Lecture №2

1. Lecture
Topic: Type of radio waves used in the medium range. The communication range
in the medium range.
Objective: to uncover type of radio waves used in the medium range. The
communication range in the medium range.
We consider the following questions:
1. Propagation of medium range.
2. Factors of medium principles of the transmitter.
1.Propagation of medium range.
The radio wave is needed to carry the signal information efficiently and
without distortion. In the case of audio frequencies, which may range from about
50Hz to 15 kHz, it would not be technically feasible to radiate the information
directly from a practical transmitter and antenna. (Try to calculate the wavelength by
the above mentioned formula with the15 kHz frequency. Then you will see the
impractical size of the antenna you need for such a transmission.)
Higher frequencies can radiate efficiently from antennas having dimensions
typically between a quarter and one wave length. Thus, practical communication
systems use a radio wave to carry the audio or other (e.g., vision or data) information
between the transmitting and receiving sites.
Three main physical mechanisms govern the propagation of radio waves:
Line of sight Ground
wave Sky wave
Each frequency range has its own propagation characteristic. The reliability of
a connection between two stations with a transmitter and a receiver depends on the
choice of the correct frequency band.
The Radio Frequency (RF) spectrum is divided in several major bands:

2. Table 1: Frequency bands
The equivalent between wavelength and frequency
Radio waves radiate at the velocity of light, 300 x 106
m per second. The
equivalent between the velocity of light (c), frequency (f) and the wavelength (A)
i.e. longer wavelength corresponded to lower frequency, shorter wavelength to
higher frequency.
f = number of cycles per second
c = velocity of light 300 x 106
meters per second (300000 km per
second) A = wavelength in meters

3. Figure 1: Example of wavelength
Table 2: Frequency ranges and their applications
Different Antennas used for specific frequencies
Different types of antennas have to correspond with the different
frequency ranges for which the antennas are used (see also 0.)
Line of sight propagation

4. 2. Factors of medium principles of the transmitter.
Figure 2: Line of sight propagation
Above about 50 MHz, propagation is essentially by line-of-sight. This is
accomplished, in the case of terrestrial radio, via the lower part of the atmosphere -
termed the troposphere - and in the case of space communication via earth-orbiting
satellites.
Figure 13: Line of sight propagation shows a stylised terrestrial radio link. In
general, the received signal is the sum of a direct signal along path a, clear of the
ground, and several reflected signals along paths such as b and c. Because a radio
signal undergoes a phase reversal at the reflection point, the theoretical situation is
that the direct and reflected signals should cancel out if the receiver antenna is at
ground level.
Since land has poor ground conductivity, total cancellation does not occur in
practice, as simple experiment with portable VHF FM receiver will show. However,
the sea is a very good conductor, which means that maritime VHF antennas should be
mounted well above the sea in order to avoid severe cancellation effects.
Ground waves and sky waves

5. Figure 3: Ground waves and sky waves
In principle, a transmitting antenna sited at the earth's surface will set up a
surface wave which follows the curvature of the earth. The distance, over which
reliable communications can be achieved by the surface, or ground wave, depends on
the frequency and the physical properties (i.e. ground conductivity and dielectric
constant) of the earth along the transmission path. A ground wave can only be
established with useful efficiency where the wavelength is greater than several tens of
meters.
Seawater has the highest conductivity and will support the propagation of a
ground wave, in much the same manner as a metal plate. At the other end of the
scale, an arid desert provides very lossy ground conditions and will not support the
efficient propagation on ground wave signal.
The significance of this for maritime communications is that long distance
working is possible at medium to low frequencies using only modest transmitter
power compared to those for broadcasting at similar frequencies over land.
Figure 13: Line of sight propagation also shows surface wave propagation
over a terrestrial radio link. In principle, the received signal will be the sum of the
line-of- sight signals and the surface wave. In practice, however, one or other of the
two components will predominate depending on the transmission frequency and
length of the radio link. Ground wave propagation predominates at MF, LF and VLF.

6. Within the frequency range of 1 - 30 MHz, ionospheric reflection is the
controlling factor in achieving long-distance communications by radio waves.