Propagation

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Propagation is simply the way radio signals travel from the transmitting antenna to the receiving antenna.

Contents

Radio Signals

Radio waves are made of an electric wave and a magnetic wave. The two waves travel in a sine wave pattern. One will travel up and down, and the other will travel side to side. Both waves travel together at the same frequency. These waves can interact with the surface of the Earth or the upper layers of the atmosphere called the ionosphere and this can affect how they travel to the receiver antenna.

Ionosphere

The ionosphere is just a bit of charged atoms, or ions, in the upper atmosphere. They get energy from the sun during the day when the sun shines, and lose that energy at night after the sun has set. The higher the sun's activity levels the more charged they become. Solar flares can cause a great increase in the ionosphere, as can sun spots. You will get the furthest range at dusk or dawn than you will at other times due to the angles between you, the sun and the other station. In general the longer distances will be towards the direction of the sun.

Currently in 2009, we are in the deepest solar minimum in the last 100 years. The sun is amazingly quiet, with over 80% of the days so far having no activity.

In winter, due to the Earth's angle to the sun, there is less sunlight hitting the ionosphere. This means that regional contacts can be a bit more difficult in winter than summer. Remember though, winter in the Northern Hemisphere is summer in the Southern Hemisphere. This can make it possible to reach a country across the equator yet you cant reach the same distance on the same side of the equator.

Polarization

As stated previously, radio signals are comprised of an electric and magnetic wave. If the antenna is horizontal, you will have horizontal polarization. Horizontal polarized signals generally have more interaction with the ground, and attenuation from them. Vertically polarized signals suffer much less attenuation from the ground.

If you transmit a horizontally polarized signal and the receiver's antenna is vertically polarized (or vice versa) you will have as much as 20dB loss, or about 99% loss. By matching the polarization between both the transmitter and the receiver you can get a much stronger signal. Likewise by being 90 degrees out of phase with other transmitters you can filter those unwanted signals out.

Ground wave

Ground wave is the path that a radio signal will travel if its coupled to the ground to some degree. As the signal travels across the ground, the edge against the ground will bend it slightly such that the wave front is angled towards the ground. This slight bending causes the signal to follow the curve of the Earth.

The Earth will also attenuate the signal. The higher the frequency the more this attenuation will be. The attenuation will eventually prevent the signal from being received with sufficient strength to be interpreted by the receiving station.

Lower frequencies will penetrate deeper into the ground, and the composition of the ground, its conductivity, etc will all affect it more. Depending on the frequency it can penetrate as deep as 100 meters below ground. Higher frequencies do not penetrate the ground nearly as much, and are usually not as effected by the composition of the ground.

Sky Wave

Sky wave is the path that a radio signal will travel if it is interacting with the ionosphere. The ionosphere is a layer of charged particles floating high above the planet. They are created by the sun's energy. The more the sun's output is the greater the ionosphere will be and the better you will be able to use skywave for long distance communications.

There are several layers of the ionosphere, the most commonly used in radio is the D, E and F layers. The D layer which is closest to the Earth, is present during the day. This generally attenuates radio signals and causes a high noise floor. You will be able to hear this with most receivers if you tune to a low frequency during the day, it sounds like static.

Higher frequencies can pass through the D layer, and reach the E and F layers. In the evening, when the sun has passed by, the D layer will fade exposing the E and F layers to the lower frequencies. VHF, UHF and higher frequencies will pass through the E and F layers and not be reflected back to the Earth.

Lower frequencies, generally in the HF and MF range will bounce off the E and F layers of the ionosphere back to Earth. This enables long range communications to occur.

During the day you may have problems getting more than a few miles with low frequencies, say the 80m ham band (3-4MHz). The 10m band (28MHz) and CB may work well as it can pass through the D layer and bounce back from the E and F layer. As the night wears on, and its been a while since the sun shined on the ionosphere, it will weaken to the point that a given band will not work. This will usually start at the higher frequencies (closer to 30MHz) and work down (towards 2MHz). As one band closes you will have to move to a lower band or call it a night.

You will see the best sky wave propagation for long range communications at dusk and dawn when the sun is shining on the side of the planet adjecent to where you are located.

Meteor showers act as a limited ionosphere as they scream through the atmosphere. It is possible to bounce signals off the tails of meteors although this method is spotty and takes a lot of patience and a good meteor storm.

Figure 1

image:Skywave_vs_Groundwave.jpg

  • (1) VHF, UHF and Microwave (above 30Mhz) sky wave
  • (2) HF and MF (below 30MHz) skywave
  • (3) Reflected signal

See Also

References

External Links

http://www.spaceweather.com Information about sun spots, solar flares and more!

http://members.cox.net/n0nbh/#MUF propagation prediction (needs JAVA)

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