The Changing Shape of Antennas in Modern Systems
By European Antennas Ltd.

For many years, from the start of radio communications, defense applications have required systems that operated at frequencies, generally high frequencies, unavailable to commercial system operators. Fortunately for us, as an antenna company, many of these frequency bands have become available and are now used for commercial operations around the world. This freedom has led to an explosion in wireless usage.

For example, back in 1992 when HTML was written, a laptop computer with 180Gb would have needed a crane, not a lap, and as for a wireless laptop computer that could connect virtually anywhere – this was something most people couldn’t envision. With the Internet and use of mobile phones came the increasing demand for high frequencies to provide greater bandwidth – broadband.

When European Antennas was formed in 1991, one of its first customers was a wireless telephone network operator with base station and subscriber antennas. Sadly, although popular, the usage wasn’t enough for that company to survive, particularly with the growth of the mobile network. But European Antennas was in the right place at the right time, and as higher frequencies became available around the world, requests were received for highly accurate, robust antennas; discreet, customized, special.

Now there’s an irony in the market, a turnaround in the way wireless communications are developing. Traditionally, antennas for military and defense applications had a high specification with a price to match. There are many applications where this is, of course, still the case. However, as the commercial wireless market has developed, European Antennas’ product range has grown and now includes many robust, high specification, accurate antennas that perform exactly as their specification, with full documentation to back them up. Examples of this type of application are those developed for F1 Grand Prix race cars, Champ cars, World Rally Championship cars, weather buoys or orbiting satellites.

These robust antennas are now part of a highly respected range of commercial-off-the-shelf (COTS) antennas – high quality, high spec, but at a commercial price – that are being requested and used by security, defense and government organizations for similar applications – rough terrain, widely varying weather and temperature conditions – all able to meet the accurate specifications required for data and voice communications.

Just one antenna?
Many systems are require coverage at 900MHz, 1700-2200MHz for DCS, PCS and UMTS. There is TETRA (around 400MHz), wireless LAN and wireless local loop (around 2.4GHz and 5.8GHz).

So, do customers want a proliferation of antennas for all these frequencies? Or just one? Which is best, one broadband antenna covering 400MHz to 6GHz (less stock, more discreet, attractive from an architectural point-of-view), a multi-band antenna (more cables) or a series of antennas? It depends on the application, of course. A more specific antenna could provide a lower cost option. Ultimately, the user has to decide which is the most important: their application, cost or performance.

A single broadband spiral antenna
Spiral antennas are frequently used in defense or security applications for direction finding systems and general threat detection – in this instance, one very small antenna can handle a variety of applications.

Technical information on broadband spiral antennas
Figure 1 shows a typical 2.0-18.0GHz cavity backed spiral antenna. Figure 2 shows an electromagnetic simulation indicating field strengths.

In these applications, a uniform pattern shape is required with respect to amplitude and phase from one antenna to another. The main beam should have a smooth curve without any points of inflection (monotonic). Figure 3 shows a typical radiation pattern for such an antenna.

It is important to control the beam shape and match the performance of the antenna from unit to unit. For commercial systems, it is important to fill an area with signal and the spiral radiating structure in the above example is suited to this.

Two types of antennas can be created using this type of structure: a bi-directional, where the spiral is allowed to radiate freely into space in both directions, or a high gain, uni-directional where a reflector plate is positioned close to the spiral so that the radiation in one direction is reflected and the forward gain is enhanced.

Figure 4 shows a bi-directional spiral. Figure 5 shows an electromagnetic simulation. Figure 6 illustrates radiation characteristics for a bi-directional spiral antenna.

When the polarization is left-circular in a forward direction, it is right-circular in the opposite direction so that in a transition region it will be linear. Such a pattern is an advantage in many deployments, such as if an antenna is installed in a corridor or hall.

Sometimes a directional broadband antenna is required for deployment in a hall or atrium. In this case, the reflector plate serves to increase gain in the forward direction.

Figure 7 shows a typical radiation pattern for a directional, low profile, spiral antenna.
Depending upon the size of spiral, coverage can range from 400 MHz and 6 GHz. These antennas are used in systems where all the communication bands must be transmitted or received by a single antenna.

Technical information on broadband bicone antennas
Bicone antennas operate effectively over a large frequency range. They produce a linear polarized signal with low azimuth ripple, meaning that the omni-directional characteristics are excellent. Depending upon the degree of input return loss degradation that can be tolerated, the effective bandwidth of a bicone antenna is in the region of two octaves.

Figures 8 and 9 show a European Antennas bicone antenna and electromagnetic simulation of the antenna.

Figure 10 shows the measured performance of the bicone, showing the low ripple in the azimuth pattern. The very low ripple in the azimuth pattern is seen clearly here (Figure 10 ). Although the high power handling capability is not always important in commercial communications systems, the small size of these structures makes them very attractive in this application.

High power handling capability of these antennas is not always important in commercial systems, but the small size makes them very attractive. For example, an antenna covering all the frequency bands from 880 MHz to 2.2 GHz can measure just 32mm (1¼ inches) diameter by 225mm (9 inches).

An alternative to broadband – single multi-band antenna
An alternative to broadband is multi-band. Most modern equipment is designed to be multi-functional, therefore a single antenna with several different frequency bands, able to work without having to adjust the operating mode of the equipment and with seamless transition between operating bands between access point and the subscriber, offers significant benefits to the customer.

If a subscriber in a system is mobile, the antenna beam pattern must be omni-directional to ensure coverage is maintained, regardless of the orientation of the handset. In a fixed system, the subscriber antenna can directed at the nearest base station.

Some systems may benefit from the use of additional antennas to ensure links are maintained - spatial diversity, polarization diversity, adaptive antennas and multiple-input multiple-output (MIMO) configurations are considerations.

About European Antennas
European Antennas Ltd undertakes antenna development projects and recognizes that communication systems may be under development before antenna specifications can be finalized. New developments are tested at the company’s on-site near field spherical test chamber, with all antennas tested during manufacture. Design, manufacture and administration are carried out at a single facility in the UK, ensuring that quality control is maintained – made in the UK. European Antennas has ISO 9001 certification.

European Antennas Ltd.
www.european-antennas.co.uk
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