Antenna Theory Primer
Edit on GitHubHow an antenna works, why length matters, and the quarter-wave rule that gives the 17.3 cm wire.
Antenna Theory Primer
The antenna is the most overlooked part of any radio. A great radio with a bad antenna is a bad radio; this page explains why.
What an antenna does
An antenna converts the electrical signal from the radio into an electromagnetic wave in space, and vice versa. It is a resonator: its physical length determines the frequency it is tuned to.
The quarter-wave rule
A simple wire antenna works best when its length is a quarter of the wavelength. At 433 MHz:
wavelength = 300 / 433 ≈ 69.3 cm
quarter-wave = 69.3 / 4 ≈ 17.3 cmThat is exactly the 17.3 cm wire in the BOM. The ground plane (or the device body) acts as the other half of the antenna.
What happens if the length is wrong
- Too short: the antenna looks capacitive, SWR rises, less power radiates.
- Too long: looks inductive, same result.
- A few millimeters off is fine at 433 MHz (the bandwidth is a few MHz); an inch off starts to hurt.
Antenna types for the beacon
| Type | When |
|---|---|
| Quarter-wave wire (17.3 cm) | Minimum build, inside the case |
| Quarter-wave whip (SMA) | Better outside the case, more efficient |
| Half-wave dipole | Best portable option, needs a balun |
| Ground plane with radials | Fixed installation, +3 dBi |
The ground plane
A quarter-wave antenna needs a counterpoise: the metal of the case, a ground plane, or radials. Without one, the antenna works poorly because the “other half” is missing. See the ground planes page for the details.
Antenna placement beats power
Doubling the power gives +3 dB. Raising the antenna from waist height to shoulder height, or moving it out of the shadow of a ridge, can easily give +10-20 dB of effective gain. Placement is the cheapest range upgrade there is.
Measuring success
Range is the honest test. The antenna testing page covers SWR and the 17.3 cm wire rule with real measurements.