires

44 The Rubber Duck Coiled wire coated in tough plastic Convenient size, rugged enough for handheld use The radio and operator make up the ground plane Small size equals compromise performance Hold vertically to maximize range Doesn t work well inside vehicles due to metal body shielding signal For mobile use, replace rubber duck with an external magnet-mount or permanent antenna

45 Start with excess length (490 / f) and adjust To raise resonant frequency, shorten each half equally Dipole Construction

46 Ground-Plane Construction Length (in feet) usually estimated 234 / frequency (in MHz) often short, start long and trim to length Thickness of whip or rod also affects calculated length Vertical ground-plane antennas are omni-directional Mount mobile whips in center of roof or trunk for best coverage

47 Ground-Plane Construction Lengthening a ¼-wavelength VHF/UHF ground-plane to 5 /8 wavelengths focuses more signal toward the horizon which usually improves range. At HF, vertical antenna size is quite large. 40 meter ¼-wavelength whip is about 32 feet Inserting an inductor makes the antenna longer electrically Reduces physical length required

48 Directional (Beam) Antennas Beam antennas focus or direct RF energy in a desired direction. Gain improves range Reduces reception in unwanted directions Reduces interference to and from other stations Directional characteristics are the same for receiving as they are for transmitting.

49 Directional (Beam) Antennas Yagi Quads

50 Directional (Beam) Antennas Used for DXing to obtain maximum range for contacts Can be used at VHF/UHF to avoid multi-path and bypass obstructions Use vertical elements for repeaters and FM simplex contacts Use horizontal elements for CW and SSB contacts to reduce ground losses

51 Directional (Beam) Antennas At microwave frequencies (above 1 GHz) it becomes practical to use a dish antenna Short wavelength High gain Small size

52 Coaxial cables Practical Feed Lines Larger diameter cables have lower loss Loss is measured in db/foot Loss increases with frequency Keep water out! Protect the jacket from cuts and cracks and ultraviolet exposure. Some cable is UV-rated

53 Common Coaxial Cables RG-174: miniature, short connections only RG-58: 0.2" OD, lossy at VHF/UHF RG-8X: 0.25" OD, good through low VHF RG-8/RG-213; 0.4" OD, used through UHF Hard line: ½" to multiple inch OD, used through microwave Most coax is 50 Ω or 75 Ω

54 Coaxial Connectors UHF SO-239/PL-259 BNC N SMA F (cable TV)

55 Installing Coaxial Connectors Soldering is the traditional way Use rosin-core solder and avoid cold solder joints See The Art of Soldering on the ARRL website Crimp connectors are becoming widely used by hams Obtain and learn to use proper crimping tools

56 Waterproofing Connectors MUST be waterproofed for use outdoors Type N are waterproof but still usually protected anyway Use good-quality electrical tape first, then a layer of self-vulcanizing tape, then another covering of electrical tape Air-core coaxial cable requires special connectors and techniques to waterproof

57 Practical Feed Lines Open-wire feed lines Flexing will eventually break conductors Vulnerable to abrasion and twisting Rain, snow, and ice do affect the line Lower loss than coax, generally Higher impedance may complicate use

58 Wattmeters SWR Meters Antenna Tuners Antenna Analyzers Feed Line Equipment

59 Wattmeters Most wattmeters are directional Sensitive to direction of power flow Read forward and reflected power Use a sensing element SWR is computed from power values Table or formula

60 Measure SWR directly by sensing power flow in the line Usually installed at the transmitter SWR Meters

61 Antenna Tuners Don t really tune the antenna Transform impedances at the end of the feed line to 50 Ω which reduces SWR to 1:1 Antenna feed point impedance unchanged Feed line SWR unchanged Also called impedance matchers, transmatches, matchboxes, other trade names

62 How to Use an Antenna Tuner Transmit a low-power signal Monitor the SWR meter Adjust the tuner until minimum SWR is achieved

63 Antenna Analyzers Low-power signal source, frequency counter, and SWR meter in one package Makes antenna and cable measurements without transmitting a full-power signal Available for HF through UHF and microwave Very handy for adjusting and troubleshooting antennas and feed lines

64 Antenna Basics Definitions: Elements conduction portion of an antenna that radiates or receives a signal Polarization refers to the orientation of the electric field radiated by the antenna Feed point impedance the ratio of RF voltage to current at an antenna s feed point Resonant when its feed point impedance is completely resistive (no reactance) Radiation pattern a graph of the signal strength in every direction or vertical angle 2015 General License Course 1

65 Antenna Basics Definitions: Azimuthal pattern signal strength in horizontal directions Elevation pattern signal strength in a vertical direction Lobes regions in the radiation pattern where the antenna is radiating a signal Nulls point at which radiation is at a minimum between lobes Isotropic antenna radiates equally in every possible direction (only a reference antenna) 2015 General License Course 2

66 Antenna Basics Omnidirectional antenna radiates a signal of equal strength in every horizontal direction Directional antenna radiates preferentially in one or more directions Gain concentration of signal transmitted toward or received from a specific direction Front-to-back ratio ratio of gain in a forward direction to the opposite direction Front-to-side ratio ratio of gain in a forward direction to directions at right angles Gain ratios are measured in db 2015 General License Course 3

67 dbd vs dbi Antenna gain is specified in decibels (db) with respect to an identified reference antenna Gain with respect to an isotropic antenna is called dbi Gain with respect to a dipole antenna s maximum radiation is called dbd Convert dbd to dbi by adding 2.15 db and from dbi to dbd by subtracting 2.15 db 2015 General License Course 4

68 Dipoles The dipole antenna is a straight conductor, usually ½ wavelengths long with a feed point in the middle A dipole radiates strongest broadside to its axis and weakest off the ends A figure-eight is the shape of the azimuth pattern for a dipole installed in free space (no ground) The feed point impedance of a center-fed dipole is approximately 72 Ω but varies depending on height The feed point impedance increases as it is moved away from center 2015 General License Course 5

69 Dipoles 2015 General License Course 6

70 Dipole In free space, ½ wavelengths (λ) in feet equals 492 divided by frequency in MHz Practical ½-wave dipoles are shorter than 492 / f because: Actual thickness - wire looks longer electrically Height above ground affects impedance, as do nearby conducting surfaces Center-fed dipoles are generally a good match to 50 or 75 Ω at common heights and on odd harmonics 2015 General License Course 7

71 Ground Planes (Verticals) The basic ground plane antenna is λ/4 element over a ground plane, radiates omnidirectionally Imagine one-half of a dipole with ground plane making up the missing half Currents in the ground plane create the effect of an electrical image of the missing half, like an electrical mirror The ground plane can be made from sheet metal or radial wires 2015 General License Course 8

72 Ground Plane Vertical Ground-mounted ground plane antennas have radial wires laid on the surface or buried within a few inches of the surface Feed point impedance of a ground plane antenna is approximately 35 Ω (half a dipole) Drooping elevated radials from 30 to 45 degrees will raise the feed point impedance to approximately 50 Ω Ground plane antenna length is 246/freq. in MHz 2015 General License Course 9

73 Mobile HF Antennas Mobile antennas for HF are often some form of a ground plane (vertical whip) Electrical loading is a technique for shortening a mobile vertical and presenting a reasonable feed point impedance A shortened antenna will have a smaller operating bandwidth without retuning. Screwdriver antennas with an adjustable coil are a good compromise between performance and convenience 2015 General License Course 10

74 Random Wires Random wire antenna is just that: a random length of wire Feed point impedance and radiation pattern is very unpredictable Random wire antennas are connected through a matching network and then to the transmitter Random wire antennas may result in significant RF currents and voltages on the station equipment and RF burns 2015 General License Course 11

75 Effects Of Height Above Ground Below ½ wavelengths in height, the antenna s feed point impedance steadily decreases until it is close to zero at ground level Above ½ wavelengths the impedance varies, eventually reaching a stable value at a height of several wavelengths Height also effects the radiation pattern Below ½ wavelengths, the dipole pattern is almost omnidirectional. Greater heights cause the pattern to develop lobes and nulls 2015 General License Course 12

76 Effects of Polarization Radio waves reflecting from the ground have lower losses when the polarization of the wave is parallel to the ground Horizontal antennas have a lower ground reflection loss than a vertical Ground-mounted vertical antennas are able to generate stronger signals at lower angles of radiation than horizontal antennas at low heights Vertical antennas are often preferred for DX contacts where it s impossible to erect tall towers and beams 2015 General License Course 13

77 How Yagis Work Yagi the most popular directional antenna Yagi antennas reduce interference: signals, noise from unwanted directions Rear and sides by more than 20 db The Yagi is an array antenna with multiple elements Array antennas create a maximum field strength in a specific direction (main or major lobe) 2015 General License Course 14

78 How Yagis Work Yagi elements: Driven element connected to the feed line Parasitic one or more elements not connected to the feed line that influence the antenna s radiated and receive pattern Parasitic arrays energy from the driven element induces a current to flow in the parasitic elements which is reradiated as part of the antenna s total signal 2015 General License Course 15

79 Yagi Structure & Function A Yagi antenna has a driven element and at least one parasitic element Driven element about the same as a resonant dipole Director elements reinforce signals in a single main lobe (shorter than the driven element) Reflectors cancel signals to the rear (longer than the driven element) Front-to-back ratio is the ratio of signal strength at the peak of the radiation pattern s major lobe to that in exactly the opposite direction 2015 General License Course 16

80 Yagi Structure The simplest Yagi is a twoelement antenna with a driven element (DE) and either a reflector (usually) or a director. DE is approx. a λ/2 dipole Reflector element is about 5% longer than the driven element and placed behind the DE by λ Director element is about 5% shorter than the driven element and placed ahead of the DE by λ 2015 General License Course 17

81 Design Tradeoffs Primary element design variables: length, spacing along the boom, thickness Placement and length of the elements affects gain, tuning, and pattern Longer boom with a fixed number of directors increases gain Larger diameter elements reduces SWR and increases SWR bandwidth Antenna modeling software used to optimize the antenna design 2015 General License Course 18

82 Impedance Matching Many Yagi designs have a feed point impedance of ohms requiring matching Gamma matching is a popular technique Section of parallel conductor transmission line using the driven element as one of its conductors A mechanical advantage - the driven element doesn t have to be insulated from the boom Tuning by adjusting length of the transmission line or a series capacitor 2015 General License Course 19

83 Loop Antennas Loops are one wavelength or more in circumference (circular, square, triangular) Quad (Delta) square (triangular) with λ/4 (λ/3) sides Loops can be mounted vertically or horizontally Strongest signal is broadside to a 1λ loop, about the same gain as a dipole Horizontal loops radiate straight up at fundamental, lower angles on harmonics, with horizontal polarization 2015 General License Course 20

84 Quad or Delta Loop Beams A two element Quad or Delta loop beam operates similarly to and has about the same gain as a 3-element Yagi Reflectors 5% longer in circumference than the driven element Attaching the feed point to the bottom or top of a quad results in horizontal polarization Attaching the feed point to the side of a quad results in vertical polarization Front-to-back and side is generally less than for a Yagi 2015 General License Course 21

85 Specialized Antennas NVIS (Near-Vertical Incidence Sky-wave) antenna radiates mostly straight up to the ionosphere where it s reflected back down to Earth over a wide area Pattern covers a geographic area of only a few hundred kilometers across Used for disaster communications Typically a simple λ/2 dipole The best height is between 1 10 and ¼ wavelengths above ground 2015 General License Course 22

86 Specialized Antennas Stacked Antennas a pair of identical antennas stacked above or beside each other to increase gain Vertical stacking increases gain and narrows the elevation beamwidth Most vertical stacks are about ½ wavelength to 1 wavelength apart dbi relative to isotropic dbd relative to a dipole (dbd = dbi 2.15) 2015 General License Course 23

87 Specialized Antennas Stacking 2 beams ½ wavelength apart increases gain by 3 db Gain another 3 db by stacking 4 beam antennas Stacking narrows the pattern beamwidth 2015 General License Course 24

88 Specialized Antennas Log Periodics are designed to have consistent radiation pattern and SWR across a wide (up to 10:1) frequency range Short elements are active at higher frequencies Longer elements are active at lower frequencies Log periodic antennas less gain or front-to-back ratio than Yagi antennas The length and spacing of the elements increases logarithmically from one end to the other 2015 General License Course 25

89 Specialized Antennas Beverage antenna receiving antenna Beverage antennas are very inefficient but do a good job of rejecting noise A Beverage antenna is a long (1λ or longer), low wire (less than 20 ft high) pointed in a preferred signal direction The Beverage works by rejecting more noise than signal (traveling wave antenna) 2015 General License Course 26

90 Multiband Antennas A half-wave dipole antenna can be used on its odd harmonics without a tuner (radiate harmonics) Trapped dipole is a common multiband antenna (40/80 meter dipole) The traps act as electrical switches to isolate parts of the antenna Act as inductors or capacitors below or above the resonant frequency Triband Yagi ( meters) trapped or separate elements for each frequency range 2015 General License Course 27

91 Types of Feed Lines 2015 General License Course 28

92 Feed Lines Characteristic impedance (Z0) is determined by the geometry of the feed line conductors and the material and distance between them The most common impedances of coax that amateurs use are 50 Ω and 75 Ω Open wire feed line has impedances from 300 to 600 Ω TV-type twin lead has a characteristic impedance of 300 Ω 2015 General License Course 29

93 Feed Lines Forward power power traveling toward a load or antenna Reflected power power reflected from an impedance mismatch at the load or antenna Standing waves interference wave pattern in a feed line from forward and reverse power Standing wave ratio (SWR) ratio between the peak and minimum voltage of the wave pattern SWR equals ratio of load or antenna Z and Z0 of the feed line, whichever is greater than 1 Short or open: SWR = and all power reflected 2015 General License Course 30

94 Calculating SWR Example 5: What is the SWR in a 50 Ω feed line connected to a 200 Ω load? SWR = 200/50 = 4:1 Example 6: What is the SWR in a 50 Ω feed line connected to a 10 Ω load? SWR = 50/10 = 5:1 Example 7: What standing wave ratio will result from the connection of a 50 Ω feed line to a non-reactive load having a 50 Ω impedance? SWR = 50/50 = 1: General License Course 31

95 Calculating SWR Example 8: What should be the SWR if you feed a vertical antenna that has a 25 Ω feed point impedance with 50 Ω coaxial cable? SWR = 50/25 = 2:1 Example 9: What would be the SWR if you feed an antenna that has a 300 Ω feed point impedance with 50 Ω coaxial cable? SWR = 300/50 = 6: General License Course 32

96 Feed Lines Impedance matching matching the feed line and load (antenna) eliminates standing waves Performed at the transmitter to match antenna system impedance to that of the transmitter 2015 General License Course 33

97 Feed Line Loss Loss is measured in db /100 ft of cable Loss increases with frequency Small coax has higher loss at a given frequency 2015 General License Course 34

98 The Ionosphere Ionosphere region beginning about 30 miles above the Earth and extending to about 300 miles The air is thin enough that solar ultraviolet (UV) radiation can break the molecules of gas into individual atoms and then knock electrons away from them (gas is ionized by the loss of an electron) Charged ions and free electrons respond to signals just like electrons in a conductor 2015 General License Course 35

99 Ionosphere Regions 2015 General License Course 36

100 Regions The main regions of the ionosphere are the D, E, F layers D Layer: miles in altitude Only present when illuminated by the Sun Disappears at night when no UV rays are present E Layer: miles in altitude Acts similarly to the D layer Disappears later at night 2015 General License Course 37

101 Regions F Layer is miles above Earth During the day it breaks into F1 and F2 layers At night it returns to a single F layer The F1 and F2 layers vary with the local time, season, latitude and solar activity The stronger the Sun s illumination, the higher the F2 layer will be (when the Sun is overhead) 2015 General License Course 38

102 Reflection & Absorption The ability of the ionosphere to bend or refract radio waves depends on how strongly the regions gasses are ionized and the frequency The stronger the ionization, the higher the bending The higher the frequency of the wave, the less it is bent (VHF & UHF are hardly bent) 2015 General License Course 39

103 Reflection & Absorption Critical Angle the highest angle at which the radio wave will be refracted back to Earth Critical Frequency the highest frequency that a signal transmitted straight up will be returned to Earth Absorption increases in the daytime when the UV is more intense (enemy of propagation) Below 10 MHz signals can be completely absorbed by the D region during the day light hours 2015 General License Course 40

104 Sky-wave Propagation 2015 General License Course 41

105 Sky-wave Propagation Hop one reflection from the ionosphere Skip (sky-wave) propagation by ionospheric refraction. The higher the reflecting region, the longer the hop F2 layer hops up to 2500 miles, E layer hops up to 1200 miles MUF (Max Useable Frequency) highest frequency at which sky-wave is available between two points. LUF (Lowest Useable Frequency) frequency below which there is too much absorption for communication Ground wave propagation along the Earth s surface 2015 General License Course 42

106 Long Path & Short Path Long path signals take the long way around the world to complete the contact (180 from short path) Short path most HF contacts are made via short path or the most direct route Round-the-world propagation occasionally you can hear your own signal coming all the way around the world (1/7 second delay echo) 2015 General License Course 43

107 The Sun Sunspots or Solar Cycle approximately an 11-year cycle Sunspot number represents the number of sunspots and groups present at a given time Sunspot numbers are useful in assessing the overall solar activity The more sunspots, the more UV is generated, creating more ionization and improving propagation conditions on the HF bands and possibly into the lower VHF range 2015 General License Course 44

108 Solar Activity High sunspot numbers mean poor conditions on 80 & 40 meters (increased absorption) Low sunspot numbers means 15, 12, & 10 meter bands are closed (no sky-wave) 20 meter band is usually good regardless of sunspot number during the daytime Sun rotates every 28 days causing sunspots to reappear if still present and HF propagation tends to repeat, as well 2015 General License Course 45

109 Measuring Solar Activity 2015 General License Course 46

110 Measuring Solar Activity Solar-Flux Index (SFI) amount of 2800 MHz (10.7 cm wavelength) energy from the Sun K index (0-9) the short term stability of the Earth s magnetic or geomagnetic field 0 is quiet, 9 is extreme storm A index (0-400) long-term geomagnetic field stability around the world Values 0 (stable) to 400 (greatly disturbed) 2015 General License Course 47

111 Assessing Propagation MUF Maximum Usable Frequency Highest frequency for propagation between two points (higher than MUF will not be refracted enough to get back to Earth) LUF Lowest Usable Frequency Lowest frequency for propagation between two points (lower than LUF will be absorbed) MUF/LUF vary with the time, season, amount of solar radiation and geomagnetic stability 2015 General License Course 48

112 Assessing Propagation MUF drops below LUF, no ordinary sky-wave propagation between those two points Check actual band conditions between two points by listening for beacon stations The NCDXF supports an international network of beacon stations that transmit continuously on meters 2015 General License Course 49

113 Solar Disturbances Solar flare a large eruption of energy and solar material on the surface of the Sun Coronal hole a weak area in the Sun s outer layer where ionized gas and charged particles escape the Sun s magnetic field Coronal mass ejection (CME) an ejection of large amounts of material from the corona CME may be directed in a narrow stream or wide spray (disruptive to radio communications) 2015 General License Course 50

114 Solar Disturbances UV & X-ray radiation from solar flares travels at the speed of light and reaches the Earth s ionosphere in about 8 minutes Increases D layer ionization and absorption dramatically, causing a radio blackout (Sudden Ionospheric Disturbance or SID) that can last from seconds to several hours Affects lower bands more than higher bands SID only impacts the sunny side of the Earth so dark side may be relatively unaffected 2015 General License Course 51

115 Solar Disturbances Geomagnetic Disturbances Sun gives off a stream of particles called solar wind Charged particles from coronal holes and CMEs take up to 20 to 40 hours to reach the Earth Particles increase ionization in the E region Causes auroral displays and geomagnetic storms 2015 General License Course 52

116 Solar Disturbances Sudden change in geomagnetic field disrupts the upper ionosphere, causing long-distance paths at high latitudes near the magnetic poles to be wiped out for a period of hours to days Auroras are actually a glow of gases ionized by the incoming charged particles as they flow vertically down into the atmosphere, guided by the magnetic field Auroral propagation is strongest on 6 and 2 meters (hiss or buzz) 2015 General License Course 53

117 Scatter Modes Scatter Characteristics Reflections are not very efficient and tend to spread out, delivering only a fraction of their signal to the receiving station Signals typically have a fluttering or wavering sound Received signals may arrive from different directions resulting in multipath interference 2015 General License Course 2

118 Scatter Modes Scatter Characteristics Signals can be heard from stations that are too far away to be received via ground wave on frequencies too high for short hop sky-wave (above MUF) Back scatter reflects signals back toward the transmitter. Back scatter helps fill in the skip zone where signals would otherwise not be heard 2015 General License Course 3

119 Ionospheric Reflections 2015 General License Course 4

120 Scatter Modes - NVIS Reflections from a signal radiating vertically are scattered back to Earth over a mile range around the transmitting station Horizontal dipoles placed close to the ground (⅛ to ¼ wavelength high) have an omnidirectional pattern at very high angles NVIS works best at 40 meters and lowerfrequency bands 2015 General License Course 5

121 NVIS Reflections 2015 General License Course 6