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Rf Waves Help (page 2)

By — McGraw-Hill Professional
Updated on Sep 12, 2011

Solar Activity

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.

Ground-wave Propagation

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.

Sporadic-e Propagation

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.

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