Propagation Radio wave

the study of radio propagation, how radio waves move in free space , on surface of earth, vitally important in design of practical radio systems. radio waves passing through different environments experience reflection, refraction, polarization, diffraction, , absorption. different frequencies experience different combinations of these phenomena in earth s atmosphere, making radio bands more useful specific purposes others. practical radio systems use 3 different techniques of radio propagation communicate:

line of sight: refers radio waves travel in straight line transmitting antenna receiving antenna. not require cleared sight path; @ lower frequencies radio waves can pass through buildings, foliage , other obstructions. method of propagation possible @ microwave frequencies , above. on surface of earth, line of sight propagation limited visual horizon 64 km (40 mi). method used cell phones, fm , television broadcasting , radar. using dish antennas transmit beams of microwaves, point-to-point radio relay links transmit telephone , television signals on long distances visual horizon, , ground stations can communicate satellites , spacecraft billions of miles earth.
indirect propagation: radio waves can reach points beyond line-of-sight diffraction , multipath propagation. diffraction allows radio wave bend around obstructions such building edge, vehicle, or turn in hall. in multipath, radio waves reflect surfaces such walls, floors, ceilings, vehicles , ground. these propagation methods occur in short range radio communication systems such cell phones, cordless phones, walkie-talkies, , wireless networks.
ground waves: @ lower frequencies, in medium wave , longwave bands, due diffraction vertically polarized radio waves can bend on hills , mountains, , propagate beyond horizon, traveling surface waves follow contour of earth. allows mediumwave , longwave broadcasting stations have coverage areas beyond horizon, out hundreds of miles. frequency drops, losses decrease , achievable range increases. military low frequency (vlf) , extremely low frequency (elf) communication systems can communicate on of earth, , submarines hundreds of feet underwater.
skywaves: @ medium wave , shortwave wavelengths, radio waves reflect off conductive ionized layer in atmosphere called ionosphere. radio waves directed @ angle sky can return earth beyond horizon; called skip or skywave propagation. using multiple skips communication @ intercontinental distances can achieved. skywave propagation variable , dependent on atmospheric conditions; reliable @ night , in winter. used during first half of 20th century, due unreliability skywave communication has been abandoned. remaining uses military over-the-horizon (oth) radar systems, automated systems, radio amateurs, , shortwave broadcasting stations broadcast other countries.




^ seybold, john s. (2005). introduction rf propagation. john wiley , sons. pp. 3–10. isbn 0471743682.