RADJATION

& PROPAGATIO

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EFFECfS OF THE ENVIRONMENT 1.

REFLECfION OF W" VES the incident ray, reflected ray, and the normal to til(' point of incidence plane, NORMAL

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/ REI'LE(."TING SURFACE both incident & roflectcd waves travel at the same wlodty, but there is .1 red uction in lilt' s igncll strength tho reflected WcWE'S strlkes the ground and bounces hack up to the f(..'(:eiving antenna. & is seriously attenuated as tl result of !>trikin~~the ground, but this is .1 bonus condition, because the wave also changes the phase by I~O (h~gn,;'t.'s.The reflected wave cancels some of the dlrl~·t wave ('ncrgy. 2.

REFRACTION takes place whcn clectrornegnetic waves PM," from om' prop,\gClting medium to ,I medium having a different density till' refraction process bends the wave d ue to 111('d i((t'f('lIn~s in th,' density of the ,IiI'. The rt'fl'Mt('(i wave does not change phase & therefore will add to the wave that clrriv(.'s by the direct path.

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DIFFRACTION the behavior of the electromagnetic waves is affected by the presence of small slits in a conducting plane or sharp edges of obstacle. HUYGEN'S PRINCIPLE - states that every point on a given (spherical) wavefront may be regarded as a source of waves from which further waves are radiated outward. INTERFERENCE occurs when two waves that left one source & traveled by different paths arrive at a point. In the difference between paths 1 & l' is lin there is complete cancellation if the ground is a perfect reflector If the difference between path 2 & 2' is 1Athere is reinforcement

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D LAYER: t \"(NJ~ ~\O"" "¥"*''t:.t ~09,I')~) exists during daytime only, disappears at night least important layer from the point of view of HF propagation L~~ reflects some VLF and LF waves & absorbs MF & HF waves to a certain extent prevents low-freq daytime skip-wave propagation, bu at night thesae low-freq waves may propagate great distance used for signals up to several megahertz E LAYER (. t£.{){) ell" - W("'Ji')I(I~ LV\'f~'() most useful at the sun's noon peak, but disappears at night due to the recombmation of the ions into molecules aids MF surface-wave propagation a little & reflects some HF waves in daytime. Used at freqs up to about 20 MHz Es LAYER (Sporadic E Layer) a thin layer of very high ionization density when it does occur, it persists during the night also does not have an important part in long-distance propagation, but it does sometimes permit unexpectedly good reception '1~', 2.4 CI 01

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FI LAYER ..- _ H"Fe~ - some HF waves are reflected from it, but most through to be reflected from the F21ayer main effect to provide more absorption for HF waves

F2 LAYER /most important reflecting medium for HF radio waves -com bines with the Fl layer at night available around the full 24 hours

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The basic idea of a sky wave is to radiate the signal toward the ionospheric layers & have it refract and return to earth a substantial distance away Some of the signal es through then layers & out into space, but enough returns to earth to be picked up by a sensitive ,receiver.

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Additional distance is possible when the signal reflects from the earth & goes back up to the ionosphere layers for another HOP. These multiple hops are what provide the capability for globe spanning comm unications.

MAXIMUM USABLE FREQUENCY (MUF) - the highest frequency that can be used for sky wave commumunications between two given points on earth. normal values: 8 - 35 MHz ~e- ....-t> ec, 1'1 ( c;n+>

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FADING - the fluctuation in signal strength at a receiver and may be rapid or slow, general or frequency selective rl 'k' (\0< (,,,~IW'{ - . it is due to interference between the two waves which left the same source but \J IJCr,\,. J' _ fU~I~) arrived at the destination by different paths. CD S QV.~c:. d.lv~~'1· f c.."__ ,,-,,, Most likely to occur at the higher freqs (i.e wave with smaller wavelengths) "v- i../f.!o. \\ ~ -'0 ~because the signal received at any instant is the vector sum of all the waves \~ -I r"'eC~ived;alternate cancellation & reinforcement will result if there is a length variation as large as a half-wavelength between any two points. ( .••_ _" ~~

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IONOSPHERIC VARIATIONS re-4v ...... -re ~,... ..... h Normal variations - seasonal height & thickness changes Abnormal variations - due mainly to the fact that the sun is a variable star * SID's (Sudden Ionospheric Disturbances) / Dellinger Dropouts - caused by solar flares - only the sunlit side of the earth is affected - VLF propagation is actually improved * Ionospheric Storms - caused by particle emissions from the sun, generally a. and 13 rays. - highest freqs are most affected '"Sporadic E Layer - when present, this layer has the twin effects of preventing long distance HF communications & permitting over-the-horizon VHF communication

waves that travel in straight Jines depends on line-of-sight conditions, thus space waves are limited in their propagation by the curvature of the earth

- RADIO HORIZON - about 4/3 as far as the optical horizon, due to the varying density of the atmosphere, & because of diffraction around the curvature of the earth

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[ A person 6 ft tall standing on a shoreline would see the horizon at a distance of 3 mi. off shore, hence the 3-m ile limit that borders every country]. Ex. A horizontally polarized antenna is placed on top of a 6\t_·S ft tower. The distance to the radio horizon is 35 mi. . A receiving antenna is 53 mi. from the transmitter would need to be raised to an elevation of 162 ft to "see" the direct path from the xmtng antenna.

The radio horizon ciTffersslightly in that radio waves have a slight bending & fill-in effect behind tall, obstructing objects. A receiving antenna immediately behind a tall hill may receive no signal from a station, but if it is moved farther from the station, the signal strength increases, this void condition is called the SHADOW EFFECT. On the other hand, any object large enough to cast a radio shadow will, if it is a good conductor, cause back reflections also, thus, in areas in front of it, a form of interference known as "GHOSTING" may be observed on the screen on a TV receiver.

SUPERREFRALTION / DUCTING : - under certain atmospheric conditions a layer of warm air may be trapped above cooler ground, often over the surface of water, the result is that the refractive index will decreases far more rapidly with higher than is usual, and this causes complete bending down of microwave freqs to take place. Microwaves are thus continuously refracted in the duct & reflected by the ground. Main requirement for formation of atmospheric ducts is the so called temperature inversion.

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TROPHOSHERIC SCAITER PROPAGATION also known as TROPOSCA ITER or FORWARD SCATIER PROPAGATION a means of beyond-the-horizon propagation for UHF signals if two directional antennas are pointed so that their beams intersect midway between them, above the horizon, at about lS km. Or 6.5 mi. from the ground, they Interact in a manner similar to ducting high transmitting power are needed since the actual proportion of forward scatter to signals incident on the scatter volume is very tiny, between -60 dB &90 dB Tropospheric scattering is a sytem of xmsn that falls in the same category as magnetism, gravity, & light energy. We can explain what happens in its presence, we can predict & control its behavior to makeit work for us, but no one really knows what it is

Tropospheric scatter propagation is subject to two forms of fading a) Rayleigh fading - caused by multi path propagation - fast occurs several times per minute, with max signal strength variations in excess of 2O-dB b) fading caused by variation in atmospheric conditions along the path To obtain best results antennas are elevated and then directed down toward the horizon. Also because of fading problems, diversity systems (space diversity, frequency diversity, quadruple diversity) are employed

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EXTRATERRESTRIAL COMM/ TRANSIONOSPHERIC SPACE-WAVE PROPAGTION / SATELLITE WAVES involves the use of various satellite relays frequencies used are well above normal critical frequencies to minimize their refraction, & be able to propagate through the ionosphere refractions becomes insignificant at freqs above 100 MHz, and atmospheric absorption is negligible up to about 14 GHz FARADAY EFFECT - problems encountered in transionospheric propagation - causes the polarization of the radio waves to rotate as it es through the ionosphere & is a complex process involving the presence of ionized particles & the earth's magnetic field - solution: use an antenna with circular polarization satellite wave systems use freqs which are much higher than the critical freq. High enough to penetrate the ionosphere without refracting back to the transmitter. Major problem: high path loss caused by the large distances. The electromagnetic energy spreads With distance & relatively little reaches the receiver.

PREPARED BY: ENGR. C. TUARIZO ECE FACULTY,