Polarization
Every electromagnetic wave has a polarization — the orientation of its electric field as the wave travels through space. This may sound like an abstract concern, but polarization has very concrete effects on how well two stations can communicate. Transmit and receive antennas with the same polarization transfer maximum signal between them. Antennas with crossed polarizations can lose 20 dB or more — the difference between a strong signal and complete inaudibility. Understanding polarization helps you choose the right antenna for each situation and explains phenomena like why your HF signal sometimes fades and recovers over a long path.
The four main polarization types. The E-field defines polarization: vertical (E-field up-down), horizontal (E-field left-right), and circular (E-field rotating as the wave travels).
View LargerWhat Polarization Means
Polarization is defined by the orientation of the electric field (E-field) of an electromagnetic wave as it propagates through space. Recall from Lesson M14A that an electromagnetic wave has both an E-field and an H-field, both perpendicular to the direction of travel. The polarization is defined by the direction of the E-field — not the H-field — as a matter of convention.
Think of a wave traveling horizontally toward you. The E-field could be pointing straight up and down (vertical polarization), pointing left and right (horizontal polarization), or rotating as the wave travels (circular polarization). The actual electromagnetic energy is the same in all three cases, but the relationship between the transmitting antenna and the receiving antenna determines how much of that energy gets transferred.
The polarization of a wave produced by an antenna is determined by the physical orientation of the antenna conductor. A vertical wire antenna (oriented perpendicular to the Earth's surface) produces a vertically polarized wave — the E-field oscillates vertically, parallel to the antenna conductor. A horizontal wire antenna produces a horizontally polarized wave. This direct relationship between antenna orientation and wave polarization is one of the elegantly intuitive results of antenna physics.
Vertical and Horizontal Polarization
Vertical polarization is produced by antennas oriented vertically — monopoles, quarter-wave verticals, ground planes, and any antenna where the radiating element is perpendicular to the ground. The E-field of the transmitted wave oscillates in the vertical plane. At the receiving end, a vertically oriented antenna (aligned with the E-field) receives maximum signal. A horizontally oriented antenna receives very little from a vertically polarized wave.
Vertical polarization has lower angle radiation for terrestrial propagation when the antenna is over a good ground plane. Vertically polarized signals also propagate well along the ground (ground wave propagation), making vertical polarization more effective at lower frequencies for short to medium distances. Mobile stations almost universally use vertical polarization because a vertical antenna is easy to mount on a vehicle and produces an omnidirectional pattern — useful when you are moving and the direction to the other station is constantly changing.
Horizontal polarization is produced by antennas oriented parallel to the ground — dipoles, beams with horizontal elements, and loops mounted in the horizontal plane. The E-field oscillates horizontally. Horizontal antennas tend to have lower radiation resistance near the ground (the ground acts as a mirror that reinforces some directions and cancels others), but at sufficient height the radiation pattern can be quite effective for long-distance HF contacts. On VHF and UHF, horizontal polarization is the standard for SSB, CW, and weak-signal work because it produces lower background noise levels than vertical (terrestrial noise tends to be vertically polarized from man-made sources).
Circular and Elliptical Polarization
In circular polarization, the E-field does not simply oscillate back and forth — it rotates continuously as the wave travels, tracing a helix through space. Imagine looking along the direction the wave is traveling: the E-field vector rotates like the hands of a clock. If it rotates clockwise (when viewed from the direction the wave is coming from), it is Right-Hand Circular Polarization (RHCP). If it rotates counter-clockwise, it is Left-Hand Circular Polarization (LHCP).
Circular polarization can be thought of as the superposition of two equal-amplitude linearly polarized waves (one vertical, one horizontal) that are 90 degrees out of phase with each other. This is how many circular polarization antennas are built — by feeding a crossed dipole pair or a pair of loops with a 90-degree phase shift between them. If the two components are not exactly equal in amplitude, the result is elliptical polarization — a more general case that is somewhere between linear and circular.
Circular polarization is valuable for satellite communications because the satellite's antenna orientation relative to the ground station changes as the satellite moves across the sky. A fixed linearly polarized ground antenna would experience significant signal variation (polarization fading) as the satellite tumbles or moves. A circularly polarized antenna on the ground avoids this problem — any linear polarization from the satellite can be received with at most a 3 dB loss compared to a circularly polarized source. This 3 dB loss is the price of using circular polarization on only one end of the link; it is the standard trade-off for LEO satellite work.
Circular polarization also reduces multipath interference in some situations. When a linearly polarized wave reflects off a flat surface (a building, a metal structure), the reflected wave is approximately reversed in handedness (RHCP becomes LHCP, and vice versa). A RHCP receiving antenna will reject the reflected LHCP wave, receiving only the direct-path wave. This multipath rejection is why circular polarization is used in some VHF/UHF terrestrial links in urban environments.
Cross-Polarization Loss
When a transmitting antenna and a receiving antenna have different polarizations, signal is lost. The amount of loss depends on the angle between the two polarization orientations. For two linear polarizations separated by an angle θ, the received power is proportional to cos²(θ):
Loss factor = cos²(θ)
where θ is the angle between the two polarization planes.
At θ = 0° (same polarization): Loss = cos²(0°) = 1 → 0 dB loss (maximum transfer)
At θ = 45° (45-degree cross): Loss = cos²(45°) = 0.5 → −3 dB
At θ = 90° (orthogonal / crossed): Loss = cos²(90°) = 0 → −∞ dB (theoretically zero signal)
In practice, a perfectly cross-polarized link (vertical transmitter, horizontal receiver, or vice versa) produces 20 to 30 dB of cross-polarization discrimination — not infinite, because in a real environment there are always some depolarizing effects (reflection off irregular surfaces, scattering, etc.) that mix the polarizations slightly. But 20–30 dB is still enormous — the equivalent of reducing a 100-watt transmitter to 0.1–1 watt from the receiving antenna's perspective.
Station A transmits with a vertical antenna at 100 watts (50 dBm). Station B receives with:
- A vertical antenna (same polarization, θ = 0°): No cross-pol loss. Full signal received.
- An antenna tilted 30° from vertical (θ = 30°): Loss = cos²(30°) = 0.75 → −1.25 dB. Small loss, barely noticeable.
- An antenna tilted 45° (θ = 45°): Loss = cos²(45°) = 0.5 → −3.0 dB. Signals are 3 dB weaker — noticeable but workable.
- A horizontal antenna (θ = 90°): Loss = cos²(90°) = 0, theoretically → −20 to −30 dB in practice. Signal is nearly gone.
Cross-polarization loss is why you should always match polarization to the other station — even a 45° mismatch costs 3 dB (equivalent to halving your transmit power).
For circular versus linear polarization, the loss is always 3 dB. A circularly polarized wave carrying 10 watts will be received as 5 watts by a vertically polarized antenna and 5 watts by a horizontally polarized antenna (either one, because the circular wave contains equal contributions of both). Two circular polarizations of the same handedness (RHCP-to-RHCP) have no polarization loss. Two circular polarizations of opposite handedness (RHCP transmitting, LHCP receiving) have theoretically infinite loss — they are orthogonal polarizations just like horizontal and vertical linear.
Polarization at HF: Why It Matters Less
At HF frequencies (3–30 MHz), long-distance contacts involve the ionosphere. Signals travel up, reflect off the ionosphere, and come back down. During this round-trip, the ionosphere does something important: it rotates the polarization of the wave. This rotation is called Faraday rotation, and it is caused by the interaction between the electromagnetic wave and the Earth's magnetic field in the ionized region.
Faraday rotation is not fixed — it depends on the ionospheric conditions at the time, which vary with solar activity, time of day, season, and frequency. The total rotation at HF can be anywhere from a few degrees to many complete rotations over a long path. The result is that the polarization of an HF signal arriving at the receive antenna is essentially random and constantly changing — it has nothing to do with how the transmit antenna was oriented.
This has a very practical implication: for ionospheric (skywave) HF contacts, polarization matching is irrelevant. Whether you use a vertical or horizontal antenna, and whether the other station uses a vertical or horizontal antenna, makes no systematic difference to signal strength — the ionosphere has already randomized the polarization by the time the signal arrives. You may hear slightly better signal at some instants with one polarization and slightly better at other instants with the other — this is polarization fading — but there is no persistent advantage to either orientation for HF DX.
Polarization does matter at HF for local ground-wave contacts (direct path, not via ionosphere) and for near-vertical incidence skywave (NVIS) where the path goes nearly straight up and down and the signal does not travel far through the ionosphere. For these purposes, matching polarization with the other station can produce a measurable improvement.
Polarization at VHF and UHF: Why It Matters a Lot
At VHF (144 MHz) and UHF (432 MHz, 1296 MHz, and above), signals travel by direct line-of-sight paths, tropospheric ducting, or troposcatter — none of which involve the ionosphere and none of which randomize polarization. What is transmitted with vertical polarization arrives with vertical polarization. What is transmitted with horizontal arrives horizontal. The cross-polarization loss of 20–30 dB between mismatched stations is fully realized, and it completely destroys communication.
This is why VHF and UHF operating has two very distinct "worlds" that rarely communicate with each other:
- FM repeater and mobile/portable operation: Uses vertical polarization exclusively. All repeater antennas, handheld radios, and mobile antennas are vertically polarized. This is the FM phone portion of VHF — handie-talkies on 2 meters, mobile to repeater, emergency communications.
- SSB, CW, and weak-signal DX operation: Uses horizontal polarization exclusively. All Yagi antennas for EME (Earth-Moon-Earth), troposcatter, meteor scatter, and SSB DX are mounted horizontally. This is the weak-signal end of VHF — the portion used for long-distance contacts and VHF contests.
These two groups use the same frequency band but with orthogonal polarizations. A vertically polarized FM mobile trying to work a horizontally polarized SSB station would lose 20–30 dB immediately — the equivalent of reducing from 50 watts to 50 milliwatts. They would almost certainly not be able to communicate even on a direct line-of-sight path.
The practical rule for VHF/UHF: always know which mode the other station is using and match polarization. If you are joining a 2-meter SSB net or contest, use horizontal. If you are talking to a repeater or working FM simplex, use vertical. Many VHF portable and fixed stations use antennas that can be rotated between horizontal and vertical to accommodate both uses.
Choosing Polarization for Different Situations
| Situation | Best Polarization | Reason |
|---|---|---|
| HF DX (ionospheric path, 3–30 MHz) | Either; match your band plan | Faraday rotation randomizes polarization; no advantage to either |
| HF NVIS (near-vertical incidence, local contacts) | Horizontal (dipole) | Traditional; matching stations easier; short ionospheric path |
| HF ground wave (medium wave, AM-style propagation) | Vertical | Vertically polarized waves propagate better along Earth's surface |
| VHF/UHF FM, repeaters, mobile | Vertical | Industry standard; all repeaters and handhelds are vertical |
| VHF/UHF SSB, CW, weak-signal DX | Horizontal | Industry standard for weak-signal work; lower man-made noise |
| Satellite (LEO, low Earth orbit) | Circular (RHCP or LHCP, match to satellite) | Avoids polarization fading as satellite moves; check satellite specs |
| Satellite (high earth orbit, geostationary) | Linear (match satellite) | Stable geometry; check satellite specifications |
| EME (Earth-Moon-Earth, moonbounce) | Horizontal or circular | Depends on station configuration; horizontal is traditional |
- Polarization is the orientation of the E-field of an electromagnetic wave. It is determined by the physical orientation of the transmitting antenna conductor.
- The four types are: vertical, horizontal, right-hand circular (RHCP), and left-hand circular (LHCP).
- Cross-polarization loss = cos²(θ) for linear polarizations at angle θ. At 90° (crossed), loss is theoretically infinite; in practice 20–30 dB.
- At HF, Faraday rotation in the ionosphere randomizes polarization — for skywave contacts, matching polarization gives no consistent advantage.
- At VHF/UHF, polarization is preserved on direct paths — mismatched stations lose 20–30 dB and may be unable to communicate.
- VHF FM uses vertical; VHF SSB/CW weak-signal uses horizontal; satellite uses circular.
Frequently Asked Questions
I can use my vertical on 40 meters and work DX stations using horizontal dipoles. How?
Because the 40-meter DX path goes via the ionosphere, and Faraday rotation randomizes the polarization of both signals during the ionospheric hop. The signal that left your vertical antenna as vertically polarized arrives at the DX station with a random polarization — which may well be mostly horizontal by the time it gets there. The DX station's horizontal dipole receives it fine. The random polarization rotation by the ionosphere is what makes it possible to use any polarization for HF skywave contacts.
My 2-meter handheld has a vertical antenna. Can I work VHF SSB stations?
With great difficulty, if at all. VHF SSB stations use horizontal antennas, typically Yagi beams. Your vertical handheld will experience 20–30 dB of cross-polarization loss, which effectively means you are transmitting with perhaps 50 milliwatts compared to your quoted 5 watts. You might succeed on a very short, strong path, but for any serious contact distance you would need to use a horizontal antenna. Some operators use small portable Yagis or turn a vertical yagi on its side for this purpose.
Why do satellite operators use RHCP rather than LHCP, or does it matter?
Whether you use RHCP or LHCP depends on the satellite's own antenna, not convention. Different satellites transmit with different handedness, and you need to match. For LEO satellites in the amateur bands (OSCAR satellites, ISS), many use RHCP on the downlink, making RHCP on the ground the right choice. Check the specific satellite's documentation. For EME (Earth-Moon-Earth), the Moon reverses the handedness of circular polarization on reflection, so some operators account for this. The critical point is that RHCP and LHCP are orthogonal polarizations — use the wrong one and you lose 20+ dB.
Test Your Knowledge
Answer the questions below to check your understanding. Every answer can be found in the lesson above.