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