Understanding Polarisation – Why It Matters for Your Antennas

When we talk about antennas, we often think about frequency, length, or height. But there’s another key aspect that governs how signals travel and how antennas “see” each other — polarisation.

What Is Polarisation?

Polarisation describes the orientation of a radio wave’s electric field (E-field). In free space, the E-field, magnetic field (H-field), and direction of travel are all at right angles — exactly as predicted by Maxwell’s equations. Most radio signals are linearly polarised, meaning the E-field stays fixed either vertically or horizontally while the wave propagates.

There’s also circular polarisation, where the E-field rotates like a corkscrew as the wave moves forward. It can rotate right-hand (RHCP) or left-hand (LHCP) depending on the sense of rotation. Many real signals are actually elliptically polarised — a mix between linear and circular — and we measure this using the axial ratio: a perfect circle has AR = 1, while nearly linear waves have AR ≫ 1.

Engineers quantify polarisation using Jones vectors for fully polarised signals or Stokes parameters (I, Q, U, V) for partially polarised waves — a framework that links directly to power, orientation, and circular sense.

Linear vs Circular

• Linear polarisation is simple and efficient for most HF use. Vertical antennas radiate vertical polarisation; horizontal dipoles radiate horizontal. For ground-wave and low-angle paths, vertical works best because horizontal fields are heavily absorbed by soil.
• Circular polarisation is common at VHF, UHF, and microwave frequencies where the antenna or platform orientation may change — think satellites, drones, or aircraft. On HF, circular or elliptical reception is used in NVIS (Near-Vertical Incidence Skywave) to reduce fading caused by ionospheric rotation.

Why Does Polarisation Matter?

• Coupling and mismatch loss: Two linearly polarised antennas misaligned by Δψ lose power according to cos²(Δψ). A 45° offset costs 3 dB; 90° ideally gives zero coupling. A linear antenna viewing a circular wave averages a 3 dB penalty. For perfect right-hand vs left-hand CP, coupling ideally vanishes.
• Fading (QSB): On HF, signals often reach you by multiple ionospheric paths. Each may carry a different polarisation and phase, so when they combine, they can cancel or reinforce — creating deep, time-varying fades.
• Faraday rotation: The ionosphere acts like a magnetised plasma and splits the wave into ordinary and extraordinary modes. The linear polarisation rotates by an angle roughly proportional to λ² and the magnetic-field–electron-density integral along the path. That’s why HF polarisation can swing dramatically during a single contact.
• Reflections flip handedness: Circular waves reverse their sense (RHCP ↔ LHCP) when reflecting off a good conductor at near-normal incidence, further randomising what reaches your receiver.
• Diversity: Using two antennas with different polarisations — e.g., vertical + horizontal or RHCP + LHCP — allows a receiver to pick whichever signal is stronger. This polarisation diversity stabilises reception and reduces fades.

NVIS & DX — Polarisation in Real-World HF Paths

NVIS (Near-Vertical Incidence Skywave)

NVIS propagation dominates short-range HF links — typically 0–500 km — by sending energy nearly straight upward and letting the ionosphere return it back down. In mid-latitudes, this ionospheric reflection isn’t a simple mirror: the magnetised plasma splits the wave into two characteristic magneto-ionic modes — the ordinary (O) and extraordinary (X) components. These are close to opposite circular polarisations (LHCP and RHCP) and propagate at slightly different phase velocities.

Field measurements and ionospheric sounder data (from the US Army CECOM, DLR, and European NVIS research networks) consistently show the returning NVIS wave to be near-circularly polarised, with mode isolation of about 25–35 dB across footprints of roughly 100 km. That means one circular hand usually dominates — but both are present and continually interact.

Handedness and Hemispheres

• Northern Hemisphere: RHCP often dominates during NVIS “sweet spots” — early morning and late afternoon on 40 m — although geomagnetic changes can shift or even invert the dominance.
• Southern Hemisphere: LHCP generally prevails in equivalent local-time windows, but local ionospheric gradients and K-index swings can upset the balance.
• Equatorial zones: High TEC gradients and scintillation cause fast, randomised polarisation shifts; both hands can swap unpredictably within seconds.

Polarisation Fading and Receive Diversity

Because the O and X modes travel at slightly different speeds and refract differently, their recombination at ground level creates polarisation flutter — a rapid, deep-fade phenomenon seen as QSB on a linear antenna. A single-polarisation receiver can lose 10–20 dB during those swings. By contrast, dual-hand circular or orthogonal linear diversity systems capture both modes, allowing the receiver to select or combine the stronger hand in real time. This approach dramatically reduces fading and stabilises SNR.

For NVIS, polarisation diversity often improves SNR more effectively than added gain — it mitigates ionospheric mode rotation rather than fighting it.

DX (Long-Haul Ionospheric Paths)

DX signals, which travel thousands of kilometres via multiple oblique refractions, experience even more depolarisation. After several reflections and traversals of differing ionospheric layers, the polarisation becomes partially random. The received field typically shows a mix of elliptical components whose orientation changes with time, frequency, and solar conditions.

For DX work, chasing “perfect polarisation alignment” offers little benefit — pattern, noise floor, and take-off angle dominate performance. The smart approach is to ensure clean feedline balance, adequate choking, and quiet surroundings; the ionosphere will take care of the rest.

TX vs RX – Should You Care?

Transmitters: Stick with linear polarisation suited to your goal — vertical for low-angle paths, horizontal for NVIS. You’ll radiate efficiently with minimal mismatch loss.

Receivers:

• Line-of-sight: Always match polarisations; otherwise, your signal meter will drop by several S-units.
• Skywave HF: Expect rotation, depolarisation, and multipath. A fixed linear antenna will sometimes be 90° “off.” Many operators assume an average 3 dB polarisation loss and prioritise quiet antennas, good grounding, and diversity systems.
• Practical tricks: Crossed dipoles, crossed loops, or dual-linear systems with phasing networks can generate any desired polarisation. Even simple A/B switching between vertical and horizontal helps tame QSB.

Build-Level Tips

• Axial ratio: For circular antennas, an AR ≤ 3 dB indicates good purity. Larger values blur circularity and reduce discrimination.
• Feedline balance: Unwanted common-mode currents tilt the intended polarisation. Proper chokes or baluns preserve purity.
• Ground effects: Soil conductivity and permittivity affect vertical and horizontal reflection differently, altering both launch angle and polarisation purity — one reason performance differs between seaside and inland sites.

Quick Science References (in words)

• Polarisation mismatch: received power = cos² Δψ; 45° → 3 dB loss.
• Linear vs circular: average coupling ≈ −3 dB; opposite-hand CP ≈ 0.
• General case (Stokes): coupling = 0.5 × (1 + dot-product of vectors).
• Faraday rotation: rotation angle ≈ RM × λ² — stronger at longer wavelengths.
• Reflection: near-normal from a good conductor flips circular handedness.

Still So Much to Discover

From early magneto-ionic theory to modern ionospheric models, polarisation remains an active research area. For everyday radio work, the rule of thumb is simple: understand the physics, build cleanly, and design your receive system to cope with what nature randomises.

Polarisation is the direction and time-evolution of a wave’s electric field. It decides how efficiently antennas couple and how signals fade. On HF it matters — but the ionosphere often has the final word.

Interested in more technical content? Subscribe to our updates for deep-dive RF articles and lab notes.

Questions or experiences to share? Contact RF.Guru.