Understanding Optimal NVIS Receive Angles
Near-vertical incidence skywave (NVIS) is a key propagation mode for regional HF communication on bands like 160m, 80m, 60m, and 40m. While often simplified as "straight-up" propagation, real-world measurements and modeling show that optimal receive angles for NVIS are not at 90° but fall within a well-defined elevation range. This article provides practical insights into the actual arrival angles of NVIS signals and their implications for antenna design.
The Misconception of 90° NVIS
It's commonly assumed that NVIS signals arrive vertically from overhead, leading many to optimize antennas for 90°. However, ionospheric reflection geometry, the curvature of the Earth, and the typical F-layer height cause signals to return to Earth at angles significantly lower than vertical. Empirical data and ionospheric modeling agree: most NVIS signal energy arrives between 50° and 75° elevation.
Typical NVIS Arrival Angles by Band
| Band | Typical Arrival Angle (Peak) | Common Range |
|---|---|---|
| 160m | 65° to 80° | 60° to 85° |
| 80m | 60° to 75° | 55° to 85° |
| 60m | 50° to 70° | 45° to 80° |
| 40m | 45° to 65° | 40° to 75° |
Implications for Antenna Design
Optimal Elevation Coverage
Antenna patterns should be designed to have strong radiation and reception in the 50° to 75° elevation range. Optimizing for only 90° leads to potential nulls or weaknesses in the actual angle range where most NVIS signals are found.
Recommended Antenna Heights
Height above ground affects the elevation lobe of an antenna. For effective NVIS coverage:
| Band | Recommended Height for Peak NVIS Reception |
|---|---|
| 160m | 10 to 15 m (approx. 0.06λ to 0.09λ) |
| 80m | 7 to 10 m (approx. 0.1λ to 0.125λ) |
| 60m | 6 to 8 m |
| 40m | 4 to 6 m |
Antennas at these heights produce broad lobes that peak in the ideal NVIS range without creating deep nulls at lower or higher elevations.
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. (Optimal Mast Height for NVIS Transmission Antennas)
Effective Antenna Types
For reception-focused NVIS performance, the following antenna types are particularly well-suited:
- Inverted V dipole at low height (0.1λ to 0.25λ)
- Horizontal or sloping doublet
- Delta loops (especially point-down)
- Compact non-resonant loops
These designs offer good elevation coverage and efficiency within the desired range.
Polarization Considerations
NVIS signals can become elliptically or circularly polarized due to the ionospheric path. In some cases, mismatches between transmitted and received polarization can cause reception losses up to 20 dB. Using antennas with circular or mixed polarization response, such as crossed dipoles or active field probes, can help mitigate this.
In the northern hemisphere, NVIS signals tend to be predominantly right-hand circularly polarized, while in the southern hemisphere, left-hand circular polarization is more common. This polarity effect is influenced by the Earth's magnetic field and ionospheric behavior.
Conclusion
Optimal NVIS reception occurs at elevation angles between 50° and 75°. Antenna designs that focus exclusively on 90° radiation miss the bulk of the usable energy. By adjusting height and antenna type to favor this practical range, one can greatly enhance the reliability and clarity of NVIS communications.
Interested in more technical content like this? Subscribe to our notification list — we only send updates when new articles or blogs are published: https://listmonk.rf.guru/subscription/form
Questions or experiences to share? Feel free to contact RF.Guru or join our feedback group!
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.