Reel 09
Field Reference
Real radioThe terms our papers and calculators keep leaning on, each written up once, properly: what it means, the formula, a worked number at 1420 MHz, and where it bites in practice.
The signal
What the hydrogen line is, where it sits, and how its frequency turns into a velocity.
- Brightness temperature and antenna temperatureT_B · T_A · antenna temperatureRadio astronomers measure power in kelvin. Brightness temperature describes the sky; antenna temperature is how much of it actually reaches your receiver through your antenna's beam.
- Hydrogen line21 cm line · HI line · 1420 MHz lineThe 1420.405751768 MHz (21.1 cm) emission line of neutral atomic hydrogen, produced when the electron in a hydrogen atom flips its spin relative to the proton.
- Local standard of rest (LSR)LSR velocity · v_LSR · kinematic LSRA reference frame that moves with the average motion of stars near the Sun. Hydrogen-line velocities are reported relative to it so that measurements from different sites and dates can be compared.
- Radial velocity and Doppler conventionsradio velocity convention · optical velocity convention · Doppler shift at 1420 MHzThe speed of a source along the line of sight, worked out from how far its observed frequency is shifted from the rest frequency. Radio astronomy uses its own 'radio convention' formula for it.
- Water holeSETI water hole · 1420–1662 MHz bandThe quiet stretch of radio spectrum between the hydrogen line at 1420 MHz and the hydroxyl (OH) lines near 1662 MHz, proposed as a natural meeting place for interstellar communication.
Antennas
Gain, beam, and the feeds you can build for a dish or use on their own.
- Antenna gain and aperture efficiencydish gain · dBi · effective apertureHow strongly an antenna concentrates its sensitivity in one direction compared with an ideal isotropic antenna, usually given in dBi. For a dish it depends on diameter, wavelength and how efficiently the feed illuminates the surface.
- Cantenna (circular waveguide feed)can feed · coffee can antenna · TE11 waveguide feedA metal cylinder closed at one end with a quarter-wave probe inside: a circular waveguide used as a dish feed or a small standalone antenna. Its diameter must suit the frequency or it won't pass the signal at all.
- Focal ratio (f/D)f/D ratio · dish focal length · feed illumination angleA dish's focal length divided by its diameter. It fixes how wide an angle the feed at the focus has to illuminate, and so which feed will work on that dish.
- Half-power beamwidth (HPBW)beamwidth · -3 dB beamwidth · FWHM beamThe angular width of an antenna's main beam between the points where sensitivity falls to half its peak. It sets how much sky you average over and how long a source takes to drift through.
- Pyramidal horn antennahorn antenna · hydrogen line horn · optimum-gain hornA rectangular waveguide flared out into a four-sided horn. It gives moderate, predictable gain with a clean beam, and it can be built from foam board and foil, which makes it the standard first antenna for the hydrogen line.
Receiver and noise
Where the noise in a receive chain comes from and how long it takes to beat it.
- Friis noise formulaFriis cascade · cascaded noise temperature · cascaded noise figureThe formula for the total noise of a chain of amplifiers, cables and filters: each stage's noise temperature is divided by the gain of everything in front of it, then they're added.
- Low-noise amplifier (LNA)LNA · preamp · SAW-filtered LNAThe first amplifier after the antenna. Its noise figure and gain set most of a receiver's sensitivity, which is why it belongs at the feed, before any cable.
- Noise figure and noise temperatureNF · noise factor · equivalent noise temperatureTwo ways of stating how much noise a component adds. Noise figure is in dB relative to a 290 K reference; noise temperature is in kelvin. Radio astronomy prefers kelvin because the numbers add directly.
- Radio-frequency interference (RFI)RFI · interference at 1420 MHz · L-band interferenceAny unwanted human-made signal in a radio telescope's band. Near the hydrogen line it comes from satellite services above and below the protected band and from electronics close to the receiver.
- Radiometer equationintegration time · radiometer sensitivity · ΔT = Tsys / √(Bτ)The relationship between system temperature, bandwidth, integration time and the smallest temperature change you can detect. It tells you how long to integrate to see a line of a given strength.
- Spectral resolution and channel widthFFT bin width · velocity resolution · channel width km/sHow finely a spectrum is divided in frequency. At the hydrogen line every channel width corresponds to a velocity width, so it decides how much detail you can see in the gas motions.
- System temperature (T_sys)Tsys · system noise temperature · SEFDThe total noise of a radio telescope expressed as a temperature: receiver noise plus sky, plus ground picked up by spillover and sidelobes. Every sensitivity calculation starts from it.
Observing
Pointing at the Galaxy, letting it drift past, and what the spectra add up to.
- Drift scantransit observation · meridian transit scanObserving with the antenna held still while the Earth's rotation sweeps the sky through the beam. It's how most small radio telescopes, and every fixed horn, work.
- Galactic coordinatesgalactic longitude · galactic latitude · l and bA sky coordinate system centred on the Milky Way: galactic longitude l runs along the plane from the Galactic centre, and galactic latitude b measures angle above or below the plane.
- Tangent-point methodterminal velocity method · galactic rotation curve from HI · Milky Way rotation curveA way to measure how fast the Milky Way rotates at different distances from its centre, using the highest-velocity hydrogen along lines of sight through the inner Galaxy.
How to use this
Every number on these pages comes from the same code as our bench tools, so you can open the matching calculator and change the inputs to your own parts. Where an example uses made-up component values we say so; nothing here is a measurement passed off as a spec.
If you're starting from nothing, read hydrogen line, then system temperature and the radiometer equation. Those three explain almost every decision in the build guides. When you're ready to point at something, the sky targets pages say where, and what the spectrum should look like when you get there. These are Earth-side radio terms. Words from the translated feed itself are in theConcordance glossary. Corrections go to contact@aliencouncil.com, and we will fix the page and say so.