types of antennas Flashcards
for v<1 GHz we can use
simple wire elements or arrays of elements
back reflector gives waves
a second chance
increases Ae
effective area can also be increased using
yagi, log periodic and helical antennas
for v> 1Ghz, more common are
horns
why are horns good
horn antenna patterns can be calculated accurately
good calibration and absolute measurements of bright sources
reflectors can increase
Ae
the feed horn must
illuminate the reflector
but illumination must taper at the edges so it doesn’t pick up the ground
theta =D/f and horn beamwidth=lambda/w so to minimise spillover
W > lambda f/ D
why is a small focal ratio (f/D) chosen
a large feed aperture (W) would block the signal
Typically, f/D is approx
0.4
which gives a more rigid structure
aperture efficiency is usually
0.6 to 0.65
cassegrain telescopes
longer effective focal length
easy access to feed
can modify secondary to improve performance
common choice for 30m dishes
no dish is perfect so it
can distort as it moves
is the rms surface error is epsilon then an rms phase error over the dish is
2pi/lambda . 2 epsilon
(factor of 2 due to reflection)
resultant amplitude is approx
ideal amplitude x cos (phase error)
antenna arrays has a larger
collecting area
antenna array has a smaller
beam
if antenna in an array are equally spaced, the array is
mathematically similar to a diffraction grating
if a > lambda, then secondary responses
appear, spaced by lambda/a
usually choose Pa so that
the grating reponses are reduced to around zero
just one beam present
there is a fourier transform relationship between
the arrangement of apertures and the resultant power pattern
the power pattern is
the squared-modulus of the fourier transform of A(x)
A(x) could either be
the position of the antenna
the electric field distribution over a wide aperture
the fourier transform of a convolution is
the product of the individual fourier transforms
concolving single aperture functions with arrays of delta function helps
construct complicated apertures
