Radio Astronomy 101 : 1-a-place-to-start

A Place to Start

Chapter:

A PLACE TO START Radio Astronomy, An Introduction (1) Jeffrey M. Lichtman (Society of Amateur Radio Astronomers)

Chapter 1
A PLACE TO START
Radio Astronomy, An Introduction (1)
Jeffrey M. Lichtman
(Society of Amateur Radio Astronomers)
(1) Signals from the Cosmos, Science Probe, 7/91)

What Is Radio Astronomy? To define Radio Astronomy, we first consult Mr. Websters dictionary. Radio Astronomy is that branch of astronomy which deals with radio waves in space in order to obtain data and information about
particular regions in the universe.

Early Historv. Radio Astronomy was born earlier in the twentieth century, to be precise, 1930. A young physicist by the name of Karl Guthe Jansky, was on the technical staff of Bell Laboratories at Holmdell, New Jersey. His
assignment was to study the arrival of atmospheric static at wave lengths of about 15 meters (ship to shore communications frequencies). Mr. Jansky constructed an elaborate rotating antenna for this task.

In a 1932 paper, Karl Jansky described three types of static found during his research, they are, intermittent crashes due to local thunder storms, weaker static due to distant thunder storms and an unknown weaker hiss from unknown origin. Needless to say, his further curiosity was peaked when he realized that the this static moved through the sky each day and was more distinct during daylight hours. He did note, that he did not think the noise was attributed to the sun.

In 1933, his second paper entitled ‘Wectrical Disturbances Apparently of Exfraterresfrial Origin”. In this paper he describes the static period of 23 hours and 56 minutes. In addition, he had noted in his paper that static came from the cooridinates of 18 hours right ascension and a declination of 10 degrees south. This was the area corresponding to the galactic center (Sagittarius.

Around 1937, Grote Reber, a graduate engineer living in Wheaton, Illinois, started to pursue his interest in the field of radio astronomy. Using spare parts and supplies, he started construction of a 31 foot parabolic reflector antenna for observations at 10 cm. Mr. Rebers first attempts at 9 cm, produced no response, so he changed his equipment to the frequency of 33 cm.

During the year of 1938, while observing at different polarization’s, he made a statement that Plancks blackbody law was not obeyed by Extraterrestrial noise.In 1939, with his new receiver operating at 1.87 meters, he detected radio emissions from the Milky Way. Mr. Reber published his first paper in 1940 entitled "Cosmic Static.

Radio Astronomy has traveled light years since the early days of Jansky and Reber. What I have described above is just an abbreviated walk through those early years. A book worth reading is "Serendipitous Discoveries in Radio
Astronomy", available from NRAO. Price and availability may be obtained by writing, NRAO, PO Box 2, Green Bank, W 24944 USA. I would like to add, that both the Jansky and the Reber antennas are well preserved and on display at
the National Radio Observatory (NRAO), Green Bank, West Virginia.

The staff at NRAO, tell us that both antennas are still in working order, Some Theow. Electromagnetic waves are constantly hitting the earth from thermal and non-thermal celestial sources. The closest and most powerful one is our sun. To go further, we must do a couple of computations. Wien's law explains that all
objects whose temperature is above absolute zero (-459 degrees F. or -273 degrees C,) emit electromagnetic radiation at a frequency, V, , that is linearly proportional to the temperature of the object, T.

Vm = 5.9 x 10" (T)
T = is in degrees Absolute (Kelvin)
Vm = is in Hertz
Example: Room temperature is approximately 75 degrees F. (25 degrees C. or
300 degrees Absolute),

If you were using output devices such as tubes, they will
radiate in the thermal range, approximately 1.8 x 1 O7 MHz. This frequency is in the far infrared range and below red light frequency (4.3 x lo7 MHz.). As temperature is increased in those output devices, the radiation increases in the frequency range to "red" hot,, "white" hot etc.
The amount of energy radiated from a body does not continue to infinity, but follows the bell shaped curve. To go further, would lead us into the area of Quantum Physics. Lets just say, that radiation emitted from any object follows the specific curve according to the temperature of that object. This is known in the physics community as "Blackbody Radiation". Blackbody Radiation is defined as radiation that an object would absorb if it were a perfect absorber. Conversely, just as in antenna theory, a good absorber is also a good radiator;
in turn, we can say that a blackbody is the most efficient radiator.

The sun has blackbody radiation which peaks at approximately 6000 degrees Absolute (frequency of 6.2 x 10 8th power) in the yellow light. The suns radiation
decreases slowly towards the radio end of the spectrum
How Do We Do It? Figure (shown in attachment photos) shows a typical total power radio telescope in block diagram form.

The incoming signal radiation is received at the antenna. Here we illustrate with a parabolic dish antenna. The antenna collects the signal The signal (lets use 400 MHz.) is then fed to the to the RF (Radio Frequency) amplifier, Bandpass Filter, a Local Oscillator (output frequency of 370 MHz.) and is injected into the mixer and mixed with the 400 MHz. The output of the mixer is 30 MHz.(difference) The 30 MHz. IF (Intermediate Frequency) is then fed to the IF amplifier which is tuned for 30 MHz. The 30 MHz. is then detected by a diode detector, integrated by a capacitor (AKA smoothing). DC amplified and sent to the recording devices
(computer and chart recorder).

Goals And Objectives. The first objective in any new undertaking is to read and collect all the data necessary to plan your goal. Consider the following:

Select the object and build the telescope.
Select The equipment based on the object (s) of interest.
Cost consideration required to establish your objective.
Other items to be considered:

Antenna space available.
Location of telescope (noise and interference).
Complexity (tracking required? operator monitoring?)

The previously mentioned factors determine the direction of
a project, and the final result is both a practical one and within your limitations.

For example:

1. Solar Observation
2, Jupiter Observation

Minimum cost for equipment (simple set-up).
Minimum space required (simple set-up).
3. Galactic Observations.
4. SET1 (Search For Extraterrestrial Intelligence).
Greater space required.

Higher cost. depending on equipment, resolution and the complexity. Which ever of the programs you select, it will all be based on the cost, complexity and the space required to meet your goals and objectives. To further help in your decision process, I will briefly describe each of the programs above.

The following paragraphs will address two types of solar observation. the first being VLF (Very Low Frequency), and the next being VHF (Very High Frequency), Solar Observations. The VLF (Very Low Frequency) system has been used with great success. Mr. Bob Sickels (Deceased and missed, 9/93) of Ft. Pierce, Florida has described the VLF radio observing area quite well, I will use his verbage.

There are a few amateurs who detect solar flares with a receiver tuned to the 30 - 80 KHz, (Kilohertz) range. The antenna is a long wire antenna (or resonant loop), and a receiver tuned to the above KHz, range. In addition to this receiver, a signal smoothing circuit and a small chart recorder are required (chart recorder set to run a 1 inch per hour). With this method, natural atmospheric noise
(equatorial lightning) is monitored and passed to a smoothing circuit which provides a relatively steady zero reference. When a major solar flare occurs, an emission of X Rays is sent fourth into the ionosphere. The X radiation drifts into the D layer of our ionosphere and causes a sudden enrichment of electrons. This enrichment causes the D layer to act like a very conductive waveguide. As
a consequence, there is a sudden enhancement of equatorial atmospheric noise, this is detected and displayed on the chart recorder, This noise rises to a peak within a few minutes and then slowly tails off, as the effects of the solar flare subsides. By these means, the flare is positively detected and its intensity is also recorded. Figure 7 illustrates a simple block diagram of this VLF set-up.

A second way of observing the sun is by the VHF (Very High Frequency) radio telescope. Referring to Figure 8, a standard short wave receiver (frequency range of 1 - 30 MHz. (Megahertz)), an RF preamplifier (detailed in Figure 8A), a loop antenna (detailed in Figure lo), a detector and integrator circuit, a chart recorder, and an audio tape are utilized.

The amateur radio bands are usually quite active. It is recommended that one tunes the receiver and finds a clear area free of any interference before observation begins.
My first radio telescope was exactly the one described above. Much useful data was received while observing in the range of 28 - 30 MHz.

Jupiter Observations. Radio noise storms from Jupiter are not always present, but hen they are, they are very powerful. The mechanism which causes these storms is believed to be the movement of the Jovian moons, lo and Ganymede through the magnetic field of Jupiter. This in turn causes great electrical storms on the planet. A book called "Evolution of Radio Astronomy" by J,S. Hey gives
an excellent account on the discovery of this phenomena.

Detection of these storms are made in the unused (vacant) shortwave bands around 18 - 24 MHz. and when Jupiter is in your local sky. Sky and Telescope magazine gives monthly locations of Jupiter and its moons. The observing system should use a rceiver capable of receiving the source you are most interested in. Additional equipment might involve a home computer with an A/D (Analog to Digital)
converter.

Galactic Observations. When observing galactic objects, one may utilize the set-up described previously. A different antenna and the addition of a high frequency converter and preamplifier.

SET1 Observations. This type of system is quite a bit more complex than the ones described previously. The expertise in the field of microwave electronics is a plus when a system of this type is attempted.

Amateur Proqrams. The Society of Amateur Radio Astronomers (SARA) is an international organization (Non-Profit Educational Incorporated Charter), dedicated to the advancement of amateur radio astronomy on all levels.
Currently, research is being done by SARA members in the areas of; receiver design, antenna design, galactic, solar, planetary, meteor detection etc.

The members report their progress to the editor, and have their findings published in the SARA Journal "Radio Astronomy". In addition, the group gathers every summer at NRAO for a three day conference and social gathering.

The material in this chapter will help those interested in the area of Radio Astronomy. Of course, this is just the tip of the subject but, one needs to start at the beginning. Any comments would be accepted.