Propagation beyond the line of sight is possible through sky waves. Sky waves are radio waves that propagate into the atmosphere and then are returned to earth at some distance from the transmitter. We will consider two cases:
propagation mode occurs when radio waves
travel into the ionosphere, a region of charged particles 50 – 300 miles above
the earth’s surface. The ionosphere is created when the
sun ionizes the upper regions of the earth’s atmosphere. These charged regions
are electrically active. The ionosphere bends
and attenuates radio waves at frequencies below 30 MHz. Above 200 MHz the
ionosphere becomes completely transparent. The ionosphere is responsible for
most propagation phenomena observed at HF, MF, LF and VLF. The ionosphere
consists of 4 highly ionized regions
D layer at a height of 38 – 55 mi
E layer at a height of 62 – 75 mi
F1 layer at a height of 125 –150 mi (winter) and 160 – 180 mi (summer)
F2 layer at a height of 150 – 180 mi (winter) and 240 – 260 mi (summer)
The density of ionization is greatest in the F layers and least in the D layer Though created by solar radiation, the ionosphere does not completely disappear shortly after sunset. The D and E layers disappear almost immediately, but the F1 and F2 layers do not disappear; rather they merge into a single F layer located at a distance of 150 – 250 mi above the earth. Recombination or charged particles is quite slow at that altitude, so the F layer lasts until dawn.
The diagram below shows the geometry of ionospheric refraction. The maximum frequency that can be returned by the ionosphere when the radio waves are vertically incident on the ionosphere (transmitted straight up) is called the critical frequency.
The critical frequency varies from place to place, and it is possible to view this variation by looking at a real-time critical frequency map
The critical frequency varies from 1 to 15 MHz under normal conditions. Most communications is done using radio waves transmitted at the horizon, to get the maximum possible distance per hop. The highest frequency that can be returned when the takeoff angle is zero degrees is called the MUF, maximum usable frequency. The MUF and critical frequency are related by the following formula:
The MUF can range from 3 to 50 MHz. You can click here to see a near real-time map of the MUF of the ionosphere.
The ionosphere also attenuates radio waves. The amount of attenuation is roughly inversely proportional to the square of the frequency of the wave. Thus attenuation is a severe problem at lower frequencies, making daytime global communications via sky wave impossible at frequencies much below 5 MHz.
The properties of the ionosphere are variable. There are 3 periodic cycles of variation:
diurnal (daily) cycle
The daily cycle is driven by the intensity of the solar radiation ionizing the upper atmosphere. The D and E layers form immediately after sunrise, and the F layer splits into two layers, the F1 and F2. The density of the layers increases until noon and then decreases slowly throughout the afternoon. After sunset, the D and E layers disappear and the F1 and F2 merge to form the F layer. Take another look at the real-time MUF map and notice the difference between the MUF numbers in the day and night regions. If you aren't sure which region is the daytime region, it has a small yellow sun icon in its center. The thick gray lines indicate the location of the terminator - the division between day and night.
Seasonal variation is linked to the tilt of the earth’s axis and the distance between the earth and sun. The effects are complex, but the result is that ionospheric propagation improves dramatically during the for the northern hemisphere during their winter, while seasonal variation in the southern hemisphere is much smaller.
The 11 year sunspot cycle exerts a tremendous effect on the atmosphere. Near the peak of the cycle (the last peak occurred in December 2001) the sun’s surface is very active, emitting copious amounts of UV radiation and charged particles, which increase the density of the ionosphere. This leads to a general increase in MUF’s and attenuation at lower frequencies. When the sun becomes extremely active, or a major solar flare occurs, the ionosphere can become so dense that global ionospheric communications are disrupted.
The maximum distance that can be covered by a single hop using ionospheric propagation is about 2500 miles. Greater distances can be covered using multi-hop propagation, in which radio waves are reflected by the ground back up to the ionosphere.
The ionosphere is not uniform and different regions refract RF differently. Multipath propagation is the result. This leads to rapid variations in the received signal amplitude known as fading.
One of the consequences of ionospheric propagation is that reception of signals on the AM broadcast band varies greatly from day to night. Click on the wave files below to hear how the disappearance of the D-layer after dark changes what is heard.
For reasons that are not clearly understood, clouds of densely ionized gases appear in the E -layer of the ionosphere. The clouds are generally relatively small and can happen at any time of day. These clouds are formed throughout the year, but are most common in the summer months. Because these clouds are so densely ionized, they can support ionospheric propagation at frequencies well above the normal MUF. Sporadic E propagation has been observed at frequencies as high as 144 MHz, and is relatively common at 50 MHz.
The E-layer is lower than the F-layer and as a result, the distance covered by a sporadic-E hop is approximately 1000-1300 miles, depending on the cloud's height. The sporadic-E clouds drift through the E-layer, adding to the unpredictability of sporadic-E propagation.
Sporadic-E propagation is not generally useful because of its unpredictability. Its main impact is negative, causing VHF-TV and FM broadcasters in different markets to interfere with each other.
Regional over the horizon communications are possible through a sky wave technique called tropospheric scatter (troposcatter or just tropo). As shown in the diagram below, the troposphere, which is the layer of the atmosphere closest to the ground, has pockets or cells of air within it that have a different water vapor content and therefore a different refractive index for radio waves. As a result, radio waves are scattered by the cells over the horizon. This scatter occurs at frequencies of 0.3 – 10 GHz. Operation above 10 GHz
is not possible because water vapor in the air strongly absorbs the signals This scattering process is not efficient and very little of the transmitted signal is scattered in the direction of the receiver. High power transmitters and sensitive receivers are required.
The troposphere contains almost all of the earth’s weather patterns, which makes the troposphere’s properties quite variable. This makes troposcatter communications subject to weather induced fading and communications blackouts. To improve the reliability of troposcatter links, a technique called diversity operation is used. There are three types of diversity:
Diversity – two frequencies simultaneously transmit the same signal
Diversity – radio waves of both polarizations are transmitted simultaneously
Space Diversity – pairs of widely separated antennas are used for transmitting and receiving
Diversity operation greatly increases the reliability of troposcatter links, but it comes at a significant cost, because at least double the amount of equipment is needed at each installation.