Tune in the cosmos: getting into amateur radio astronomy

Whole Earth Review, Fall, 1990 by Jeff Lichtman

IT IS ESTIMATED that all the energy which has fallen upon Earth's radio telescopes would not equal the kinetic energy in a single snowflake. Yet radio astronomers have so refined the sensitivity of their equipment that these small powers are not only detected, but also evaluated into information about the Universe which is both illuminating and exciting.

Radio astronomy has been described as the examination of ripples on waves riding upon an ocean of noise. One begins with as large an antenna as can be achieved, to trap as much energy as possible from the desired object. Most discrete radio objects are very weak, so receivers must have low internal noise and very high gain. Happily, the design of such equipment has been made easy with the arrival of very-low-noise amplifiers and receivers 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. Mass production of these devices has brought their cost down to within the budget of the average radio astronomy amateur.

Basically, amateur efforts in this discipline fall into two general categories:

1. Indirect methods - for studying solar phenomena, meteor infall and Jupiter noise storms, for example. This is usually done at the low radio frequencies, with relatively narrow-band receivers. It does not involve sharp imaging of the radio noise source. The readout instrument is usually a strip chart recorder or a computer.

2. Imaging radio astronomy - this work makes up the bulk of amateur radio astronomy efforts. It is, by its nature, best practiced at higher frequencies, with broad-band receiving equipment.

The purpose of the Society of Amateur Radio Astronomers (SARA) is to provide sufficient technical information to enable amateurs to do this kind of work. Such information circulates freely within the society and is regularly published in SARA's monthly 24-page journal, Radio Astronomy Additional specific information is available from SARA's technical advisors, many of whom are radio engineers.

You are invited to survey the potential of each radio band, and to evaluate your own potential. Specific design information may be secured from the SARA journal office, or from SARA's technical advisors.

(The following band descriptions are adapted from the Radio Astronomy Handbook by R. M. Sickels, 1986.)

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 long-wires, 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 $30 to $60. 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 to $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. 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.

18-24 MHz:

This band is used by amateur radio astronomers to monitor radio noises from the planet Jupiter. 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. It is believed that at least some of this noise originates as material ejected from lo's volcanoes interacts with Jupiter's very powerful magnetic field.

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 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 shortwave transmissions and therefore monitoring fadeouts is also useful. 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. 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. This is a very inexpensive way to get started in radio astronomy.