|Fig 1. A European Antennas 2-18 GHz cavity-backed spiral antenna
Fig 2. Electromagnetic simulation of a cavity-backed spiral antenna
Fig 3. Typical radiation pattern for a cavity backed spiral antenna
Fig 4. A bi-directional spiral antenna. Note the additional proprietary features used to extend the frequency coverage.
Fig 5. Electromagnetic simulation of a bi-directional spiral antenna
Fig 6. Typical radiation pattern for a bidirectional spiral antenna
Fig 7. Typical radiation pattern for a unidirectional spiral antenna
Fig 8. A European Antennas bicone antenna
Fig 9. Electromagnetic simulation of a bicone antenna
Fig 10. Typical radiation pattern for a bicone antenna
|Technical article published in Microwave Journal, January 2007
The Use of Broadband Military Antenna Technology in Modern Communications Systems
by Chris Walker, Technical Director, European Antennas Ltd
For many years military applications have required broad bandwidth communication links with antennas that must operate over a frequency range of several octaves. Consequently antenna structures that have multi-octave capability have been designed and these techniques are now proving useful in designing antennas for modern commercial communications systems. This article considers the evolution of commercial communications systems, particularly the increasing demand for new frequency bands before focusing on two types of antenna spiral and biconical.
MODERN COMMUNICATIONS REQUIREMENTS
Many communication systems are required to provide coverage in the 900 MHz frequency range and as networks evolve additional bands have been utilised at 1700-2200 MHz for DCS, PCS and UMTS. Coverage is often required for TETRA (around 400 MHz), wireless LAN and wireless local loop (2500 MHz, 3500MHz and 5500 MHz). To satisfy all these requirements it is necessary for the antennas to provide effective coverage from 400 MHz to 6 GHz.
In some applications this may be accomplished by a series of individual antennas each assigned to a portion of the band. However, to avoid many antennas being required, and for a more discreet appearance, it is preferable to provide coverage over the entire frequency range from a single antenna. Other benefits of using a single antenna are, apart from general aesthetics it can help obtain planning permission and satisfy architectural requirements by avoiding a proliferation of antennas cluttering a new building.
Multi-functionality is part of the appeal of many modern systems. Antennas can include a multi-band capability that may be fixed or mobile, access point or base station. The professional user needs the equipment to work without having to adjust the the equipment. The transition between operating bands, therefore, needs to be seamless for both the access point and subscriber.
If a subscriber in a system is to be mobile, then the antenna radiation pattern will be omni-directional so that coverage is assured, regardless of the orientation of the subscriber with respect to access points. In a fixed system, the subscriber antenna may be need to be directional, oriented towards the nearest base station, for optimum performance.
Base stations can be deployed in a number of configurations. Sometimes an omni-directional base station is placed in the centre of a coverage area, or a cluster of sector antennas may be deployed at a single location with each covering a sector of the area around the base station. Systems typically employ sector angles ranging from 30º to 180º.
Also, the polarisation used in systems varies. It may be linear vertical, horizontal, 45° slant or circular, each having advantages which vary according to system protocols and architecture.
There are further system benefits through the use of additional antennas. The provision of spatial diversity, polarisation diversity, adaptive antennas, or multiple-input multiple-output (MIMO) configurations all serve to increase the statistical likelihood that a link can be maintained with acceptable signal to noise ratio for successful operation.
The scope of this article is to consider the performance of a single antenna system, rather than looking at the further benefits that might be obtained by combining several antennas.
The spiral antenna has long been used in defence applications for direction finding systems and general threat detection. Figure 1 shows a typical 2-18 GHz cavity backed spiral antenna and Figure 2 shows an electromagnetic simulation performed on such an antenna, indicating the field strengths present on various parts of the structure.
In these applications the antennas generally require a uniform pattern shape with respect to amplitude and phase from one antenna to another. The main beam should exhibit a smooth curve without any points of inflection - monotonic.
A typical radiation pattern for such an antenna is shown in Figure 3. It is more important in these applications to control the beam shape and match the performance of the antenna from unit to unit than to maximise the antenna gain.
For many commercial communication systems it is more important to fill an area with signal than to produce a precise beam shape. The spiral radiating structure used in the above example is suited to this, but no longer needs to be used in conjunction with a cavity loaded with absorber. Two types of antenna can then be created using this type of structure; a bi-directional structure where the spiral is allowed to radiate freely into space in both directions normal to its place, or a higher gain, unidirectional structure, where a reflector plate is positioned close to the spiral so that the radiation in one direction is reflected and the forward gain is therefore enhanced. Figure 4 shows a bi-directional spiral antenna while Figure 5 illustrates an electromagnetic simulation of a similar type.
The radiation characteristics for a bi-directional spiral antenna such as this are shown in Figure 6. It shows that when the polarisation is left hand circular in a forward direction, it is right hand circular in the opposite direction. Thus, in a transition region in between it will be linear. Such a pattern is an advantage in many deployments such as where an antenna is to be placed in a corridor or hall, then a bi-directional antenna of this type is ideally suited.
Sometimes a directional antenna is desired whilst retaining its broadband properties. This would typically be for a deployment in the corner of the area to be covered, such as a large hall or atrium. In this case, the reflector plate increases the gain in the forward direction. Figure 7 shows a typical radiation pattern for such a unidirectional low profile spiral antenna.
Depending upon the size of spiral selected, coverage can be provided across the bands between 400 MHz and 6 GHz. These antennas are used in systems where all the communication bands are required to be transmitted or received by a single antenna, either when originally installed or at a later date.
Bicone antennas can be designed to operate effectively over a large frequency range. These antennas produce a linearly polarised signal, and exhibit extremely low azimuth ripple, so that the omni-directional characteristics are excellent. Depending upon the degree of input return loss degradation that can be tolerated, the effective bandwidth of such a structure is in the region of two octaves. This will depend upon the extent to which the structure has been miniaturised by the choice of transition region from a cone to a cylinder, or if an entirely conical section has been used. Figure 8 and Figure 9 show a European Antennas bicone antenna and its simulated results.
Figure 10 shows its measured performance and the very low ripple in the azimuth pattern can clearly be seen. Although the high power handling capability is not always important in commercial communications systems, the small size of these structures makes them attractive in this application. For example, a product capable of covering all the frequency bands from 800 MHz to 2.2 GHz can be packaged in a structure of 32 mm diameter by 225 mm long.
It is possible to achieve wider bandwidths, but a compromise has to be made in terms of the diameter of the product. A larger diameter could extend the frequency of operation.
This type of antenna structures retains a radiation pattern characteristic (shown in Figure 10) over this wide frequency range making them suitable for microcell and picocell applications where the antennas can provide a signal boost such as in a railway terminus. A central deployment would be the most effective for antennas with this kind of radiation pattern.
There is an increasing demand for new frequency bands to be added to modern communications networks.
As these new frequency bands become available for commercial use, system antennas have to be upgraded to cover these additional frequencies, however if an existing antenna was not designed with sufficient bandwidth, it will be unable to maintain its performance over the wider frequency.
However, design variations applied by European Antennas to traditional spiral and bicone broadband antennas has resulted in commercial products that are using technology originally developed for military applications and now provided significant commercial benefits.
Chris Walker is technical director of European Antennas Ltd with responsibility for the assessment, modeling and development of antennas for future technologies. He graduated from Cambridge University with a Bachelor's degree in physics with theoretical physics in 1982 and he has worked in the design of microwave devices ever since. He has worked with EEV, Litton and CPI developing vacuum tubes for radar and communications systems. The most significant projects being related to the development of injection locked magnetrons for use in pulsed Doppler radar systems. Other development work included developing piezoelectric tuning mechanisms, long life cathode technology and miniaturised magnet packages for magnetrons. During these activities he was awarded five patents relating to aspects of magnetron design. He joined European Antennas, part of the Cobham Antennas Division of Cobham plc, in 1998, where he has been involved in the company’s design and manufacture of directional flat panel antennas, base station sector antennas and omni-directional antennas within the range 250 MHz to 40 GHz for military and commercial applications within satellite, data, WiMAX, WLAN, telemetry, security, surveillance and broadcast systems.