Quarter Wave Antenna
Edit on GitHubThe quarter-wave whip: how it works, how long it is at 433 MHz, and why it needs a ground plane.
Quarter Wave Antenna
The quarter-wave vertical is the default antenna for handheld UHF gear, and for good reason: it is short, simple and works well against a ground plane.
The physics in one paragraph
A quarter-wavelength vertical radiates because the feed point sits at a current maximum and a voltage minimum, which presents a low, convenient impedance (about 36 ohms, close enough to 50 that no matching network is needed). The missing quarter wave is supplied by its image in the ground plane below.
Length at 433 MHz
The free-space wavelength at 433 MHz is about 692 mm. A quarter wave is 173 mm. Because of end effects (the “velocity factor” of a real wire), the practical length is 0.95 to 0.97 of the ideal:
| Band | Ideal quarter wave | Practical whip |
|---|---|---|
| 433.05 - 434.79 MHz | 173 mm | 165 - 168 mm |
| 446 MHz (PMR446) | 168 mm | 160 - 163 mm |
The ground plane requirement
A quarter-wave whip without a ground plane has no reference half: the current has nowhere to return, the pattern collapses and the SWR climbs. The ground plane can be:
- The metal body of a case
- Four radial wires of the same length, spread at 45 degrees
- A metal plate under the antenna
The beacon’s standard case includes a small ground plane under the antenna mount for exactly this reason.
Tuning
Cut the whip a few millimeters long, then trim while watching the SWR (see SWR Explained). At 433 MHz, 1 mm is about 2 MHz of resonance shift, so cut conservatively.
Related pages
- Antenna Design for the general theory
- Antenna Lengths for the full table
- Ground Planes and Counterpoises for the reference half