# MODES OF PROPAGATION OF ELECTROMAGNETIC WAVES:→
The communication is done from one radiating antenna to receiving antenna with the help of propagation of radio waves in that region. So, some methods of propagation of radio waves are given below:-
(i) Surface Wave Propagation.
(ii) Sky Wave propagation
(iii) Space Wave propagation.
(i.) SURFACE WAVE PROPAGATION:- An electromagnetic wave that travels over the surface of the earth, this propagation of wave is known as surface wave propagation.
It is propagated by the charge induced in the earth when the waves move over earth surface. These charges move with wave and produce the current. But the earth offers a resistance to the flow of current, so energy is dissipated and the wave gets attenuated and propagates towards the earth’s surface .Surface wave used at frequencies as low as 15 KHz.
At high frequency, surface wave can not be used for long distance transmission because attenuation is more.
APPLICATIONS→ 1.) Surface wave used for ship to ship and ship to shore communication.
2.) It is used for radio navigation and maritime mobile communication.
DISADVANTAGES→
(1) Surface wave required a high power transmission.
(2) These waves are limited to very low level and medium frequencies requiring large antennas.
(3)These losses in these waves may be increase or decrease with surface material.
ADVANTAGES→
(1) Surface waves are not affected by changing weather condition.
(2) At enough power it can be used to communicate between any two locations in the world.
(ii.) TROPOSCATTER PROPAGATION or SPACE WAVE PROPAGATION:- In space wave propagation, the electromagnetic wave travels of earth’s atmosphere. Space waves may be both ground and reflected waves.
Direct wave travel in straight line between the transmitting and receiving antennas.
Space wave propagation with direct waves is called line-of-sight transmission (LOS). Therefore direct space wave is limited by the curvature of earth.
Ground reflected waves are the waves reflected by earth’s surface as they propagate between transmit and receive antenna.
The space wave propagation between two antenna is shown in figure:-
The field intensity at the receiver antenna depends upon the distance between the two antennas. The curvature of earth presents horizon to space wave propagation called radio horizon.
The refraction of wave is caused by the troposphere because of changes in its density, temperature ,water vapour content and relative conductivity,
The region of radio horizon can be increased by elevating the transmit and receive antenna above the earth’s surface with tower or by placing them on the top of mountain or high building.
The space wave and radio horizon is shown in figure . 
Let d=distance to radio horizon(miles)
h=height of antenna above sea level.
as d = dt +dr
where, dt = distance to radio horizon for transmit antenna
dr = distance to radio horizon for receiving antenna
ht = height of transmit antenna , & hr = height of receiving antenna.
Duct Propagation:- When the density of lower atmosphere is such that electromagnetic wave are trapped between lower atmosphere and earth’s surface then this type of propagation of wave is known as duct propagation.
The layer of atmosphere acts as duct, and hence the electromagnetic waves can propagate for a large distance around the curvature of earth with in this duct as shown in figure.
(iii) SKY WAVE PROPAGATION or IONOSPHERIC PROPAGATION :- When the electromagnetic wave propagates in ionosphere region of earth’s atmosphere and these wave either reflected or reflected back to earth by the ionosphere, then this type of wave propagation is known as sky wave or ionospheric propagation.
The Sky Wave Propagation is shown in fig.
The ionosphere located approximately 50km to 400km above the earth’s surface.
When a radio passes through ionosphere the electric field of the wave exerts a force on the free electrons causing then to vibrate.
These vibrating electrons decrease the current which is equivalent to reducing the dielectric constant.
By reducing the dielectric constant, the velocity of propagation is increases and hence the electromagnetic wave bent away from ionosphere toward the earth’s surface.
