At the heart of the system is an...

Hills Limited
By Rolf Roelfsema, Wireless Products Dev Manager, Hills Industries Limited
Monday, 10 January, 2005


My dictionary tells me that an antenna is an insect's feeler or an aerial. This article will take a how and why look at aerials as devices used for sending and receiving radio waves.

The design and construction of antennas has provided fascination to those involved in the radio industry and a mystery to those who do not understand the associated theory.

With the recent increasing proliferation of wireless connectivity for phones, entertainment, data networks and security there is a whole new generation of people asking, "How does it do that?"

The appearance of antennas has changed from the basic models produced by radio pioneers as advances in electronic equipment technology have enabled operation at ever higher frequency bands.

The fundamental starting point for the design of antennas is to know the length of one radio wave in metres by dividing the speed of light 300,000,000 m/s by the proposed operating centre frequency in hertz.

The required target area for the proposed radio path influences the wavelength and length in the design of the antenna. Then start adding reflectors with directors and you will realise that antenna engineers deserve their pay.

As antennas are required to focus on smaller target areas, much greater attention must be given to the rigidity of construction and the immobility of the antenna mounting structure.

Antenna designers must also consider the influence of the earth or ground plane as it becomes a part of the design path objective.

A whole related design requirement to provide safety for operating personnel and protection for equipment from lightning strikes is too much to include here.

My first contact with antennas and the realisation that there are invisible powerful radio signals surrounding us, was demonstrated to me when my dad used a 10 m end fed long wire antenna to feed a 'crystal set' detector with high impedance earphones so that I could listen to AM radio without using amplifiers, batteries or solar power.

Amateur radio groups must also be given a great deal of credit for fostering new entrants into the radio communications industry with a willingness to try ideas that often progress to commercial production.

Many readers may not be aware that through government groups such as the CSIRO and companies such as Amalgamated Wireless Australasia, Hills Industries and others, Australians have been pioneers in many antenna designs.

Antennas are essential elements in communication without the use of interconnecting wires or optical fibres as they radiate electromagnetic waves, which are then intercepted by other antennas to retrieve as much as possible of the original information.

This transmission and reception process has enabled communication between individual people, mass broadcasts such as TV, vehicles, ships, submarines, aircraft, satellites and space craft.

The devices are key elements in navigation aids for ships or aircraft and personal navigation tools like the satellite-based global positioning system. Some low frequency antenna systems such as those used for communication with submarines are so tall that they become physical hazards for aircraft and transmit such high energy levels that fences for kilometres around those sites become energised like antennas unless they are adequately earthed.

Antennas are required for radio astronomy and, when combined with the wide open space and the low RF noise in rural Australia, enable giant steerable dish antennas to watch the stars and be used as part of the international space program.

Terrestrial yagi and satellite dish television antennas would be the most common types of antennas seen in the residential suburbs but people are becoming aware that those long panels and large dishes on the mobile phone base site towers are also antennas.

Most mobile phone users are probably not aware that they are holding a cleverly integrated antenna in their hand many times each day.

The question most often asked about antennas is how far will this one take my signal?

Consultants use special databases and formulas to calculate radio paths but some manufacturers provide estimates of performance of their equipment.

Apart from the antenna being designed to operate in the required frequency band, government bodies such as the Australian Communications Authority provide regulations and controls on the amount of RF energy specific antenna installations are allowed to transmit for efficient use of the radio frequency spectrum.

To maintain the expected performance of a radiocommunications link, the antenna designated to receive the signal must remain within the target pattern of the transmitting antenna or the signal quality usually drops off dramatically.

Designers are being requested to produce antennas at ever higher frequencies, so it is fortunate that many designs can be scaled down or up for rapid response to commercial requirements.

Antenna manufacturing companies that have invested in R&D engineering staff and calibrated testing facilities can usually be first to market with new professionally built antennas.

To achieve radio path distance and to punch through the RF background noise focused antennas with high front to back signal ratios are desirable and the reduction of RF side lobes helps minimise co-channel interference.

The use of horizontal or vertical polarisation of antennas signals also helps increase isolation between antennas on similar frequencies at the same site.

Designers use a variety of techniques to produce antennas from calculated layout of tracks on a printed circuit board, to folded metal tubing, to phased arrays, to slots cut in RF wave guides. These designs are often coupled with parallel reflector elements or parabolic focusing grids or dishes.

Antennas due to their size and location often high above the ground experience high wind loadings, therefore they need to be mounted on physically stable structures such as rigid poles or towers with guy wires.

These types are considered necessary but are often a visual pollutant, which has resulted in many separate antenna installations sharing the same support structures.

At the other end of the scale, 'rubber duck' type antennas on mobile radios need to be very flexible to withstand multiple impacts.

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