Chapter 7Propagation

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 sky-wave 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.