Antennas and noise Flashcards
antennas
devices that turn incident radiation into a corresponding electrical signal
astronomical signals are
noise-like
total power received is roughly proportional to bandwidth
why we simply add the powers received from each part of the source to get the total power
the radiation is from incoherent sources
the signals received from different parts of the source are uncorrelated
front end of antenna
antenna and low noise preamplifier
determines the strength of the signal and instrumental noise
back end of the antenna
cable, local oscillator, mixer, detector
performs the signal processing
as well as the noise-like signal itself, there is additional unwanted noise from
equipment
ground
atmosphere
galaxy
(sum of which is usually much stronger than the signal)
in a well-designed system,
no further noise is generated by the back end
large antennas are more
sensitive and directive
can think loosely of an antenna beam
the solid angle over which it collects signal
normalised power pattern
p(theta, thi) normalised so that Pmax=1
if we have an antenna that detects only one polarisation (1/2 the total power from an unpolarised source)
need to define effective area Ae
power received = 1/2SAe delta v
aperture efficiency
n= Ae/Ageo
if the source is not on the axis of the antenna but at (theta, thi) then power=
1/2 S Ae P(theta, thi) delta v
reciprocity theorem
antenna power patterns are the same for transmitters and receivers so can think in terms of either
simplest antenna - the dipole
E-field of radiation sets up currents in the antenna –> voltages over the resistor
these are relatively short and only sensitive to one polarisation
can a reversible antenna be sensitive to all the radiation
no
this would contradict the second law of thermodynamics
consider a resistor at temp T - thermal motion of the electrons in the resistor…
generate a fluctuating voltage over its end
Nyquist noise theorem
available power = kbT
very useful
in equilibrium, there can be no
overall transfer of power between the two cavities
the noise power (per unit frequency) generated by the antenna is
kbT
if an antenna’s beam is filled with a blackbody source of temp T, the power the source delivers over a small bandwidth is
w=kbT delta v
a dipole (or any antenna) in a cavity at T produces a power of
kbT delta v
we can do away with the cavity if
the source at temp T fills the beam of the antenna
antenna temp = true temp of the source if and only if
the source fills the beam
the source is blackbody, with a Planck spectrum
antenna temp
just a useful unit with which to measure the power Hz^-1 received by a telescope
if Ta=Tb then Ae=
lambda^2 / omega A
the effective area and beam solid angle are
inversely related
big antennas have
small beams
small antennas have
wide beams
directive gain
angular selectivity an antenna has over an isotropic antenna
beam efficiency
power in main beam lobe / total power
ie excludes sidelobes
sidelobes are a worry because
they pick up stray radiation eg from the ground
