Antennas and Noise Flashcards
Antennas definition
devices that turn the incident radiation into a corresponding electric signal
Antenna signals are
noise-like, random fluctuations in the system
Radiation from incoherent sources
have no correlation between signals from different parts of the sky and thus add powers from these parts
Power through area A formula
w = SAΔv measured in watts
power through the effective area
when an antenna only detects 1 polarisation
w = 1/2 S AeΔv
for one polarisation
1/2 the total power
aperture efficiency
ŋ = Ae/Ag
the power through an area where the source is not on the axis of the antenna
w = 1/2SAe*P(θ,Φ)Δv
signal from an extended source =
w = 1/2Ae∫B(θ,Φ)P(θ,Φ)dΩ*Δv
the beam solid angle Ωa =
∫B(θ,Φ)P(θ,Φ)dΩ
the reciprocity theorem
Antenna power patterns are the same for transmitters and recievers
Simple antenna - the dipole diagram
check notes for diagram
E-field of radiation sets up currents in the antenna => voltage over the resistor
must be kept short (<λ)
only sensitive to one polarisation (E// dipole)
a single antenna sensitive to all the radiation cannot be designed as it
would contradict the 2nd law of thermodynamics
Thermal motions of electrons in a resistor generate a
fluctuating voltage over its ends
Nygnist’s Noise Theorem
P = kT W Hz^-1
if an antenna’s beam is filled with a blackbody source at temperature T, the power it delivers is
w = kTΔv measured in watts
antenna temperature
Ta = w/kΔv
Ta = true temperature of source if
1) the source fills the beam
2) the source is thermal, with a Planck spectrum
expression for antenna temperature derivation
2kTa = S*Ae
S = ∫ B(θ,ϕ)P(θ,ϕ)dΩ
Ta = Ae/λ² * ∫ Tb(θ,ϕ)P(θ,ϕ)dΩ
if the source is not small compared with the beam , we must include P(θ,ϕ)
antenna temperature in terms of sky brightness temperature
Ta = Ae/λ² * ∫ Tb(θ,ϕ)P(θ,ϕ)dΩ
if the sky brightness is uniform then
Ta = Tb
effective area formula
Ae = λ²/Ωa
and is true for any antenna
directive gain
4π/Ωa
system temperature
measure of the power per unit bandwidth contributing to the noise in a radio telescope system
antenna temperature
how much noise an antenna produces in a given environment
is the component of this coming from the radio source of interest
in a phased array the signal
from the antennas are added.
in a correlating interferometer the phased array of the signal
are multiplied
dish surface accuracy
the dish needs a surface accuracy that is a small fraction of a wavelength to maintain a high efficiency
observing bandwidth
for a blackbody thermal noise source a wider bandwidth will increase the power recieved, so give a stronger signal.
to distinguish the noise signal from the system noise given the dish cannot move
chopping a secondary reflector on and off the source.
why use a large collecting area of small dishes rather than one large dish
1) impractical to make a steerable single dish
2) has a very small primary beam and thus a small field of view
3) we want to spread the area out over large baselines to get good angular resolution
4) we want to preform interferometry and this can only be done with multiple recievers
