Helical Antenna
Edit on GitHubThe loaded helical antenna: how a coil of wire makes a short antenna resonant, and its trade-offs for the beacon.
Helical Antenna
A full-size quarter wave at 433 MHz is 17 cm. A helical (coiled) antenna squeezes the same electrical length into a few centimeters of rubber-coated coil. That is why your key fob and many small transmitters work with such short antennas.
How loading works
An antenna does not need to be physically resonant if it is loaded. Winding the wire into a coil adds inductance along its length, which slows the current wave and makes the structure electrically longer than it is physically. The coil acts as a “loading coil” distributed along the radiator.
The trade-offs
| Property | Helical vs straight whip |
|---|---|
| Physical length | 3 - 8 cm vs 17 cm |
| Efficiency | Lower (coil losses, narrow band) |
| Bandwidth | Narrower |
| Durability | Better in a pocket |
| Radiation pattern | More omnidirectional, less vertical gain |
A typical small helical at 433 MHz achieves 50 - 70% efficiency versus a good whip. That is 1.5 - 3 dB of loss: noticeable on the link budget but often acceptable for a beacon that is heard at close-to-medium range.
Choosing one for the beacon
Use a helical when the beacon must fit in a pocket or survive abuse. Use a whip when range is the priority. The beacon’s case design accommodates both via the standard SMA mount, so the antenna is a field-swappable decision, not a build-time one.
Testing a helical
Helicals are narrow band: check the SWR across the whole 433 MHz band, not just the center. A helical tuned for 433.92 MHz may show poor SWR at 433.05. The Antenna Testing and Tuning page covers the procedure.
Related pages
- Antenna Design for the general theory
- Antenna Selection for comparing options
- SWR Explained for the measurement