NVIS, DX and Local Reception: Understanding Polarisation in HF
NVIS, DX & Local Reception: Understanding Polarisation in HF
Polarisation strongly influences HF reception and fading. Between ionospheric Faraday rotation, magneto-ionic mode splitting, and multi-hop mixing, a signal that leaves the transmitter linearly polarised may arrive as linear, elliptical, or circular — and it can change rapidly with time, frequency, and path.
Faraday Rotation — Why Polarisation Changes
As a wave traverses the magnetised ionospheric plasma, its plane of polarisation rotates. The rotation angle θ scales roughly as 1/f² and increases with electron density, magnetic field component along the path, and path length. At low HF (1.8–7 MHz) the rotation can be large enough to scramble linear polarisation on receive.
Practical takeaway: a fixed linear receive antenna may see deep periodic fades as the incoming polarisation rotates through its null.
NVIS & Polarisation
NVIS (0–500 km) uses near-vertical incidence. In mid-latitudes, the ionosphere splits energy into two characteristic magneto-ionic waves (ordinary & extraordinary) which are close to opposite circular polarisations (LHCP/RHCP). Measurements and field reports frequently show the down-coming NVIS wave to be near-circular with mode isolation ~25–35 dB over ~100 km footprints.
Handedness and hemispheres
- Northern Hemisphere: RHCP is often dominant during the NVIS “sweet spots” (early morning / late afternoon on 40 m), though geomagnetic conditions can flip the balance.
- Southern Hemisphere: Observations mirror the North with LHCP dominance in equivalent windows, but local ionospheric dynamics matter.
- Equatorial regions: High TEC gradients and scintillation lead to very rapid polarisation dynamics; both hands may swap unpredictably.
Polarisation fading in NVIS
Because the two modes have different phase velocities, their superposition produces polarisation flutter. With a single linear antenna this appears as rapid deep fades; with dual-hand circular or orthogonal linear diversity the fades are greatly reduced.
DX Paths (20 m and Above)
Faraday rotation weakens at higher frequencies, yet multi-hop DX introduces ellipticity via unsynchronised mode contributions, oblique reflection, and ground/terrain scattering. Arrival polarisation becomes a time-varying mix of linear/elliptical/circular. Polarisation mismatch can cost several dB of link budget and increase flutter.
Mitigations for DX
- Diversity: Cross-polar linears or dual-circular (RH/LH) into diversity combining (analog or SDR) minimise fades.
- Antenna orientation: For fixed linears, align broadside to the dominant arrival when possible; for beams, consider switchable polarisation.
- Signal processing: Polarisation-aware combining/MRC in SDRs can track the dominant component and reject the orthogonal one.
Summary & Takeaways
| Key factor | Enhanced view |
|---|---|
| NVIS signature | Down-coming wave typically near-circular; mode isolation ~25–35 dB |
| Fading driver | Mode superposition causes polarisation flutter; worst with single linear receive |
| Diversity solutions | Dual-circular or cross-polar linear diversity (XPD ≳ 12 dB) recommended |
| 20 m+ DX | Weaker Faraday rotation but multi-hop mixing → elliptical/dynamic arrival |
| Hemispheric effects | RHCP often dominant in NH; LHCP in SH — subject to ionospheric/geomagnetic variability |
- Below ~10 MHz, Faraday rotation is strong; expect rapid polarisation changes.
- NVIS is best received with circular or diversity to suppress flutter.
- On 20 m+, fades are smoother but diversity still helps on long-haul, low-angle paths.
- Multi-hop DX ends up quasi-random in polarisation; plan for elliptical reception.
Designing for polarisation pays off. Dual-hand (RH/LH) converters, cross-polar arrays, and diversity combiners (analog or SDR) markedly improve readability for NVIS, regional, and DX paths.
Mini-FAQ
- Do I need circular polarisation for NVIS? — It’s not mandatory, but dual-circular (RH/LH) or cross-polar diversity dramatically lowers fades.
- Why does 40 m fade so hard at times? — Strong Faraday rotation and mode beating cause polarisation flutter; linear-only receive hits deep nulls.
- Is polarisation important for 20 m DX? — Yes. Rotation is weaker, but multi-hop mixing produces elliptical/dynamic arrivals; diversity still adds SNR margin.
- Which hand (RH/LH) should I use? — NH often favors RHCP, SH often LHCP, but it varies with time, path, and geomagnetic state — use both for reliability.
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