Notes4-2 - Notes #4, ECE594I, Fall 2009, E.R. Brown...

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A critical aspect of any remote sensor is the coupling from the circuit (or transmission line) medium of the sensor to the external medium in which the target is embedded (usually free space), and/or the coupling of the external medium to the sensor • The component that carries out this coupling is traditionally called the “antenna”. • RF sensor antennas generally fall into one of two categories: (1) wire antennas, and (2) aperture antennas. • At the low end of the RF spectrum, roughly up to 10 GHz, the wire antennas take on the form of dipoles, spirals, helices and other simple shapes. The aperture antennas usually take on the form of parabolic or elliptical dishes. • At the high end of the RF spectrum, the wire antenas usually occur on substrates in the form of patches, slots, or other “printed-circuit” antennas. The aperture antennas usually have the form of feedhorns or small dishes. Coupling of THz Radiation to Free Space: Antennas* *Good reference on antennas: R.S. Elliott, “Antenna Theory and Design,” (Prentice Hall, Englewood Cliffs, 1981). All antennas are also classified by their electromagnetic properties (i.e. radiation or beam “pattern” in the external medium) and their circuit properties (i.e., the impedance) in the internal medium. Because the antenna is a reciprocal passive element, the properties in reception are related to those in transmission, but the transmit case is easier to do first Electromagnetic Properties in Transmit (1) The radiated electric field at a far distance from the antenna will tend to display a modified spherical-wave of the form r e F r E jkr ) , ( ) , , ( φ θ where k is the free-space propagation constant (= ω /c = 2π/λ ) and F is the (normalized) intensity pattern function, F |S(r, θ , φ )|/S max with S being the Poynting vector and S max is its maximum magnitude, wherever in space that occurs. • All antennas display a limited direction in space where F( θ,φ ) is large and other regions where it is negligible, in contrast to isotropic (point) sources. Therefore, a useful metric is the directivity, D. B d F D = ∫∫ / 4 ) , ( 4 1 4 π where B is the beam solid angle. Conceptually D defines how much greater the intensity is at the peak of F compared to the isotropic radiator emitting the same total power, for which B = 4 π and D = 1. 83 Notes #4, ECE594I, Fall 2009, E.R. Brown
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In this case it is useful to approximate F( θ,φ ) by an equivalent spherical cone or sector having a symmetry axis along θ P , φ P , and polar angular width (or widths) equal to the full-widths at the half-maximum points β(φ) of the real major lobe . Throughout the cone or sector, F ( θ,φ ) =1.0 If the pattern has perfect conical symmetry (generally true for parabolic dishes and lenses, and often the design goal for feedhorns), then one finds )] 2 / cos( 1 [ 2 sin ) , ( 2 / 0 2 0 4 β π θ φ = ∫∫ d d d F B ) 2 / cos( 1 2 D and In the limit of a narrow “pencil” beam where β is small (<< 1 rad), one can
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This note was uploaded on 12/02/2009 for the course ECE 000 taught by Professor O during the Spring '09 term at UCSB.

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Notes4-2 - Notes #4, ECE594I, Fall 2009, E.R. Brown...

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