Propagation of Radio waves

           Propagation of Radio waves

Radio waves launched from a transmitting antenna travel outward and are not markedly affected by the surrounding atmosphere, rain, snow etc. These waves readily pass through nonmetallic substances but are greatly attenuated by metals. Their movement in free space is best, and no material medium is required for their propagation.

Electromagnetic waves with frequencies extending from about 10 kHz to 300 GHz are classed as radio waves. These wave are subdivided into smaller ranges for convenience, as shown in

Band -- Frequency range -- wavelength range – Principle of application

Very low frequency (VLF) – 10 to 30 kHz – 30 to 10 km – Direct long range communication

Low frequency (LF) – 30 to 300 kHz – 10 to 1 km – Marine, navigational aids

Medium frequency (MF) – 300 kHz to 3 MHz – 1km to 100m – Broadcasting

High frequency (HF) – 3 to 30 MHz – 100 to 10m – all types of communication

Very high frequency (VHF) – 30 to 300 MHz – 10 to 1 m – TV, FM, radar, short wave communication

Ultra high frequency (UHF) – 300 MHz to 3 GHz – 1m to 10cm – Microwaves communication, radar

Super high frequency (SHF) – 3 to 30 GHz – 10 to 1 cm – Radio, relay, navigation, radar, satellite communication

Extremely high frequency (EHF) – 30 to 300 GHz – 1cm to 1mm -- Experimental

Depending primarily on the frequency, a radio wave travels from the transmitting to the receiving antenna in several ways. On the basis of the mode of propagation, radio waves can be broadly classified as

1.      Ground or surface wave

2.      Space or tropospheric wave

3.      Sky wave

These are three main types of propagation are discussed below:

1.      Ground or surface wave:

In ground wave propagation, radio  wave are guided by the earth and move along its curved surface from the transmitter to the receiver. As the wave move over the ground, they are strongly influenced by the electrical properties of the ground. As high frequency waves are strongly absorbed by ground, ground wave propagation is useful only at low frequencies. Below 500 kHz, ground waves can be used for communication within distance of about 1500km from the transmitter. AM radio broadcasts in the medium frequency band cover local areas and take place primarily by the ground wave. The ground wave at higher frequencies employed by frequency modulation (FM) and television (TV) are increasingly absorbed and therefore become very weak beyond a distance of several kilometers from the transmitter. Ground wave transmission is very reliable whatever be   the   atmospheric conditions.

2.      Space or tropospheric wave:

In space wave propagation, radio waves   move in the  earth  tropospheric within about 15km over the surface of the earth. The space wave is made up of two components: (a) a direct or line of sight wave from the transmitting to the receiving antenna and (b) the ground reflected wave traversing from the transmitting antennas to the ground and reflected to the receiving antenna these two components     of the space wave. The ground reflected wave suffers a phase shifts of 180⁰ upon reflection at the point of incidence of the earth surface. So, when the path length of the ground reflected and the direct waves from the transmitting to the receiving antenna differs by a wavelength, the two waves reach the receiving antenna in opposite phase and annul each other. But when the path lengths differ by half a wavelength   the two waves reach in phase and reinforce each other. By changing the height of the receiving antenna, the path length of the two waves can be varied. In this way, alternate enhancement and reduction of signal strength can be achieved. This phenomenon is known as selective fading.

The space wave is not continuously absorbed by the earth’s surface. Hence it can cover a greater range than the ground wave. FM and TV  programmes in the VHF band are propagated by direct waves with contribution from ground-reflected waves. In the UHF band transmission occurs entirely by the direct waves. Interestingly, transmission around the globe cannot be obtained by the direct wave because the curvature of the earth obstructs a direct view of the transmitting antenna from a receiving antenna positioned on the other side of the earth. However, due to refraction or scattering of the wave by the earth’s atmosphere and diffraction by the roughness of the earth’s surface, the signal can be received over a little distance beyond the direct line-of sight. This extra distance is referred to as the shadow zone. If the moisture content of the air is very high at the earth surface and falls off rapidly with increasing height, the signal strength in the shadow zone increases. The  sharp  fall of the moisture content cause a rapid decrease of the refractive index of the air with increasing height. Therefore, the curvature of the rays travelling through the atmosphere can become larger than that of the earth. Consequently, the rays originally moving parallel to earth’s surface are trapped and traverse around the curved surface of the earth through successive reflection. This phenomenon is called duct propagation.

3.      Sky wave:

In sky wave propagation, radio wave transmitted from the transmitting antenna reach the receiving antenna after reflection from ionosphere, i.e., the ionized layers laying in the sky earth’s upper atmosphere. Short wave transmission around the globe is possible through sky waves via successive reflection at the ionosphere and the earth surface.