20 August 2004

Radio astronomy

Part 1. Early history and basic systems

Jeffrey M. Lichtman
Radio Astronomy Contributing Editor

While in High School (1964), I stumbled on "Radio Astronomy for Amateurs," a book by British amateur radio astronomer Frank W. Hyde. This book was had a major impact on my life long interest in science.

With the excitement that grows with new found knowledge, I began collecting anything electronic. I quickly filled my parent's basement with an assortment of TV sets and radios.

When Dad asked, "What are you going to do with all this junk?" I replied, "I'm going to build a radio telescope!"

Dad, being a radio operator during World War Two, gave me an encouraging smile. Back then, there wasn't much information available to a fledgling radio astronomer, and life had other demands. Consequently, my enthusiasm for radio astronomy was put on hold until after completing military service in the U.S. Army. My training was in radar and the Nike Missile System (Redstone Arsenal, Huntsville, Alabama). This training provided the educational spark that expanded my interest in radio telescopes.

One lesson I learned in those early years was not to rush into any endeavor until becoming familiar with the subject, having adequate funds, and formulating an observing objective.

So, in this introductory article about radio astronomy, where should we start?

It's first essential to become familiar with the basics, so let's begin with some early history.

Early history of radio astronomy

Early in the 1930s, Bell Labs was interested in the static (i.e., background transmission noise) picked up on trans-Atlantic communication cables. A young physicist, Karl Jansky, was assigned the task of investigating its origin. This research involved building a merry-go-round type antenna that would operate at a frequency of 15 meters.

During his observations, Jansky noted a particularly intense disturbance when the antenna was pointed south at an elevation of 26 degrees declination (close to the galactic center).

Grote Reber, a young radio engineer and amateur radio operator (W8GFZ), was inspired by the work of Jansky. Reber built the first amateur radio telescope at his home in Wheaton, Illinois. His home built, 9.8-meter (32-feet) diameter antenna coupled to his nine-centimeter radio receiver detected solar and galactic radiation.

During World War II, two British radar engineers noted signal interference that might have been the work of German jamming systems. What they detected turned out to be the effects of solar flares.

After the war, many of the radar units were no longer needed and due to be scrapped. Sir Bernard Lovell, and his colleagues, acquired some of the left over equipment. These systems were soon converted into radio astronomy receivers, with antennas mounted on surplus gun mounts. Some of the first signal detections came from the constellation of Cygnus. This radio source was then cataloged as Cygnus A.

Planetary radio emissions

In January 1955, radio astronomers Burke and Franklin detected strong radio bursts from the planet Jupiter. It was found that synchrotron radiation was being emitted at the frequency of 18 -- 24 MHz. Further investigation showed that one of the Jovian moons, Io, might be responsible for triggering the radio bursts, whenever it entered the magnetic field of Jupiter.

One of the most active Jupiter radio observation programs is conducted at the University of Florida, which has an active Jupiter radio observatory. Dr. Fracisco Reyes, a member of the Society of Amateur Radio Astronomers (SARA), manages the university's radio astronomy web site.

Amateurs and educators can easily participate in Jupiter observations via NASA's Radio Jove Program.

Jim Sky, a SARA Member, offers Radio-SkyPipe, other Jupiter observation and prediction software and other radio astronomy products at Radio-Sky Publications. Radio-SkyPipe is an internet enabled strip chart data logger. With this software, the amateur or educator can observe Jupiter noise storms or bursts as they happen.

Other real-time observations can be monitored on the internet. Richard Flagg, of RF Associates, a SARA member and a noted Jupiter researcher, developed the initial design for the Jove receiver, which is also used in the NASA program. This receiver will work with the Radio-SkyPipe software mentioned above.

The single antenna radio telescope

Antenna -- The antenna collects incoming cosmic signals and focuses the signal into a feedhorn. The feedhorn is also part of the antenna. Inside the feedhorn there is a small probe called a monopole. The monopole is sized for the operating frequency of the receiver.

Low Noise Amplifier -- The Low Noise Amplifier (LNA) receives the weak cosmic signal via coaxial cable and amplifies it for further processing.

Receiver Backend -- The receiver backend is made up of specific circuits to further amplify the signal from the LNA, and then process the signal through various Analog to Digital Converters (A/D) or Digital Signal Processors (DSP).

Computer and Display -- The computer is responsible for displaying the processed signal received from the radio telescope.

Signal Output

The processed signal appears as the radio source transits the antenna. At right is an example of a 21 cm (1420.405 MHz) spectral hydrogen signal received by a single antenna radio telescope.

The two antenna interferometer radio telescope

The interferometer version of the radio telescope is very similar to the single antenna system. The only difference is the addition of a second antenna, an additional LNA and a signal combiner. The antennas are placed along an east-west baseline. The resultant signal shows up as interference fringes that are a function of the separation of the two antennas at the specific wavelength being received.

Signal Combiner -- The signal combiner is a zero phase module which combines the signals from the two antennas and routes it to the Receiver Backend.

To be continued. In Part 2 we will look at some notable discoveries in radio astronomy. We will also introduce the Society of Amateur Radio Astronomers (SARA).