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