By Saneeju m salu

Radio waves are one form of electromagnetic radiation RADIO WAVES

RADIO FREQUENCY BANDS

5 PROPAGATION MECHANISMS Reflection Propagation wave impinges on an object which is large as compared to wavelength - e.g., the surface of the Earth, buildings, walls, etc. Diffraction Radio path between transmitter and receiver obstructed by surface with sharp irregular edges Waves bend around the obstacle, even when LOS (line of sight) does not exist Scattering Objects smaller than the wavelength of the propagation wave - e.g. foliage, street signs, lamp posts

6 RADIO PROPAGATION EFFECTS Transmitter d Receiver hbhb hmhm Diffracted Signal Reflected Signal Direct Signal Building

7 FREE-SPACE PROPAGATION The received signal power at distance d: where P t is transmitting power, A e is effective area, and G t is the transmitting antenna gain. Assuming that the radiated power is uniformly distributed over the surface of the sphere. Transmitter Distance d Receiver hbhb hmhm

8 ANTENNA GAIN For a circular reflector antenna Gain G =  (  D / ) 2  = net efficiency (depends on the electric field distribution over the antenna aperture, losses, ohmic heating, typically 0.55) D = diameter thus, G =  (  D f /c ) 2, c = f (c is speed of light) Example: Antenna with diameter = 2 m, frequency = 6 GHz, wavelength = 0.05 m G = 39.4 dB Frequency = 14 GHz, same diameter, wavelength = m G = 46.9 dB * Higher the frequency, higher the gain for the same size antenna

9 LAND PROPAGATION The received signal power: where G r is the receiver antenna gain, L is the propagation loss in the channel, i.e.,L = L P L S L F L F - Fast fading L S - Slow fading L P - Path loss

10 FADING Signal Strength (dB) Distance Path Loss Slow Fading (Long-term fading) Fast Fading (Short-term fading)

Fast Fading : - The signal from the transmitter may be reflected from objects such as hills, buildings, or vehicles Slow Fading : - The long-term variation in the mean level is known as slow fading (shadowing or log-normal fading). This fading caused by shadowing

12 DOPPLER SHIFT Doppler Effect: When a wave source and a receiver are moving towards each other, the frequency of the received signal will not be the same as the source. When they are moving toward each other, the frequency of the received signal is higher than the source. When they are opposing each other, the frequency decreases. Thus, the frequency of the received signal is where f C is the frequency of source carrier, f D is the Doppler frequency. Doppler Shift in frequency: where v is the moving speed, is the wavelength of carrier. MS Signal Moving speed v 

13 DELAY SPREAD When a signal propagates from a transmitter to a receiver, signal suffers one or more reflections. This forces signal to follow different paths. Each path has different path length, so the time of arrival for each path is different. This effect which spreads out the signal is called “Delay Spread”.

14 MOVING SPEED EFFECT Time V1V1 V2V2 V3V3 V4V4 Signal strength

15 DELAY SPREAD Delay Signal Strength The signals from close by reflectors The signals from intermediate reflectors The signals from far away reflectors

16 INTERSYMBOL INTERFERENCE (ISI) Caused by time delayed multipath signals Has impact on burst error rate of channel Second multipath is delayed and is received during next symbol For low bit-error-rate (BER) R (digital transmission rate) limited by delay spread  d.

17 INTERSYMBOL INTERFERENCE (ISI) Time Transmission signal Received signal (short delay) Received signal (long delay) Propagation time Delayed signals

PROPAGATION MODES 1.Ground-wave propagation 2.Sky-wave propagation 3.Line-of-sight propagation

19 Types of Waves Transmitter Receiver Earth Sky wave Space wave Ground wave Troposphere ( km) Stratosphere ( km) Mesosphere ( km) Ionosphere ( km)

GROUND WAVE PROPAGATION

Follows contour of the earth Can Propagate considerable distances Frequencies up to 2 MHz Example : AM radio

SKY WAVE PROPAGATION

Signal reflected from ionized layer of atmosphere back down to earth Signal can travel a number of hops, back and forth between ionosphere and earth’s surface Reflection effect caused by refraction Examples : HF radio communication (between 3 and 30 MHz)

LINE-OF-SIGHT PROPAGATION

Transmitting and receiving antennas must be within line of sight Satellite communication – signal above 30 MHz not reflected by ionosphere Ground communication – antennas within effective line of site due to refraction Refraction – bending of microwaves by the atmosphere Velocity of electromagnetic wave is a function of the density of the medium When wave changes medium, speed changes Wave bends at the boundary between mediums

LAYERS OF THE IONOSPHERE Lowest part: D layer has enough collisions to cause it to disappear after sunset Remaining ions and electrons recombine, without sunlight new ones are no longer produced Layer return at sunrise

D LAYER AND RADIO TRANSMISSION  Is the innermost layer, 50 km to 90 km above the surface of the Earth.  Low frequencies ( below 10MHz) absorbed - high frequencies pass through  More ionized = more radio wave absorption  Maximum usable frequency ( highest frequency that can be refracted) : 16 MHz Maximum usable frequency  Optimal usable frequency: 13.6 MHz  A common example of the D layer in action is the disappearance of distant AM broadcast band stations in the daytime.

E LAYER OF IONOSPHERE  Is the middle layer, 90 km to 120 km above the surface of the Earth  Ionized gas  Reflects medium frequency waves, causes radio waves to be propagated beyond horizon  At night the E layer begins to disappear because the primary source of ionization is no longer present.  The increase in the height of the E layer maximum increases the range to which radio waves can travel by reflection from the layer.

E LAYER AND RADIO TRANSMISSION  Refracts radio signals and causes them to skip back to earth  Weakest at night - radio signals pass right through  Maximum usable frequency : 28 MHz  Optimal usable frequency : 23.8 MHz  Most abundant molecule: O 2  Few seasonal or daily differences for transmission

F LAYER OF IONOSPHERE  Most important in terms of high frequency communications  During the day- 2 layers; combines into one layer at night  Thickest  Most reflective of radio on the side of the Earth facing the sun

F LAYER AND RADIO TRANSMISSION  Ionized all night  Refracts higher frequencies by day, but passes them through at night  Low frequencies ( 10-15MHz) are refracted back to earth at night  Maximum usable frequency : 16 MHz  Optimal usable frequency : 13.6 MHz  Most abundant molecules present: Nitrogen in F1 sub layer and Oxygen in F2 sub layer.

RADIO WAVES THROUGH THE ATMOSPHERE D layer disappears at night- low frequencies can now be used ( AM vs. FM) E Layer weak at night F sublayers combine into one layer at night SunspotsSunspots can increase the ionosphere’s ability to refract high frequency radio waves Solar flares Solar flares can increase the amount of radio wave absorption, thus hurting radio communications

RADIO WAVES PATHS