Amateur Radio Astronomy

By Peter Chapin

Disclaimer

My knowledge and experience with amateur radio astronomy is very limited. Although I do have a background in electrical engineering with a special interest in electromagnetic radiation, I don't yet have any significant experience with astronomical observing at radio frequencies. It is my hope to change this situation. This web page is the first step.

The Electromagnetic Spectrum

Optical observers also see the universe using electromagnetic radiation. The only difference between optical and radio observing is the frequency of the radiation used to make the observations. Visible light has a frequency that ranges from about 4.3 x 10¹⁴ Hz to 7.5 x 10¹⁴ Hz. This is a fairly narrow range of frequencies -- less than one octave. Yet the universe produces radiation over a range of frequencies that spans many orders of magnitude. There are many wonderful sights to behold in the sky that are quite invisible to our eyes!

As we turn down the frequency dial of a hypothetical "super" receiver, we would first go from visible light to infrared light. Although the boundary between infrared light and high radio frequencies is fuzzy, I think it's reasonable to say that infrared light goes all the way down to approximately 3.0 x 10 ¹² Hz (100 micron wavelength). This is 3 THz or 3000 GHz. Below that frequency we enter the domain of millimeter waves. Microwave frequencies kick in around 100 GHz and extend all the way down to, say, 1 GHz. Below 1 GHz we are strictly in the domain of "radio" frequencies. However when I talk about radio observing in this document, I mean (at least potentially) observing at any frequency below infrared frequencies. That is, I mean any frequency below 3000 GHz.

Observing the Radio Sky

Since our eyes are not sensitive to radiation outside their limited range, we can not directly observe the sky using radio frequencies. Instead we have to rely on electronic (and possibly expensive) equipment to convert the radio information to a form that we can accept: sound, charts, graphic display, etc. This is not really very different than what goes on today among optical observers. Many serious observers rely on CCD devices and computers to collect and process optical information. Many optical observers are also looking at their results on a computer screen!

When one thinks about radio astronomy one usually thinks about large, impressive radio telescopes and expensive, high speed computers. In general it does not sound like something an amateur could get into. I believe that is incorrect. The purpose of this page is to show how an interested amateur, using reasonably modest equipment, can do some exciting radio astronomy. As I write this, I don't know the details of how this will work either. However, it is my hope to figure that out as I go along.

I am one of the advisors to the Astronomy Club at Vermont Technical College. This page is linked into the VTC Astronomy Club page. While amateur radio astronomy is something that interests me personally, it is also something that I'm doing in connection with the VTC Astronomy Club.

Issues

To make useful radio observations, there are several issues that must be considered.

Atmospheric Transparency

The atmosphere is mostly transparent to visible light (except when it's cloudy!). However, the transparency of the atmosphere varies wildly with the frequency of the radiation that is trying to propagate through it. At some frequencies, the atmosphere absorbs the radiation. The signal from the sky thus attenuates massively before it reaches the ground. At other frequencies the ionosphere reflects the radiation back into space. (This "mirror" effect is also what allows some radio broadcasts to be heard at locations that are far over the horizon from the transmitter). To do effective radio observations, one must choose an frequency that will propagate through the atmosphere effectively.

"Light" Pollution

The problems optical observers have with light pollution are well known in some circles. Radio observers have the same problems. Human activity generates a great deal of radio noise on a wide range of frequencies. Some frequencies have been specifically set aside by international agreement for radio astronomy. These "clear channel" frequencies should, in theory, have a minimal amount of noise on them. Of course the reality is not always like that. Some human produced radio noise is very broadband and will disrupt observations at virtually any frequency.

Large Antennas, Low Resolution

Because light has such a short wavelength, optical observers have the luxury of being able to build small, very high gain parabolic antennas (called "telescope mirrors" by the optical community). Since radio frequencies are many orders of magnitude lower than optical frequencies, very large antennas are needed to obtain any "reasonable" amount of gain. Yet even so, the resolution that a modest radio telescope can expect to achieve is much less than what a similarly modest optical telescope could obtain. In short: the "views" that can be obtained from a radio telescope are very fuzzy at best.

Expensive Equipment

Optical observing can be done with no equipment at all. Naked eye observing can be both fun and effective. With $1000 or so worth of telescope, the range of objects that can be observed optically increases dramatically. Radio astronomy requires complex and potentially expensive equipment before anything can be observed at all. In addition to antennas and receivers, observers need to convert the incoming radio information into a form suitable for presentation. This extra complexity can be daunting.

Radio Meteor Observations

It is possible to make interesting observations of meteor showers using a standard FM receiver (88 MHz to 108 MHz range). Since this doesn't require any special equipment it seems like a good way to get started with amateur radio astronomy. Check out the Observing Meteors by Radio page for more information on the theory and technique involved.

Observing meteors by radio requires that you point your FM antenna toward a distance station that you don't normally receive. Locate a frequency that is clear on your dial. What stations use that frequency? Consult the FCC's FM Radio Database Query Program to find out!

Links to Other Amateur Radio Astronomy Pages

There are quite a few pages on the web pertaining to amateur radio astronomy. I direct you particularly to the home page of the Society of Amateur Radio Astronomers (SARA). As you might expect, this site contains quite a bit of information about amateur radio astronomy. SARA's page "Interesting Web Sites for Radio Astronomy Enthusiasts" contains numerous links to other sites of interest. Many amateur radio astronomers have put up web pages detailing the design of their telescopes. Check them out!

Return to the Amateur Astronomer's Notebook.