Beschreibung
What it does
A 3D-printed helix antenna tuned to the 1525–1559 MHz Inmarsat downlink band. Two form factors selectable at checkout:
- 3 turns, LHCP (default) — designed as a feed for an offset satellite dish (~50–80 cm). Mounts in the LNB position via the Electronics Cover accessory (sold separately), which provides the standard 40 mm clamp interface that fits common TV-SAT LNB holders. Best for stationary GEO reception of Inmarsat-3 / Inmarsat-4.
- 10 turns, RHCP (default) — direct reception on an el/az rotor, no dish needed. Higher gain on low-elevation GEOs and suitable for both GEO and LEO satellites.
Both variants share the same connector (N-type) and PETG construction. Polarization can be flipped on either form factor (LHCP ↔ RHCP) at extra cost.
Specifications
| Parameter | 3-turn (offset dish) | 10-turn (direct) |
|---|---|---|
| Center frequency | 1525 MHz | 1525 MHz |
| Polarization (default) | LHCP | RHCP |
| Helix turns | 3 | 10 |
| Wire diameter | 3.5 mm | 3.5 mm |
| Connector | N-type | N-type |
| Length × Ø | 186 × 180 mm | 521 × 180 mm |
| Weight | 289 g | 650 g |
| Mount | 40 mm LNB clamp via Electronics Cover (sold separately) | 6 × M4 bolts (universal) |
| Material | PETG, UV-resistant | PETG, UV-resistant |
Measured performance
Live demonstration on the Inmarsat L-Band WebSDR — receiving Inmarsat-4 (Alphasat) 25°E and Inmarsat-3 F5 54°W from grid square JN58UA using the 7-turn and 10-turn variants in production today.
Polarization & dish use
The 3-turn variant is designed for use as a dish feed. A parabolic reflector inverts the circular polarization on each reflection, so a 3-turn feed should be the opposite handedness of the desired received signal — pick LHCP if you need to receive RHCP off the dish, and vice versa. The 10-turn variant is a standalone antenna with no dish, so feed polarization equals received signal polarization.
Typical use cases
- Inmarsat STD-C and AERO message decoding (with JAERO + RTL-SDR / Airspy + L-band LNA)
- Iridium telemetry monitoring
- Mobile LEO downlink experiments (10-turn on rotor)
Required for dish mounting (3-turn)
The 3-turn variant is a bare helix on an N-type baseplate — it does not include the 40 mm clamp body needed to drop it into a TV-SAT LNB holder. To mount on a dish arm you need the Electronics Cover accessory, which encloses the LNA and provides the 40 mm clamp interface. Order both together for a complete dish-feed assembly.
Compatible chain
- L-band LNA (~+25 dB) — required since the antenna is passive
- SDR with at least 2 MHz bandwidth (RTL-SDR v3, Airspy Mini, or HackRF)
- Elevation clamp + Cross clamp for a 40 mm pipe build
- Electronics Cover — required for 3-turn dish-feed use (provides 40 mm LNB clamp body + LNA housing). Optional but recommended for the 10-turn standalone build.
- Tripod adapter for portable Manfrotto setup
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 Antenna Updates — design iteration notes
- L-Band WebSDR — public reception demo
- Design narrative — offset / prime focus / direct geometry comparison
Reviews & demos
- Tech Minds — L-Band Helix Antenna (3-turn LHCP with offset dish, YouTube)
- Tech Minds — High-Gain 10-Turn Helix For L-Band & Inmarsat (YouTube)
- RTL-SDR.com — Testing an Inmarsat L-Band Helix for Offset Satellite Dishes
Custom polarization or color combinations available on request — please use the contact form for a quote.
