Optimal Mast Height for NVIS Transmission Antennas
NVIS (Near Vertical Incidence Skywave) communication is essential for regional HF coverage, especially in mountainous terrain or where line-of-sight paths are blocked. To optimize NVIS propagation, the antenna must radiate at high elevation angles—typically above 60 degrees. A crucial factor in achieving this is the height of the antenna above ground.
Why 0.18 to 0.22 λ?
For NVIS antennas, particularly horizontal dipoles or inverted-Vs, the optimal height is relatively low: between 0.18 and 0.22 times the wavelength (λ). This range provides a strong upward radiation lobe and minimizes low-angle radiation, which is less useful for short-range communication.
Placing the antenna too low (below 0.15λ) increases ground losses and reduces efficiency, while too high (above 0.25λ) results in flatter radiation angles, which undermines the NVIS effect.
Recommended Heights per Band
The table below shows the recommended mast height range for typical NVIS setups for the most commonly used HF bands.
| Band | Frequency (MHz) | Wavelength (λ) | Optimal Height Range (meters) |
|---|---|---|---|
| 30m | 10.1 | 29.7 m | 5.3 m – 6.5 m |
| 40m | 7.1 | 42.3 m | 7.6 m – 9.3 m |
| 60m | 5.3 | 56.6 m | 10.2 m – 12.5 m |
| 80m | 3.6 | 83.3 m | 15.0 m – 18.3 m |
| 160m | 1.9 | 157.9 m | 28.4 m – 34.7 m |
These values represent the feedpoint height above ground. Inverted-V antennas can have sloping ends, but the feedpoint should still conform to the ideal height range to ensure effective NVIS radiation.
This again demonstrates that reciprocity does not hold in practice for NVIS scenarios—contrary to claims in traditional sources like Rothammel and the ARRL Handbook. Receive performance is optimized at lower heights, while optimal transmit heights are significantly higher. (Understanding Optimal NVIS Receive Angles)
Practical Tips
- Trees or portable masts can be used to achieve suitable heights.
- Ground conductivity plays a role: in poor soil, slightly higher placement may help.
- For multi-band NVIS, a fan dipole or doublet at the 60m optimal height (~11m) is a practical compromise.
- Avoid high dipole placements (e.g., >0.3λ) if your goal is short-range communication.
Proper NVIS setup improves SNR on local QSOs and is ideal for emergency, military, and net operations where consistent regional coverage is key.
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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.