- Polarisation

While polarisation may seem insignificant compared with other considerations in setting up a communications system, choosing the right type can make significant improvement in performance.

Looking at the basic physics, an antenna radiates energy in the form of an expanding sphere in what are known as wave fronts. These wave fronts travel at an angle of 90 degrees to the direction of travel of the radiation. All surface energy is in phase and although it initially progresses symetrically, the further the wave fronts travel, the less spherical the radiation becomes, eventually forming a plane surface or plane waves.

The radiation field consists of both electrical and magnetic lines of force at right angles to each other. Waves are polarised in the direction of the electrical lines of force. When the electric lines are perpendicular to the earth, polarisation is vertical, when they are parallel to the earth, polarisation is horizontal.

Thus, at distances far enough from an antenna to form plane waves, a signal becomes polarised in the direction of the antenna elements, so that an antenna with elements placed vertically to the ground will result in vertical polarisation. Similarly horizontal elements parallel to the ground will produce horizontal polarisation or horizontal lines of force. Vertical and horizontal polarisation are known as linear polarisation.

The characteristics of arrays with elements placed in parallel do not differ in polarisation from single element antennas. However, if both horizontal and vertical elements are used in the same plane and radiate in phase, the polarisation becomes tilted between horizontal and vertical, and when fed out of phase, elliptical polarisation is produced with waves rotating continually, somewhat at random. This gradual shift in polarisation is known as Faraday rotation. Harmonic antennas are polarised in the direction of the antenna axis and are subject to becoming elliptical in most directions.

Elliptical polarisation is subject to fading due to cross-polarisation, or radiation that is different from the polarisation in which the antenna was intended to be used. Cross-polarisation in linear polarised antennas is at right angles to the intended polarisation.

In space, waves are not subject to the influence of the earth and the concepts of vertical and horizontal polarisation have no meaning, resulting in elliptical polarisation (as above). Circular polarisation is frequently used for satellite communications to harness the random rotations of elliptical polarisation. Arrays of crossed linear elements, such as dipoles or yagis, helicals or quadrifilar helix antennas, like the Moonraker type QH, all exhibit circular polarisation. (to be continued)

In circular polarisation, the wave front appears to rotate every 90 degrees between vertical and horizontal, making a complete 360 degree rotation once every period, thus overcoming the effects of Faraday rotation.

The signal is rotated either clockwise or anti-clockwise from the transmit antenna and the receive antenna is oriented to this direction of circularity. Cross-polarisation occurs when the receive antenna is oriented in the opposite direction to the transmitted signal. In terrestrial communications, there is little use for circular polarisation. When the transmit antenna is relatively close to the ground, propagationally speaking, vertically polarised waves tend to be stronger, while horizontally polarised waves are stronger high above ground. While polarisation does not generally alter over short distances, this does not remain so. Over long distances the change is usually small at low frequencies but can become quite rapid at high frequencies.

Sky wave transmission usually changes the polarisation of travelling waves so that antennas do not need to have the same polarisation at both ends of a circuit larger than a few miles. Choice of polarisation is usually determined by required radiation characteristics together with practical considerations, like the height of available antenna supports and possibility of signal interference from local radio or television broadcasts or man-made RF noise.

Vertical antennas are often favoured over long distances as they are omnidirectional and exhibit low angle radiation. Horizontals, provided the site is large enough, can be preferable over short to medium ranges, and, depending on height above ground, can also be used for longer circuits. The degree to which they are omnidirectional is also directly related to antenna height above ground.

Tilted antennas are favoured for close-in local area communications to minimise the skip distance as they exhibit high angle radiation with both horizontal and vertical patterns. As horizontal polarisation produces practically no ground wave, a ground wave (LF to low HF), of necessity must be vertically polarised. So, the radiation from an antenna that is to produce a good ground wave must also be vertically polarised with mostly vertical antenna elements, as in the Moonraker T Top.

Electric lines touching the surface of a good conductor must do so at right angles. At frequencies below around 10 MHz most ground is an adequate conductor. So waves at these frequencies are mainly vertically polarised. Over partially conducting ground, the wave front tilt forwards increasingly as more and more energy is absorbed by the ground, thus attenuating the signal. Above 10MHz the wave fronts have become so tilted that the range of the ground wave ceases to be of practical use. (to be continued)

At VHF and above the direct wave signal is much shorter and less likely to change polarisation as it travels. Therefore, in order to avoid cross-polarisation and achieve maximum absorption of energy from the electromagnetic fields, it is necessary for a receiving antenna to have the same polarisation as the transmit antenna otherwise some loss of signal strength will occur.

Where antennas are of differing polarisation, discrimination is quite high, usually in excess of 20dB.

Horizontal polarisation predominates in other communications on 50 MHz and higher. There is some evidence that above 100 MHz, where polarisation shifts may occur due to terrain obstructions or reflections, vertical polarisation shifts to horizontal more readily than horizontal to vertical. Therefore horizontals may be preferable for weak signal communications, as higher signal levels can be achieved over irregular terrain. Also man-made noise, especially ignition interference, is less likely to be horizontally polarised.

Vertically polarised antennas are more popular with FM and repeaters, as they are much simpler to use in omnidirectional systems and in mobile work. Practically all VHF mobiles are now handled with vertical systems due to the physical advantages of mounting on vehicles and at sea. In addition it is easier to obtain omnidirectional radiation with a vertical antenna than it is with horizontal one. There is no need for mounting at a specific height above ground.

Generally it is not a good idea to use circular polarisation for earth circuits where the arriving polarisation is random, as there will be a -3dB loss (half power) compared with a single plane polarisation of any kind. The choice of polarisation/antenna type for space communications depends on the characteristics of the individual satellite and its orbit.

Helically wound antennas are frequently used for both terrestrial and space communications as they exhibit different characteristic depending on the ratio of length to diameter. If the diameter of the helix approaches 1/3 wavelength, at the frequency of operation, the antenna functions as a travelling wave type antenna with a directional circularly polarised wave that rotates about the axis of the antenna. It has a broad bandwith with high gain providing predictable radiation pattern, gain and impedance characteristics over a wide frequency range and is suitable for use with circularly polarised satellites but only those in geostationary orbit, unless azimuth/elevation rotators are used.

Where the ratio is small, however, the antenna functions as a normal vertically polarised monopole with omnidirectional patterns in the azimuth plane, such as Moonraker types that are used for hand held transceivers.

Polarisation is increasingly important where receiving antennas are concerned. Today, radio and television broadcasts are allocated increasingly higher frequencies where shorter wavelengths make quality reception more of a challenge, especially in terrain that is not flat. Signals at UHF frequencies are far more subject to polarisation reversal. This is why our new MkII TV Dome features multiple UHF arrays that can receive both horizontally and vertically polarised signals.

This all goes to prove that the more advances we make, the more we need to make just to keep up!