THE ELECTRONIC JOURNAL OF THE ASTRONOMICAL SOCIETY OF THE ATLANTIC
Volume 1, Number 7 - February 1990
GETTING STARTED IN AMATEUR RADIO ASTRONOMY
by Jeffrey M. Lichtman, SARA President
The Society of Amateur Radio Astronomers (SARA) regularly surveys
each of its members regarding their interests in the field of radio
astronomy, as well as how SARA may address these interests. Invariably,
most every new member asks the question: "How do I get started?" It
is to these people that this article is addressed. We will deal with
both general and specific information and recommendations.
Basically, amateur efforts in this discipline fall into two
general categories:
1. Indirect method studies of solar phenomena, meteor infall, and
Jupiter noise storms, for example. This type work is usually done at
the low radio frequencies, with relatively narrow band receivers. It
does not involve sharp imaging of the radio noise source. This work
is conducted mainly with communications-type receivers, requiring
only a minimal need for auxiliary equipment. The expansion equipment
usually takes the form of a strip chart recorder or computer as a
readout instrument, and a suitable DC (Direct Current) amplifier
required to drive the readout. This work, of course, does require
a quiet radio band in the spectrum of interest.
2. Imaging radio astronomy. This work makes up the bulk of
amateur radio astronomy efforts. It is, by its very nature, best
practiced in the VHF, UHF, and EHF radio spectra with receiving
equipment of relatively broadband design. The reason for the
broadband receivers is that all discrete radio objects radiate
over a very broad spectrum, and the bandwidth of the receiver
equates to the energy received from the object.
Discrete radio sky objects are very weak emitters. A power flux
unit has been adopted for radio astronomy. It has to do with the tiny
incremental power falling from the sky upon one square meter of Earth
surface, per cycle per second. This unit is called the Jansky, after
the original radio astronomy pioneer. By common accord, one Jansky
is defined as 10-26 watt/(meter2*Hertz), a very small flux indeed.
Upon examination, one would think this infinitesimal amount of power
impossible of detection at all. Radio astronomy has indeed been
described as the examination of ripples riding upon waves, above an
entire sea of noise. It is estimated that all of the energy which
has fallen upon Earth's radio telescopes would not equal the energy
in a single snowflake.
Yet radio astronomers have refined the sensitivity of their
equipment such that these small powers are not only detected, but
also evaluated into information about the Universe which has been
both illuminating and exciting. This, despite the fact that the
receivers used to make these measurements typically generate as much
as a million times the noise signal as the energy from the desired
object. How is this accomplished? The assault on the problem is
multi-directional and is conducted in the following ways:
One begins with as large an antenna as can be achieved, in order
to trap as much energy as is possible from the desired object. This
usually involves a radio-quiet location, but does not necessarily
require huge single antennas. The problem may be successfully
addressed with phased antenna arrays.
The receiver is designed to be of low internal noise, very
high gain, and of wide bandwidth. The stability of such receivers
represents a continual challenge to the radio design engineer.
Happily, the design of low-noise radio equipment has been made
easy with the arrival of very low-noise receiving equipment using
gallium arsenide field effect transistors (GaAsFETs). The large
market generated by ham radio operators and television receive-only
satellite stations has encouraged manufacturers to invest in this type
of research. Input noise temperatures of GaAsFET antenna amplifiers
typically fall to 25 degrees Kelvin at room temperature and without
any attempt at cryogenic cooling of the devices. The noise temperature
of the input amplifiers pretty well determines the sensitivity of the
total instrument. Mass production of these devices has brought their
cost down to well within the budget of the average radio astronomy
amateur.
Additionally, the balance of the radio astronomy receiver is
designed such that the internal noise is canceled out. This is
usually accomplished by converting all the receiver noise, plus the
desired signal, into a fluctuating DC voltage. A counter voltage is
then introduced such that the internal receiver noise is canceled
out. The residual desired signal is then amplified to a very high
level, in order that it may be measured by the readout device. In
practice, the cancellation of the receiver noise is accomplished in
one of two ways:
1. In so-called total power receivers, the full power of the
instrument is delivered to the DC amplifier, and the receiver noise
is canceled out by the introduction of a back-biasing voltage at
this point. This permits the DC amplifier to greatly amplify what is
left, which is, of course, the desired signal. This practice works
quite well as long as there is no appreciable drift of gain in the
receiver. Long-term observations will inevitably show gain drift of
the receiver. In such cases where the zero reference line deviates,
a known calibration signal is introduced at the start, sometimes
during, and at the end of the observation. This permits quantitative
evaluation of the received data.
2. There is yet another type receiver which is designed to
automatically cancel out its own internal noise. In practice, this
is accomplished by circuitry which causes the receiver to alternately
"look at" the signal plus the noise, then at its own internal noise
only. This is usually done with the introduction of a square wave
generator, which functions as an on-off switch. In one instant of
time, the receiver is connected to the antenna system; at another
instant the receiver input is terminated into a load resistor such
that only the internal noise is present at the receiver output. A
phase-sensitive detector circuit, driven by the same square wave
generator, is then employed to deliver the difference to the DC
amplifier used to drive the readout instrumentation. Here, again,
this difference represents the desired signal. This so-called Dicke
switching method improves the receiver sensitivity by one to two
orders of magnitude. Because the receiver only looks at this
difference, the effects of gain drift are largely erased.
In consideration of all of the above, it becomes obvious that the
design of radio astronomy receivers has a great deal to do with just
what the observer is after in the data. It therefore follows that
each project must be begun with a firm idea of just what the observer
has in mind as a project. The equipment is either acquired or built,
and tailored to do the job. The story of all modern science, regard-
less of the specific discipline, proceeds as follows:
1. Conceive the project.
2. Build or otherwise acquire the instrumentation to do the work.
3. Conduct the measuring of observations in a clear-cut and
methodical way, giving attention to all observing parameters.
4. Analyze the data without the introduction of personal bias.
5. Publish the results.
Are negative observing data useful? The answer is most assuredly
yes; if for no other reason than to prevent other observers from
duplicating effort which is unlikely to bear fruit.
The purpose of the Society of Amateur Radio Astronomers is to
provide sufficient technical information to enable amateurs to do this
kind of work, commensurate with the antenna aperture which may be
acquired. This involves the free circulation within the society of
technical information. Such information is regularly published in
SARA's monthly 24-page journal, RADIO ASTRONOMY. Additional specific
information is also available from SARA's technical advisors, many of
whom are radio engineers. The technical advisory staff is regularly
published on page two of each journal issue. In addition to the
above, SARA also operates a nonprofit laboratory (SARALAB), which
continually develops state-of-the-art receiving equipment. The
services of the lab are offered free of charge to SARA members both
in an advisory capacity and also for the rendering of assistance in
helping observers to get their equipment into usable operation.
For the benefit of those who are still trying to define a
receiving/observational project which fits the individual's span of
expertise, the balance of this publication is devoted. We invite you
to survey the potential of each radio band, and to evaluate your own
technical potential. Specific design information may then be secured
from the SARA Journal office, or from any of SARA's many technical
advisors. Please use the address at the end of this article for
obtaining more information on SARA.
The tabled information below is taken from the RADIO ASTRONOMY
HANDBOOK, 1986, by R. M. Sickels.
Which Band? Which Receiver? Which Observing Program?
At the turn of the Twentieth Century, anyone listening to a modern-
day all-wave receiver would have heard nothing but natural noises;
static from lightning, and at very high frequencies the noise of the
Milky Way Galaxy. This may have been punctuated by radiation from
some man-made machinery, but little else. Today, however, the world
has gone information crazy and the radio spectrum is almost entirely
filled up with some kind of radio broadcast. An alien radio astronomer
looking at this planet from interstellar space would find it brighter
than the Sun in some regions, due to the very high megawatt power of
television and radar transmitters operating at about one meter (3.3-
foot) wavelengths and below. Add to that the motor brush noise of
our appliances, the arcing of power insulators, ignition noise from
automobiles, and even the neighbor's lawn mower, and the situation
seems hopeless.
Nevertheless, there are some clear radio bands allocated to radio
astronomy. In addition, there are radio bands which are unused in
the VHF and UHF TV spectrum. Anyone operating transmitters in these
unassigned bands is in violation of federal law.
Bands Allocated for Radio Astronomy Use:
25.55 - 25.67 MHz
37.5 - 38.5
73.00 - 74.60
406.1 - 410 MHz
608 - 614
1400 - 1427 (21 cm hydrogen radiation)
1660 - 1670 (OH molecule radiation)
2655 - 2700
4990 - 5000
10680 - 10700
15350 - 15400
22210 - 22500
23600 - 24000
31300 - 31800
51400 - 54250
58200 - 59000
64000 - 65000
86000 - 92000
105000 - 116000
Of course, some of these extremely high frequency bands are out of
the question for the average radio astronomy observer, unless one also
happens to be a microwave engineer. Nevertheless, amateurs are now
beginning to explore the 21 and 23 centimeter radiation bands of
neutral hydrogen and the oxygen/hydrogen molecule with equipment of
considerable sophistication.
Let us now explore the entire spectrum of radio frequencies with
the idea of just what kind of work can be usefully done, and the type
of receiving equipment necessary to do the job.
20-100 kHz
This noisy radio band is useful in observing solar flares. The
plan involves simple receivers of very inexpensive design and which
are usually home-built. Antennas may be longwires, loops, and in some
instances amplified whip antennas for those who lack the space for
more elaborate arrays. The cost of the basic receiver may range from
thirty to sixty dollars. To this must be added the cost of a strip
recorder, which may be bought quite cheaply at some of the ham radio
flea markets, but may range from $350-$700 if purchased new. The
observing technique involves the continual monitoring of Earth-
produced atmospheric noise (mainly equatorial lightning discharges)
for any enhancements due to solar flares. This is an indirect method
of doing solar studies, but nevertheless a very effective one. These
observations are regularly conducted by a dedicated group loosely
affiliated with SARA (the VLF Experimenter's Group), and the data
are useful to professional solar observatories and to all others
who have an interest in our closest star.
Another observing technique in this band is to tune up on a
marginally received radio beacon and to observe any enhancement of
the signal due to a solar flare. Either of these basic methods is
equally effective and the results are identical. The flare is recog-
nized on strip charts as a sudden enhancement of signal rising to full
amplitude in seconds and slowly decaying as the effect of the flare
diminishes and the ionosphere once again reaches its state of equili-
brium. This is also very interesting work if conducted as a team
effort with someone who has an optical telescope coupled to an H-alpha
red filter. Here, the effects of the flare may be simultaneously
observed in the radio as well as the optical window. Delayed effects
from large flares are also observed as heavy particles arrive at
Earth's surface 24 to 36 hours later. These not only produce radio
enhancements but also the well-known auroras. The data are also of
interest to ham radio broadcasters because the condition of the
ionosphere determines the distance of received transmissions.
18-24 mHz:
This band is used by amateur radio astronomers to monitor radio
noises from the planet Jupiter. These noises are not always present
and are sporadic in nature. It is quite possible that anyone who owns
a modern day sensitive shortwave receiver has already heard these
sporadic noises without realizing the source. When present they have
a characteristic wavering structure not unlike the rushing of a rapid
ocean surf. This is punctuated by a wavering sub-second structure.
These noises when present are of very high intensity and may be
detected with communications type receivers tuned to an inactive
portion of this band. Antennas used are identical with any antenna
system resonant at this frequency. The noises are so powerful that
the antenna need not necessarily be resonant. Most communications
receivers nowadays have a control to resonate any antenna in use.
There are at least four mechanisms proposed for the production of this
noise. Three of these involve the effect of the giant planet on its
innermost Galilean moon, Io. It is believed that at least some of
this noise originates as material ejected from Io's volcanoes interacts
with Jupiter's very powerful magnetic field. Data gathering in this
band may be gathered approximately eight months of the year, when
Jupiter is not too close to the Sun from our perspective on Earth.
10-26 mHz and 28-80 mHz
The reader will note that the 27 mHz band has been deleted due
to the very high level of Citizen's Band (CB) traffic. Solar flare
monitoring in these bands may be conducted with shortwave communi-
cations receivers and appropriate antenna systems. Two methods are
in common use. Enhancements of radio noise may mark an event.
Flares also cause fadeouts of shortwave transmissions and therefore
monitoring fadeouts is also useful. The radio receiver used must be
operated without automatic gain control or any other filtering which
would mask the effect of a flare. The data are gathered either by
strip recorder, computer, or both. Here again, the data are of
interest to professional solar observatories and to hams. The Sun
is continually studied and all of our knowledge has been mainly
derived from phenomena occurring on the Sun's surface. Carefully
prepared and evaluated data are always useful and frequently outlive
the observer.
88-108 mHz
This may be recognized as the commercial FM radio band. There
are local portions of this band which are unassigned for transmission.
If a simple conversion is made to change a standard FM set to AM
reception, the receiver, together with a suitable antenna and low
noise amplifier, may be used for solar flare studies and also crude
imaging of some of the more powerful discrete radio sources such as
Cassiopeia A and Cygnus A. In this work a clear band is sought out
and no limiters of any kind are used in the receiver. The antennae
used are usually Helicals or Yagis (Dishes only become viable at
frequencies above 400 mHz). This is a very inexpensive way to get
started in radio astronomy with the intelligent modification of a
cast-off FM receiver. The cost of suitable recording equipment must
of course be added to the instrumentation budget.
The overall gain is boosted by the use of a low-noise antenna
amplifier and the quality of this device also determines the
sensitivity of the instrument. Operation of a converted FM receiver
as a radio telescope in this band produces typical sky resolution of
about thirty degrees of arc, a very broad observing beam indeed.
Nevertheless, the poor resolution is at least partially offset by the
ease of detection of some of the discrete powerful radio objects.
Cassiopeia A and Cygnus A are very strong radio emitters at these
frequencies, and are therefore quite easily detected. Scintillations
are also observed as these point sources are disturbed by Earth's
atmosphere. The galactic arms and the center of the Milky Way Galaxy
are very strong and extended sources of radiation which are quite
easily detected in this radio band. This project would make an
inexpensive and thoroughly worthwhile science fair type effort,
and also provide useful experience in the taking of data.
75 mHz
This may be recognized as the aircraft beacon band. If a suitable
receiver and directional antenna system are tuned up in this band to
a marginally received aircraft beacon, the arrival of an infalling
meteor will be recognized as a characteristic "ping" sound after a
simple conversion to audio output. This method of meteor detection
produces tenfold the optical visual count. It is also useful in the
daylight hours when optical counts are impossible. Directional antenna
systems might permit ranging of a large meteorite's fall to Earth.
These objects are of very high monetary and scientific value to
museums and research institutions, who study them for clues to the
chemical composition of the early solar system. The data are also
of importance to the American Meteor Society (AMS), an organization
wholly devoted to these phenomena.
88-890 mHz
The high frequencies, very high frequencies, and ultra high
frequencies are useful bands for solar burst detection with suitable
AM receivers. The bursts are usually most easily detected at the
lower frequencies. As the observational frequency becomes higher,
improved sky resolutions result from the typical amateur antenna
systems, making possible the imaging of discrete radio sources. Use
of the VHF and UHF bands where they are unoccupied by local broadcast
allows the saving of money on some components such as I.F. amplifiers
designed for television sets, because of their low cost in mass
production. Antennas used are Yagis and Helicals at the low end of
the spectrum, and paraboloid dishes at frequencies above about 400
mHz. Use of a dish permits the observer to predict his circular
beam resolution by a simple formula.
1-4 gHz
Though not formerly used by amateurs because of equipment cost,
this band is opening up due to the ready availability of equipment
designed for TV satellite reception. Encoding of desirable movie
channels is causing enough disapproval that amateurs will soon reap
a bonanza of dishes and low-noise receiving equipment designed for
satellite TV reception. This band also encompasses the 1420 and 1660
mHz spectral line channels. Amateur and professional SETI (Search
for ExtraTerrestrial Intelligence) observations are also conducted
in these bands, due to the belief that advanced alien life would
choose to announce their presence in the so-called "water hole",
where galaxy noise is at its minimum. The sky background noise is
very low in this "hole". Antennas used are mainly dishes, although
arrays of smaller antennae are quite viable. Reduction of data in
these bands can keep a computer hacker very busy.
Very inexpensive analog to digital conversion techniques have
recently been developed by SARALAB which enable an observer to cheaply
interface a microcomputer to the radio telescope output. Discrete
radio sources, due to the synchrotron mechanism of radiation, become
weak emitters at the extremely high frequencies, and thus require
suitable antenna aperture to detect. This problem is partially offset
by the increased resolution at these very short wavelengths, with
the consequent rejection of surround-sky noise. Thermal radiators
increase dramatically in radiated power as the observational frequency
increases. This makes possible good imaging of the Sun, which is
observed mainly in its very hot corona. Interferometry also makes
possible sectional imaging of the solar area.
About the Author -
Jeffrey M. Lichtman, a long-time amateur radio astronomer and
active Society member, is president of the national Society of
Amateur Radio Astronomers (SARA), an organization of nearly 250
radio hobbyists. For more information on SARA, please contact
Jeffrey at the following address:
1425 Parkmont Drive
Roswell, Georgia 30076
Telephone: (404) 992-4959