![]() Point at Cygnus and you’ll receive a strong signal from the local arm of the Milky Way very near the expected 1420.4-MHz frequency. This plot distinctly showed a hydrogen “line” (really a fat bump) when I pointed my horn at the star Deneb, which is a convenient guide star in the constellation of Cygnus. After getting more familiar with the HDSDR software, I figured out how to time-average the signal and focus on the spectral plot, which I adjusted to display average power. Spiral Arms: The antenna can detect emissions from the hydrogen gas in nearby arms of the galaxy: Dark green (above) represents the signal from the sky light green shows the baseline system response with no signal.Īt my instrument’s “ first light,” I was able to detect the neutral hydrogen line with just a little squinting. The dongle just sits on the ground next to my horn and is attached to a Windows laptop through a USB extension cable. The dongle itself provides power for the amplifier through the coaxial cable that connects them, a 30-cm (12- inch) length of coax purchased on ($9). I purchased the dongle from ($37) because that company had also recently started selling a gizmo that seemed perfect for my application: It contains two low-noise amplifiers and a surface-acoustic-wave (SAW) filter centered on 1420 MHz ($38). (The software was chosen on the basis of a report from two amateur radio astronomers in Slovenia who had used it to good effect.) ![]() So that’s where I drilled a hole to accommodate an N-type coaxial bulkhead connector that I had purchased on for $5, along with an N-to-SMA adapter ($7).įor my receiver, I went with a USB dongle that contains a television tuner plus a free software-defined radio application called HDSDR. An online tutorial and another calculator showed the appropriate distance from the base to be 68 mm. However, in this case the relevant wavelength isn’t 21 centimeters but what’s called the guide wavelength, which corrects for the difference between how the signal propagates in free space versus inside the waveguide. The tricky part is figuring out where to place it in the can-it needs to be a quarter of a wavelength from the base. Many cantenna tutorials say to make the pin a quarter of a wavelength long, which in this case works out to 53 millimeters. But none of the projects’ documentation showed exactly how to construct the feed’s “pin”: the part that picks up signals inside the waveguide and passes them to the telescope’s receiver. Some folks contributing to Open Source Radio Telescopes were using similar cans. ![]() Signal Plumbing: The signal is picked up by a wire projecting into the waveguide (top) and sent to a receiver via a coaxial cable attached to a bulkhead connector (bottom). A handy online waveguide calculator told me this feed would have an operating range that nicely brackets the neutral-hydrogen line frequency of 1420 megahertz. The empty can serves as a waveguide feed at the base of the horn antenna. I also purchased a 1-gallon can of paint thinner ($9) and gave away its contents. Attaching a square of ordinary foam board (not the aluminized kind) to the open end made it plenty robust. Some hours with snips and aluminized HVAC tape ($8) resulted in a small horn antenna. ![]() An online calculator showed that a horn of those dimensions would have a respectable directional gain of 17 decibels. The roll was 10 feet (3 meters) long, which limited the length of the four sides to 75 cm. The width of my roll determined the aperture of my horn’s wide end. In any case, I abandoned the board and for $13 purchased a roll of 20-inch-wide (51-centimeter-wide) aluminum flashing-the thin sheet metal used to weatherproof tricky spots on roofs. That experiment still perplexes me, because people definitely do build radio telescopes out of aluminized foam board. ![]()
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