Radio Wave Propagation ppt full detailed ECE Branch BTech

1. Chapter Four
•Radio Wave propagation
Dr. Ali

2. Introduction
•Before discussing how ionosphere influences radio propagation, it is
necessary to describe some related concept, like the radio refractive index,
interaction of radio waves with the Ionosphere, reflection of radio waves by
the Ionosphere.
•The electromagnetic radiation of the Sun is continuum; it spans radio waves
through the infrared, visible, ultraviolet, x-ray, and beyond.
•Table (1.1) below specifies the designated nomenclature for those
frequencies used in communications.

3. Frequency Rang Designation Classification
30-300 Hz ELF Extreme Low Frequency
3-30 KHz VLF Very Low Frequency
30-300 KHz LF Low Frequency
0.3-3 MHz MF Medium Frequency
3-30 MHz HF High Frequency
30-300 MHz VHF Very High Frequency
0.3-3 GHz UHF Ultra High Frequency
3-30 GHz SHF Super High Frequency
30-300 GHz EHF Extremely High Frequency
Table (1) Electromagnetic Spectrum Nomenclature

4. • Radio Refractive Index of the Ionosphere
•If the collisions between electrons and neutral molecules are neglected as a first
approximation, an ionized gas is treated as a perfect dielectric then the radio
refractive index of an ionized gas could be expressed by the following
relationship:
2
8
.
80
1
f
N



N is the number of electrons per cubic centimeter, and f is the frequency of the
radio waves.
•This expression for the refractive index is unusual in more than one respect.
1. First, the absolute value of the refractive index is less than unity, whereas all
solids, liquids and gases have a refractive index more than unity.
2. Second, the refractive index of the ionospheric media is frequency dependent.

5.  Absorption of Radio Wave Energy Passing through the Ionosphere
1. When a radio wave enters the ionosphere and encounters a significant
concentration of free electrons, some energy of the radio wave transferred
to the electrons, which are thus set into oscillation at the same frequency
as that of the radio waves.

6. 3. Appreciable attenuation of radio waves can occur in the D-region where
collisions of electrons with neutral particles is more due to the higher
density of neutral molecules as compared to upper regions of the
ionosphere like E and F layers.
2. The electrons can lose some of this energy a result of collisions with
neutral particles (atoms or molecules) in the upper atmosphere and it results
in attenuation of radio waves when passing through the ionosphere. But if
there are no such collisions, the oscillating electrons will reradiate
electromagnetic waves at the same frequency and restore the original
radio waves without loss.

7.  Refraction and Reflection of Radio Waves in the Ionosphere
1. It is evident from the equation of Refraction index of a radio wave passing
through an ionized media, have N electrons, that for a given frequency "f " the
refractive index decreases when radio waves passes from a medium of lower to
higher electron density.
2. Hence a beam of radio waves going upward the ionosphere will be refracted
downward, (i.e. the angle with the vertical increases when the electron density
increases and hence a radio wave will refracted downward).

8. The Law of Refraction (snell’s law)

9. 3. Now, consider the radio wave of frequency f enters the ionosphere in
which the electron density increases with altitude. Firstly, the electron
density N, in equation (1) is very small and the velocity is near the velocity
of light c.
4. As N increases, with constant f, the phase velocity Vp must increases
while the refractive index decreases. Eventually, when the argument of the
square root in equation (1) becomes zero, the refractive index will be zero,
and in such case, N will represent critical electron density Nc.

10. 5. At this density, the radio wave can no longer propagated in the
forward (upward) direction and it will reflected back to the Earth. The
critical frequency fc of a layer is given by:

12. •Types of HF Propagation
High Frequency (3 to 30 MHz) radio signals can propagate to a distance
receiver via two methods, these method are:
1. Ground-Wave Propagation
A propagated ground wave takes three separate paths to the receiver. They
are the direct wave, the ground-reflected wave, and the surface wave, as shown
in Figure (1). The effectiveness of ground waves depends on the radio frequency,
transmitter power, transmitting antenna characteristics, electrical characteristics
(conductivity and dielectric constant) of the terrain, and electrical noise at the
receiver site. Low and very low frequencies were propagated much better by
surface path than are higher frequencies. When high-powered transmitters and
efficient antennas are used, the surface path has a maximum range, of about 500
km, at 2MHz. Surface path range decreases as frequency increases. The distance
80km represents the usual minimum range.

13. Figure (1) The propagation paths of radio waves

14. 2. Sky-Wave Propagation
Radio waves in the low frequency (LF) and medium (MF) ranges may also
propagate as ground waves, but suffer significant losses or attenuated, particularly
at higher frequencies. But as the ground wave mode fades out, a new mode
develops, it is called the sky wave. Sky waves are reflected from the
Ionosphere. While the wave is in the ionosphere it is strongly bent, or refracted,
ultimately back to the ground. For a long distance way this appears as a
reflection. Long ranges are possible in this mode, up to hundreds of miles. Sky
waves in this frequency band are usually possible at night only, when the
concentration of ions is not too great. At night, there are just enough ions to reflect
the wave but not reduce its power too much.

15. The HF band operates almost exclusively with sky waves. The higher
frequencies have less attenuation and less refraction in the ionosphere as
compared to MF. At the high end, the waves completely penetrate the
Ionosphere and become space waves. At the low end, they are always
reflect. The HF band operates with both these effects almost all of the time.
The characteristics of the sky wave propagation depend on the conditions in
the ionosphere, which in turn are dependent on the activity of the Sun.