Rf Waves Help (page 3)
An EM disturbance is called a radio-frequency (rf) wave if its wavelength falls within the range of 100 km to 1 mm. This is a frequency range of 3 kHz to 3000 GHz.
Formal RF Band Designators
The rf spectrum is split into eight bands , each representing one order of magnitude in terms of frequency and wavelength. These bands are called very low, low, medium, high, very high, ultrahigh, superhigh , and extremely high frequencies . They are abbreviated, respectively, as VLF, LF, MF, HF, VHF, UHF, SHF, and EHF. These are depicted in Table 18-1 in terms of the frequency and the free-space wavelength.
Table 18-1 The Bands in the Radio-frequency (rf) Spectrum. Each band spans one mathematical order of magnitude in terms of frequency and wavelength.
These bands have alternative names. Energy at VLF and LF is sometimes called longwave radio or long waves . Energy in the HF range is sometimes called shortwave radio or short waves (even though the waves aren’t short compared with most EM waves in wireless communications used today). Superhigh-frequency and extremely-high-frequency rf waves are sometimes called microwaves .
Radio-frequency waves propagate through the Earth’s atmosphere and through space in various ways, depending on the wavelength. Some waves are affected by the ionosphere; this is especially true at VLF, LF, MF, and HF. The troposphere can bend, reflect, or scatter waves at VHF, UHF, SHF, and EHF.
The atmosphere of our planet becomes less dense with increasing altitude. Because of this, the energy received from the Sun is much greater at high altitudes than it is at the surface. High-speed subatomic particles, UV rays, and x-rays cause ionization of the rarefied gases in the upper atmosphere. Ionized regions occur at specific altitudes and comprise the ionosphere . The ionosphere causes absorption and refraction of radio waves. This makes long-distance communication or reception possible at some radiofrequencies.
Ionization in the upper atmosphere occurs in four fuzzy layers. The lowest region is called the D layer . It exists at an altitude of about 50 km (30 mi) and ordinarily is present only on the daylight side of the planet. This layer does not contribute to long-distance radio communications, and sometimes impedes them. The E layer , about 80 km (50 mi) above the surface, also exists mainly during the day, although nighttime ionization is sometimes observed. The E layer can facilitate medium-range radio communications at certain frequencies. The uppermost layers are called the F1 layer and the F2 layer . The Fl layer, normally present only on the daylight side of the Earth, forms at about 200 km (125 mi) altitude. The F2 layer, which exists more or less around the clock, is about 300 km (180 mi) above the surface. On the dark side of the Earth, when the F1 layer disappears, the F2 layer is sometimes called simply the F layer .
Figure 18-3 illustrates the relative altitudes of the ionospheric D, E, F1, and F2 layers above the Earth’s surface. All these layers have some effect on the way radio waves travel at very low, low, medium, and high frequencies. Sometimes, ionospheric effects can even be observed into the VHF portion of the radio spectrum. These layers not only make long-distance wireless communications possible between points on the Earth’s surface; they also prevent radio waves at frequencies below approximately 5 MHz from reaching the surface from outer space.
The number of sunspots is not constant but changes from year to year. The variation is periodic and dramatic. This fluctuation of sunspot numbers is called the sunspot cycle . It has a period of approximately 11 years. The rise in the number of sunspots is generally more rapid than the decline, and the maximum and minimum sunspot counts vary from cycle to cycle.
The sunspot cycle affects propagation conditions at frequencies up to about 70 MHz for F1- and F2-layer propagation and 150 to 200 MHz for E-layer propagation. When there are not many sunspots, the maximum usable frequency (MUF) is comparatively low because the ionization of the upper atmosphere is not dense. At or near the time of a sunspot peak, the MUF is higher because the upper atmosphere is more ionized.
A solar flare is a violent storm on the surface of the Sun. Solar flares cause an increase in the level of radio noise that comes from the Sun and cause the Sun to emit an increased quantity of high-speed subatomic particles. These particles travel through space and arrive at the Earth a few hours after the first appearance of the flare. Because the particles are electrically charged, they are accelerated by the Earth’s magnetic field. Sometimes a geomagnetic storm results. Then we see the “northern lights” or “southern lights” (aurora borealis or aurora australis , often called simply the aurora) at high latitudes during the night and experience a sudden deterioration of ionospheric radio-propagation conditions. At some frequencies, communications can be cut off within seconds. Even wire communications circuits are sometimes affected.
Solar flares can occur at any time, but they seem to take place most often near the peak of the 11-year sunspot cycle. Scientists do not know exactly what causes solar flares, but the events seem to be correlated with the relative number of sunspots.
In radio communication, the ground wave consists of three distinct components: the direct wave (also called the line-of-sight wave) , the reflected wave , and the surface wave . The direct wave travels in a straight line. It plays a significant role only when the transmitting and receiving antennas are connected by a straight geometric line entirely above the Earth’s surface. At most radiofrequencies, EM fields pass through objects such as trees and frame houses with little attenuation. Concrete-and-steel structures cause some loss in the direct wave at higher frequencies. Earth barriers such as hills and mountains block the direct wave.
A radio signal can be reflected from the Earth or from certain structures such as concrete-and-steel buildings. The reflected wave combines with the direct wave (if any) at the receiving antenna. Sometimes the two are exactly out of phase, in which case the received signal is weak even if the transmitter and receiver lie along a direct line of sight. This effect occurs mostly at frequencies above 30 MHz (wavelengths less than 10 m).
The surface wave travels in contact with the Earth, and the Earth forms part of the circuit. This happens only with vertically polarized EM fields (those in which the electrical flux lines are vertical) at frequencies below about 15 MHz. Above 15 MHz, there is essentially no surface wave. At frequencies from about 9 kHz up to 300 kHz, the surface wave propagates for hundreds or even thousands of kilometers. Sometimes the surface wave is called the ground wave , but technically this is a misnomer.
At certain radio-frequencies, the ionospheric E layer occasionally returns signals to the Earth. This effect is intermittent, and conditions can change rapidly. For this reason, it is known as sporadic-E propagation . It is most likely to occur at frequencies between approximately 20 and 150 MHz. Occasionally it is observed at frequencies as high as 200 MHz. The propagation range is on the order of several hundred kilometers, but occasionally communication is observed over distances of 1,000 to 2,000 km (600 to 1,200 mi).
The standard FM broadcast band is sometimes affected by sporadic-E propagation. The same is true of the lowest television (TV) broadcast channels, especially channels 2 and 3. Sporadic-E propagation is sometimes mistaken for effects that take place in the lower atmosphere independently of the ionosphere.
Auroral And Meteor-scatter Propagation
In the presence of unusual solar activity, the aurora often reflect radio waves at some frequencies. This is called auroral propagation . The aurorae occur in the ionosphere at altitudes of 25 km (40 mi) to 400 km (250 mi) above the surface. Theoretically, auroral propagation is possible, when the aurorae are active, between any two points on the Earth’s surface from which the same part of the aurora lies on a line of sight. Auroral propagation seldom occurs when either the transmitter or the receiver is at a latitude less than 35 degrees north or south of the equator. Auroral propagation can take place at frequencies well above 30 MHz and often is accompanied by deterioration in ionospheric propagation via the E and F layers.
When a meteor from space enters the upper part of the atmosphere, an ionized trail is produced because of the heat of friction. Such an ionized region reflects EM energy at certain wavelengths. This phenomenon, known as meteor-scatter propagation , can result in over-the-horizon radio communication or reception.
A meteor produces a trail that persists for a few tenths of a second up to several seconds depending on the size of the meteor, its speed, and the angle at which it enters the atmosphere. This amount of time is not sufficient for the transmission of very much information, but during a meteor shower , ionization can be almost continuous. Meteor-scatter propagation has been observed at frequencies considerably above 30 MHz and occurs over distances ranging from just beyond the horizon up to about 2,000 km (1,200 mi) depending on the altitude of the ionized trail and on the relative positions of the trail, the transmitting station, and the receiving station.
The lowest 13 to 20 km (8 to 12 mi) of the Earth’s atmosphere comprise the troposphere . This region has an effect on radio-wave propagation at certain frequencies. At wavelengths shorter than about 15 m (frequencies above 20 MHz), refraction and reflection can take place within and between air masses of different density. The air also produces some scattering of EM energy at wavelengths shorter than about 3 m (frequencies above 100 MHz). All these effects generally are known as tropospheric propagation , which can result in communication over distances of hundreds of kilometers.
A common type of tropospheric propagation takes place when radio waves are refracted in the lower atmosphere. This is most dramatic near weather fronts, where warm, relatively light air lies above cool, more dense air. The cooler air has a higher index of refraction than the warm air, causing EM fields to be bent downward at a considerable distance from the transmitter. This is tropospheric bending . It is often responsible for anomalies in reception of FM and TV broadcast signals.
At frequencies above about 100 MHz, the atmosphere has a scattering effect on radio waves. The scattering allows over-the-horizon communication at VHF, UHF, and microwave frequencies. This is called tropospheric scatter , or troposcatter . Dust and clouds in the air increase the scattering effect, but some troposcatter occurs regardless of the weather. Troposcatter takes place mostly at low altitudes where the air is the most dense. Some effects occur at altitudes up to about 16 km (10 mi). Troposcatter can provide reliable communication over distances of several hundred kilometers when the appropriate equipment is used.
Figure 18-4 shows tropospheric scatter and bending. The transmitting station is at the lower left. There is a temperature inversion in this example; it exaggerates the bending. If the boundary between the cool air near the surface and the warm air above is well defined enough, reflection can occur in addition to the bending. If the inversion covers a large geographic area, signals can bounce repeatedly between the inversion boundary and the surface, providing exceptional long-range communication, especially if the surface is salt water.
Fig. 18-4 . The troposphere can bend and scatter radio waves at some frequencies.
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