Last Updated on May 10, 2026 by John Berry
When excited by a radio frequency current, an antenna creates both an electric field and a magnetic field. Collapsing and recovering fields realise a radio wave. Wave polarisation is defined by the orientation of the electric field relative to the Earth’s surface. Generally the longest dimension of the antenna driven element sets this polarisation. This orientation (and polarisation) determines how your antenna interacts with the passing electromagnetic wave. Understanding polarisation distortion helps you optimise your station’s performance and link reliability. When attempting to receive with an orthogonal orientation there will likely be a cross polarisation discrimination and associated signal loss. But there may be significant beneficial polarisation distortion when propagating via the ionosphere.
Linear and circular polarisation
Most amateurs use linear polarisation. This is either vertical or horizontal. A vertical antenna radiates a vertically polarised wave. Conversely, a horizontal antenna radiates a wave that oscillates parallel to the ground.
Circular polarisation occurs when the wave rotates as it travels. We call this Right-Hand Circular Polarisation (RHCP) or Left-Hand Circular Polarisation (LHCP). Right or left here describes the direction of rotation. Satellite operators often use circular polarisation to combat signal fading caused by satellite spinning.
It’s also possible to set up a field that generates a wave that is of mixed polarisation – with both vertical and horizontal components. To receive this efficiently, the antenna must be able to receive either vertically or horizontally polarised waves or both. This is a technique often used in broadcasting. Mixed polarisation also occurs in ionospheric polarisation.
Polarisation discrimination
Cross-polarisation discrimination (XPD) measures an antenna’s ability to reject signals of the opposite polarisation. We express XPD in decibels (dB). For example, a horizontal antenna would receive a vertical signal with significant loss.
In practice, antennas are never perfect. Some energy always “leaks” into the unintended plane. XPD values often exceed 25 dB though this might reduce in practical amateur installations.
Tropospheric propagation
The troposphere maintains the polarisation of the launched wave as it propagates from transmitter to receiver. There is no shift (or distortion) from vertical toward horizontal or the like. Hence, if the wave is launched as vertical, it will arrive vertically polarised. Clearly this is the ideal state.
The image I’ve included below gives a neat allegory of cross-polarisation discrimination. The rope oscillations on the left (a) (simulating vertical polarisation) pass through the vertical slot antenna easily. Conversely, the rope oscillations on the right (b) (simulating horizontal polarisation) are attenuated.
When a wave reflects off the ground, its polarisation can change. The conductivity and permittivity of the soil dictate this shift. This effect can be exploited during EME operations.
Ionospheric propagation
If the wave propagates through a significant volume of ionised media, significant polarisation distortion may occur. The ionosphere comprises free electrons under the influence of the Earth’s magnetic field. As a linearly polarised wave enters the ionosphere, it splits into two independent modes. These modes travel at different speeds.
When they recombine on exiting the ionosphere, the resulting E-field orientation may have rotated. This is Faraday rotation. The degree of rotation depends on the Total Electron Content (TEC) of the ionosphere. It also varies with the frequency of the signal.
This is exploited in HF propagation via the ionosphere. Often propagation via the ionosphere results in mixed polarisation. One station receiving horizontal and the other receiving vertical suffer no relative disadvantage.
You can find values of polarisation discrimination, between vertical and horizontal, for example, from antenna specification sheets.