Description
What it does
A 10-turn RHCP helix tuned to 1420 MHz, the rest frequency of the 21 cm neutral-hydrogen line. Used for amateur radio-astronomy work — observing galactic hydrogen distribution, tracking spiral-arm rotation curves, and building a personal H1 radio telescope without a parabolic dish.
Specifications
| Parameter | Value |
|---|---|
| Center frequency | 1420 MHz |
| Polarization | RHCP |
| Helix turns | 10 |
| Wire diameter | 3.5 mm |
| Connector | N-type |
| Length × Ø | 521 × 180 mm |
| Weight | 650 g |
| Mount | 6 × M4 bolts (universal) for el/az tracking |
| Material | PETG, UV-resistant |
| Color | Orange/White (default), Custom on request |
Typical use cases
- Detection of the galactic 21 cm hydrogen line from the Milky Way disc
- Doppler-shift mapping of galactic-arm rotation
- Single-dish radio-astronomy education projects
Compatible chain
- Hydrogen-line LNA (e.g. SAWbird+ H1, ~25 dB gain, integrated bandpass) — required, since the antenna is passive
- SDR with at least 2 MHz bandwidth (RTL-SDR v3 / Airspy Mini / HackRF)
- Outdoor / dark-sky site away from cellular and WiFi RFI
- SDR# / Gqrx / GNU Radio for capture; H-line specific software for averaging
Choosing turns and polarization
The 3-turn option is optimised as a feed for small offset dishes (~1 m): its broader beam illuminates a small dish far better than the high-gain 10-turn, which suits larger dishes or standalone use. On polarization, a single dish reflection reverses the circular sense — so for a normal prime-focus or offset dish choose RHCP, and the dish then receives LHCP on-sky, rejecting the RHCP L-band satellites (GNSS, Iridium, Inmarsat). Choose LHCP only for a direct feed or an even number of reflectors.
Color options
The Color option applies to the 3D-printed parts only — the baseplate and rim. The radome pipe is always white, regardless of the Color choice.
- Orange/White — orange baseplate and rim with the standard white radome pipe.
- Custom — custom color for the baseplate and rim. White is a valid choice; pick Custom if you want a uniform white antenna.
Documented in the Lab
- L-Band Helix Antenna for Radio Astronomy — A Practical Guide — full setup, results, RFI mitigation
Custom polarization or color combinations available on request — please use the contact form for a quote.
Seen in the field
Volker Tympel uses this antenna to detect the galactic 21 cm hydrogen line from a city balcony in Jena, with a full signal chain and reproducible galaxy transits. Read the write-up: Urban Radio Astronomy with the L-Band H1 Helix.
