Understanding Polarisation – Why It Matters for Your Antennas
When we talk about antennas, we often focus on frequency, length, or height. But there’s another “quiet boss” that decides how efficiently antennas couple ... and why signals sometimes fade for no obvious reason: polarization.
At a Glance
- Line-of-sight (VHF/UHF, repeater, FM, microwave): polarization matching matters a lot. Mismatch can cost several S-units.
- HF skywave (NVIS & DX): the ionosphere rotates and mixes polarization. Don’t obsess over “perfect alignment” ... focus on noise, take-off angle, and clean feedlines.
- Rule of thumb: two linear antennas offset by Δψ lose power ≈ cos²(Δψ). 45° ≈ 3 dB. 60° ≈ 6 dB.
- Best practical upgrade on HF receive: polarization diversity (vertical + horizontal, or dual-hand circular) often beats “more gain”.
If you only remember one sentence: match polarization for direct paths; expect chaos on skywave and design your RX to cope.
What Is Polarization?
Polarization describes the orientation (and time behavior) 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 ... just as Maxwell’s equations predict.
Plain-language picture: imagine you’re holding a jump rope and flicking it forward.
- If the rope moves up/down in one fixed plane, that’s linear polarization.
- If the rope tip traces a circle as it moves forward, that’s circular polarization.
- If it traces an oval, that’s elliptical polarization (the most common “real-world” case).
Most day-to-day stations use linear polarization: verticals produce (mostly) vertical; horizontal dipoles produce (mostly) horizontal. Circular polarization is common where orientation changes or reflections are harsh (satellites, aircraft, drones, some microwave links).
Engineers quantify polarization using Jones vectors (fully polarized waves) or Stokes parameters (I, Q, U, V) for partially polarized waves ... a neat framework that ties directly to power, linear angle, and circular “sense”.
Axial Ratio: “How Circular” Is Circular?
For circular polarization, we talk about axial ratio (AR): a perfect circle is AR = 1 (often expressed as 0 dB). As AR grows, the wave becomes “more linear”.
- Good CP purity: AR ≤ 3 dB (common design target).
- So-so CP: AR around 4–6 dB (works, but weaker discrimination).
- Nearly linear: AR ≫ 6 dB (it behaves mostly like a linear signal).
Linear vs Circular (and when each shines)
| Type | What it means | Common antennas | Where it’s usually best |
|---|---|---|---|
| Linear (vertical) | E-field stays in one fixed plane (vertical). | ¼λ vertical, ½λ vertical, ground-plane, collinear. | Ground-wave, mobile, many VHF/UHF “vertical” systems, low-angle HF when noise allows. |
| Linear (horizontal) | E-field stays in one fixed plane (horizontal). | Dipole, inverted-V, Yagi (mounted horizontal), horizontal loop sections. | Many HF base stations, NVIS-style installations, lower receive noise in many locations. |
| Circular (RHCP / LHCP) | E-field rotates like a corkscrew. Right-hand (RHCP) or left-hand (LHCP) rotation. | Helix, crossed Yagi with phasing, patch arrays, turnstile, some crossed-loop systems. | Satellites, platforms that roll/tilt, multipath-heavy paths, some NVIS receive diversity ideas. |
| Elliptical | Anything between linear and circular (most “real” signals after reflections/ionosphere). | Often the result of the path, not a deliberate antenna choice. | HF skywave in general, urban multipath, mixed reflection environments. |
Why Does Polarization Matter?
Coupling and mismatch loss (the “math that actually matters”)
Two linear antennas misaligned by an angle Δψ lose power approximately as cos²(Δψ). That’s why rotating a Yagi from horizontal to vertical can make a strong signal collapse (and vice versa).
| Misalignment Δψ | Coupling cos²(Δψ) | Loss (approx.) | What you’ll “feel” on the air |
|---|---|---|---|
| 0° (perfect match) | 1.00 | 0 dB | Full signal |
| 30° | 0.75 | ≈ 1.25 dB | Small but noticeable |
| 45° | 0.50 | ≈ 3 dB | About half power |
| 60° | 0.25 | ≈ 6 dB | Often ~1 S-unit (rule-of-thumb) |
| 90° (cross-polarized) | 0 (ideal) | ∞ dB (ideal) | In practice, you still hear “something” due to reflections, tilt, and imperfect antennas |
Note: S-units aren’t perfectly calibrated across rigs. The classic “1 S-unit ≈ 6 dB” is a useful mental model ... not a law of physics.
Linear versus circular penalties (quick rules)
- Linear receiving a circular wave: average coupling ≈ 0.5 → about −3 dB (you get “half the wave” on average).
- RHCP vs LHCP: ideally no coupling (very strong isolation). In the real world, purity (AR), reflections, and path effects reduce that isolation.
Fading and QSB are often “polarization drama”
On HF, signals can arrive via multiple ionospheric paths. Each path can have a different polarization and phase. When those fields add at your antenna, the result can swing between reinforcement and cancellation ... that’s QSB.
Faraday rotation twists linear polarization
The ionosphere behaves like a magnetized plasma. A linearly polarized wave effectively splits into two characteristic modes that travel differently, so the plane of polarization can rotate during propagation.
- Key scaling: rotation tends to be stronger at longer wavelengths ... roughly proportional to λ².
- Practical outcome: on HF skywave, your “vertical” or “horizontal” assumption can be wrong within seconds.
Reflections can flip handedness
Circular waves can reverse their sense (RHCP ↔ LHCP) when reflecting from a good conductor at near-normal incidence. Add multiple reflections and oblique angles, and the received polarization becomes a moving target.
Diversity is the “cheat code” for stable receive
Polarization diversity means using two antennas with different polarizations (vertical + horizontal, or RHCP + LHCP) and letting the receiver choose or combine the stronger one. This can turn deep fades into minor ripples.
NVIS & DX — Polarization in Real-World HF Paths
NVIS (Near-Vertical Incidence Skywave)
NVIS dominates short-range HF links (often 0–500 km) by sending energy nearly straight up and receiving it back down. But the “reflection” isn’t a simple mirror: the ionosphere supports two magneto-ionic modes that can behave close to opposite circular senses.
Practical NVIS takeaway: the returning signal can be close to circular/elliptical, and it can change quickly.
- If you RX only on a single fixed polarization, you can see big swings (10–20 dB) during mode mixing.
- With diversity (two polarizations), you often “ride through” those swings with much less drama.
For NVIS receive, diversity frequently improves SNR more reliably than squeezing out a few extra dB of antenna gain.
Handedness, hemispheres, and reality
Operators sometimes report one circular “hand” seeming to dominate at certain times and locations. Treat this as an observation, not a guarantee: the ionosphere is dynamic, and local time, geomagnetic conditions, and path geometry can change what you receive.
Advanced corner: what’s happening physically?
In magneto-ionic theory, the wave in a magnetized plasma decomposes into two characteristic modes (often described as ordinary and extraordinary). They can have different refractive indices and absorption. When they recombine at the receiver, the resulting polarization can “flutter” ... and a purely linear antenna may see rapid fades as the polarization ellipse rotates relative to the antenna.
If you like symbols: Faraday rotation is commonly written as rotation angle ≈ RM × λ², where RM depends on electron density and the magnetic field along the path.
DX (long-haul ionospheric paths)
DX signals travel thousands of kilometers via multiple oblique refractions and often multiple hops. After several traversals through different ionospheric regions plus ground/ocean reflections, polarization is commonly partially randomized (elliptical, time-varying, frequency-dependent).
Practical DX takeaway: chasing “perfect polarization alignment” is rarely the highest-payoff effort.
- Pattern & take-off angle usually dominate.
- Noise floor often dominates even more (a quieter antenna can beat a “better” antenna).
- Feedline cleanliness (baluns/chokes, symmetry, grounding) prevents unintended polarization tilt and local noise pickup.
TX vs RX — Should You Care?
Transmit
- Line-of-sight paths (VHF/UHF): match the system. FM/repeaters are commonly vertical; many weak-signal SSB/CW setups are horizontal.
- HF ground-wave / low-angle work: verticals can be very effective if the site/noise allows.
- HF NVIS: a low horizontal dipole (often around 0.1–0.25 λ high) is a classic, effective choice.
- HF skywave in general: don’t expect your transmit polarization to arrive unchanged ... the ionosphere will rotate/mix it.
Receive
- Line-of-sight: polarization mismatch can be a “silent killer”. If a signal is mysteriously weak, check polarization first.
- HF skywave: expect rotation, depolarization, and multipath. Many operators assume an average polarization loss around a few dB and prioritize quiet receive antennas.
- Practical tricks that work: A/B switch between a vertical and a horizontal; use crossed dipoles/loops; or use two receivers / diversity combining if you’re serious about stability.
Build-Level Tips (and “why is my polarization weird?”)
- Feedline balance matters: unwanted common-mode currents can tilt your intended polarization and distort patterns. Use proper baluns and chokes.
- Keep the antenna what you designed: a “vertical” with the coax running along the radiator, or a dipole fed without choking, can become a mixed-polarization radiator.
- Ground effects are real: soil conductivity/permittivity influence reflection and launch angle. Coastal sites can behave very differently from inland rock/poor soil.
- For circular antennas: aim for AR ≤ 3 dB if you want good RHCP/LHCP discrimination. Poor AR turns “circular” into “kinda circular-ish”, reducing the benefit.
A quick station test you can do today
- Pick a stable signal: a local beacon, repeater, or a steady station on VHF/UHF (line-of-sight).
- Change polarization: rotate a handheld, swap from vertical to horizontal, or rotate a small Yagi 90°.
- Watch the drop: if you see ~6–20 dB changes, that’s polarization doing its thing (plus local multipath).
- Now repeat on HF skywave: you’ll often see swings that don’t “make sense” ... that’s the ionosphere and multipath mixing polarizations and phases.
Quick Science References (in words)
- Linear mismatch: received power ≈ cos²(Δψ). Example: 45° → ~3 dB loss.
- Linear vs circular: average coupling ≈ 0.5 → ~−3 dB.
- Opposite-hand circular: ideally ~0 coupling (real systems limited by AR, reflections, and the path).
- Faraday rotation: rotation angle often written ≈ RM × λ² (stronger at longer wavelengths).
- General (Stokes) view: coupling can be written as 0.5 × (1 + dot-product of normalized polarization vectors) ... useful when waves are partially polarized.
Glossary (fast definitions)
- E-field: the “electric” part of the EM wave. Antennas mostly respond to this.
- H-field: the “magnetic” part of the EM wave, at right angles to the E-field.
- Linear polarization: E-field stays in one plane (vertical or horizontal).
- Circular polarization (RHCP/LHCP): E-field rotates as the wave moves forward (right-hand or left-hand sense).
- Elliptical polarization: the general case between linear and circular (very common after propagation).
- Axial ratio (AR): how close to perfect circular a wave/antenna is (AR ≤ 3 dB is “good CP”).
- QSB: fading on HF caused by interference of multiple paths/modes (amplitude changes over time).
- Faraday rotation: ionospheric effect that rotates the plane of linear polarization.
- NVIS: short-range HF using near-vertical radiation and ionospheric return.
- Diversity: using two different antennas (often different polarizations) to reduce fades by selecting/combining signals.
Still So Much to Discover
From early magneto-ionic theory to modern ionospheric modeling, polarization remains an active and fascinating topic. For everyday radio work, the rule stays practical: build cleanly, match polarization when the path is direct, and on HF design your receive system for the polarization that nature delivers ... not the one you wish it delivered.
Polarization 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.
Mini-FAQ
- Does circular polarization help on HF? — Sometimes on receive (especially for NVIS diversity ideas). On transmit, the ionosphere often rotates/mixes it anyway.
- How big is the loss if my antennas are cross-polarized? — For linear-to-linear, a 45° error is ~3 dB; 60° is ~6 dB; 90° is “ideally zero”, but real-world reflections and imperfections usually prevent infinite loss.
- Why do I hear a station better on the “wrong” antenna? — Because the path can rotate polarization (ionosphere) or create multipath (buildings/terrain), so the arriving wave may match your “wrong” antenna better.
- Can ground reflections change polarization? — Yes. Reflections and lossy ground can mix components and alter both elevation pattern and polarization purity.
- Is Faraday rotation predictable? — Only roughly. It scales with λ² and varies with time of day, season, solar activity, and geomagnetic conditions.
- What’s the best “one upgrade” for stable HF receive? — Often: diversity (vertical + horizontal, or two orthogonal antennas) plus good choking/baluns to keep your system honest.
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