World of Ham Radio 1997/WOHR97_AmSoft_(1997-02-01).iso/swl/docs/propa.doc

Text File | 1997-02-01 | 16KB | 298 lines

RADIO PROPAGATION.
    How does that radio wave travel half-way around the
world to your antenna?  How come I can hear radio stations
farther away at night?  Why is it that I can hear AM radio
stations hundreds of miles away, while FM stations fifty
miles away are inaudible?  These are some of the first
questions asked by my students in my Propagation Block at
the school I teach at.  In this file I'll be attempting to
give you a simple primer on Radio Propagation which you can
use to make better decisions on when and where to listen to
enhance your listening skills.  If you are worried about
your lack of knowledge, put your mind at ease.  I'll be
covering working knowledge, not esoteric theory.  With that
said, lets get started.
    What is a Radio wave?  Simply put, it is a combination
of electric and Magnetic fields that were originally
generated at the transmitting antenna by passing current
through a conductor.  These fields are at right angles to
each other, an effect caused by a simple law of electricity,
known to electronic technicians around the world as the
'Left-Hand Rule'.  When looking at a Dipole antenna, the
Electric field is parallel to the plane of the conductor,
while the Magnetic field is at right angles to the
conductor.
                            ^
       -> -> -> -> -> -> -> ^ -> -> -> electric field.
       ---------------------^------------------  Wire
                            ^
                            ^
                            ^
                     Magnetic Field

    So Radio waves, like Light, has polarity.  For best
reception one must arrange the receiving antenna's wire to
be in the same plane as the transmitting antenna.  By doing
this you arrange for the magnetic field to induce maximum
current in the antenna wire.
    How important is 'Polarity'?  For Ground waves, a
receiving antenna that is at right angles to the
transmitting antenna will suffer a 6dB power loss, a
difference that is definitely audible.
    Now that the radio wave has left the transmitters
antenna, it will travel through space until it is completely
absorbed or attenuated to nothingness by distance.

    Radio waves act differently depending on a combination
of frequency and the media it is passing through.  Since
those waves we will be interested in generally travel
through the atmosphere, we will break down the propagation
effects into frequency bands.

    VLF;  Very Low Frequency is that band of frequency's
that range from 0 to 150 kHz.  Frequency's this low are
propagated entirely by Groundwave, that is, the radio waves
travel close to the earth.  In fact, frequency's this low
will actually follow the curvature of the earth, completely
circling the globe.
    Two properties stand out at these frequency's which
make them uniquely useful.  First, since they follow the
curvature of the earth without being reflected from
anything, there is only a single path the radio waves can
take from the transmitter to the receiver.  The distance of
this path is easily calculated, being the Great Circle
distance from the transmitter to the receiver.  By measuring
the time difference between the transmission of a radio
pulse and its reception, or the time difference between the
reception of pulses from three different VLF transmitters
whose exact location is known, you can determine the
distance that your receiver is from the transmitters, and by
drawing arcs at those distances from the transmitters, find
your location.  This is the basis of the LORAN C
Radionavigation system, one of the three prominent services
in this band.
    The second prominent service on this band also rely on
VLF's easily calculated propagation delay.  These are Time
Signal/Frequency Standard Stations, which allow the highly
accurate setting and calibration of Atomic Clocks and
Frequency Standards in remote locations.
    The third major service relays on the second major
property of VLF frequency's.  Unlike all higher radio
frequency's, VLF signals penetrate the earth and water to
substantial distances.  This makes these frequency's
uniquely useful to the military by allowing them to be able
to communicate to submarines as deep as four hundred feet
beneath the waves.  The U.S. Navy has several transmitters
between the frequency's 17.8 and 26.1 kHz.

    LF; Low Frequency is that band running from 150 kHz to
500 kHz.  Like VLF, signals at this frequency propagate
mainly by ground-wave.  However, they do not follow the
curvature of the earth as far, only about a thousand miles
or so.  Prior to 1930 this band was packed with radio
services, such as Ship-to-Ship and Ship-to-Shore stations,
as well as International Broadcasters.  At the time it was a
well known fact that the farther you wanted to transmit, the
lower your radio frequency had to be.  Until a fellow named
Heaviside found a Joker in the Propagation deck.  Now it is
mainly a dead band, with only the scattered remains of its
former glory evident in a few endangered Marine and
Aeronautical Radio Direction-finding beacons and a couple of
die-hard European Broadcasters.

    MF; The Medium wave band is the most familiar to
laymen.  It spans the range of 500 kHz to 3000 kHz.  The
lower half, 500 kHz to 1600 kHz, contains the AM Broadcast
band, while the upper half is used by the Tropical Broadcast
Band, the old LORAN A radionavigation system, and Ship
communications.  Propagation is mainly limited to
ground-waves with a range of a hundred miles or so, with
some highly attenuated single-hop skywave propagation at
night adding about 600 more miles of range (a subject we
will get into deeper in the Shortwave frequency range).
    The AM Broadcast band is used worldwide for domestic
broadcasting, except in the Tropics, where atmospheric
effects and high noise make it useless.  In the Tropics two
higher bands are used, 2300-2495 kHz and 3200-3400 kHz,
giving these two bands their nickname of the Tropical Bands.

    HF; The Shortwave or High Frequency Band spans the
range of 3000 kHz to 30,000 kHz.  Prior to 1930 frequency's
above 3 MHz (3,000 kHz) were thought to be totally useless
for long-range radio communication.  Propagation was limited
to just slightly greater than line-of-sight, less than 100
miles.  Then in 1926 Radio Amateurs discovered that there
was a Joker in the deck.  Banished to these useless
frequency's, they discovered that they were suddenly able to
do something that had eluded them on the lower frequencies.
They could cross the Atlantic!  Unfortunately, secrets that
good are hard to keep, and before long it was general
knowledge that there was some kind of radio mirror in the
heavens that reflected these short waves back to earth
several thousand miles away.
    The mirror was the Ionosphere, or the Ozone layer that
has been so prominent in the news lately.  This effect
introduced a new propagation mode, called Sky-wave
Propagation.  As the Sun hits the Earths atmosphere, the
Ultraviolet radiation strips the oxygen atoms apart in the
upper atmosphere.  This forms an ionized layer in the upper
atmosphere.  To frequency's below a certain frequency, the
LUF (Lowest Usable Frequency, a frequency which changes from
hour to hour, day to day), radio signals penetrating the
Ionosphere are mainly absorbed, the lower the frequency, the
greater the absorption.  The little power that is left is
either refracted back to earth, or into space (which
explains why Medium wave frequencies, which are nearly
always below the LUF manage to get reflected back to earth
at night, although greatly attenuated.).  As the radio waves
frequency increases, the attenuation is reduced, but the
Ionosphere progressively looses its ability to refract the
signal back to earth.  Finely a point is reached where there
is not enough signal refracted back to earth to be
considered useful.  The frequency at which this occurs is
called the Maximum Usable Frequency, or MUF.  At this point
most of the signal exits the other side of the Ionosphere
and continues out to space.  Between these two frequency's
radio signals are refracted back to earth hundreds to
thousands of miles from the transmitter with little
attenuation.  Often a radio signal may 'bounce' from the
Ionosphere to earth and back to the Ionosphere to be
refracted back to earth again.  Sometimes a radio signal may
'bounce' up to six times before being attenuated into
uselessness.  This effect is what makes Shortwave
frequencies so effective for worldwide communications.
    As the Ionosphere plays such an important part in our
hobby, lets delve deeper into its workings.
    The Ionosphere displays two basic forms.  The first is
during the Daytime, when energy is constantly pouring into
the Ionosphere from the Sun.  This energy input causes the
Ionosphere to split into four separate layers, From bottom
to top they are generally referred to as the 'D' layer, the
'E' layer, the 'F1' layer, and the 'F2' layer.
    The 'D' layer, being only 40 to 60 miles up, is in a
relatively thick section of the atmosphere.  The Ionized
atoms are very volatile as other atoms are always nearby to
recombine with.  Because of this the 'D' layer forms just
after sunrise, reaches its peak density at noon, then
quickly disappears at sunset, when the energy source is
removed.  As far as radio propagation is concerned, the 'D'
layer mainly acts to absorb radio frequencies below 14 MHz,
making the lower frequencies useless during most of the day.
Also, it never is really thick enough to effectively refract
radio waves at any frequency, so it is just a general pain
in the preamp.
    The `E` layer, about 65 miles up, is much the same as
the 'D' layer.  It also quickly forms after sunrise, peaks
at noon, and quickly disappears at sunset.  Although this
layer can refract radio waves in the range of 14 to 50 MHz,
this is relatively rare, and it generally just absorbs
frequencies below 14 MHz.
    The next layer, the 'F1' layer, is a relatively weak
layer that splits off of the next higher layer, the 'F2'
layer, during the daylight hours.  It is about 100 miles up,
and generally has little effect on radio wave propagation.
    The highest, thickest, and most useful layer is the
'F2' layer.  It is about 100 to 300 miles high ( its height
varies, depending on the season, latitude, time of day, and
how the Cubs are doing this year.).  At this altitude the
atmosphere is so rarefied that recombination of ionized
atoms occurs quite slowly.  In fact this altitude is quite
popular for spy satellites which need to remain up for only
a week or so.  As the Sun comes up, the ionization level
increases until it reaches a peak about 14:00 local time.
Since recombination takes place so slowly, the ionization
level doesn't reach a minimum until shortly before sunrise.
As the level of ionization increases, this layer becomes
capable of refracting higher and higher frequencies,
sometimes as high as 70 MHz.  After sunset, the strength of
this layer begins to decrease, and the frequency it can
successfully refract back to earth goes down.  However, the
lower layers, which only act to attenuate the radio signal,
disappear.  So, on balance, Sky-wave propagation is best in
the early evening.
    Many factors affect the stability and strength of this
`F` layer, and thus its ability to refract back radio waves.
The most prominent is the local time of day at the point the
radio wave is being refracted at.  As we discussed before,
during the daylight hours the maximum frequency it can
refract back goes up to about 25-50 MHz.  After sunset, it
starts to de-ionize, and the maximum frequency goes down,
reaching a minimum of about 7 MHz just before sunrise.
    Another factor is the stability of the Sun.  Sunspots,
and the resulting outpouring of Solar wind, disturbs the
thickness and stability of the 'F' layer.  This can cause
the 'F' layer to loose its ability to reflect radio waves
from periods ranging from minutes to days.  Magnetic Storms
have the same effects.
    The third and more subtle effect is the so-called Solar
Cycle.  The average MUF increases and decreases on an eleven
year cycle.  During the trough years the MUF may only rarely
exceed 15 MHz.  1986-1987 are good examples.  During peak
years ( to which we are heading now) the MUF may reliably
exceed 50 MHz, going as high as 70 MHz on many days.  There
is also growing evidence of an even longer cycle, about 33
years long, which, if true, means that this coming peak may
equal the amazing peak of the 1950s.
    To sum it up, you can use the following rules to
determine which bands are probably open to 'Skip'.  During
the Daylight hours, listen high, above about 14 MHz.  In
late afternoon, skip frequencies began to decrease from the
east, passing west during the early evening.  So the higher
frequency's from Europe fade out before sunset, while
signals from the Pacific stay high into the early evening.
As the evening continues, the 25 meter band will fade first,
followed by the 31 meter band.  By midnight, only the 41 and
49 meter bands will still receive skip.  In the morning,
start listening for Europe on the higher bands, while the
Pacific will remain dead until 11:00 AM or so.
    Seasonal changes also occur, although this is more an
effect of thunderstorms increasing background noise than
anything else.  So the background noise during the Summer
months requires a stronger signal to overcome it than in
Winter.

    VHF;  The Very High Frequency Band ranges from 30 MHz
to 300 MHz.  At these high frequencies the Ionosphere can no
longer refract the radio waves back, and there is no
appreciable Ground-wave action.  Propagation is limited to
Line-of-Sight only.  In other words, if you can't 'see'
them, you can't hear them.  This band, along with higher
ones, are populated with local broadcasts, such as TV
stations, FM stations, Aircraft, police, Delivery trucks,
Taxi Services, Railroads, Military, etc..  Range is rarely
more than 50 miles.
    As with all rules there are exceptions which extend the
range of these signals far in excess of normal.  The most
common is the effect called 'Ducting'.  This is where a dry
layer of air is sandwiched between two layers of air with a
higher humidity.  Under these conditions, radio waves get
trapped between them and can travel many hundreds of miles
before exiting.  This effect is quite common along the Gulf
Coast, and along the Atlantic and Pacific Coasts at the
lower latitudes.  When I was a child in Texas, my Father was
the first person in the block to get a TV set, in fact the
first person on the entire Air Force Base.  Although there
was not a TV station within 200 miles, Ducting was so common
that we could watch the TV station in Atlanta Georgia nearly
every day!
    Other esoteric modes are Troposcatter (where very high
power transmitters beam the signal up into the Atmosphere so
that an over-the-horizon receiver can pick up the minute
amount of signal scattered back from the Troposphere
boundary).  and Meteor Scatter (where the signal is
reflected off the ionized trail of entering meteorites).

    UHF; Ultrahigh frequencies comprise the range from 300
MHz to 1000 MHz (or 1 GHz).  Propagation at these
frequencies is directly Line-of-Sight, no If's, And's, or
But's about it.  In fact, most communications at these
frequencies are Point-to-Point rather than broadcast in all
directions.  At these frequencies the terrain becomes very
important as even small hills between the transmitter and
receiver can block reception.

    This concludes our little lesson on Propagation.  In
the interest of simplicity, I have told a few white lies,
but the scope of this file was to give a layman a general
feel on how radio waves propagate over different
frequencies.  In that I feel that I have succeeded.