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350lect10 - 10 Antenna gain beam pattern directivity A...

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10 Antenna gain, beam pattern, directivity A dipole antenna (or a closely related monopole to be studied in Lecture 18) is a “near perfect” radiator for purposes of “broadcasting” — that is, sending waves of equal amplitudes in all directions to reach out multiple targets or receivers. However dipole is a poor choice when the objective is to radiate the power P rad in a specific direction (i.e., towards a specific receiver), as in – communication with deep space probes or orbiting satellites, or with – radar beacons where the objective is to determine the direction of a moving target. In such applications we need high-gain and directive antennas, as opposed to low-gain and non-directive antennas such as a single dipole. Qualitatively speaking, gain and directivity of an antenna measures its ability to confine its radiated wave fields within a narrow field of view called the antenna beam or beam pattern . when a narrow antenna beam is achieved, and all the radiated power P rad of the antenna is conveyed through this beam, the power density of the waves is naturally high within the beam. 1
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Arrays of dipoles can serve as high-gain antennas needed in beaming applications as we will learn in the next lecture. In this lecture we will focus on the definition of antenna gain and directivity as well as the related concept of beam solid angle. Quantitatively, the gain of an antenna is simply the average power density ratio G ( θ , φ ) | E ( r , t ) × H ( r , t ) | P rad 4 π r 2 where the denominator is what the transmitted power per unit area at distance r would have been if the antenna radiated isotropically.
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