The CBSS Tiny Horn Antenna Radio Telescope (THART)
Van de Hulst, a graduate student in Leiden predicted the existence of the radio emission of neutral hydrogen at 21 cm (1420.406 MHz) and in 1951 Ewen and Purcell succeeded in detecting this emission line with a horn antenna at Harvard University. A horn antenna is an antenna that consists of a flaring metal waveguide shaped like a horn to direct radio waves in a beam. Horns are widely used as antennae at UHF and microwave frequencies, above 300 MHz and they can operate over a wide range of frequencies. The pyramidal horn antenna is a type of horn antenna with the horn in the shape of a four-sided pyramid, with a rectangular cross section.
The CBSS Tiny Horn Antenna Radio Telescope (THART) is a portable and compact full-fledged radio telescope designed and constructed by a team of CBSS astronomers using locally available materials with the aim of detecting and measuring this spectral line from neutral hydrogen clouds in our Milky Way galaxy. It comprises a pyramidal horn antenna, front-end electronics and a SDR (software defined radio) based receiver.
Left: The original horn antenna built by H. Ewen and E. Purcell (1951) to detect the Galactic hydrogen 21 cm line. Middle: H. Ewen and the receiver used for this detection. Right: The first detection of the this line from the Milky Way, obtained on 9 April 1951. The spectrum was obtained in frequency switching mode. Image Credit: https://www.gb.nrao.edu/fgdocs/HI21cm/ephorn.html.
The CBSS Tiny Horn Antenna Radio Telescope (THART).
How the CBSS Tiny Horn Antenna Radio Telescope (THART) works.
The CBSS Big Horn Antenna Radio Telescope (BHART)
This is a scaled up version of the CBSS Tiny Horn Antenna Radio Telescope (THART) with improved sensitivity but with the same front-end electronics and receiver system. This antenna has an aperture dimensions of 75cm x 60cm and a beam width of 20° in the H plane and 24° in the E-plane. The unique design w adopted in assembling the four sides of the antenna is such that it allows for easy de-coupling and re-assembly, hence making the antenna portable. We further added another great advantage of our antenna over similar ones in the web by making it’s sides foldable like a book. This ability to be foldable reduces the area taken by the antenna sides by a factor of two, hence making it so compatible that it can be enclosed in transportable box.
The CBSS Big Horn Antenna Radio Telescope (BHART) on a German Equatorial optical telescope mount
In general, small radio telescopes with diameter ranging from 1 to about 3m operating at frequency of 1420.405 MHz for 21 cm hydrogen line studies are widely used in developed nations for education and outreach. The line produced by clouds of neutral hydrogen in the interstellar medium, though discovered about 70 years ago, is still the most important and significant spectral lines in radio astronomy. It provides radio astronomers with a very useful probe for studying the differential rotation of spiral galaxies.
A pyramidal horn is much simpler to design and build compared to a parabolic antenna which is usually purchased ready-made. This provides students and tutors of astronomy in Nigerian universities the rare opportunity to build and a fully-functional low cost radio telescope. Most importantly, a wide spectrum of activities with this horn antenna (ranging from 10mins to hours of observation time) can be easily incorporated into their curricula such as a radio astronomy laboratory course. Such activities were delivered to the participants (university lecturers) of the IAU-OAD sponsored workshop (Held in December 2022) on the design, assembly and use of the CBSS horn antenna in their universities.
For researchers in CBSS, a long term project with the horn antenna has been designed and will be initiated in March 2023. It is titled Creating a 3D Map of the Milky Way Galaxy at 21cm with the CBSS’s Pyramidal Horn antenna and will take about 3 – 6 months to complete depending on the level of continuity and consistency of our observations. Through the observation of the 21cm line at galactic longitudes along our galactic plane one can show that the angular velocity increases as youlook at points closer to the galactic center The aim of the long term project is two-fold (a) to produce a rotational curve for our galaxy using the 21-cm spectral line observations obtained at different galactic longitudes along the galactic plane (galactic latitude of 0°) (b) to create a 3D-map by performing such observations at several galactic longitudes (from 0° – 360°) and different galactic latitudes. An example of such map obtained from 1656 spectra collected from 23 days of observation is shown in the next figure.
An example hydrogen line map created using a 1.5m diameter radio telescope and resulted from 1656 spectra collected from 23 days of observation. The observation site is in Netherlands where declination of -20° to 90° can be accessed.
Image credit: https://www.rtl-sdr.com/
The CBSS Yagi-Uda Antenna Radio Telescope
Between 1926 and 1929, Shintaro Uda, an assistant professor at Tohoku University Japan experimented with a new kind of antenna array, leading to his papers entitled, “On the Wireless Beam of Short Electric Waves”. His work was primarily done with a single parasitic reflector, a single parasitic director, a reflector, and up to 30 directors on the array. In 1926, Hidetsugu Yagi (Figure 9), a professor of electrical engineering at Tohoku University Japan collaborated with Uda on a paper presented to the Imperial Academy entitled “Projector of the Sharpest Beam of Electric Waves”. After this, funding was given to Yagi to continue working on the array, and after several tours of the Pacific and United States, the antenna array became known as a Yagi. However, in recent years, credit has been given to Uda and all of the work he contributed to the creation of the antenna, thus becoming the “Yagi-Uda Array”.
The Yagi-Uda antenna is the most popular and easy-to-use type of antenna with better performance, which is famous for its high gain and directivity and lower fabrication cost. Therefore a considerable attention has been given to design and implement them for the aim of many engineering applications by the research community over the years. Outside radio astronomy, Yagi-Uda antennas are designed specifically for HF (3-30 MHz), VHF (30-300 MHz), UHF (300-3000 MHz) where they are used in various types and structures on radar, satellite, RFID, jamming and wireless communication applications. In Nigeria, Yagi-Uda antennae can be seen on top of almost every house hold where they used for radio and TV (eg GOTV) reception.
This antenna (Figure 10) was successfully simulated, designed and constructed in CBSS for 21cm radio astronomy. It’s worth mentioning that the construction materials were fully sourced from Nsukka. The striking feature of this radio telescope is that all of its components can be easily packaged in a travelling case measuring 30 X 7 X 4 inch (L x W x H).
Figure 9: Hidetsugu Yagi with one of the early antennas that he and Uda developed
The simulation of the antenna was done with P. McMahon’s YagiCAD3 software. The table below gives the dimensions of the 22 element Yagi computed with this software. For the elements we adopted 1.6mm copper wires cut to the lengths specified in this table and for the boom we adopted a 1 inch diameter PVC pipe. The front-end electronics is the same as that of the horn antenna radio telescopes described in earlier sections. Figure 10 displays the rectangular metal box that houses these electronics and a cross-section of the antenna during construction.
Figure 10: The CBSS Yagi-Uda Antenna mounted on an optical telescope’s GEM mount (Top) and packaged in a travelling case measuring 30inch X 7inch X 4inch.
The CBSS Three-Element Radio Interferometer
The future of radio astronomy unarguably relies heavily on interferometers as evidenced by the presence of ALMA, SKA, EVLA, VLTI and other current and planned radio interferometry array. Access to these professional interferometers for university courses can not be guaranteed if at all possible due to it’s high demand. Educational interferometers that can be used for hands-on experiments are critical for a full understanding of the basic concepts of interferometry taught in undergraduate and graduate courses. In Nigeria, the lack of such educational tools in universities offering astronomy courses remains an obstacle to training the future generation of astronomers.
In the light of the above, CBSS astronomers embarked on a multi-phase project to design and build a 3-element radio interferometer for the purpose of education and small scale research in radio astronomy. Such project will, in the absence of a professional radio telescope in Nigeria, offer researchers and students a great opportunity to acquaint themselves with techniques used in radio astronomy observation, data acquisition and analysis as the design and functionality of the 3-element interferometer look similar to that of professional massive telescopes except that dish size and sensitivity are more restricted. Furthermore, experiences gained in this kind of project will beneft not only students in astronomy but also those in engineering (electronics and mechanical) and computer science.
The Interferometer System (Figure 11) comprises mainly three TV satellite parabolic dish antennas, the front-end electronics for each dish and the back-end receivers and PCs. The components of an element of the CBSS 3-element Radio Interferometer are displayed in Figure 12. A novel feature of our interferometer is the provision of a backup power in situations where other source of power (EEDC, generator and solar) fail. In other words, the system can be operated entirely off grid with 5V DC rechargeable high capacity lithium ion batteries. When fully charged, the batteries can power the front-end electronics for 12 hours. Short-duration observations and students’ demonstrations can thus be carried out without worrying about power supply.
Figure 11: The CBSS Three-Element Radio Interferometer
Figure 12: Top Left: A close-up view of its first stage front-end electronics housed in a water-proof box referred. Top Right: Close-up view of the Front-End Box. Bottom Left: Bottom view of the same Box showing its RF and DC connectors. Bottom Right: The same Box 2 when the frst inner door is closed to reveal a layer of single-board PC and LED display used for single-dish observations and trouble shooting.