(Draft) NVIS, DX, and Local Reception: Understanding Polarisation in HF

Polarisation plays a critical role in HF (High Frequency) radio reception, often making the difference between intelligible signals and complete fades. This article dives into the various aspects of polarisation in HF, including the impact of Faraday rotation, NVIS (Near Vertical Incidence Skywave) propagation, long vs short hop behaviour, and how different modes such as DX and regional NVIS are affected differently.

Faraday Rotation: The Root of Polarisation Change

Faraday rotation occurs when a linearly polarised electromagnetic wave travels through the ionosphere, which contains a magnetised plasma. The plane of polarisation rotates progressively with distance, and the angle of rotation (theta) can be described by:

θ = (2.36 × 10^4 / f²) × ∫₀ᴸ Ne × B × cos(α) dl

Where:

  • f = frequency in Hz
  • Ne = electron density (electrons/m³)
  • B = magnetic field strength (Tesla)
  • α = angle between propagation direction and magnetic field vector
  • L = path length through the ionosphere

This rotation is most pronounced at lower frequencies (e.g. 1.8–7 MHz) and can lead to complete polarisation scrambling on receive.

Implication: Signals that start as vertically or horizontally polarised may arrive as any mix of linear, elliptical, or even circular polarisation.

NVIS and Polarisation Effects

NVIS is used for short-range HF communication (0–500 km) by directing signals nearly vertically into the ionosphere, where they are refracted back down.

Dominant Polarisation in NVIS

In mid-latitude Northern Hemisphere locations during the NVIS “sweet spot” (typically early morning or late afternoon on 40m), received signals often show a dominance of RHCP (Right-Hand Circular Polarisation). This is due to:

  • The geomagnetic field configuration causing preferential absorption or delay of LHCP (Left-Hand) waves
  • Mode splitting in the ionosphere: the ordinary and extraordinary waves correspond roughly to LH and RH polarisation
  • Absorption being higher for the ordinary (LH) wave, especially in the D-layer at low angles

In the Southern Hemisphere, the situation is similar but mirrored due to the magnetic field's reversed inclination. As a result, LHCP dominance may occur in equivalent latitudes and conditions. However, this can vary depending on geomagnetic activity and local ionospheric dynamics.

Characteristic Waves and Circular Polarization

Mid-latitude NVIS experiments consistently observe near-perfect circular polarization of downward-propagating waves, confirming strong separation between ordinary and extraordinary mode contributions. These characteristic waves maintain circular polarization over distances of approximately 100 km. The isolation often exceeds 25–35 dB.

Implication for NVIS coverage:

  • At distances up to approximately 200 km, the received polarization is reliably circular, with RHCP often dominating in the Northern Hemisphere during the NVIS sweet spots.
  • Strong deep fades are mainly seen with mismatched linear or cross-hand antennas.

Polarisation Fading in NVIS

The superposition of the ordinary and extraordinary modes, which travel at different phase velocities, leads to differential phase delay and interference, causing rapid polarisation changes. This is observed as polarisation fading, particularly when receiving with a single linear antenna. In extreme cases, the signal may rotate through all polarisations in less than a second.

Mitigation

  • Polarisation diversity (e.g., dual circularly-polarised antennas) statistically reduces fades by receiving both characteristic waves.
  • HF-MIMO or SIMO architectures improve performance via decoupled channels, with XPD values exceeding 12 dB.

Role of Antennas and Systems

Antenna design is critical:

  • Dual-circular (RHCP + LHCP) or cross-polar linear arrays match ionospheric modes and are proven effective.
  • Diversity combiner hardware (analog or digital) is necessary to harness both polarisation channels—especially for NVIS reliability.

Faraday Rotation in NVIS Context

Though Faraday rotation rotates plane-polarised waves, in NVIS where circular polarization dominates, its impact is mostly on the relative amplitude and phase of ordinary versus extraordinary modes. This causes polarisation flutter rather than full scrambling. With linear-only reception, this can lead to deep fades as the polarization plane rotates through nulls.

Regional and Geomagnetic Variation

  • Northern Hemisphere: RHCP dominance is typical due to the configuration of the Earth's magnetic field. This applies particularly during early morning and late afternoon, when the incidence angles and D-layer absorption favor ordinary wave suppression.
  • Southern Hemisphere: The situation is essentially mirrored. The inclination of the geomagnetic field favors LHCP dominance in similar NVIS time windows (morning/evening), but reversed handedness compared to the Northern Hemisphere. Field observations in South America, South Africa, and Australia confirm this behavior. However, exact effects vary with geomagnetic latitude, solar cycle, and ionospheric activity.
  • Equatorial Regions: Near the geomagnetic equator, the magnetic field is primarily horizontal, producing a unique set of conditions. This region—known as the Equatorial Anomaly Zone—features high electron densities, irregular TEC distributions, and strong ionospheric scintillation. Faraday rotation rates can be extremely high, sometimes exceeding 10° per kilometer, leading to very dynamic and unpredictable polarisation states. Mode separation may still exist, but transitions between RHCP and LHCP can occur rapidly within minutes. NVIS behavior here is far less predictable.
  • Middle Latitudes: Between the equatorial and auroral zones, the ionospheric behavior is typically more stable. These zones experience classic NVIS conditions with predictable RHCP/LHCP dominance depending on hemisphere and diurnal cycle. This is often the most favorable region for consistent NVIS communication.
  • Polar Regions: Polar cap absorption (PCA) events during solar storms heavily affect propagation. The ordinary wave can be completely suppressed for hours, and only the extraordinary mode may reach the ground. Polarisation becomes strongly skewed and unpredictable. Circular and elliptical components dominate, but fading can be deep and prolonged.

Polarisation Behaviour on 20m and DX Paths

On higher frequencies like 20m (14 MHz) and up to 30 MHz, Faraday rotation is less severe (as θ ∝ 1/f²), but polarisation changes still occur, especially on long-haul DX paths. Multiple ionospheric hops cause complex polarisation mixing:

  • Ellipticity increases with each hop due to unsynchronized ordinary/extraordinary mode contributions
  • Arrival polarisation may be partially circular, elliptical, or dynamically rotating

Polarisation Shifts in DX Signals

DX signals are particularly affected by dynamic polarisation shifts due to the cumulative effects of multiple propagation phenomena:

  • Reflections and Scattering: Radio waves reflecting off the ionosphere, ground, or large objects can change their polarisation state. These changes are especially pronounced at non-normal angles of incidence.
  • Multipath Propagation: Signals often take multiple paths to the receiver, each with different lengths and conditions. The result is a combination of wavefronts arriving with various polarisation states and relative phases, causing constructive and destructive interference.
  • Ionospheric Effects: While less severe above 10 MHz, the ionosphere still induces Faraday rotation and can differentially affect the ordinary and extraordinary modes, particularly during disturbed conditions or near dawn/dusk transitions.
  • Terrain and Obstacles: Natural and man-made terrain features cause scattering and localised depolarisation, particularly for lower take-off angles.
  • Antenna Orientation: Misalignment between the transmitting and receiving antenna polarisations reduces signal coupling efficiency, especially with narrow-beam or polarisation-specific antennas.

Impact of Polarisation Shifts

  • Signal Fading: As the polarisation angle of the incoming signal rotates, the alignment with the receiving antenna may periodically reach nulls, causing severe fading.
  • Signal Distortion: Phase-shifted multi-path and mismatched polarisation can reduce signal integrity, increasing bit error rates or lowering SNR in analog reception.
  • Reduced Link Efficiency: A mismatched polarisation can attenuate received power by several dB, effectively degrading link budget and reducing the usable distance or reliability.

Compensating for Polarisation Shifts

  • Diversity Reception: Using multiple antennas with orthogonal or circular polarisations (e.g. LHCP and RHCP) increases the probability of coherent signal capture.
  • Adaptive Antennas: Some systems can dynamically adjust their polarisation response to match the incoming wave. While uncommon in amateur setups, this principle is used in radar, satellite, and advanced digital communication systems.
  • Signal Processing Techniques: Polarisation diversity combiners and digital filtering can extract the dominant signal component and reject mismatched or interfering polarised contributions.
  • Understanding Propagation Conditions: Predictive awareness of propagation paths, especially at sunrise/sunset or during geomagnetic disturbances, allows operators to choose better alignment and antenna configurations.

Summary and Takeaways

Key Factor Enhanced View
Signature Polarization Near-circular downward NVIS waves, high mode isolation (~25–35 dB)
Polarisation Fading Rapid flutter from mode superposition, mitigable by diversity techniques
Diversity Solutions Dual‑circular or cross‑polar systems with XPD > 12 dB recommended
Antenna Architecture Supports RHCP/LHCP reception and diversity combining
Environmental Variables Latitudinal/hemispheric differences; use diversity in disturbed conditions
  • Faraday rotation causes dynamic polarisation changes across all HF bands, especially below 10 MHz
  • NVIS is prone to strong polarisation fading; RHCP tends to dominate in Northern Hemisphere morning/evening NVIS
  • On 20m and above, fading is smoother but still benefits from polarisation-aware reception
  • Multi-hop paths result in near-random final polarisation — elliptical reception is typical
  • Diversity and dual-polarisation strategies improve intelligibility and reduce fading loss
  • Understanding magneto-ionic effects is critical to antenna design, especially for military, emergency, and high-reliability communications

This knowledge is especially useful for those designing receive arrays, building RH/LH converters, or implementing diversity systems. Innovations such as our PolarFLIP, dynamic phase combining, and cross-polar SDR decoding can significantly enhance signal robustness in all HF scenarios.

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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.

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.