By Flavio Falcinelli
The line at 21 centimeters (1420.40575 MHz), due to the almost monochromatic emission of the "cold" hydrogen that populates the interstellar space (it is the most abundant element in the universe), was predicted theoretically by Van De Hulst in 1944 and discovered by H.I. Ewen and E.M. Purcell in 1951 as they watched a region of the Milky Way.
The event stimulated the search for other substances in the interstellar medium: so far, many complex molecules, including the organic ones, have been catalogued.
The study of 21 cm line is, rightfully, the first major and independent success of radio astronomy: through radio telescopes it was possible to "see" the spiral structure of the Galaxy, impossible for optical instruments because of the absorption of interstellar clouds.
Using radio-spectrometers is possible to determine with considerable accuracy the profile of the line as a function of frequency and, by applying Doppler techniques to the analysis of the data, you can achieve important information on the dynamics of the movement of large emitting masses of gas.
The "radio" study of the Galaxy structure is accessible at the amateur level: it is very interesting and instructive to define the shape of our galaxy investigating the distribution of gas and the characteristics of the 21 cm line profile. The relative transparency of the galactic disk at this frequency enables the "exploration" as a whole, while with optical means the observations are limited to a small region close to the solar system because of the strong absorption due to interstellar gas.
The images show our first telescope built to test this research possibility at amateur level. Browsing the web there are many well-documented examples of radio astronomy projects for the study of the 21 centimeters line.
The system consists of an antenna horn built with aluminum foil (we were inspired by the project detailed in http://www.setileague.org/articles/horn.htm) and supported by a wooden support, adjustable in elevation, assembled according to a classic amateur "philosophy" which favors the use of inexpensive and readily available materials (as long as you can ...).
The signal picked up by the antenna is amplified and filtered (LNA) to limit local interference, subsequently applied to the receiver (a prototype) that analyzes a portion of the bandwidth of the system, centered on the nominal value at rest of the hydrogen line (1420.40575 MHz).
The observations will highlight the doppler shifts of the rest frequency of the line, indicative of the relative motion of the gaseous masses with respect to the observer. For proper evaluation of the speed of movement, it is essential that the receiver is very stable in frequency: in fact a radio-spectrometer, which analyzes the spectrum of the signals within its bandwidth, must minimize the measurement errors caused by the receiver’s and local oscillator’s own drift (if the receiver is at frequency conversion).
This is the prototype antenna we built to study the sky at the neutral hydrogen frequency 1420 MHz. It is a typical amateur realization: the antenna is made with aluminum foil and installed on a wooden support manually orientable in elevation (fixed azimuthal orientation to the south). The support has been realized with a simple structure, recycling material from pallets normally used in the transportation of goods.
Amplifier (LNA) inserted between the antenna and the receiver. The device was constructed by connecting in cascade a band-pass filter centered on 1420 MHz, two commercial broadband amplifiers (low noise and high dynamic), further band-pass filter, identical to the previous one, connected to the output. The picture shows the frequency response of the amplifier.
In our prototype, called RALSpectrum, we treated carefully this requirement by using a local oscillator precise and heat stabilized (the receiver structure is at single frequency conversion, with quadrature mixer), coupled to the time reference signal from the GPS satellite network via an auxiliary receiver.
The first test is to orient the antenna on the zenith and... wait: if the system works, it should be clearly visible profile of the hydrogen line. The signal level will be minimal, given that the antenna “observes" a region of the sky away from the Milky Way where the gas is thickened.
The following images show some test recordings: more details on the structure of the radio telescope and the recordings can be found in the following animations.
The purpose of our experiment was to test the functionality of an amateur radio telescope suited to the study of the profile of the neutral hydrogen line at 21 centimeters: in our tests we used, and built for the occasion, an antenna horn able to provide sufficient gain for the purpose.
Obviously, this is not the only or the best possible solution: larger antennas as, for example, parabolic reflectors with a diameter of about three meters, allow an easy reception of the hydrogen "monochrome" emission.
Particular attention was paid to the development of a radio-spectrometer characterized by a high sensitivity and stability, as requested by these applications. As we shall see, a receiver of this type is used for other purposes such as, for example, in the development of an amateur SETI listening program.