Aircraft antennas

It may not be apparent from an inspection of the external profile of an aircraft that most large aircraft carry several dozen antennas of different types. What should be apparent from this is that many of the antennas are of the low profile variety which is essential to reduce drag.

Antennas are used both for transmission and reception. Transmitting antennas convert the high frequency electrical energy supplied to them into electromagnetic energy which is launched or radiated into the space surrounding the antenna.

Receiving antennas capture the electromagnetic energy in the surrounding space and convert this into high frequency electrical energy which is then passed on to the receiving system. The law of reciprocity indicates that an antenna will have the same gain and directional properties when used for transmission as it does when used for reception.

Isotropic radiator

The most fundamental form of antenna (which cannot be realised in practice) is the isotropic radiator. This theoretical type of antenna is often used for comparison purposes and as a reference when calculating the gain and directional characteristics of real aircraft antennas.

Isotropic antennas radiate uniformly in all directions. In other words, when placed at the centre of a sphere such an antenna would illuminate the internal surface of the sphere uniformly. All practical antennas have directional characteristics. Furthermore, such characteristics may be more or less pronounced according to the antenna's application.

Half-wave dipole antennas

The half-wave dipole is one of the most fundamental types of antenna. Half-wave dipole antennas consist of a single conductor having a length equal to one-half of the length of the wave being transmitted or received.    

The conductor is then split in the centre to enable connection to the feeder. In practice, because of the capacitance effects between the ends of the antenna and ground, the antenna is cut a little shorter than a half wavelength, rather than its actual physical length. End effects, or capacitance effects at the ends of the antenna require that we reduce the actual length of the aerial and a 5% reduction in length is typically required for an aerial to be resonant at the centre of its designed tuning range.

The current is maximum at the centre and zero at the ends. The voltage is zero at the centre and maximum at the ends. This implies that the impedance is not constant along the length of aerial but varies from a maximum at the ends (maximum voltage, minimum current) to a minimum at the centre.

Dipole antennas have directional properties. A vertical dipole will have omnidirectional characteristics whilst a horizontal dipole will have a bi-directional radiation pattern.