Este formata dintr-un conductor gros. rasucit in spirala, si seamana cu bobina unui radioemitator de putere. Dimensiunile sale sint determinate de frecventa de lucru.
Diametrul spirelor se ia egal cu λ/3, iar pasul bobinajului (spiralei) este egal cu λ/4, lungimea depinzind de numarul de spire. Antena prezinta urmatoarele particularitati functioneaza pe frecvente ce pot fi cu pina la 20% mai mari sau mai mici decit frecventa pentru care este calculata, avind deci o banda larga de lucru si dimensiuni necritice.
Pe de alta parte, aceasta antena emite unde electromagnetice fara o polarizare definita, care pot fi captate de orice antena, indiferent de pozitia ei intr-un plan vertical, perpendicular pe directia de radiatie.
Permite receptia atit a undelor polarizate vertical, cit si a celor polarizate orizontal. In general, antena spirala se asaza in fata unui reflector plan, obtinind o radiatie omnidirectionala. Antena prezentata in fig. 252 este calculata pe frecventa de 180 MHz, putind lucra in banda 144—225 MHz.
Pe 144 MHz largimea diagramei de radiatie pentru o atenuare pina la 6 dB este de 60° si cistigul antenei circa 11 dB.
Pe 180 MHz, largimea fasciculului este de 50° si cistigul 13 dB.
Pe 220 MHz, largimea fasciculului este de 40° si cistigul 15 dB.
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