Ham Radio Antenna Polarisation: Horizontal, Vertical & Cross-Pol

Polarisation is the orientation of the electric field vector of the radio wave your antenna radiates. Match polarisation at both ends of a link and everything works. Mismatch it by 90° and you lose 20–30 dB — effectively cutting the other station out entirely. Understanding polarisation is essential for VHF/UHF line-of-sight work, satellite operation, and making sense of the complex polarisation behaviour of HF signals after ionospheric propagation.

A radio wave is a transverse electromagnetic wave — its electric field (E-field) and magnetic field (H-field) oscillate perpendicular to the direction of propagation and perpendicular to each other. Polarisation describes the orientation of the electric field vector as the wave propagates through space. If the E-field oscillates in a fixed vertical plane, the wave is vertically polarised. If it oscillates in a fixed horizontal plane, the wave is horizontally polarised. If the E-field rotates as the wave travels — tracing a helix in space — the wave is circularly or elliptically polarised.

An antenna's polarisation is determined by its physical orientation relative to the earth. A horizontal dipole radiates horizontally polarised waves in the directions broadside to its elements. A vertical whip radiates vertically polarised waves. A yagi mounted with its boom horizontal and elements horizontal is horizontally polarised. The same yagi rotated 90° about its boom axis — elements now vertical — becomes vertically polarised. The directional properties of the antenna do not change; only the polarisation of the radiated wave changes.

When two antennas in a link have different polarisations, not all of the transmitted power can be received — some is lost to polarisation mismatch. This loss is called cross-polarisation loss or polarisation mismatch loss. It depends on the angle between the E-field orientations of the two antennas, and follows a cosine-squared relationship:

The implications of this formula are profound and often misunderstood. A 45° polarisation mismatch — one antenna horizontal, the other at 45° slant — causes exactly 3 dB of loss. This is the same as halving your transmitter power. A 90° mismatch — one antenna horizontal, the other vertical — causes theoretically infinite loss (cos 90° = 0). In practice, real antennas are not perfect and there is some residual coupling even at 90° cross-polarisation, typically 20–30 dB of isolation rather than truly infinite. But 20–30 dB is an enormous loss — effectively eliminating the link entirely for all practical purposes.

At HF frequencies, signals travel to the ionosphere and are refracted back to earth. During this transit, the ionosphere — a magnetised plasma — causes the polarisation of the wave to rotate continuously. This effect is called Faraday rotation. The amount of rotation depends on the electron density in the ionosphere, the path length, and the frequency. At 14 MHz the polarisation may rotate several complete turns during a single hop. At 7 MHz, rotation can be even more extensive.

The practical consequence is that by the time an HF signal arrives at the receiving antenna, its polarisation is essentially random — unrelated to the polarisation of the transmitting antenna. It could be horizontal, vertical, 37°, or anything else, and it changes from moment to moment as the ionosphere fluctuates. This means that polarisation matching between antennas is essentially irrelevant for most HF sky-wave communication. A horizontally polarised dipole receives just as well as a vertically polarised dipole from a distant HF station, because the arriving signal has no fixed polarisation reference.

This is why HF operators use horizontal Yagis, horizontal dipoles, and all manner of antenna orientations without worrying about polarisation mismatch — the ionosphere randomises polarisation on every path. The choice of horizontal vs. vertical at HF is driven by radiation angle, ground interaction, and convenience, not by polarisation matching to the distant station.

At VHF and UHF, propagation is primarily line-of-sight (LOS) or near-LOS, with signals reflecting from terrain, buildings, and the troposphere. Faraday rotation is negligible at these frequencies. Polarisation is therefore preserved from transmitter to receiver — a vertically polarised signal arrives as a vertically polarised wave, and a horizontally polarised signal arrives horizontally polarised. Polarisation mismatch between two stations directly and predictably degrades the link.

The 2 m and 70 cm band convention

By convention, most amateur VHF and UHF FM simplex and repeater operation uses vertical polarisation — antennas are omnidirectional verticals or gain verticals (collinears). This allows mobile and handheld stations with whip antennas to communicate effectively. SSB and weak-signal DX operation on VHF/UHF uses horizontal polarisation by convention — fixed stations use horizontal Yagis, and this choice is supported by some practical arguments: horizontal antennas at typical VHF heights have slightly different ground reflection characteristics, and horizontally polarised signals may experience somewhat different multipath behaviour than vertical signals in built-up environments.

The consequence is that a vertically polarised station (FM handheld) and a horizontally polarised station (SSB Yagi operator) on the same frequency will suffer 20–30 dB of cross-polarisation loss and will barely hear each other even if they are physically close. This cross-band polarisation mismatch is a frequent source of confusion for newer operators who cannot understand why a nearby strong signal is barely audible on their horizontal Yagi.

EME (Earth-Moon-Earth) — Faraday rotation returns at VHF

At 144 MHz, the path to the moon and back passes through the ionosphere twice, and at some times of day and solar conditions, significant Faraday rotation can occur even at VHF. EME operators sometimes use circular polarisation to avoid polarisation-dependent fading — if the signal arrives with random polarisation, a circularly polarised antenna recovers it with only a 3 dB loss (circular-to-linear mismatch) regardless of the rotation angle, whereas a linearly polarised antenna may suffer 20+ dB loss at the worst polarisation angle.

Circular polarisation (CP) is produced when two equal-amplitude waves are combined with a 90° phase difference between them — one in the horizontal plane and one in the vertical plane. The resulting wave's E-field rotates continuously as it propagates. If the rotation is clockwise when viewed in the direction of propagation, it is Right Hand Circular Polarisation (RHCP). Counter-clockwise is Left Hand Circular Polarisation (LHCP). Both have the same power as a linearly polarised wave — the energy is simply distributed into the rotating field rather than a fixed plane.

Circular polarisation in satellite work

Amateur satellites transmit and receive using circular polarisation (most commonly RHCP) for several important reasons. The signal path through the ionosphere causes Faraday rotation that varies continuously as the satellite moves and the ionospheric path length changes. A circularly polarised antenna is immune to polarisation rotation — RHCP transmitted becomes LHCP after reflection or after passing through an odd number of Faraday rotations, but the loss is fixed at 3 dB (RHCP-to-LHCP mismatch) rather than varying from 0 to 30+ dB as with a linear antenna facing random polarisation rotation.

The standard satellite ground station antenna for linear satellites (those using FM or SSB with linear polarisation) is typically a crossed Yagi — two identical Yagis mounted at 90° with their elements in orthogonal planes, fed with a 90° hybrid coupler to produce RHCP or LHCP output. Switching the phasing between +90° and −90° selects between RHCP and LHCP, accommodating the polarisation sense reversal that occurs at different points in the satellite pass.

Antennas that produce circular polarisation

• Helical antenna: The most common dedicated RHCP or LHCP antenna — a helically wound conductor over a ground plane. RHCP results from right-hand winding (follow the helix in the direction the wave propagates and your right hand curls in the direction of E-field rotation). Used widely for satellite ground stations and 23 cm EME.
• Turnstile / crossed dipole: Two dipoles at 90° fed with 90° phase difference. Produces omnidirectional circular polarisation in the broadside direction — used for satellite downlink reception and NOAA weather satellite reception. 3 dB lower gain than a single dipole in any one linear polarisation direction.
• Crossed Yagi: Two Yagis at 90°, fed with 90° phasing coupler. Directional CP with Yagi gain. The standard satellite ground station antenna. Switching the 90° coupler between +90° and −90° selects RHCP or LHCP.
• Patch antenna: A square or circular microwave patch antenna with a single corner feed produces circular polarisation from a linearly symmetric structure. Used in GPS receivers, APRS trackers, and some satellite uplink applications.

VHF/UHF weak signal (SSB/CW DX)

Use horizontal polarisation. This is the established convention for 144 MHz, 432 MHz, and higher band SSB DX work. Both stations should use horizontal Yagis or equivalent. The 20–30 dB isolation between horizontal and vertical polarisation modes on VHF means that horizontal weak-signal operators will not be troubled by vertically polarised FM activity on the same band — they are essentially invisible to each other except in strong-signal situations.

FM repeater and simplex — VHF/UHF

Use vertical polarisation. All mobile, handheld, and base FM stations use vertical polarisation. Attempting to access a vertical-pol repeater with a horizontally polarised Yagi will result in 20–30 dB of loss — the repeater may still be accessible at close range with a high-gain antenna, but it is a poor configuration. Similarly, a horizontally polarised beam antenna used for FM will work only when pointed directly at the repeater with a strong signal.

Satellite — LEO amateur satellites

Use circular polarisation if possible (crossed Yagi or helix). Accept 3 dB loss if using a linear antenna, and be prepared for polarisation fading as the satellite moves through different ionospheric geometries. For low passes (below 30° elevation) from the horizon, polarisation fading from Faraday rotation can be rapid and severe — a circular polarisation antenna eliminates this problem entirely.

HF contesting and DX

Polarisation is not a meaningful consideration for HF sky-wave paths. Focus on antenna gain, take-off angle, and feedline efficiency. The only HF situation where polarisation matters directly is comparison of two antennas at the same site for local ground-wave contacts — where vertical polarisation has a propagation advantage — or for NVIS operation where the angle of incidence at the ionosphere can influence whether the signal returns in a useful polarisation state.

Portable and emergency communication

In emergency communication situations, polarisation mismatch can significantly reduce effective range. A horizontally deployed wire dipole (H-pol) communicating with a mobile station's vertical whip (V-pol) suffers up to 20 dB of polarisation mismatch loss on a VHF or UHF line-of-sight path. In an emergency, a simple solution is to orient a portable dipole at 45° — slant polarisation — which introduces only 3 dB of loss to both horizontal and vertical stations simultaneously, providing a workable compromise when the polarisations of other stations are unknown.

In practice, neither linear nor circular polarisation is perfectly achieved by real antennas. A horizontally polarised Yagi has some small vertical component due to manufacturing imperfections, boom effects, and asymmetric feed systems — its cross-polarisation isolation is finite, typically 20–40 dB. A circularly polarised turnstile or crossed Yagi departs from perfect circular polarisation because the 90° hybrid coupler is not perfectly phase-matched across all frequencies and because the two orthogonal antennas may not have exactly equal gain in all directions.

The result is elliptical polarisation — the E-field traces an ellipse rather than a perfect circle or a perfectly straight line as the wave propagates. The quality of a circular polarisation system is characterised by its Axial Ratio (AR) — the ratio of the major to minor axis of the polarisation ellipse, expressed in dB. A perfect circle has AR = 0 dB. A practical crossed Yagi might achieve AR = 1–2 dB across its operating bandwidth. An AR below 3 dB is generally considered acceptable for satellite work. Higher AR means the antenna behaves increasingly like a linearly polarised antenna with its associated sensitivity to polarisation angle.