types of antennas

Question 1

for v<1 GHz we can use

Answer 1

simple wire elements or arrays of elements

Question 2

back reflector gives waves

Answer 2

a second chance

increases Ae

Question 3

effective area can also be increased using

Answer 3

yagi, log periodic and helical antennas

Question 4

for v> 1Ghz, more common are

Answer 4

horns

Question 5

why are horns good

Answer 5

horn antenna patterns can be calculated accurately

good calibration and absolute measurements of bright sources

Question 6

reflectors can increase

Answer 6

Ae

Question 7

the feed horn must

Answer 7

illuminate the reflector

but illumination must taper at the edges so it doesn’t pick up the ground

Question 8

theta =D/f and horn beamwidth=lambda/w so to minimise spillover

Answer 8

W > lambda f/ D

Question 9

why is a small focal ratio (f/D) chosen

Answer 9

a large feed aperture (W) would block the signal

Question 10

Typically, f/D is approx

Answer 10

0.4
which gives a more rigid structure

Question 11

aperture efficiency is usually

Answer 11

0.6 to 0.65

Question 12

cassegrain telescopes

Answer 12

longer effective focal length
easy access to feed
can modify secondary to improve performance
common choice for 30m dishes

Question 13

no dish is perfect so it

Answer 13

can distort as it moves

Question 14

is the rms surface error is epsilon then an rms phase error over the dish is

Answer 14

2pi/lambda . 2 epsilon

(factor of 2 due to reflection)

Question 15

resultant amplitude is approx

Answer 15

ideal amplitude x cos (phase error)

Question 16

antenna arrays has a larger

Answer 16

collecting area

Question 17

antenna array has a smaller

Answer 17

beam

Question 18

if antenna in an array are equally spaced, the array is

Answer 18

mathematically similar to a diffraction grating

Question 19

if a > lambda, then secondary responses

Answer 19

appear, spaced by lambda/a

Question 20

usually choose Pa so that

Answer 20

the grating reponses are reduced to around zero

just one beam present

Question 21

there is a fourier transform relationship between

Answer 21

the arrangement of apertures and the resultant power pattern

Question 22

the power pattern is

Answer 22

the squared-modulus of the fourier transform of A(x)

Question 23

A(x) could either be

Answer 23

the position of the antenna

the electric field distribution over a wide aperture

Question 24

the fourier transform of a convolution is

Answer 24

the product of the individual fourier transforms

Question 25

concolving single aperture functions with arrays of delta function helps

Answer 25

construct complicated apertures