Polarization of Electromagnetic waves
Polarisation of Electromagnetic Waves
The polarisation of a radio wave describes the direction of its electric field as it travels away from the antenna. In practical amateur radio terms, polarisation affects signal strength, fading, line-of-sight performance, and the radiation pattern produced by an aerial.
Quick visual guide
The direction of the aerial element usually determines the main polarisation of the transmitted wave. These simple drawings show the basic idea.
Horizontal
A horizontal wire produces mainly horizontal polarisation.
Vertical
A vertical radiator produces mainly vertical polarisation.
Slanted
A sloping wire produces both horizontal and vertical components.
Circular
Crossed elements with 90-degree phasing can produce circular polarisation.
Radiation angle made simple
What is polarisation?
Electromagnetic waves can be radiated in different forms, including horizontal, vertical, slanted, elliptical, or circular polarisation. The polarisation is mainly determined by the physical orientation of the radiating element of the antenna.
Horizontal Polarisation
If the antenna wire or element runs horizontally, parallel to the earth, the transmitted signal is mainly horizontally polarised. A typical HF dipole is a good example.
Vertical Polarisation
If the radiating element is vertical, at right angles to the earth, the transmitted signal is mainly vertically polarised. A quarter-wave vertical is a common example.
Slanted Polarisation
A sloping or angled wire produces a mixture of horizontal and vertical components. In practice, many real-world antennas are not perfectly horizontal or perfectly vertical.
Circular Polarisation
Circular polarisation can be produced by crossed elements fed with the correct phasing, normally with a 90-degree phase difference. This is often used for satellite and space communications.
Circular polarisation and satellites
Radio amateurs working orbiting satellites on VHF, UHF, and microwave frequencies often use circular polarisation. This helps reduce fading caused by the satellite changing position, rotating, or presenting a constantly changing antenna angle.
Circular polarisation can be right-hand circular polarisation or left-hand circular polarisation. For best results, both stations should normally use the same sense of circular polarisation, although reflections and satellite design can alter what is received.
Practical point: For satellite work, circular polarisation can be more forgiving than a fixed horizontal or vertical antenna because the relative angle between the satellite and the ground station is constantly changing.
Does polarisation matter on HF?
On HF, where signals are reflected or refracted by the ionosphere, the original polarisation of the signal is often changed before it reaches the receiving station. This is one reason why opposite antenna polarisations are not always a major problem on long-distance HF contacts.
As the signal passes through the ionosphere, the polarisation can slowly rotate. This effect can contribute to fading, known by radio amateurs as QSB.
Important: Polarisation matters much more for direct line-of-sight communication, such as VHF, UHF, microwave, local FM, repeaters, and point-to-point links. A horizontal antenna working a vertical antenna can suffer a large signal loss.
Polarisation and radiation angle
The orientation and height of the antenna affect both the polarisation and the radiation angle. The radiation angle is important because it influences whether the signal works better for local, medium-distance, or long-distance contacts.
Horizontally polarised HF antennas at ordinary amateur heights usually produce stronger high-angle radiation. This is useful for shorter-distance contacts, especially on the lower HF bands. They still produce some low-angle radiation, but it is usually weaker unless the antenna is installed much higher.
For strong low-angle radiation from a horizontal antenna, the aerial normally needs to be a significant fraction of a wavelength above ground. A height of around one wavelength is excellent, but this is difficult on the lower HF bands.
| Band | Approx. One Wavelength | Practical Meaning |
|---|---|---|
| 80 metres | About 280 feet | Very difficult for most amateur stations to achieve. |
| 40 metres | About 140 feet | Possible only with very tall supports or towers. |
| 20 metres | About 70 feet | More achievable, but still a good height for most stations. |
Simple rule: High-angle radiation is normally better for nearer stations. Low-angle radiation is normally better for DX.
Horizontal versus vertical antennas
A horizontal dipole at ordinary height is often very effective for local and medium-distance HF work because it tends to produce useful high-angle radiation. This can be helpful for contacts within a few hundred miles, especially on 80 metres and 40 metres.
A vertical antenna usually produces stronger low-angle radiation, which can be useful for DX. However, a vertical also depends heavily on a good ground system, radials, or counterpoise. Without this, efficiency can be poor, especially on the lower bands.
For stations less than roughly 500 miles away, a low horizontal antenna may often outperform a vertical because the vertical’s high-angle radiation is usually weaker. For long-distance DX, especially over good ground or with a good radial system, the vertical can be very effective.
Key points to remember
- A horizontal wire produces mainly horizontally polarised radiation.
- A vertical conductor produces mainly vertically polarised radiation.
- A sloping wire produces a mixture of horizontal and vertical radiation.
- Circular polarisation is useful for satellite and space communication.
- On HF, ionospheric propagation can rotate polarisation, causing fading or QSB.
- For line-of-sight work, matching polarisation is very important.
- High-angle radiation is better for nearer stations; low-angle radiation is better for DX.
- Vertical antennas can be excellent for DX, but they need a good ground or radial system.