Radio Wave Propagation1

8/12/2019 Radio Wave Propagation1

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Radio Wave Propagation


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VLF ( 330 KHz) and

LF (30300 KHz) Propagation


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Introduction

The dominant factor in VLF and LF propagation is the

extremely large wavelength of the waves.

l ~ 110 km (VLF)

l ~ 0.11 km (LF)

Because the wavelength is so large, horizontal antennas are

not practical (imagine trying to construct a dipole 5km

long that is 5km above ground) and only vertical

polarization is used. Although amateurs in the US do not have an LF allocation,

some European countries do, at 137 KHz.


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Guided Waves

Most VLF and LF propagation occurs via guided wave.

The ground and the ionosphere are highly conductive at

this range of frequencies, and they form the walls of a

spherical waveguide.

Although amateurs in the US do not have an LF allocation,



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Introduction



l ~ 110 km (VLF)

l ~ 0.11 km (LF)







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Introduction



l ~ 110 km (VLF)

l ~ 0.11 km (LF)







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MF (0.33 MHz) Propagation


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HF (330 MHz) Propagation


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Introduction

The HF region is the one of two regions of RF frequencies

that consistently supports long distance propagation.(the

other is the VLF/LF/MF) region

The HF region includes:

International broadcasting at on the 120, 90, 60,

49,41,31,25,19,16, and 13 meter bands.

Amateur Radio Service operations on the 80, 40, 30, 20, 17, 15,

12, and 10 meter bands. Citizens Band operation on 11 meters (27 MHz)

Point-to-point military and diplomatic communications


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Overview of HF Propagation

Characteristics of HF radio propagation Propagation is possible over thousands of miles.

It is highly variable. It has daily and seasonal variation, aswell as a much longer 11 year cycle.

HF radio waves may travel by any of thefollowing modes: Ground Wave

Direct Wave (line-of-sight)

Sky Wave


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Ground Waves

In the HF region, the ground is a poor conductor and

the ground wave is quickly attenuated by ground losses.

Some ground wave communication is possible on 80m,

but at frequencies above 5 MHz, the ground wave isirrelevant.


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Direct Waves

Direct waves follow the line-of-sight path between

transmitter and receiver. In order for direct wave

communication to occur, antennas at both ends of the

path have to have low angles of radiation (so they cansee each other). This is difficult to do on the lower

bands, and as a result, direct wave communication is

normally restricted to bands above 20m. Its range is

determined by the height of both antennas and generallyless than 20 miles.


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Sky Waves

Sky waves are waves that leave the transmitting antenna

in a straight line and are returned to the earth at a

considerable distance by an electrically charged layer

known as the ionosphere. Communication is possiblethroughout much of the day to almost anywhere in the

world via sky wave.


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The Ionosphere

Created by ionization of the upper atmosphere

by the sun.

Electrically active as a result of the ionization. Bends and attenuates HF radio waves

Above 200 MHz, the ionosphere becomes completely

transparent

Creates most propagation phenomena observed at HF,

MF, LF and VLF frequencies


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The Ionosphere

Consists of 4 highly ionized regions

The D layer at a height of 3855 mi

The E layer at a height of 6275 mi

The F1 layer at a height of 125150 mi (winter) and 160180mi (summer)

The F2 layer at a height of 150180 mi (winter) and 240260mi (summer)

The density of ionization is greatest in the F layers and

least in the D layer


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The Ionosphere


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The Ionosphere

Though created by solar radiation, it does not

completely disappear shortly after sunset.

The D and E layers disappear very quickly after

sunset.

The F1 and F2 layers do not disappear, but merge

into a single F layer residing at a distance of 150

250 mi above the earth.

Ions recombine much faster at lower altitudes. Recombination at altitudes of 200 mi is slow

slow that the F layer lasts until dawn.


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The D-Layer

Extends from 3855 milesaltitude.

Is created at sunrise, reaches maximum density at noon,

and disappears by sunset.

The D layer plays only a negative role in HFcommunications.

It acts as an attenuator, absorbing the radio signals,

rather than returning them to earth.

The absorption is inversely proportional to the square ofthe frequency, severely restricting communications on

the lower HF bands during daylight.


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The E layer

Extends from 3855 milesaltitude.

Is created at sunrise, reaches maximum density at noon,

and disappears by sunset.

It can return lower HF frequencies to the Earth, resulting in

daytime short skip on the lower HF bands.

It has very little effect on higher frequency HF radio

waves, other than to change slightly their direction of

travel.


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The F Layers

The F1 layer extends from 125150 mi (winter) and 160

180 mi (summer)

The F2 layer extends from 150 180 mi (winter) and 240

260 mi (summer)

The F layers are primarily responsible for long-haul HF

communications.

Because there is only F layer ionization throughout the

hours of darkness, it is carries almost all nighttime

communications over intercontinental distances.


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The Critical Frequency (fc)

When radio waves aretransmitted straight uptowards the ionosphere(vertical incidence), the radio

wave will be returned to earthat all frequencies below thecritical frequency, (fc) .

The critical frequencydepends on the degree ofionization of the ionosphere,as shown in the followingequation:

1010*24.1

ecr

Nf


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Maximum Usable Frequency (MUF)

Generally, radio waves leave the transmitting antenna at angles

of 0 to 30 degrees and hit the ionosphere obliquely, requiringless bending be returned to earth, thus frequencies above thecritical frequency can be returned.

The maximum frequency returned at a 0 takeoff angle is calledthe maximum usable frequency (MUF). The critical frequencyand the MUF are related by the following equation:

where R = earths radius and h = height of the ionosphere:

Typical MUF values:

1540 MHz (daytime)

314 MHz(nighttime)

2

1

hR

R

fMUF cr


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Maximum Usable Frequency (MUF)


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Hop Geometry

The longest hoppossible on the HFbands isapproximately

2500 miles

Longer distancesare covered bymultiple hop

propagation. When

the refracted radiowave returns toearth, it is reflected

back up towardsthe ionosphere,which beginsanother hop.


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Daily Propagation Effects Shortly after sunrise, the D and E layers are formed and the F

layer splits into two parts. The D layer acts as a selective absorber, attenuating low frequency

signals, making frequencies below 5 or 6 MHz useless during the dayfor DX work.

The E and F1 layers increase steadily in intensity from

sunrise to noon and then decreases thereafter. Short skip propagation via the E or F1 layers when the local time atthe ionospheric refraction point is approximately noon.

The MUFs for the E and F1 layers are about 5 and 10 MHzrespectively.

The F2 layer is sufficiently ionized to HF radio waves andreturn them to earth. For MUFs is above 5 - 6 MHz, long distance communications are

possible.

MUFsfalls below 5 MHz, the frequencies that can be returned by theF layer are completely attenuated by the D layer.


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Daily Band Selection

During the daylight hours:

15, 12, and 10m for multi-hop DX.

40, 30, 20 and 17m, for short skip.

After dark

80, 40, 30 and 20m for DX.

Noise levels on 80m can make working across

continents very difficult.


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Seasonal Propagation Effects

During the winter months, the atmosphere is colder anddenser.

The ionosphere moves closer to the earth increasing the

electron density.

During the the Northern Hemisphere winter, the earthmakes its closest approach to the sun, which increases the

intensity of the UV radiation striking the ionosphere.

Electron density during the northern hemisphere winter

can be 5 times greater than summers.

Winter MUFsare approximately double summers.


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Seasonal Band Selection

During Winter: 20, 17,15, 12, and 10m for daytime DX.

80, 40, 30 and possible 20m for DX after dark.

During Summer 20, 17 and 15m for daytime DX.

40, 30m and 20m after dark.


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Geographical Variation

The suns ionizing radiation is most intense in the

equatorial regions and least intense in the polar regions.

Daytime MUF of the E and F1 layers is highest in the

tropics. Polar region MUFs for these layers can be three

times lower.

The F2 layer shows a more complex geographical MUF

variation. While equatorial F2 MUFsare generally higher

that polar F2 MUFs, the highest F2 MUF often occurssomewhere near Japan and the lowest over Scandinavia.


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Effects of Sunspots

A sunspot is a cool region on the suns surface thatresembles a dark blemish on the sun.

The number of sunspots observed on the suns surface

follows an 11 year cycle.

Sunspots have intense magnetic fields. These fieldsenergize a region of the sun known as the chromosphere,

which lies just above the suns surface. More ultraviolet

radiation is emitted, which increases the electron density in

the earthsatmosphere.

The additional radiation affects primarily the F2 layer.

During periods of peak sunspot activity, such as December

2001 or February 1958 the F2 MUF can rise to more than

50 MHz.


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Effects of Sunspots

During sunspot maxima, the highly ionized F2

layer acts like a mirror, refracting the higher HF

frequencies (above 20 MHz) with almost no loss. Contacts on the 15, 12 and 10m bands in excess of

10,000 miles can be made using 10 watts or less.

During short summer evenings, the MUF can stay

above 14 MHz. The 20 m band stays open to some

point in the world around the clock.


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Effects of Sunspots

When the sun is very active, it is possible to have

backscatter propagation either from the ionosphere or

the auroral regions.

Backscatter communication is unique in that the stationsin contact do not point their antennas at each other, but

instead at the region of high ionization in the ionosphere

or towards the north (or south in the other hemisphere)

magnetic pole. During periods of high solar activity, the auroral zone

may expand to the south, approaching the US-Canadian

border in North America, and covering Scandinavia in

Europe.


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The Northern Auroral Zone


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Effects of Sunspots

During a sunspot minimum, the chromosphere is veryquiet and its UV emissions are very low.

F2 MUFsdecrease, rarely rising to 20 MHz

Most long distance communications must be carried out on

the lower HF bands. During periods of high sunspot activity:

The best daytime bands are 12 and 10m.

At night, the best bands are 20, 17 and 15m.

At the low end of the solar cycle,

The best daytime bands are 30 and 20m.

After dark, 40m will open for at least the early part of theevening.

In the early morning hours, only 80m will support worldwidecommunications


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Propagation Disturbances

A solar flare is a plume of very hot gas ejected from thesunssurface.

It rises through the chromosphere into the corona, disturbingboth regions.

X-ray emission from the corona increases, which reaches Earthin less than 9 minutes. If they are intense enough, theionosphere will become so dense that all HF signals areabsorbed by it and worldwide HF communications are blackedout.

Large numbers of charged particles are thrown out into spaceat high velocity, reaching Earth in 2-3 days. The particles are

deflected by the geomagnetic field to the poles, expanding theauroral zones. Signals traveling through the auroral zone areseverely distorted, in some cases to the point ofunintelligibility

Generally speaking, ionospheric disturbances affect the

lowest HF bands most. Occasionally communications on


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Propagation Indices K index a local index of geomagnetic activity computed every

three hours at a variety of points on Earth. The K scale is shownbelow.

The best HF propagation occurs when K is less than 5. A K index

less than 3 is usually a good indicator of quiet conditions on 80 and40m.

K Index Ap Index

0 Inactive

1 Very Quiet

2 Quiet

3 Unsettled

4 Active

5 Minor Storm

6 Major Storm

7 Severe Storm

8 Very Severe Storm

9 Extremely Severe Storm


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Propagation Indices

Ap index - a daily average planetary geomagnetic activity

index based on local K indices. The A scale is shownbelow:

Good HF propagation is likely when A is less than 15,particularly on the lower HF bands. When A exceeds 50 ,ionospheric backscatter propagation is possible on 12 and10m. When A exceeds 100, auroral backscatter may be

possible on 10m.

Ap Index Geomagnetic Condition

0-7 Quiet

8-15 Unsettled

16-29 Active

30-49 Minor Storm

50-99 Major Storm

100-400 Severe Storm


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Propagation Indices

The K and Ap indices are related as follows:

K Index Ap Index

0 0

1 32 7

3 15

4 27

5 48

6 80

7 140

8 240

9 400


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Propagation Indices

Solar Flux This index is a measure of 10.7 cmmicrowave energy emitted by the sun. A flux of 63.75corresponds to a spot free, quiet sun. As the flux number

increases, the solar activity increases. Single hop HFpropagation is normally possible on bands below 20mwhen the flux is greater than 70. Multi-hop propagation is

possible on 80 20m when the flux exceeds 120.Openings on 15 and 10 meters are common when the flux

exceeds 180. Should the flux exceed 230, multi-hoppropagation is possible up into the VHF region.


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Propagation Indices

Sunspot Number (Wolf Number) This is the oldestmeasure of sunspot activity, with continuous records

stretching back into the 19th century. The sunspot

number is computed multiplying the number of sunspot

groups observed by 10 and adding this to the number ofindividual spots observed. Because the sun rotates and

different areas of the sun are visible each day, it is

common to use 90 day or annual average sunspot

numbers. The lowest possible sunspot number is 0. Thelargest annual average value recorded to date was 190.2

in 1957. As with solar flux, higher sunspot numbers

equate to more solar activity.


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Propagation Indices

Solar Flux and Sunspot Number for the past 15 years

(September 1986March 2002):


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Propagation Indices

Actual monthly sunspot number

Smoothed Sunspot Number

This chart shows the solar flux for the past 15 years

(September 1986March 2002):


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VHF (30300 MHz)

Propagation


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VHF Propagation Modes

Every type of propagation is possible in the VHFrange: Line of Sight (LOS)

Tropospheric Propagation (tropo)

Sporadic E Meteor Scatter

Auroral Scattering

Transequatorial F

Ionospheric F2 LOS and tropo occur throughout the VHF range,

while the other modes are most frequentlyobserved below 150 MHz


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Line of Sight (LOS) and

Tropospheric Propagation

Line of Sight LOS coverage is determined primarily by the height of the transmitting and

receiving antennas

For typical amateur 6 m stations LOS coverage is about 20 miles

LOS propagation is unaffected by solar conditions, time of day or the seasons

Tropospheric Propagation Variations in the humidity of the troposphere cause RF to be scattered over

the horizon. This is known as tropospheric scatter

Temperature inversions (warm dry air located above cool moist air) refract

RF in the VHF range back towards the earth. Temperature inversions occurdaily in the middle latitudes at sunrise and sunset. Communications arepossible over a ranges up to 600 miles

Over the oceans, stable temperature inversions can create a duct, throughwhich VHF can travel without significant loss up to 2500 miles


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Tropospheric Scatter

Tropospheric Ducting


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Sporadic E (ES) Clouds of high density ionization form without warning in the ionospheres E

layer

ESis not dependent on solar activity. It may occur any time, but is most frequentbetween May and August, with a smaller peak of activity in December

Single hop EShas a range of ~1400 mi

Double hop ES has a range of ~ 2500 mi

Cause of Sporadic E is not known: high altitude wind shear may be responsible.


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Sporadic E (ES)

The ionized clouds that cause sporadic E propagation canmove. This animated sequence shows grid squares contactedin hour intervals during an ESopening beginning at 0500Z, 10 June 2001


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Meteor Scatter As meteors are vaporized in the upper atmosphere, they leave behind

ionized trails at heights of 6070 miles that are sufficiently dense to reflectVHF

A long trail lasts only 15 seconds so contact must be made quickly on SSB

SSB QSOs via meteor scatter are usually possible only during a meteorstorm

Short trails that occur continuously may be used for high speed CW QSOs(> 100 wpm)

Best time for meteor scatter is after midnight or during a meteor storm

A (A )


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Aurora (Au)

During periods of intense

auroral activity, chargedparticles in the auroral zonecan scatter 50 MHz RF

The RF interacts stronglywith the aurora, resulting insignificant distortion of the

signal. Only narrow bandmodes such as CW are usedduring Au openings

To work Au, the transmitterand receiver point theirantennas at the auroral

zone, not each other.


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Transequatorial F (TE)

The ionospheres F layer ismost intense in the region ofthe geomagnetic equator.

Stations within about 2500miles of the geomagneticequator can launch 50 MHz

RF into these regions. TheRF is refracted and travelsacross the equator and intothe other hemispherewithout scattering from theground

Stations using TE must be atapproximately equaldistances from thegeomagnetic equator


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Closing Comments

This is meant to be a brief overview of RF propagation.

There have been many books written on this subject and a

there are many computer resources available, particularly

for propagation forecasting. The Radio Society of GreatBritain has an interesting website devoted to propagation,

www.keele.ac.uk/depts/por/psc.htm

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