Chapter 7
Propagation
The Ionosphere
• The ionosphere is the region of the upper
atmosphere where the Sun’s ultraviolet radiation
can ionize oxygen molecules to create a positive
ion and a free electron.
• The ionosphere protects us from excessive
ultraviolet radiation.
• There are three main layers of the ionosphere.
– D layer is 30 to 60 miles in altitude. The ions in this
layer recombine at night.
– E layer is 60 to 70 miles in altitude. The ions
in this layer also recombine at night, but last
longer after sunset.
– F layer is 100 to 300 miles and is the least
dense so it can stay partially ionized all night.
• The F layer splits into the F1 and F2 layers during
the day. The layers combine into a single F layer
at night.
• When the Sun is directly overhead the F2 layer will
be at its highest altitude.
• The ionosphere is a weak conductor
because of the ions and free electrons.
– The ionosphere can cause radio waves to
bend (refract)
– The more dense the ionization the higher the
degree of refraction, and at higher
frequencies.
– VHF and higher radio waves usually pass
through the ionosphere into space.
– HF radio waves are most affected by
refraction.
• The virtual height is the height from which
the radio wave appears to be reflecting.
• The critical angle is the angle at which a
radio wave must hit the ionosphere to
reflect back to the Earth.
• The critical frequency is the highest
frequency that a radio wave transmitted
straight up will return to the Earth.
• Radio waves that pass through the denser
D and E regions are partially absorbed.
Frequencies below about 10 MHz are
completely absorbed.
– 160, 80, 60, and 40 meters provide the best
DX at night when the D layer is gone.
– Shortwave broadcasters move to lower
frequencies at night.
Sky-Wave and Ground-Wave Propogation
• Sky-wave propagation refers to radio wave
propagation via the ionosphere. Each
reflection from the ionosphere is a hop.
Reception of sky-wave propagation is
called skip.
• The higher the region in the ionosphere
where the hop occurs, the greater the
distance the wave can travel.
– F2 skip can travel up to 2500 miles
– E skip can travel up to 1200 miles
• Sky-wave propagation can include multiple
hops between the Earth and the
ionosphere.
• If the ionosphere is very dense, then the
critical angle is high and short skip is
possible.
– Short skip distances are much shorter than
the usual skip distances.
– Short skip on the 10 M band is a good
indicator that sky-wave propagation is
possible on 6 M.
• Sky-wave signals due to fluctuations in the
ionosphere which can create multiple
paths for the signal (multipath). The
combination of multipath signals can
cause some distortion or fading.
• Ground-wave signals travel along the
surface of the Earth. The ground is a poor
conductor so ground wave losses are high
and increase with frequency.
• The skip zone is the region between the
maximum ground-wave and minimum skywave where a station can not be heard.
Long Path and Short Path
• Short path is the shorter distance between your
location and the other station. Long path is
longer path, and is 180 degrees away.
• Propagation over both long and short path can
occur. The longer delayed signal will create an
echo.
The Sun
• The amount of UV radiation generated by
the Sun is related to the sunspot activity of
the Sun.
• Sunspots are a magnetic phenomenon
that occurs at the top of the photosphere
(the visible layer of the Sun, or solar disk).
– The sunspot cycle, or solar cycle, is about 11
years.
– The sunspot cycle also has a 22 year cycle
where the magnetic orientation of the
sunspots returns to the same N-S
configuration.
• The sunspot number is the number of
sunspots on the solar disk at a given time.
• At a sunspot peak, the more intense UV
radiation creates a denser ionization layer
that allows sky-wave propagation into the
upper HF and lower VHF frequencies.
– Propagation on 80 M and 160 M are worse
during a sunspot peak.
– Propagation on 10 M can stay open well into
the night at a sunspot peak.
– During a sunspot minimum, the 15 M and
higher bands are “closed” and the lower
bands have good propagation.
– The 20 M band, being in the “middle”, tends to
have good propagation almost all the time.
• The Sun rotates once every 28 days.
Actually the Sun is not a solid and rotates
fastest at the equator and slowest at the
poles. This differential rotation is what
gives rise to the sunspots.
– Propagation cycles are on about a 28 day
cycle as sunspots rotate out of view, and then
back into view.
• There are three primary indices for
measuring solar activity:
– Solar Flux Index (SFI) is a measure of the
amount of 2800 MHz (10.7 cm) radiation
coming from the Sun.
• The K index measures the short term stability of
the Earth’s magnetic field.
• The A index is based on the previous eight K
index values and measures the long term
stability of the Earth’s magnetic field.
• Maximum usable frequency (MUF) is the highest
frequency at which propagation exists between
two points. Frequencies higher than the MUF
pass through the ionsphere into space.
• Lowest usable frequency (LUF) is the lowest
frequency at which propagation exists between
two points. Frequencies lower than the LUF are
absorbed in the ionosphere.
• The MUF and LUF are affected by:
–
–
–
–
Time of day
Season
Amount of solar radiation
Ionospheric stability
• If the LUF is greater than the MUF, then
sky-wave propagation is not possible
between the two points.
• Listening to beacon stations is an
excellent way to determine propagation
conditions.
Solar Disturbances
• Solar flares are an explosion associated
with a disturbance in the magnetic field.
Solar flares are associated with sunspots.
• Coronal holes are a weak spot in the
Sun’s corona (the outer, irregular, layer
seen during a solar eclipse) through which
ionized gasses can escape.
• Coronal Mass Ejections (CME) can occur
during a solar flare and represent a very
large ejection of material from the Sun.
• UV and X-rays from a solar flare move at
the speed of light and impact the Earth 8
minutes later.
– Ionization in the ionosphere increases rapidly
and causes an increase in absorption. This is
sudden ionospheric disturbance (SID).
– HF communications, particularly the lower
bands, can be cut off for minutes or hours.
– SID’s only affect the lit side of the Earth, so
night time DX is not affected.
• The Sun gives a stream of ionized
particles called the solar wind. The
magnetosphere is formed from the
interaction of the solar wind with the
Earth’s magnetic field.
• A geomagnetic storm occurs when there is
a large amount of charged particles
ejected from the Sun (during a CME, for
instance).
– Charged particles from the Sun take from 20
to 40 hours to reach the Earth
– The charged particles travel along the
magnetic field lines toward the poles of the
Earth. They increase ionization in the E layer
and cause auroral displays.
– During a geomagnetic storm, HF propagation
at high latitudes and near the poles is wiped
out for hours or days.
– Auroras are caused by the ionization of
atmospheric gasses by the incoming charged
particles. VHF and UHF signals can be
bounced off of auroras. The rapidly changing
aurora modulates the reflected signals
rendering voice modulation unusable, but CW
can still be heard.
Scatter Modes
• The ionosphere is not a uniform set of layers.
Both density and orientation can vary widely.
– Sporadic E propagation on the VHF bands is an
example of a scatter mode.
• Signals from HF scatter are weaker because
only a small fraction of the transmitted signal will
scatter.
• Multiple paths cause signal distortion and
fluctuating signal strength.
• Scatter and backscatter can fill in the skip zone
where sky-wave and ground-wave propagation
do not reach.
Near Vertical Incidence Sky-Wave
• Near Vertical Incidence Sky-Wave (NVIS)
can occur for radio waves below the
critical frequency, which is usually below 5
MHz for most locations.
• Horizontally polarized antennas must be
mounted between 1/8 to ¼ of a
wavelength above the ground to use
NVIS.
• The resulting skip is usable up to 200 to
300 miles.