Arrays and reflectors Antenna (radio)

rooftop television yagi-uda antennas these used @ vhf , uhf frequencies.

the amount of signal received distant transmission source geometric in nature due inverse-square law, , leads concept of effective area. measures performance of antenna comparing amount of power generates amount of power in original signal, measured in terms of signal s power density in watts per square metre. half-wave dipole has effective area of 0.13 



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. if more performance needed, 1 cannot make antenna larger. although intercept more energy signal, due considerations above, decrease output due moving away resonant length. in roles higher performance needed, designers use multiple elements combined together.

returning basic concept of current flows in conductor, consider happens if half-wave dipole not connected feed point, instead shorted out. electrically forms single  ⁄2-wavelength element. overall current pattern same; current 0 @ 2 ends, , reach maximum in center. signals near design frequency continue create standing wave pattern. varying electrical current, standing wave in element, radiate signal. in case, aside resistive losses in element, rebroadcast signal similar original signal in both magnitude , shape. if element placed signal reaches main dipole in-phase, reinforce original signal, , increase current in dipole. elements used in way known passive elements.

a yagi-uda array uses passive elements increase gain. built along support boom pointed toward signal, , sees no induced signal , not contribute antenna s operation. end closer source referred front. near rear single active element, typically half-wave dipole or folded dipole. passive elements arranged in front (directors) , behind (reflectors) active element along boom. yagi has inherent quality becomes increasingly directional, , has higher gain, number of elements increases. however, makes increasingly sensitive changes in frequency; if signal frequency changes, not active element receive less energy directly, of passive elements adding signal decrease output , signals no longer reach active element in-phase.

it possible use multiple active elements , combine them transmission lines produce similar system phases add reinforce output. antenna array , similar reflective array antenna consist of multiple elements, half-wave dipoles, spaced out on plane , wired transmission lines specific phase lengths produce single in-phase signal @ output. log-periodic antenna more complex design uses multiple in-line elements similar in appearance yagi-uda using transmission lines between elements produce output.

reflection of original signal occurs when hits extended conductive surface, in fashion similar mirror. effect can used increase signal through use of reflector, placed behind active element , spaced reflected signal reaches element in-phase. reflector remain highly reflective if not solid; gaps less  ⁄10 



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have little effect on outcome. reason, reflectors take form of wire meshes or rows of passive elements, makes them lighter , less subject wind-load effects, of particular importance when mounted @ higher elevations respect surrounding structures. parabolic reflector perhaps best known example of reflector-based antenna, has effective area far greater active element alone.