Radio waves belong to a family of waves called the electromagnetic spectrum.
They are waves of energy travelling through the vacuum of space at
the speed of light (3 x 10 8 meters/second) made from waves in the electric and magnetic fields.
The length of the wave from crest to crest is called the wavelength. How many wave
crests pass by you per second is called the frequency, measured in Hertz (Hz),
thousands of Hertz (kHz) or millions of Hertz (MHz). Thermal radio waves are produced by
hot objects. Synchrotron radio waves are produced by tiny charged
particles (electrons and protons) spiralling through magnetic fields.
Microwaves and infrared waves are sometimes described as types of radio wave.
The radio band of the electromagnetic spectrum is classified as follows:
0 to 30 kHz, VLF
30 kHz to 300 kHz, LF or long wave
300 kHz to 3 MHz, MF or medium wave
3 MHz to 30 MHz, HF or short wave
30 MHz to 300 MHz, VHF
300 MHz to 3GHz, UHF or microwaves if above 1 GHz
3 GHz - 30 GHz, SHF
Remember c = f x l, where
c is the speed of light, 3 x 10 8 m/s
f is the frequency of the radio waves, Hertz
l is the radio wavelength, metres
A radio telescope detects radio waves from objects in space. As the earth rotates, a radio source in the sky will pass across in front of the radio telescope. A dish or aerial
focuses the radio waves from the source upon a small metal dipole or into a metal horn. Here the radio waves induce an
electrical signal at the same frequency as the waves, called the
astronomy signal. This passes immediately into the preamplifier where this tiny signal is amplified before being passed down the coaxial feeder cable.
Unfortunately, unwanted noise signals from the sky background, the preamplifier and the feeder cable are all added in with the original astronomy signal.
The total unwanted noise and the astronomy signal, or "total signal", is now sent along the long
feeder cable to the receivers. Before it enters the main radio receiver, the frequency of the total signal will be
converted to a lower frequency since the main receiver amplifies the total signal considerably,
sometimes making it ten thousand million times bigger. It is technically easier to amplify lower frequency signals. The amplified total
signal is then converted into a dc signal by a diode, a process known as detection.
The size of this dc signal directly relates to the size of the total astronomy signal and the unwanted noise
signal. The dc signal is now integrated (smoothed) and measured by a computer
with an analogue to digital converter (ADC), recorded and stored. Sometimes the dc signal is
plotted out against time on a chart recorder. The results can be stored on computer disc.
High-speed computers can process many results and display a radio image or picture of the source.
Not initially. A radio receiver appropriate for observing the Sun or Jupiter can take the form of
a conventional communications receiver. In radio astronomy, it is the strength (or amplitude) of the
carrier wave that needs to be determined. To do this, the radio must be set to detect amplitude modulated waves (AM) and the dc output
from the audio output smoothed and measured. This technique does however restrict the bandwidth
over which the signal is observed to an audio bandwidth, typically 6 to 12 kHz.
Another important point is that the main radio receiver must
not have automatic gain control (AGC), otherwise the dc output after the detector will be kept constant.
More expensive receivers have the facility for switching the AGC off.
For more advanced radio astronomy work,
a custom made radio receiver with a wide bandwidth of several Megahertz is required to maximise receiver sensitivity, allowing weaker sources in the sky
to be observed. With wide bandwidth receivers, one can sadly encounter the problem of unwanted radio frequency interference (RFI) from terrestrial sources.
Most commercial communications receivers have narrow bandwidths though some expensive
models have the option to switch to a wider bandwidth.
Yes, but not easily. Professional radio astronomers produce radio images by combining
many results from a radio telescope known as a radio interferometer. This arrangement
requires 2 aerials or dishes and observations of the same source with different distances
between the aerials. The results must be stored on a computer. The results are then
added together to make a radio image or radio picture using a process known as Fourier transforming.
The main factor which decides how many radio sources you can "see" with a radio telescope
is the size of the main aerial or dish. Larger dishes collect more radio waves and so
can "see" weaker sources. This is just the same for normal optical telescopes. Larger telescope mirrors or
lenses collect more light.
Bandwidth is very important. A radio telescope receiver system tunes to a particular frequency,
say 151 MHz. However, it does
not just detect radio waves at exactly this frequency. It would for example detect radio waves
having radio frequencies from 150 to 152 MHz. This is actually called a radio frequency 'band'
and this one is reserved specifically for radio astronomy. The bandwidth of a typical AM radio
is about 10 kHz. Radio astronomers require wide bandwidth,
normally about 2 MHz or more. Increasing the bandwidth of a radio receiver increases the
sensitivity of a radio receiver permitting weaker radio sources to be detected. However, wide bandwidth radio
receivers are also more prone to radio interference from terrestrial sources and so a compromise must be sought.
Radio receivers designed to contribute a very small amount of
unwanted receiver noise signal are also good at revealing astronomy signal from faint
radio sources.
A sensitive radio telescope system needs a large aerial or dish, a low noise preamplifier,
a large bandwidth main receiver and a long integration time of the dc signal.
A radio telescope measures the power of radio waves from radio sources in space.
The unit is Watts received per square meter (on Earth's surface) per Hertz of bandwidth. One Jansky or 1 Jy, the most popular unit,
is equal to 10 -26 W/m2/Hz.
Radio power is related to the square of the amplitude of the incoming radio waves.
The essence of the radio telescope lies in its ability
to detect and measure a dc voltage which can be related to the power of the
incoming radio waves.
Radio waves are emitted from hot objects (thermal emission),
and the power of the radio waves is proportional to the actual temperature of the source (Rayleigh-Jeans approximation).
Often in radio astronomy, the equivalent (not necessarily actual) temperature of a source, or object,
or even a preamplifier, is used to measure the radio power it produces. This why a radio receiver
designed for radio astronomy is known as a radio thermometer or "radiometer".
The radio frequency power induced by the incoming radio waves in the aerial is amplified
considerably and then detected by a diode. The size of the final dc voltage, sometimes called the noise level,
is directly related to the power of the incoming radio waves. A voltmeter, computer
with ADC or chart recorder can measure this voltage.
Due to the random nature of radio waves, the dc noise level fluctuates. It is therefore 'integrated'
to smooth out these variations, allowing any astronomy signal to stand out above it. If these fluctuations, basically ac in form, are amplified through
an audio amplifier and played through a speaker, then they will be heard as a constant hiss. This is sometimes
called white noise, or just 'noise'.
The ability of any telescope to see detail is called its resolving capability. The larger
the diameter of the dish or the shorter the wavelength, the better the resolution of the
radio telescope.
This is the same for optical telescopes. Larger mirrors and lenses in optical telescopes
or viewing in blue light (which is shorter wavelength than red light) allow more detail
to be seen. As radio waves have relatively large wavelengths, the resolution of
radio telescopes is poor unless very large aerials or dishes are used.
A pair of dishes spaced a distance apart (an interferometer) with the signals combined will
improve resolution since they then
function together as the extremities of a very large dish. With a single aerial system with less than
a few metres effective diameter operating at 1420 MHz, only the wide spread milky way itself can be resolved.
At the Taunton Radio Astronomy Observatory (TRAO), over 2000 radio sources
can be seen with their radio telescopes. Many are not
visible to the naked eye. The sun is the strongest emitter of radio waves. The sun is very
hot and so always emits thermal radio waves. Charged particles from solar flares and around sunspots
also move violently through the solar magnetic field
and these produce strong synchrotron radio waves,
easily detected on earth even when cloudy. The moon also emits radio waves and radio
telescopes can measure these to determine the temperature of its surface.
Jupiter emits synchrotron radio waves due to the interaction of charged particles from Io's
active volcanoes interacting with the strong Jovian magnetic field.
In the constellation of Cassiopeia, a supernova remnant emits strong synchrotron radio waves
(Cassiopeia A). In the constellation of Cygnus, a radio galaxy also emits strong synchrotron
radio waves (Cygnus A). Hydrogen gas in our own galaxy also emits radio waves and these can be
measured and used to map out the spiral structure of our milky way, not possible with normal
optical telescopes.
At the TRAO, radio waves
have been detected from quasars, the comet Shoemaker Levy 9
collision with Jupiter, the Andromeda galaxy, Orion nebula, Crab nebula, to mention
but a few. Operational frequencies include upto 30 kHz, 21.4 MHz, 151 MHz, 1420 MHz, 5 GHz and
10 GHz. They are currently working on a system to observe radio waves from pulsars at 406 MHz.
Amateur radio enthusiasts are also interested in radio waves from the sun.
They indicate the amount of solar activity and this in turn decides how far they can talk
to people around the world since solar activity affects the earth's ionosphere.
The condition of the earth's ionosphere determines how far certain transmitted radio waves
will bounce and travel around the earth. A simple radio telescope system can therefore benefit
an amateur radio station.
There is the possibility that intelligent life exists elsewhere in the universe.
If it wishes to make its presence known, then it may try to communicate using
electromagnetic waves. The easiest waves to exploit are light and radio.
Lasers would be ideal since they emit such intense and directional light.
However, very important radio frequencies exist in a band from 1400 upto 1670 MHz.
This is known as the waterhole band since hydrogen atoms emit at 1420 MHz
and hydroxyl molecules emit around 1667 MHz. Water is essential to virtually
all known forms of life and is made from hydrogen atoms and hydroxyl molecules.
For this reason, some believe that intelligent life may try to transmit
radio messages at frequencies within the waterhole. Thus a search for extra-terrestrial
intelligence (SETI, pronounced settee) could take the form of using a radio telescope
system that scans frequencies in the waterhole looking for some type of encrypted or
encoded message.
An impressive SETI system would consist of a steerable dish system,
a scanning radio receiver and a computer to analyse the output. Complex computer
programs would help to analyse the large amount of radio data so permitting
rapid scanning of the sky. Currently available via the Internet courtesy of the
Arecibo Radio Telescope is a computer screen saver that allows you to analyse real data from space.
In the film Contact staring Jodie Foster, aliens contacted the earth
at a frequency of p times hydrogen
(p x 1420 MHz = 4.462 GHz).
There are reserved radio bands for radio astronomers. Here are some suggestions for some
useful radio frequencies to observe:
0 to 30kHz (very low frequency or VLF) for studies concerning the earth's ionosphere.
Around 21 MHz (high frequency or HF) for observing Jupiter Io emissions.
151 MHz (Very High Frequency or VHF) for observing the sun and milky way.
406 MHz for observing the sun.
1400 to 1427 MHz for observing galactic hydrogen in the milky way and SETI.
5 and 10 GHz for thermal radio waves from the Sun, Moon and Orion nebula.
To observe fainter sources like the supernova remnants Cassiopeia A, Taurus A, the radio
galaxies Cygnus A and Hercules A, pulsars and quasars, large dishes or radio interferometers working at
151, 406 or 1420 MHz are essential.
The frequency of galactic hydrogen emission around 1420 MHz varies by about +/- 1 MHz
due to the Doppler effect. This is because hydrogen gas in some directions of our galaxy could be
moving towards or away from us. This emission can be studied and our galaxy mapped using a narrow bandwidth
scanning receiver.
World-wide official Radio Astronomy band allocations useful for amateur radio astronomy are as follows:
It would be advisable to start with a single aerial radio telescope (total power system)
and then progress to a radio interferometer (requiring two aerials or dishes). If starting out,
it is best not to be too ambitious if one lacks experience, since failure to achieve
any results can be disappointing. Only groups, societies and larger organisations normally have access to the land
necessary for large radio interferometers. It is best to start with a total power system in the
HF and VHF band such as a 21 MHz system for observing Jupiter Io emissions or
perhaps a 151 MHz system for sun and milky way observations. An excellent tool
for radio astronomy would be a 2 to 4 metre steerable dish operating at 1420 MHz.
Computers can be adapted to steer these systems and monitor and store the data.
Smaller steerable satellite dishes and commercial TV equipment can be used to monitor
around the 5 and 10 GHz bands.
The choice of antenna is important. For ionospheric studies around 30 kHz, large wire loops are
adequate. For Jupiter work around 20 MHz, a ground based half wave dipole
is required. For work up to 406 MHz, multi-element yagis are best. For work above
610 MHz it becomes viable to use simple wire or mesh dishes. The rule of thumb is that
dish efficiencies of 50% or better will be attained for dishes that are at least
10l in diameter, where the wire spacing is no less than l/10, and where
the irregularities in the dish surface are no more than l/16.
Symbol l is the operational
wavelength. For frequencies above 2 GHz, solid dishes are recommended.
The radio telescope receiver system will require a preamplifier
mounted close to the aerial system. A long run of
coaxial cable will be required to take the signal indoors.
A frequency converter may be required to change the frequency to suit that of
the main radio receiver. Following the main radio
receiver and diode detection will be a dc amplifier with an integrator to smooth the dc.
Finally there will be a voltage measuring facility like an ADC connected to a computer or just
a chart recorder.
The unwanted noise signal of the preamplifier must be
minimised otherwise this could dominate the total signal output. The preamplifier must
be mounted as close to the aerial or dish as possible though this is not quite so critical around 21 MHz
and below. The low noise preamplifier
should contain a decent filter, reducing unwanted radio interference, and have a gain of
about 25 dB. The run of coaxial cable to the frequency converter should be
kept less than 25 metres though again this is only really important above 20 MHz.
The frequency converter will need a gain around 10dB and a bandwidth commensurate with
the main radio receiver bandwidth, typically 2 MHz or less.
The main radio receiver
should offer manually controlled gains of up to 90dB and a bandwidth equal to or less
than the operational bandwidth. Narrow bandwidth crystal filters can make the main
radio receiver particularly selective (avoiding unwanted radio frequency interference)
though radio astronomers tend to prefer larger bandwidths to maximise sensitivity
(the ability to see faint radio sources). Ambient temperature variations can change
the gain of the main radio receiver. This causes the dc signal to vary and consequently
confused with any
small variations due to genuine astronomy signal.
All systems and cables should be matched to ensure maximum transfer of radio power.
All system losses should be minimised as these will manifest as an increase in the
total system unwanted noise level. All coaxial cable impedances and input and output
impedances of receivers, filters and connectors should be 50W.
A 10-fold increase in radio power is 10 decibels. A 100-fold increase in radio power is
20 decibels, a 1000-fold increase in radio power is 30 decibels, a 10-fold reduction in
radio power would be -10 decibels and so on.
The decibel is abbreviated to 'dB'.
In fact the decibel measures a ratio:
Ratio in decibels = 10 x log (new power/old power)
As electrical power is related to the square of electrical voltage, it can also be shown that:
Ratio in decibels = 20 x log (new voltage/old voltage)
For instance, a voltage amplifier that makes the electrical voltage 10-times bigger and thus the
electrical power 100-times bigger, would have a gain of 20dB.
Preamplifier noise level is also quoted in decibels.
It is calculated as follows:
Noise level in decibels = 10 x log [(Tn-290)/290]
where Tn is the preamplifier equivalent noise temperature in Kelvin.
For instance, a preamplifier with an equivalent noise temperature of 50K
would have a noise figure of 0.7dB.
Here are some ideas for projects. They are in order of increasing difficulty:
Frequencies upto 30 kHz VLF can be interesting. By tuning to known terrestrial stations in this band,
one can listen to enhancement and fading of the signal during solar storms. This can provide information about
how solar activity affects the earth's ionosphere.
Several stations operating at 151 MHz and simultaneously observing solar radio emissions during solar storms and flares
could correlate their observations. This would provide an understanding of the absorption and refraction of the 151 MHz radio waves as
they pass through the earth's ionosphere.
By observing constantly at different frequencies, one could look for sudden enhancements associated with extra-galactic high energy bursts.
A satisfying project is to work out the weakest signal that your radio telescope could detect. This would be its theoretical
design limit. One would then identify a source in the sky with a radio power at the limit of detection, and strive to detect it.
This leads to a thorough understanding of the practical aspects (and limitations) of your radio telescope system.
By observing the moon at 1420 MHz, one could deduce the temperature of the lunar soil just below the surface, and see how it varies with phase of the moon.
A well calibrated radio telescope system at 151 MHz could be used to measure the strength of Cassiopeia A, the supernova remnant, and
see if the source strength is falling from one year to the next.
Using a radio interferometer at say 151 or 1420 MHz, one could measure the visibility functions for various base-line separations and synthesise an approximate
one dimensional radio image.
