Half Wave Dipole
Edit on GitHubThe half-wave dipole for the beacon: balanced feed, no ground plane needed, and when it beats the quarter-wave whip.
Half Wave Dipole
A dipole is two quarter-wave elements fed in the middle. It is the reference antenna of radio engineering, and it has one property the beacon builder should know about: it needs no ground plane.
Why no ground plane
The quarter-wave vertical relies on the ground plane for its missing half. The dipole contains both halves in its own two wires, so it is a complete, balanced antenna on its own. This makes it attractive when the beacon sits on a backpack or a rock with nothing metallic under it.
Length at 433 MHz
Two quarter-wave elements of 165 - 168 mm each (see Quarter Wave Antenna) give a total length of about 33 cm tip to tip. The feed impedance at the center is about 73 ohms, close enough to 50 that a simple 1:1 balun or even a direct feed works acceptably for field use.
Pattern and orientation
A dipole radiates broadside to its axis and has nulls off the ends. For a beacon on a backpack, hang the dipole vertically: the pattern then covers the horizon in all directions, which is exactly what a search receiver needs.
When to choose it over the whip
| Situation | Better choice |
|---|---|
| Metal case under antenna | Quarter-wave whip |
| No metal nearby (backpack, snow) | Dipole |
| Hanging from a pole or tree | Dipole |
| Handheld, in a pocket | Quarter-wave whip |
The folded variant
A folded dipole (the two halves joined at the ends with a third wire) raises the feed impedance and is mechanically tougher. It is slightly wider band and is a good choice for a rugged field antenna, at the cost of a bit more wire to break.
Related pages
- Antenna Design for theory
- Antenna Selection for the comparison
- Homebrew Antenna for building one