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SPACE Digest                                      Volume 11 : Issue 17

Today's Topics:
       Electronic Journal of the ASA, Vol. I, No. VII.
----------------------------------------------------------------------

Date: 7 Feb 90 17:44:36 GMT
From: mailrus!uflorida!mephisto!eedsp!chara![e-mail]  (Donald J. Barry)
Subject: Electronic Journal of the ASA, Vol. I, No. VII.


                        THE ELECTRONIC JOURNAL OF
                THE ASTRONOMICAL SOCIETY OF THE ATLANTIC

                    Volume 1, Number 7 - February 1990

                       ###########################

                            TABLE OF CONTENTS

                       ###########################

         * ASA Membership Information

         * Radio Astronomy: A Historical Perspective - David J. Babulski

         * Getting Started in Amateur Radio Astronomy - Jeffrey M. Lichtman

                       ###########################

                       ASA MEMBERSHIP INFORMATION

        The Electronic Journal of the Astronomical Society of the
    Atlantic (EJASA) is published monthly by the Astronomical Society
    of the Atlantic, Inc.  The ASA is a non-profit organization dedicated
    to the advancement of amateur and professional astronomy and space
    exploration, and to the social and educational needs of its members.

         Membership application is open to all with an interest in
    astronomy and space exploration.  Members receive the ASA Journal
    (hardcopy sent through U.S. Mail), the Astronomical League's REFLECTOR
    magazine, and may additionally purchase discount subscriptions to
    SKY & TELESCOPE, ASTRONOMY, DEEP SKY, and TELESCOPE MAKING magazines.

         For information on membership, contact the Society at:

          Astronomical Society of the Atlantic (ASA)
          c/o Center for High Angular Resolution Astronomy (CHARA)
          Georgia State University (GSU)
          Atlanta, Georgia 30303
          U.S.A.

         or use the ASA network address at:

          [e-mail]   -or-   [e-mail]

         or telephone the Society recording at [phone]

         ASA Officers and Council -

         President - Don Barry
         Vice President - Bill Bagnuolo
         Secretary - Ken Poshedly
         Treasurer - Alan Fleming
         Board of Advisors - Bill Hartkopf, Edward Albin, Jim Bitsko

         EJASA Editor - Larry Klaes
         Observatory Co-Chairs - Michael Wiggs, Max Mirot
         Observing Coordinator - Eric Greene
         Georgia Star Party Chairman - Patti Provost
         Advertising Committee - Paul Pirillo
         Community Coordinator - Becky Long
         Regional Planetary Society Coordinator - Jim Bitsko
         Society Librarians - Julian Crusselle, Toni Douglas

         Telephone the Society Info Line at [phone] for the
    latest ASA News and Events.

         ARTICLE SUBMISSIONS -

         Article submissions on astronomy and space exploration to
    the EJASA are most welcome.  Please send your on-line articles to
    Larry Klaes, EJASA Editor, at the following net addresses:
    [e-mail], or ...!decwrl!wrksys.dec.com!klaes,
    or [e-mail], or [e-mail],
    or [e-mail]

         If you cannot send your articles to Larry, please submit them
    to Don Barry, ASA President, at the following net addresses:
    [e-mail], or chara![e-mail], or [e-mail]

         You may also use the above net addresses for EJASA backissue
    requests and ASA membership information.

         Please be certain to include either a network or regular mail
    address where you can be reached, a telephone number, and a brief
    biographical sketch.

         DISCLAIMER -

         Submissions are welcome for consideration.  Articles submitted,
    unless otherwise stated, become the property of the Astronomical
    Society of the Atlantic, and although they will not be used for profit,
    are subject to editing, abridgment, and other changes.  Copying or
    reprinting of the EJASA, in part or in whole, is encouraged, provided
    clear attribution is made to the Astronomical Society of the Atlantic,
    the Electronic Journal, and the author(s).  This Journal is Copyright
    (c) 1990 by the Astronomical Society of the Atlantic.


                RADIO ASTRONOMY: A HISTORICAL PERSPECTIVE

                          by David J. Babulski

        Reprinted with author's permission from HANDBOOK OF RADIO
    ELECTRONICS.

        When you step outside on a clear night and look up at the
    darkened sky, you see a myriad of stars punctuating the otherwise
    velvety blackness of the night sky.  However, what you are really
    seeing in the night sky is energy, emitted from the outer atmospheres
    of the stars, centered in a narrow band called the "visible" portion
    of the electromagnetic spectrum.  Human eyes can only respond to
    this narrow visible band and are blind to energy produced in the
    remainder of the electromagnetic spectrum.

        For hundreds of years, astronomers have concentrated their
    observations in the visible portion of the electromagnetic spectrum.
    Windows into the Universe, other than the visible window, were
    suspected by astronomers and scientists as early as the Nineteenth
    Century, but remained unobserved until an accidental discovery in
    1932.  During the 1920s and 1930s, transoceanic communications
    was handled largely by powerful shortwave radio transmitters.
    Engineers working for the large communications companies noticed
    that, at certain times of the year, excessive amounts of static-like
    interference was encountered on the shortwave communication links.
    One of these large communications companies, Bell Telephone
    Laboratories, assigned a radio engineer named Karl Jansky to
    investigate the source of this periodic radio interference.  Jansky
    put together a large steerable radio antenna tuned to a wavelength
    of 14.6 meters (48 feet).

        Jansky discovered that thunderstorms, both local and at large
    distances, contributed to the interference, but he also noticed a
    steady radiation level that was not caused by his receiver and that
    varied with both direction and time.  Although he knew nothing of
    astronomy when he started, Jansky was able to eliminate the Sun as the
    source of the mysterious radiation.  After having collected several
    months' worth of data, he concluded that the time difference in the
    reception of maximum intensity of the radiation (this was about four
    minutes per day) meant that the radiation had its origin outside the
    solar system.  The radiation was strongest in the direction of the
    constellation Sagittarius.  The center of our Milky Way Galaxy is
    located in the direction of Sagittarius, and Jansky correctly
    concluded that the radiation was coming from the Milky Way Galaxy
    itself.  He also noted that the characteristics of the noise were
    similar to those created when electric current flows through a
    resistor.

        Based on these observations, Jansky suggested that the source
    of the galactic radiation were very hot charged particles in the
    interstellar medium.  Countless radio astronomical observations
    conducted since then have demonstrated that Jansky was correct.
    As important as Jansky's discovery was, it was not the first radio
    astronomy observation.

        To find these first radio astronomical observations, we must go
    back to the year 1887, when Heinrich Hertz produced and measured James
    C. Maxwell's electromagnetic waves.  In fact, it was Thomas Edison
    who, in 1890, proposed an experiment involving a radio telescope
    weighing in the megaton range!  Edison reasoned that since the Sun was
    seen to produce disturbances in visual light, these disturbances might
    also radiate at radio wavelengths.  He planned to place a loop of
    telephone wires around a huge field of iron ore in New Jersey!  The
    iron ore was magnetite, a mineral that becomes magnetized by induction
    of electric currents.  Edison thought that electromagnetic disturbances
    from the Sun would magnetize the iron ore.  This in turn would cause
    an induced electric current in the wire coil, which could then be
    recorded or listened to.

        There is no record of the experiment having taken place, which
    is not surprising, as solar radiation would not have been detected.
    Edison's rather large detector would not have been sufficiently
    sensitive, plus the fact that radiation of wavelengths long enough
    to be picked up by this equipment would not have penetrated Earth's
    ionosphere.  Several other scientists also looked for radio radiation
    from the Sun from 1887 to the early 1900s, but without success.
    Jansky's successful experiment in 1932 provided the spark that was
    to eventually ignite the science of radio astronomy.

        Little was done in the field of radio astronomy until 1937, when
    an electrical engineer and radio amateur by the name of Grote Reber
    became interested in Jansky's research.  Reber reasoned that Planck's
    Law predicts that for radio waves at any probable temperatures, the
    intensity per unit bandwidth is proportional to the square of the
    frequency.  In addition, he knew that the higher the frequency, the
    better the resolution.  Therefore, a very high frequency (or short
    wavelength) was required.  Using his own money and the occasional
    help from the village blacksmith, Reber assembled a nine-meter
    (thirty-foot) parabolic antenna in his backyard in Wheaton, Illinois.
    Initially, the antenna was designed for operation at a wavelength
    of nine centimeters (four inches).  After several fruitless months
    of observations, Reber concluded that Planck's black body law was
    not valid for celestial radiation.

        Reber moved up to a wavelength of 33 centimeters (thirteen
    inches).  It is interesting to note that this is a wavelength forty
    times smaller than that used by Jansky and a sensitivity several
    hundred times greater.  Again, there was no success after several
    frustrating months of observations.  Reber concluded that "perhaps
    the relationship between intensity and frequency was opposite from
    Planck's Law."  Not being one to give up, Reber moved up again to a
    wavelength of 187 centimeters (75 inches).  This particular wavelength
    was selected because a circular waveguide could be constructed from a
    standard length of aluminum tubing!  At this wavelength, Reber was
    finally successful.  Over a period of years and many painstaking
    observations, Reber was able to construct a radio map of the Milky
    Way Galaxy at a wavelength of 187 centimeters (75 inches).  This
    translates to a frequency of about 150 MegaHertz (MHz).  In addition,
    Reber was also the first scientist to detect radio emission from the
    Sun.

        Until after the conclusion of World War Two in 1945, Reber
    remained one of the only active radio astronomers.  RADAR (RAdio
    Detecting And Ranging) technology was born as a result of this war.
    It was also partially responsible for increasing the activity in this
    new subdiscipline of astronomy.  During the war years, J. S. Hey in
    England, working with meter wavelength radar, and Southworth in the
    United States, working at a wavelength of three to ten centimeters
    (one to four inches), discovered that the Sun was a powerful and
    highly variable radio source.  On one particular day in 1942, all
    British radar systems suddenly found themselves severely jammed.
    The jamming was so complete that some people feared a major attack
    by the Axis forces.  However, the "enemy" turned out to be the Sun!

        After the war, the field of radio astronomy expanded rapidly.  The
    wartime radio and radar engineers found themselves with laboratories
    full of equipment with no prospective enemy in sight.  Many of these
    scientists and engineers turned their attention to the new science of
    radio astronomy.  In 1946, the University of Manchester in England
    built a 65-meter (218-foot) parabolic dish-type radio telescope.
    Later, the now well-known fully steerable 75-meter (250-foot) radio
    telescope at Jodrell Bank in England was built.  A great number of
    discoveries were made during the postwar period between 1945 and 1951.

        One scientist in particular deserves special mention during this
    period.  He is J. S. Hey, a British wartime radar researcher.  Hey
    discovered and described the radiation emitted from the Sun.  He also
    discovered that meteor trails produce radar echoes and detected new
    streams of meteors in the process; and he was the first to detect a
    small source of radio emission from the extragalactic source named
    Cygnus A.  In addition to Hey, J. W. Phillips and S. J. Parsons in
    England began in 1946 a detailed survey of the radio sky at a
    frequency of 250 MHz.  In Australia, the Commonwealth Scientific
    and Industrial Research Organization (CSIRO) switched to peacetime
    research in this field.

        In 1945, J. L. Pawsey and his co-workers started to study the Sun
    at radio wavelengths.  For this research they used a unique instrument
    called a Cliff Interferometer.  An old wartime radar antenna was
    mounted on a cliff overlooking the sea.  This arrangement created a
    Lloyd's Mirror interferometer, which received both the direct rays
    from a rising or setting object, and the rays reflected by the sea
    which traveled a longer path.  This interferometer provided high
    angular resolution, and in 1946, Pawsey was able to locate some strong
    radio disturbances over a huge sunspot.  About this same time in
    Australia, John C. Bolton and Gordon Stanely, after reading about
    Hey's discovery of a radio source (Cygnus A), put their own Cliff
    Interferometer to work.  They found that Cygnus A was less than seven
    minutes of arc in diameter.  These same workers went on to discover
    several other discrete radio sources.  During this period, Martin Ryle
    and his group in England developed the two-element interferometer.
    Ryle's work is of great importance to radio astronomy, as interfero-
    metry is ultimately the only way that high resolutions can be reached.
    Another development in radio interferometry during this period was the
    construction of the Cross Interferometer Radio Telescope, invented by
    B. T. Mills at Sydney University in Australia.

        At a symposium held in Leiden, The Netherlands, in 1944, H. C. Van
    de Hulst predicted that the spin transition of the neutral hydrogen
    atom should be observable at a wavelength of 21 centimeters (eight
    inches).  This translates to a frequency of 1,420 MHz.  It was not
    until 1951 that Ewen and Purcell at Harvard University, Muller and
    Oort in Holland, and Christianson at Sydney all detected the 21-
    centimeter (eight-inch) line of neutral hydrogen.  This discovery
    opened the door to large-scale investigations of both the structure
    of the Milky Way Galaxy and the subject of cosmology.  It is also
    the wavelength most used by scientists listening for any radio
    signals from extraterrestrial civilizations.

        In the United States and Puerto Rico, many professional radio
    telescopes are operated by a number of organizations, which include
    Cornell University, the National Astronomy and Ionospheric Center, the
    National Radio Astronomy Observatory, and the Department of the Navy.
    The largest radio telescope in the continental United States is the
    VLA (Very Large Array) interferometer, located in Socorro, New Mexico.
    This large telescope array actually consists of 27 individual parabolic
    dish antennas arranged along three radial lines forming a Y-shaped
    array.  Two of these arms are 21 kilometers (thirteen miles) long and
    the third is nineteen kilometers (11.6 miles) across.  This large radio
    telescope array is capable of providing the resolution of an optical
    telescope.

        After the VLA, the second largest radio telescope is operated
    by Cornell University, in cooperation with the National Science
    Foundation, and is located in Arecibo on the island of Puerto Rico.
    This radio telescope is made up of a three hundred-meter (one
    thousand-foot) diameter spherical reflector nestled in a natural
    limestone crater.  The reflective surface is made from over 38,000
    individual aluminum panels and covers an area of over fourteen acres.
    The receiving antennas are mounted to a cable supported platform,
    which is suspended over the reflective surface by three concrete
    towers.  By moving the platform between the three supporting towers,
    the antenna can effectively be steered to various sectors of the sky.
    Because the antenna structure is fixed to the surface of Earth, the
    rotation of Earth is used to sweep the antenna beam across the sky.

        Another major United States radio telescope is operated under the
    direction of the Naval Research Laboratory of the Department of the
    Navy.  The laboratory now operates the Maryland Point Observatory,
    which consists of a 25.5-meter (85-foot) parabolic dish antenna
    designed for operation at frequencies as high as fifty GigaHertz
    (GHz), and a 25.2-meter (84-foot) antenna designed for frequencies
    as high as two GHz.

        A number of American universities also have radio telescope
    installations.  In addition the U.S. radio astronomy observatories
    just mentioned, there are many international observatories.  Just
    about every industrial nation has at least one radio telescope in
    operation.

        Among the larger international radio observatories is the Jodrell
    Bank radio telescope in Great Britain.  This radio telescope was among
    the earliest constructed and has contributed much basic information
    to the discipline.  Another major radio observatory is located in
    Narrabri, Australia.  This observatory is unique in that it is
    designed for solar observations at radio wavelengths.  Another major
    radio telescope is located at the Itopatinga Radio Observatory at Sao
    Paulo in Brazil.  This radio telescope installation is used for a wide
    variety of observations, including ionospheric studies of solar radio
    emissions, and centimeter-wavelength radio observations.

        The following is a summary of some of the major events in the
    history of radio astronomy:

        1932 - Karl Jansky discovers radio emissions from the center of
               the Milky Way Galaxy.

        1940 - Grote Reber maps the radio sky at 160 MHz.

        1942 - Radio emissions from the Sun are detected in England and
               the United States.

        1945 - Radio emissions from the Moon are detected.

        1948 - The ten strongest radio sources are known.

        1949 - The Crab Nebula (Messier 1), a supernova remnant, and two
               galaxies are identified as emitters of radio signals.

        1951 - The 21-centimeter (eight-inch) emission line of neutral
               hydrogen is detected from interstellar gas.

        1953 - Cygnus A is found to be a double radio source, the
               archetypical radio galaxy.

        1955 - Radio bursts from the planet Jupiter are detected.

        1960 - Project Ozma, the first radio search for extraterrestrial
               signals, is begun by Frank Drake.  Two Sol-type stars,
               Epsilon Eridani and Tau Ceti, are investigated, with
               negative results.

        1963 - Quasars are discovered by the identification of 3C273
               with a distant starlike object.

        1964 - Microwave background radiation is discovered as a remnant
               from the beginning of the Universe.

        1965 - Radio emission from an interstellar MASER is discovered.

        1967 - Pulsars (rotating neutron stars) are discovered as
               precisely periodic radio emissions.  The first is found
               in the Crab Nebula.

        1973 - The first radio red shift of a distant quasar is measured.

        1974 - The most precise measurement of the bending of
               electromagnetic waves by the Sun is made with a radio
               interferometer and gives a new confirmation of Albert
               Einstein's theory of relativity.

        1981 - Completion of the VLA (Very Large Array) in New Mexico.
               This is the first truly image-forming radio telescope with
               an angular resolution better than an optical telescope.

        1985 - Project META (Megachannel ExtraTerrestrial Assay) is begun
               in Harvard, Massachusetts using a 25.2-meter (84-foot)
               radio telescope.  Developed from Project Sentinel by The
               Planetary Society, META is the most sophisticated SETI
               (Search for ExtraTerrestrial Intelligence) program yet
               implemented.

        Suggested further reading:

        Abell, George O., and D. Morrison, EXPLORATION OF THE UNIVERSE,
    Saunders College Publishing, New York, 1987.

        Kraus, J. D., RADIO ASTRONOMY, Penguin Books, Baltimore,
    Maryland, 1966.

        Smith, F. G., RADIO ASTRONOMY, McGraw-Hill, New York 1960.

        About the Author -

        David J. Babulski is an amateur astronomer and professional
    science writer, with particular interest in radio astronomy and
    rocketry.


                 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: [phone]


     THE ELECTRONIC JOURNAL OF THE ASTRONOMICAL SOCIETY OF THE ATLANTIC

                       February 1990 - Vol. 1, No. 7

                         Copyright (c) 1990 - ASA


--
Donald J. Barry       [phone]         | [e-mail]
Center for High Angular Resolution Astronomy | President, Astronomical
Georgia State University, Atlanta, GA 30303  | Society of the Atlantic

------------------------------

End of SPACE Digest V11 #17
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