scars technician / general license course week 4 · radio wave propagation: ... •the troposphere...

SCARS

Technician / General

License Course

Week 4

Radio Wave Propagation:

Getting from Point A to Point B• Radio waves propagate in 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

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.

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 line-

of-sight.

• Range of a few hundred miles on bands used by

amateurs.

Reflect, Refract, Diffract

• Diffraction occurs

when a wave

encounters a sharp

edge (knife-edge

propagation) or

corner

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.

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

“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 ducts that trap and conduct VHF and UHF

radio waves for hundreds of miles

The Ionosphere

• 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

Ionospheric Levels

• Higher ionization refracts

or bends radio waves

more strongly

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.

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.

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.

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.

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.

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.

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.

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.

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: NOT radioactivity! An antenna emitting

electromagnetic waves.

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

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

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 sky-

wave) any antenna will respond at least partially.

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.

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.

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


• 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

Azimuth

Pattern

Elevation

Pattern


• 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.

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 loss and

it increases with frequency.

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

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

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

Characteristic Impedance

• The impedance presented to a wave traveling

through a feed line

• Given in ohms (Ω), symbolized as Z0

• Depends on how the feed line is constructed and

what materials are used

• Coax: 50 and 75 Ω

• OWL: 300, 450, and 600 Ω

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 meter or SWR bridge

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).

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

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

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.

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

• Most basic antenna

• 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

The Dipole

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-space picture

The Ground-Plane

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 ground-plane, including the ground!

• Car roof or trunk, or other metal surface

• Radial wires

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

Dipole Construction

• Start with excess length (490 / f) and adjust

• To raise resonant frequency, shorten each half equally

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

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

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.


Yagi

Quads

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


• At microwave frequencies (above 1 GHz) it

becomes practical to use a dish antenna

• Short wavelength

• High gain

• Small size

Practical Feed Lines

• Coaxial cables

• 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

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

Coaxial Connectors

• UHF

• SO-239/PL-259

• BNC

• N

• SMA

• F (cable TV)

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

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

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

Feed Line Equipment

• Wattmeters

• SWR Meters

• Antenna Tuners

• Antenna Analyzers

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

SWR Meters

• Measure SWR directly

by sensing power flow

in the line

• Usually installed at the

transmitter

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

How to Use an Antenna Tuner

• Transmit a low-power signal

• Monitor the SWR meter

• Adjust the tuner until minimum SWR is achieved

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

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

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)


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


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


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


Dipoles


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


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


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


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


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


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


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


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)


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


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


Yagi Structure• The simplest Yagi is a two-

element 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 0.15-0.2 λ

• Director element is about 5%

shorter than the driven

element and placed ahead of the DE by 0.15-0.2 λ


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


Impedance Matching

• Many Yagi designs have a feed point impedance of 20-25 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


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


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


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



• 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)



• 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



• 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



• 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)


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 (10-15-20 meters) trapped or separate elements for each frequency range


Types of Feed Lines


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


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


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 Ωimpedance?

• SWR = 50/50 = 1:1


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 = 6:1


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


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


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


Ionosphere Regions


Regions• The main regions of the ionosphere are the

D, E, F layers

• D Layer: 30 – 60 miles in altitude

• Only present when illuminated by the Sun

• Disappears at night when no UV rays are present

• E Layer: 60 – 70 miles in altitude

• Acts similarly to the D layer

• Disappears later at night


Regions

• F Layer – is 100 – 300 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)


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 hardlybent)


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


Sky-wave Propagation


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


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)


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


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


Measuring Solar Activity


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)


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


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 20-10 meters


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)


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


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


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)


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


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

Ionospheric Reflections

4

Scatter Modes - NVIS

• Reflections from a signal radiating vertically

are scattered back to Earth over a 200-300 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 lower-frequency bands


NVIS Reflections


scars technician / general license course week 4 · radio wave propagation: ... •the troposphere...

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