Yagi
Understanding Yagi Antennas
The Yagi-Uda antenna is one of the most recognized antenna designs, visible on rooftops worldwide as TV reception antennas. Despite its simplicity, it provides excellent directivity and gain in a compact, lightweight structure. Its principle of operation uses parasitic coupling to create a directional radiation pattern.
How It Works
- Driven element: Half-wave dipole connected to the feedline. The only directly energized element.
- Reflector: Slightly longer than the driven element, placed behind it. Re-radiates energy forward, improving front-to-back ratio.
- Directors: Slightly shorter than the driven element, placed in front. Each director adds about 1 dB of gain and narrows the beam. More directors = higher gain.
Yagi Parameters
| Elements | Gain (dBi) | Beamwidth | F/B Ratio |
|---|---|---|---|
| 3 (D+R+1Dir) | 7-8 | 70 deg | 15 dB |
| 5 | 9-11 | 55 deg | 18 dB |
| 10 | 12-14 | 40 deg | 20 dB |
| 15+ | 14-17 | 30 deg | 22 dB |
Frequently Asked Questions
What is a Yagi antenna?
A Yagi antenna is a directional antenna using a driven dipole element, a reflector behind it, and one or more director elements in front. The parasitic elements focus radiation into a beam toward the directors, providing 7-17 dBi gain depending on the number of elements.
How many elements does a Yagi need?
As few as 3 (driven + reflector + 1 director) for about 7 dBi gain. Each additional director adds roughly 1 dB. For TV reception, 10-15 elements providing 12-15 dBi is typical. Above 15 elements, additional gain per element diminishes.
What frequency range does a Yagi cover?
A Yagi typically covers a 5-10% bandwidth centered on its design frequency. Wideband Yagis using log-periodic elements or special feeding techniques can cover wider bands. They are most commonly used from 30 MHz (HF) to 3 GHz (S-band).