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Lecture 7- antennas and receivers (Beam patterns (Diagram, Side lobes,…
Lecture 7- antennas and receivers
Satellite tv antennas
Prime focus system, antenna is parabolic
offset configuration so dish is apparently not pointing in direction of satellite
path of radiation not blocked by focus arrangements, less loss and lower sidelobes
Green bank
Schematic diagram
Collects emission from limited range of angles (beam)
turns emission into electric signal, a fluctuating voltage which is amplified filtered and its power measured
Front end
Amplifies emission collected by feed, cooled to <20K to reduce noise
Back end
contains filters and further amplifiers which need not be cooled
diagram
Lots of types and diagrams
Beam patterns
Diagram
Side lobes
Main beam
Antennas collect powwer over limited range of angles
main beam width is FWHM on either side of the straight ahead direction
FWHM is lam/D where D is dimension of the antenna aperture
power also enters via the sidelobes
Equations for main beam efficiency and fraction of beam solid angle from sidelobes
Antenna Beams
Diffraction pattern from aperture field
the far field beam pattern is the fourier transform of aperture plane electric field distribution
diffraction pattern on right of diagram is square of amplitude pattern, only detect the power of the incident radiation not the complex time varying amplitude pattern
Rayleigh distance
gives boudary between spherical waves (Fresnel regine)
and plane waves (Fraunhofer regime)
Fraunhofer Diffraction at an aperture
Angular pattern of electric field = fourier transform of illuminating field
Maths
Autocorrelation
Maths and convolution
Wiener-Khincine Theorum
Fourier transform of auto-correlation of function is equal to fourier transform of the function squared
General maths formula
FT [ACF of aperture distribution]=antenna power beam pattern
when applied to circular aperture w/ uniform illumination gives airy disk
Beam efficiency
Affected by
Illumination
blockage losses
surface imperfection
effect is multiplicative
Illumination effects
Uniform illumination never achieved
Impossible to arrange a sharp transition from full illumination to zero at the edge of the aperture without spillover of radiation from the ground
ground adds to system noise
Tapered illumination usually adopted
Seek to maintain power collected while minimising the spillover noise
Ad:lower sidelobes
Dis:wider beam
Blockage losses
Physical blockage from focal legs, receiver cabin, reduces signal captured and scatters power into sidelobes
reduces efficiency
Surface imperfections
Irregularities in reflecting surface cause delay or phase errors in wavefronts
because of irregularities, waves arrive from surface at different times
loss of coherance and signal
Ruze formula
maths
Good efficiency
0.63