Propagation

Question File Number 28



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The Spectrum

Amateur Radio is all about the transmission of radio waves from place-to-place without wires. Signals travel from the transmitting antenna to the receiving antenna in different ways depending on the frequency used. Some frequencies use the ionosphere to bounce signals around the world while other frequencies can only be used for line-of-sight operations.

Radio waves are part of the spectrum of electromagnetic radiation, with infrared, light, ultraviolet, x-rays and cosmic rays at the extreme upper frequencies. Radio waves further subdivide into different frequency ranges. See Frequencies for the various band divisions.

All electromagnetic radiation travels at the same speed, commonly referred to as the speed of light, c = 3 x 108 metres per second or 300 000 km per second.

Electromagnetic radiation consists of two waves travelling together, the magnetic and the electric, with the planes of the two waves perpendicular to each other.

The polarisation of a radio wave is determined by the direction of the electric field. Most antennas radiate waves that are polarised in the direction of the length of the metal radiating element. For example, the metal whips as used on cars are vertically polarised while TV antennas may be positioned for either vertical or horizontal polarisation. Polarisation is important on VHF and higher but is not very important for HF communications because the many reflections that a skywave undergoes makes its polarisation quite random.

The Path

The simplest path to understand is the direct path in a straight line between transmitter and receiver. These are most important for communication on frequencies above 50 MHz. The signal might be reflected off buildings and mountains to fill in some shadows, but usually communication is just line-of-sight.

On lower frequencies the ionosphere is able to reflect the radio waves. The actual direction-change in the ionosphere is closer to refraction but reflection is easier to envisage.

For simplicity, we will use the reflection word here, but remember that the mechanism is more truly refractive. Similarly, again for simplicity, we will consider the regions where the change-of-direction takes place to be layers although they are more strictly regions.

The signal reflected off the ionosphere is referred to as the skywave or ionospheric wave. The groundwave is the signal that travels on the surface of the earth and depends upon the surface conductivity.

Groundwaves are the main mode of transmission on the MF bands (e.g. AM broadcast band), but they are not very important for amateur use - except perhaps on the only amateur MF band, 160 metres, 1.8 MHz. The groundwave is usually attenuated within 100 km.

On VHF and higher frequencies, variations in the atmospheric density can bend the radio waves back down to the earth. This is referred to as the tropospheric wave.

The Skywave

The skywave is the primary mode of long distance communication by radio amateurs and is usually of the most interest. A skywave will go farther if it can take longer hops. For this reason, a low angle (< 30 ) radiation is best for DX (long distance) communication as it will travel farther before reflecting back to earth. Antennas that produce low angle radiation include verticals or dipoles mounted high (at least half a wavelength) above the ground.

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The Sun And The Ionosphere

The ionosphere refers to the upper region of the atmosphere where charged gas molecules have been produced by the energy of the sun. The degree of ionisation varies with the intensity of the solar radiation. Various cycles affect the amount of solar radiation with the obvious ones being the daily and yearly cycles. This means that ionisation will be greatest around noon in the summer and at minimum just before dawn in the winter.

The output from the sun varies over a longer period of approximately 11 years. During the maximum of the solar sunspot cycle, there is greater solar activity and hence greater ionisation of the ionosphere.

Greater solar activity generally results in better conditions for radio propagation by increasing ionisation. However, very intense activity in the form of geomagnetic storms triggered by a solar flare can completely disrupt the layer of the ionosphere and block communications. This can happen in minutes and communications can take hours to recover.

Ionospheric Layers

Ionosphere

The ionosphere is not a homogenous region but consists of rather distinct layers or regions which have their own individual effects on radio propagation. The layers of distinct interest to radio amateurs are the E and F layers.

The E layer at about 110 km is the lower of the two. It is in the denser region of the atmosphere where the ions formed by solar energy recombine quickly. This means the layer is densest at noon and dissipates quickly when the sun goes down.

The F layer is higher and during the day separates into two layers, F1 and F2 at about 225 and 320 km. It merges at night to form a single F layer at about 280 km.

The different layer of the ionosphere can reflect radio waves back down to earth which in turn can reflect the signal back up again. A signal can hop around the world in this way. The higher the layer, the longer the hop. The longer the hop the better since some of the signal's energy is lost at each hop.

Lower angle radiation will go farther before it reflects off the ionosphere. So to achieve greatest DX, one tries to choose a frequency that will reflect off the highest layer possible and use the lowest angle of radiation. The distance covered in one hop is the skip distance. For destinations beyond the maximum skip distance the signal must make multiple hops.

The virtual height of any ionospheric layer at any time can be determined using an ionospheric sounder or ionosonde, in effect a vertical radar. This sends pulses that sweep over a wide frequency range straight up into the ionosphere. The echoes returned are timed (for distance) and recorded. A plot of frequency against height can be produced. The highest frequency that returns echoes at vertical incidence is known as the critical frequency.

Absorption

The ionosphere can also absorb radio waves as well as reflect them. The absorption is greater at lower frequencies and with denser ionisation. There is another layer of ionisation below the E layer, called the D layer, which only exists during the day. It will absorb almost all signals below 4 MHz - i.e. the 80 and 160 metre bands. Short-range communication is still possible using higher angle radiation which is less affected. It travels a shorter distance through the atmosphere. The signal can then reflect off the E layer to the receiver. The D and E layers are responsible for you hearing only local AM broadcast stations during the day and more distant ones at night.

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Attenuation

The attenuation of a signal by the ionosphere is higher at lower frequencies. So for greater distance communication one should use higher frequencies. But if the frequency used is too high, the signal will pass into space and not reflect back to earth. This may be good for satellite operation but is not useful for HF DX working.

For DX working on HF, one should try to use the highest frequency that will still reflect off the ionosphere. This varies with solar activity and time of day. It can be calculated with various formulas given the current solar indices. This frequency is referred to as the Maximum Usable Frequency (MUF). In the peak of the solar cycle it can often be over 30 MHz and on rare occasions up to 50 MHz. At other times, during the night, it can drop below 10 MHz.

At the low end of the spectrum, daytime absorption by the D layer limits the possible range. In addition, atmospheric noise is greater and limits the Lowest Usable Frequency (LUF). This noise and absorption decreases at night lowering the LUF at the same time as the MUF is lowered by the decrease in solar excitation of the ionosphere. This usually means that by picking the right frequency, long range communication is possible at any time.

Fading

Radio waves can travel over different paths from transmitter to receiver. If a path length varies by a multiple of half the wavelength of the signal, the signals arriving by two or more paths may completely cancel each other. This multi-path action causes fading of the signal. Other phenomena can cause this. Aircraft, mountains and ionospheric layers can reflect part of a signal while another part takes a more direct path.

Sometimes fading may be so frequency-dependent that one sideband of a double-sideband (AM) signal may be completely unreadable while the other is good copy. This is known as selective fading. It will often be observed just as a band is on the verge of closing, when reflections from two layers are received simultaneously.

Fading can also occur when a signal passes through the polar regions, referred to as polar flutter, caused by different phenomena. The ionosphere is much more disorganised in the polar regions because of the interaction of solar energy with the geomagnetic field. The same phenomena that cause aurora can cause the wavering of signals on polar paths.

Other Atmospheric Effects

Other atmospheric effects can affect radio propagation and may often extend the transmission of VHF and higher signals beyond the line-of-sight. The lowest region in the atmosphere, the troposphere, can scatter VHF signals more than 600 km - tropospheric scatter. Ducting is a phenomenon where radio waves get trapped by a variation in the atmospheric density. The waves can then travel along by refraction. Ducting usually occurs over water or other homogenous surfaces. This is more common at higher frequencies and has permitted UHF communication over distances greater than 2500 km.

Another phenomenon, sporadic E skip, is a seasonal occurrence, usually during the summer. A small region of the E layer becomes more highly charged than usual, permitting the reflection of signals as high in frequency as 200 MHz. This highly-charged region soon dissipates. Sporadic E propagation will occur for only a few minutes to a few hours.

Communication can be achieved by bouncing signals off the ionised trails of meteors. Meteor scatter communication may only last a few seconds so it is feasible only when large numbers of meteors enter the atmosphere, particularly during times of meteor showers.

Skip Zone

Skip

Amateurs are usually concerned about working to the maximum possible distances but there are times when one can talk to people thousands of kilometres away but cannot talk to someone only 500 km away. A skip zone can be created by the ionosphere reflecting signals from a shallow angle. Waves at a higher angle pass directly through and are lost into space. The critical angle varies with the degree of ionisation and generally results in larger skip zones at night. The area between the limit of maximum range by direct wave or ground wave, and the maximum skip distance by skywave is known as the skip zone.




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Compiled Sun Nov 28 2010 at 8:41:44pm


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