Antennas and noise

Question 1

antennas

Answer 1

devices that turn incident radiation into a corresponding electrical signal

Question 2

astronomical signals are

Answer 2

noise-like

total power received is roughly proportional to bandwidth

Question 3

why we simply add the powers received from each part of the source to get the total power

Answer 3

the radiation is from incoherent sources

the signals received from different parts of the source are uncorrelated

Question 4

front end of antenna

Answer 4

antenna and low noise preamplifier

determines the strength of the signal and instrumental noise

Question 5

back end of the antenna

Answer 5

cable, local oscillator, mixer, detector

performs the signal processing

Question 6

as well as the noise-like signal itself, there is additional unwanted noise from

Answer 6

equipment
ground
atmosphere
galaxy

(sum of which is usually much stronger than the signal)

Question 7

in a well-designed system,

Answer 7

no further noise is generated by the back end

Question 8

large antennas are more

Answer 8

sensitive and directive

Question 9

can think loosely of an antenna beam

Answer 9

the solid angle over which it collects signal

Question 10

normalised power pattern

Answer 10

p(theta, thi) normalised so that Pmax=1

Question 11

if we have an antenna that detects only one polarisation (1/2 the total power from an unpolarised source)

Answer 11

need to define effective area Ae

power received = 1/2SAe delta v

Question 12

aperture efficiency

Answer 12

n= Ae/Ageo

Question 13

if the source is not on the axis of the antenna but at (theta, thi) then power=

Answer 13

1/2 S Ae P(theta, thi) delta v

Question 14

reciprocity theorem

Answer 14

antenna power patterns are the same for transmitters and receivers so can think in terms of either

Question 15

simplest antenna - the dipole

Answer 15

E-field of radiation sets up currents in the antenna –> voltages over the resistor

these are relatively short and only sensitive to one polarisation

Question 16

can a reversible antenna be sensitive to all the radiation

Answer 16

no
this would contradict the second law of thermodynamics

Question 17

consider a resistor at temp T - thermal motion of the electrons in the resistor…

Answer 17

generate a fluctuating voltage over its end

Question 18

Nyquist noise theorem

Answer 18

available power = kbT

very useful

Question 19

in equilibrium, there can be no

Answer 19

overall transfer of power between the two cavities

Question 20

the noise power (per unit frequency) generated by the antenna is

Answer 20

kbT

Question 21

if an antenna’s beam is filled with a blackbody source of temp T, the power the source delivers over a small bandwidth is

Answer 21

w=kbT delta v

Question 22

a dipole (or any antenna) in a cavity at T produces a power of

Answer 22

kbT delta v

Question 23

we can do away with the cavity if

Answer 23

the source at temp T fills the beam of the antenna

Question 24

antenna temp = true temp of the source if and only if

Answer 24

the source fills the beam

the source is blackbody, with a Planck spectrum

Question 25

antenna temp

Answer 25

just a useful unit with which to measure the power Hz^-1 received by a telescope

Question 26

if Ta=Tb then Ae=

Answer 26

lambda^2 / omega A

Question 27

the effective area and beam solid angle are

Answer 27

inversely related

Question 28

big antennas have

Answer 28

small beams

Question 29

small antennas have

Answer 29

wide beams

Question 30

directive gain

Answer 30

angular selectivity an antenna has over an isotropic antenna

Question 31

beam efficiency

Answer 31

power in main beam lobe / total power ie excludes sidelobes

Question 32

sidelobes are a worry because

Answer 32

they pick up stray radiation eg from the ground