SIXMILE RUN
Glossary

Reel 08

Bench Tools

Real radio

The calculators we kept rewriting on scrap paper, cleaned up and put online. They run in your browser, they're free, and they're for anyone pointing an antenna at the hydrogen line, whatever you think about what we found there.

  1. TOOL-01Hydrogen-line Doppler and LSR calculator"What velocity is that peak, and where should I tune?"Turn an observed frequency near 1420.405 MHz into a radial velocity, with the local standard of rest correction worked out for your site, date and pointing.Open the tool →
  2. TOOL-02Dish, cantenna and horn sizing calculator"How big does the antenna need to be, and how do I build the feed?"Gain, beamwidth and feed geometry for a dish at 1420 MHz, plus cantenna dimensions and an optimum-gain pyramidal horn you can cut from sheet metal.Open the tool →
  3. TOOL-03Receive-chain noise and integration time calculator"Where is my noise coming from, and how long until I see the line?"A Friis cascade of cable, LNA, filter and SDR that gives system temperature, then the radiometer equation for how long to integrate.Open the tool →
  4. TOOL-04Galactic-plane transit planner"When does the galactic plane pass through my beam?"When each stretch of the Milky Way crosses your meridian over the next day, and how high it gets. Built for fixed and drift-scan antennas.Open the tool →

Build guides

Six papers that take you from parts list to a rotation curve. Each one has its worked numbers run through these calculators, with a link to open the tool already filled in.

  1. 01 · LAB-008The $150 Hydrogen-Line Receiver, End to EndA complete parts list and build sequence for the cheapest receive chain we'd trust to show the 1420 MHz hydrogen line: RTL-SDR Blog V3, SAWbird+ H1 LNA, a foil-lined pyramidal horn with a WR-650 throat, and the adapters nobody remembers to buy.
  2. 02 · LAB-009First Light: Detecting the Hydrogen Line with an RTL-SDR V3 and a HornA step-by-step first observing session for a horn-and-RTL-SDR hydrogen-line receiver: picking the night, pointing at the inner Galactic plane, setting up the software, taking on and reference spectra, and knowing when you've actually seen the line.
  3. 03 · LAB-010Cantenna, Horn or Dish: Choosing a Hydrogen-Line AntennaWhat each of the three common amateur 1420 MHz antennas actually buys you in gain, beamwidth and effort, why a small horn sees the hydrogen line better than its gain suggests, and when a dish is worth the trouble.
  4. 04 · LAB-011LNA Placement and Coax Loss at 1420 MHzWhy the low-noise amplifier goes at the feed, what feedline loss really costs in kelvin and integration time, when a second amplifier helps, and the four placement mistakes we see most, each worked through the Friis cascade.
  5. 05 · LAB-012Reading Your First Hydrogen-Line SpectrumHow to turn a raw 1420 MHz power spectrum into something you can interpret: removing the bandpass, converting frequency to LSR velocity, recognising real emission versus artifacts, and what the shape of the line says about where in the Milky Way the hydrogen is.
  6. 06 · LAB-013From Spectra to a Galactic Rotation CurveHow to measure the Milky Way's rotation curve with an amateur hydrogen-line telescope using the tangent-point method: which longitudes to observe, how to pick the terminal velocity off each spectrum, the arithmetic, and the errors that matter.
Sky targetsWhere to point once the antenna worksTen standard pointings, from Cygnus and l = 30° in the north to Norma and Crux in the south, each with coordinates, how high it gets at your latitude, transit times by month, and the velocity range the spectrum should cover.Open the sky targets →

The order we'd use them in

Size the antenna first with the dish and horn calculator, then check the chain behind it with the noise calculator, which will tell you whether your LNA is in the right place before you've bought any coax. On the night, the transit planner says when the plane comes through your beam, and the Doppler calculator turns the peaks you record into velocities you can compare with anyone else's.

If you're starting from nothing, LAB-008 is the parts list and LAB-009 the first night out. The hardware these assume is written up in LAB-006, and the reason for the frequency in LAB-005. The math is ordinary radio astronomy and every formula is printed on the page that uses it. The terms behind them, from LSR to system temperature, each have a page in the field reference. If you find an error, tell us at contact@aliencouncil.com. We'd rather be corrected than quoted wrong.