15.2 Instrument Background
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15.2 Instrument Background
A radio telescope consists of two components: the antennas that collect the radio
signal and the hardware that turns it into an oscillating electrical signal, with the
frequency of oscillation being the same as that of the signal. This change occurs
due to the electromagnetic characteristics of light, with the magnetic field inducing
a current in the conducting antenna.
This current propagates down coaxial cabling, the transmission line, to the
receiver, which measures the voltage being received, which is proportional to the
signal strength and can therefore be taken as a measure of how loud the signal is.
Within the transmission line, there will often be one or more low-noise amplifiers,
designed to enhance the signal on its way to the receiver.
All antennas have five defining properties: impedance, directional characteristics,
forward gain, polarisation, and beam pattern.
The impedance is the measure of the opposition that the antenna presents to the
induced current. Impedance should be matched throughout the system, as mismatching can cause internal reflections and signal loss.
Some antennas, such as the one on your Wi-Fi router, are omnidirectional.
Although some radio telescopes use omnidirectional antennas, for example LOFAR,
it is generally not a desirable property of a radio telescope antenna. Hence, most
radio telescope antennas are designed to receive, to the extent possible, radio waves
from only one direction.
The forward gain indicates just how efficient the antenna is when pointed to
a source as compared to a standard dipole antenna. It is measured in dB. A 3 dB
forward gain means that your antenna is twice as sensitive as a standard antenna,
while 6 dB gain would indicate an antenna that was four times as sensitive. Similarly,
the backwards gain is the amount of signal that is lost as it leaves the antenna.
Basically, with regard to forward gain, the bigger the better.
As we have previously discussed, electromagnetic waves consist of an electric
wave at right angles to a magnetic wave. If the orientation of the electric field is
always the same for all photons from a source, the source is said to be polarised. In
effect, most astronomical sources have only limited levels of polarisation, although
most antennas are polarised. Hence, half the photons are lost. This feature can be
used to block unwanted terrestrial sources.
The beam pattern is the spread of radiation that would be caused if we used our
antenna to transmit rather than receive. Do not worry if this seems an odd concept.
Trust me—the mathematics works. We find that there is a main beam extending out
from the antenna related to its field of view. However, there will be some other beams,
known as side lobes, extending as pairs on either side of the main beam. You should
be aware that you can detect a source in a side lobe, but the side lobe will have lower
sensitivity.
The most common antenna is the dipole, which consists of two lengths of wire,
tube, or rods that are linked to the radio receiver via the transmission line. The most
common dipole is the half wave, which, as its name suggests, has antennas half the
207
15.2 Instrument Background
A radio telescope consists of two components: the antennas that collect the radio
signal and the hardware that turns it into an oscillating electrical signal, with the
frequency of oscillation being the same as that of the signal. This change occurs
due to the electromagnetic characteristics of light, with the magnetic field inducing
a current in the conducting antenna.
This current propagates down coaxial cabling, the transmission line, to the
receiver, which measures the voltage being received, which is proportional to the
signal strength and can therefore be taken as a measure of how loud the signal is.
Within the transmission line, there will often be one or more low-noise amplifiers,
designed to enhance the signal on its way to the receiver.
All antennas have five defining properties: impedance, directional characteristics,
forward gain, polarisation, and beam pattern.
The impedance is the measure of the opposition that the antenna presents to the
induced current. Impedance should be matched throughout the system, as mismatching can cause internal reflections and signal loss.
Some antennas, such as the one on your Wi-Fi router, are omnidirectional.
Although some radio telescopes use omnidirectional antennas, for example LOFAR,
it is generally not a desirable property of a radio telescope antenna. Hence, most
radio telescope antennas are designed to receive, to the extent possible, radio waves
from only one direction.
The forward gain indicates just how efficient the antenna is when pointed to
a source as compared to a standard dipole antenna. It is measured in dB. A 3 dB
forward gain means that your antenna is twice as sensitive as a standard antenna,
while 6 dB gain would indicate an antenna that was four times as sensitive. Similarly,
the backwards gain is the amount of signal that is lost as it leaves the antenna.
Basically, with regard to forward gain, the bigger the better.
As we have previously discussed, electromagnetic waves consist of an electric
wave at right angles to a magnetic wave. If the orientation of the electric field is
always the same for all photons from a source, the source is said to be polarised. In
effect, most astronomical sources have only limited levels of polarisation, although
most antennas are polarised. Hence, half the photons are lost. This feature can be
used to block unwanted terrestrial sources.
The beam pattern is the spread of radiation that would be caused if we used our
antenna to transmit rather than receive. Do not worry if this seems an odd concept.
Trust me—the mathematics works. We find that there is a main beam extending out
from the antenna related to its field of view. However, there will be some other beams,
known as side lobes, extending as pairs on either side of the main beam. You should
be aware that you can detect a source in a side lobe, but the side lobe will have lower
sensitivity.
The most common antenna is the dipole, which consists of two lengths of wire,
tube, or rods that are linked to the radio receiver via the transmission line. The most
common dipole is the half wave, which, as its name suggests, has antennas half the