Debunking the Height Myth: Why a 20M Yagi May Not Be Your Best Choice
Many amateur radio operators believe that a high-mounted Yagi antenna is the ultimate DX solution. However, the real-world performance of a Yagi depends on more than just height. Factors like takeoff angle, nearby obstructions, and terrain effects can drastically alter its effectiveness. In many cases, a simple vertical antenna may outperform a Yagi, especially when considering cost and practical constraints. This article analyzes the optimal height for a 20M Yagi, how obstructions impact performance, and why a vertical may often be the better choice.
1. Height Above Ground and DX Radiation Angles
The height of a Yagi above ground directly affects its radiation pattern and takeoff angle. The ideal height for maximizing DX performance depends on achieving the correct elevation angle for short-haul and long-haul propagation.
| Height (m) | Height (wavelengths) | Main Lobe Takeoff Angle |
|---|---|---|
| 5m | 0.25λ | 30°-40° |
| 10m | 0.5λ | 20°-25° |
| 15m | 0.75λ | 15°-20° |
| 20m | 1.0λ | 10°-15° |
| 25m | 1.25λ | 8°-12° |
- Lower Heights (5m-10m): Good for regional contacts (NVIS effect), less ideal for long-haul DX.
- Medium Heights (15m-20m): Best balance between DX and local contacts.
- Higher Heights (20m-25m+): Low takeoff angle, excellent for long-haul DX.
Key Takeaway: A 15m mast places the Yagi at 0.75λ, resulting in a good DX takeoff angle (15°-20°). However, the effective height depends on the environment.
2. Impact of Nearby Objects on Takeoff Angle
Even if your mast is 15m high, obstructions such as houses and trees affect performance. Large objects within 1-2 wavelengths (20-40m) can cause signal diffraction, pattern distortion, and unwanted reflections.
Minimum Clearance from Objects
| Obstruction Type | Minimum Distance (λ) | Minimum Distance (m) |
| Small trees | 0.5λ | 10m |
| Large trees | 1λ | 20m |
| Buildings | 1.5λ | 30m |
| Metal structures | 2λ | 40m |
- Metallic Objects: Reflect signals and create pattern nulls.
- Houses and Trees: Absorb and scatter RF energy, increasing ground loss.
- Solution: Keep the antenna clear of obstacles by at least one wavelength (20m for 20M Yagi).
Key Takeaway: A 15m mast does not guarantee a clear takeoff. If a house or tree is within 20m, radiation angles and efficiency may degrade.
3. Terrain and Ground Effects
- Flat Open Ground: Provides the best performance, minimizing ground loss.
- Hilly Terrain: Affects low-angle radiation. A Yagi positioned on a hilltop benefits from natural elevation.
- Water or Wetlands: Enhances low-angle radiation due to improved conductivity.
Example: Hilltop vs. Urban Setting
| Location | Effective Takeoff Angle |
| Hilltop (20m) | 8°-12° |
| Suburban (15m) | 15°-20° |
| Urban (10m) | 20°-30° |
Key Takeaway: Even a high-mounted Yagi in an urban area can suffer from higher takeoff angles due to reflection and absorption.
4. Why a Vertical Antenna May Be the Better Choice
A properly installed quarter-wave vertical (with a good radial system) often achieves lower takeoff angles than a Yagi at an impractical height.
Comparing a 20M Yagi vs. a Vertical
| Antenna Type | Typical Takeoff Angle | Performance Considerations |
| 20M Yagi at 15m | 15°-20° | Requires a tall mast, prone to obstructions |
| 20M Yagi at 10m | 20°-25° | Higher takeoff angle, less effective for DX |
| 20M Vertical | 10°-15° | Lower cost, low-angle DX, requires ground radials (unless it is a half-wave vertical, which does not require radials) |
Advantages of a Vertical Antenna
- Low Takeoff Angle: Consistently radiates at 10°-15°, perfect for long-haul DX.
- Less Affected by Obstructions: Nearby trees and buildings have less impact.
- Lower Cost: A Yagi requires a tall tower and rotator; a vertical is much cheaper.
- Smaller Footprint: Ideal for small spaces where a large tower isn’t feasible.
Key Takeaway: Unless you can install a Yagi at 20m+ height with clear surroundings, a vertical antenna may provide better DX results at a lower cost.
5. Conclusions and Recommendations
- A 15m mast is NOT equivalent to 15m effective height. Nearby obstructions and ground effects must be considered.
- For best DX results, aim for 1λ (20m height) if possible. If limited, keep at least 1λ distance from large objects.
- Vertical antennas often outperform Yagis at typical installation heights. Their lower takeoff angle makes them more efficient for DX.
- Cost-effectiveness matters. A Yagi on a small mast may not justify the expense compared to a well-installed vertical.
Final Thought: Before investing in a Yagi, consider whether a vertical might be a more practical and cost-effective solution for your DX needs.
Written by Joeri Van Dooren, ON6URE – RF, electronics and software engineer, complex platform and antenna designer. Founder of RF.Guru. An expert in active and passive antennas, high-power RF transformers, and custom RF solutions, he has also engineered telecom and broadcast hardware, including set-top boxes, transcoders, and E1/T1 switchboards. His expertise spans high-power RF, embedded systems, digital signal processing, and complex software platforms, driving innovation in both amateur and professional communications industries.