NVIS, DX and Local Reception: Understanding Polarisation in HF

I would not choose an HF receive polarisation from a hemisphere slogan or a band label. First ask which path is carrying the signal. A nearby station received by direct or ground wave, an F-layer NVIS circuit and a multihop DX signal can reach the same antenna through completely different field geometries.

A transmitted linear wave can arrive linear, elliptical or nearly circular. Its state can also change during a contact. That does not make polarisation random by definition: it means the path supports several propagation mechanisms and each has its own amplitude, phase, delay, Doppler and angle of arrival.

Local, NVIS and DX Are Different Path Questions

Local reception may be dominated by direct wave, a surface-related ground wave, diffraction or reflections from buildings and terrain. If ionospheric energy is negligible, the transmitting antenna, receiving antenna and local environment dominate the polarisation. ITU-R P.368 treats ground-wave field strength separately from ionospheric sky wave for exactly this reason. Calling every short HF path “NVIS” hides the mechanism we need to diagnose.

NVIS launches useful energy at steep elevation angles so ionospheric refraction returns it at comparatively short range. The ionosphere is not a polished mirror. Its electron density and geomagnetic field vary continuously with height, position and time, bending the ray until it returns or passes through. A virtual reflection height is a useful prediction device, not a physical reflecting surface.

DX normally uses lower-angle sky-wave modes and may involve several ionospheric hops plus ground or sea interactions between them. It can also include multiple layers, ordinary and extraordinary modes, lateral or equatorial scattering and several simultaneous arrival angles. “Twenty metres and above” is not a physical polarisation boundary. Frequency matters through refraction, absorption, gyrofrequency, critical frequencies and the path’s operating MUF, but the transition is not tied to one amateur band.

The Ionosphere Supports Two Characteristic Modes

In a cold magnetised plasma, the Appleton-Hartree relation gives two characteristic magneto-ionic solutions. Radio engineers call them the ordinary and extraordinary modes. They can have different refractive index, absorption, group delay, Doppler and limiting frequency, so they may follow related but non-identical paths.

The characteristic polarisations are generally elliptical. In the quasi-longitudinal case—propagation sufficiently close to the geomagnetic-field direction—they can approach opposite circular senses. Near-vertical mid-latitude paths often make that approximation useful, but elevation angle alone does not guarantee perfect circularity. The angle to the geomagnetic field, frequency, electron density, collisions and ionospheric gradients all matter.

This is why a fixed claim such as “RHCP dominates in the Northern Hemisphere” is not an operating rule. A mid-latitude Northern Hemisphere experiment may identify a downward ordinary wave as one hand and the extraordinary wave as the other under its declared convention. That result does not prove that the same hand will be stronger on another frequency, path, hour, latitude or antenna.

Faraday Rotation Is Differential Phase, Not a Mirror Trick

A linearly polarised field can be resolved into two oppositely rotating components. In a magnetised plasma those components propagate with different phase constants. Their differential phase rotates the orientation of the recombined linear field: this is Faraday rotation.

The familiar inverse-square frequency dependence is useful for a wave passing through an ionosphere that is effectively transparent, where rotation can be related to integrated electron content and the geomagnetic-field component along the path. ITU-R P.531 uses that regime for Earth-space paths above the relevant critical frequency. A refracted HF sky-wave circuit near a turning region is not described completely by that shortcut. The ray path, Appleton-Hartree modes, collisions, absorption and spatial electron-density profile must also be considered.

Faraday rotation can turn a linear wave relative to a fixed linear antenna and therefore cause polarisation mismatch. It is not the only fading mechanism. Different ordinary and extraordinary paths, several hop orders, ground reflections, ionospheric irregularities, Doppler spread and changing arrival angles can produce time- and frequency-selective fading. Those mechanisms may also depolarise an observed field: no single stable polarisation state then describes every component in the receive bandwidth.

NVIS Has No Fixed Circularity, Isolation or Footprint

Careful NVIS experiments show why circular reception is worth investigating. Witvliet and colleagues separated the characteristic modes with a dual-circular antenna on a declared mid-latitude path. One report demonstrated at least 13 dB isolation; subsequent work with a specific 0.5 by 0.5 wavelength antenna reported more than 25 dB separation and an 8–11 dB fading-margin reduction over a 110 km test path.

Those are measured results for those antennas, calibrations, frequencies, sites and intervals—not a universal 25–35 dB mode isolation or a 100 km coverage law. Another 5.4 MHz, 95 km experiment found time-varying differences in mode availability and evaluated selection and equal-gain combining. Its outcome is evidence that both modes can be useful, not proof that one mode or one circular hand always wins.

The NVIS service area follows the launched elevation-angle distribution, usable ionospheric layer and height, frequency relative to the path MUF, absorption, terrain and required SNR. A high-angle antenna can fill the short-range skip zone when the ionosphere supports the frequency, but it does not draw a fixed-radius circle on the ground.

Multihop DX Mixes More Than Polarisation

On a long path, each supported mode accumulates its own phase and delay. Ground or sea interactions between hops treat field components differently, and the next ionospheric encounter can launch a new mixture of characteristic modes. Several paths may arrive together from different azimuths and elevations.

The result can be a changing linear, elliptical or near-circular field, but “multihop becomes random” is too strong. A stable dominant mode may exist for part of the circuit, and a beam’s pattern, the ground below it and the receive environment can bias what is observed. Measure the actual path rather than assuming a universal 20 m threshold or a guaranteed several-decibel mismatch.

For a nearby direct path, start with the transmitted and received antenna orientations. For NVIS, examine high-angle O/X-mode support and absorption. For DX, add hop structure, arrival angles, ground interactions and Doppler. That path-first sequence is more useful than choosing horizontal, vertical or circular from the contact distance alone.

Orthogonal Antennas Do Not Automatically Create Diversity

Two nominally orthogonal antenna ports can be valuable because together they sample more of the incident electric field. Yet orthogonal geometry at the drawing board does not guarantee low correlation at the receiver. Pattern overlap, imperfect axial ratio or cross-polar discrimination, mutual coupling, feedline common mode, local noise and unequal receiver channels can make the two branches strongly correlated or simply make one branch worse.

Diversity gain appears when the second branch contains useful signal during fades of the first and when its added noise and interference do not cancel that advantage. Selection combining chooses the branch with the better declared metric. Equal-gain combining needs phase alignment and can add a noisy branch. Maximal-ratio combining requires channel estimates and weights each calibrated branch according to its signal and noise. None of those methods guarantees a benefit on every path or at every instant.

A Bounded Polarisation-Diversity Test

My practical conclusion is not “always use circular” or “diversity always wins.” It is this: HF polarisation changes because the complete path changes. Two calibrated, sufficiently independent field samples give a receiver more evidence to work with. Whether that produces one decibel or ten—or nothing useful at all—is a result to measure.

Primary Technical References

• ITU-R P.533: Method for the Prediction of the Performance of HF Circuits
• ITU-R P.531: Ionospheric Propagation Effects, Including Faraday Rotation
• ITU-R P.368: Ground-Wave Propagation from 10 kHz to 30 MHz
• Witvliet et al.: Circular Polarisation for Diversity Reception and MIMO in NVIS
• Witvliet et al.: Characteristic-Wave Diversity in NVIS Propagation
• Witvliet et al.: Probes for Magneto-Ionic NVIS Propagation
• Ordinary and Extraordinary Ionospheric Modes for NVIS Digital Channels

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