Radioastronomy

Amateur radio astronomy, by Jeffrey Lichtman, SARA

Frequencies (II)

At the turn of the XX^th 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') 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 national laws.

To check : Officials frequencies bands allocated to radioastronomy

Astrophysical spectral lines

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 some spectral lines, like the 21 and 23 centimeter lines 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 (400-800 euros) if purchased new or, an A/D (Analog to Digital converter) and computer. Note however that Radio Astronomy receivers must be capable of AM detection with the use of a Square-Law detector.

VLF

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 recognized on strip charts or on the computer display 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 equilibrium. 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, which arrive at Earth's surface, 24 to 36 hours later. These not only produce radio enhancements but also the well-known auroras for which you will find an interesting page on this web (in French). The data is also of interest to ham radio broadcasters, reason being, the condition of the ionosphere determines the distance of received transmissions.

18 - 32 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 (Audio tapes containg this cosmic noise may be obtained from SARA or Radio Astronomy Supplies). When present they have a characteristic wavering structure not unlike the rushing of a rapid ocean surf, bubbling oil or popping sounds. 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, 28 - 80 MHz

The reader will note that the 27 MHz band (26.9-27.5 MHz) 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 short-wave communications receivers and appropriate antenna systems. Two methods are in common use. Enhancements of radio noise may mark an event. Flares also cause fadeouts of short-wave 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, (Square Law Detector at the output of the last IF stage) 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 in-falling 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".

As we told about space communications with Mars, antennas used are mainly dishes, although arrays of smaller antennas are quite viable. Reduction of data in these bands can keep a computer hacker very busy. Very inexpensive analog to digital conversion techniques have been developed by amateurs and commercial vendors 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.

Radio astronomy projects

The above list of frequency bands confirm that the radio spectrum is by far less used than its visible counterpart. It is a challenge for an amateur to get information from this so-called "noise from space" and be interested in such activities. The next list gathers potential subjects which confirm however there are a lot of thing to do even using small infrastructures.

Some of them are ambitious but well in the range of the capabilities of an amateur radio telescope.

In parallel to these activities if you can do visual observation of the same areas, you can try to correlate these observations with radio records. Then you will begin to understand where is the true utility of your work.

The solar activity

Being the most powerful radiosource, the Sun is of course our best observation subject. Your equipment only requests a broad beamed radio telescope, optionaly fixed on an equatorial mount. In a period of a few months you can detect the periodicity of the signals induced by its rotational rate. With some accuracy you can also detect the non-uniformity of this rate according to the solar latitude.

Jupiter storms

These electrical storms we can heard in the HF bands between 18-24 MHz 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.

Note that from time to time, when severe storms are expected on Jupiter, NASA provides a link to its receivers through the Internet.

The shape of the Milky Way

Second by its power, the Milky Way is also a subject of great interest. Its shape is easily discernable by a simple radio telescope. The easiest method to detect it is using a drift scan. While the sky rotates, the angular distance to the galactic center will vary and the signal strenght will be modulated according to the angular separation from the galactic core.

With an more accuracy system the inclination of the galactic equator to the ecliptic can also be detected.

Thermal emissions

Remember Jansky's pioneer work. With a radio telescope relatively sensitive you can operate in the Very High Frequency bands of microwaves to detect the temperture of the subject. The electromagnetic energy of the blackbody is everywhere present and can easily be recorded. This more demonstrative use of the radio telescope can be tested on various subjects, from a human body to the nigh sky or in presence of a warm object. Then you can compare your output signal with the true temperature of the subject analyzed.

With application of appropriate formulas and knowing the characteristics of the antenna pattern you can also estimate the temperature of the Sun and the Moon.

Interstellar Hydrogen clouds

Hydrogen clouds emission can be detected using a quite sensitive antennna and an oscilloscope display (completed with a ramp voltage generator and a downconverter stage). As the sources can be very short is sizes, the bandwidth of your receiver must be narrow enough to resolve hydrogen lines but not too much to preserve the sensibility. The frequency must also be steady to be able to pratice some calibration and in the bast cases to observe the doppler shift of the lines.

This very interesting activity give you the chance to map the relative motions of bright hydrogen clouds distributed all over the sky.

Pulsars

Using either stacked antenna or a dish, amateurs fans of SDR (Software-Defined Radio) can take avantage of this technology to detect emissions of pulsars, these small neutron stars which size is about 15-30 km of radius emitting a beam of electromagnetic radiation that seems to pulse according to their rotation rate and their orientation vs. the Earth.

As explained on RTL-SDR website, users were able to detect and measure the rotation period of the Vela pulsar. Also, from their data they were able to estimate that the minimum dish aperture required to observe the Vela pulsar would be 6 m, noting that the Vela pulsar is probably the strongest pulsar ever detected. They also confirm that using 5 RTL-SDRs to gather 10 MHz of bandwidth together with some processing, the minimum required dish aperture could be reduced to 3.5 m.

To download : Pulsar software for SDR

High energy pulses

High energy pulses or HEP have been reported for years by amateurs with some spots in the center part of our Milky May. Recent gamma rays observations have revealed some of these HEP have indeed been correlated with observations did by amateurs. But not all are accociated with galactical sources. Therefore a confirmation of their origin is required as well as the coorelation between their radio signal and a potential gamma burst. This work requests the cooperation of many amateurs, widely separated, sharing their observations and correlating them with the simultaneous observations from professionals.

Of course such project request a high accurate timing calibration but works fine with a standard amateur beam able to track the celestial sphere drift along the night.

With all these data in mind, we are now ready to built our first radio telescope.

Pour plus d'informations

Suppliers

How does a Radio Telescope Work?, CSIRO

Radioastronomy supplies (formerly Nitehawk)

Weak signal communication resource, Nitehawk

Pulsar software for SDR

Primaluce Lab (radiotelescopes and accessories)

Radio-Sky Publishing

Radioastronomy projects

Amateur pulsar observations with an RTL-SDR, RTL-SFDR.com, 2017

Build a Homebrew Radio Telescope (PDF), WA8SME, QST, June 2009