Скачать или смотреть RADIO WAVE PROPAGATION - Ground wave, Sky wave and Space wave

electromagnetic waves are oscillations produced due to crossing
over of an electric and a magnetic field. The direction of the propagation of such waves is perpendicular to the direction of the force of either of
these fields.
Properties of the EM waves
• EM waves travel at the speed of light.
• EM waves do not require any medium for propagation.
• EM waves travel in a transverse form.
• EM waves are not deflected by electric or magnetic field.
• EM waves can be polarized.
• EM waves undergo interference and diffraction.
These layers are used for propagation of EM waves and that EM
waves travel basically in any one of the three methods given below:
• Ground Wave
• Space Wave
• Sky Wave
1. Ground Wave Propagation
To radiate signals with high efficiency, the antennas should have a size comparable to the wavelength λ of the signal (at least ~ λ/4).
• At longer wavelengths (i.e., at lower frequencies), the antennas have large physical size and they are located on or very near to the ground.
• In standard AM broadcast, ground based vertical towers are generally used as transmitting antennas. For such antennas, ground has a strong
influence on the propagation of the signal.
• The mode of propagation is called surface wave propagation and the wave glides over the surface of the earth.
Properties of Ground Wave Propagation
 Used for a low-frequency range transmission, mostly less than 2MHz.
 Employs the use of large antennas order of which is equivalent to the
wavelength of the EM waves and uses the ground or Troposphere for
its propagation.
 Signals over large distances are not sent using this method.
 It causes severe attenuation which increases with increased
frequency of the waves. The maximum range of coverage depends on
the transmitted power and frequency (less than a few MHz).
2. Sky Wave Propagation
In the Frequency range 300KHz–30MHz, long distance communication can be achieved by ionospheric reflection of radio waves back towards the earth.
• The sky wave, often called the ionospheric wave, is radiated in an upward direction and returned to Earth at some distant location because of refraction from the ionosphere.
• This form of propagation is relatively unaffected by the Earth's surface and can propagate signals over great distances. Usually the high frequency (HF) band is used for sky wave propagation.
Properties of Sky Wave Propagation
• Used for the propagation of EM waves with a frequency range of
300KHz – 30MHz.
• Make use of the ionosphere so called due to the presence of charged
ions in the region of about 60 to 300 km from the Earth surface.
These ions provide a reflecting medium to the radio or
communication waves within a particular frequency range.
• Use of the property of the ionosphere for long-distance transmission
of the waves without much attenuation and loss of signal strength.
• Examples
• o Amateur radio , Citizens Band (CB) radio , International broadcasts
3. Space Wave Propagation
• Another mode of radio wave propagation is by space waves.
• The space wave follows two distinct paths from the transmitting
antenna to the receiving antenna :
 one through the air directly to the receiving antenna,
 the other reflected from the ground to the receiving antenna
At frequencies above 40 MHz, communication is essentially limited to
LOS paths. At these frequencies, the antennas are relatively smaller
and can be placed at heights of many wavelengths above the ground.
Because of LOS nature of propagation, direct waves get blocked at
some point by the curvature of the earth as illustrated in Fig.
• If the signal is to be received beyond the horizon then the receiving
antenna must be high enough to intercept the LOS waves.
Properties of Space Wave Propagation
• Space waves are used for LOS communication as well as satellite
communication.
• Space satellite communication and very high-frequency waves use
this propagation method.

Summary
• Radio wave signals are mainly used for communication purpose.
• Transmission of broadcast signals to the communication with
aircrafts, every mode of communication and transportation is
dependent on signals transmitted in radio wave frequency.
• The main objective of radio wave propagation is to transmit signals
securely without any error.
• It is concluded that Radio wave propagation is an important branch of
communication studies.