Radiating Cable Antenna Systems
By Tom Goodwin and Ray Schwartz, M/A-COM

Personal communication devices are ubiquitous in today’s world. With the plethora of cellphones, PDAs, and wireless laptops operating on different standards over widely varying frequencies, the demands placed on antennas required by system providers are extreme in order to achieve the goal of universal coverage. Additionally, wireless communication systems antennas are more and more frequently being installed not only on cell towers, but also inside tunnels and buildings, on trains and other vehicles, even including aircraft, so that besides the required Ultra Wide Bandwidth (UWB) capability, physical constraints also challenge the antenna designer.

In order to address these challenges, M/A-COM has developed a series of Radiating Cable Antenna products that are designed to replace conventional antennas for applications that require very wide bandwidths or for use in physical spaces that provide extreme challenges to traditional antenna solutions. These unique antenna assemblies use a special low loss flexible coaxial cable construction which includes radiating apertures in the cable outer conductor.  The amount of radiation can be controlled by adjusting the aperture parameters, allowing for performance to be easily tailored for a wide range of application-specific requirements. The low weight and small minimum bend radius expedite installation in locations with restricted access.

M/A-COM’s Radiating Cable Antennas operate over broad frequency ranges and they can be used for transmit, receive and simultaneous Tx/Rx applications. The antenna construction is rugged and is suitable for airborne, vehicular or stationary ground applications.  Antenna lengths are available up to 200 feet. These products are designed to address a wide range of system applications and environments.

Antenna Requirements and Capabilities
While specific antenna requirements must be tailored to each particular application, we have produced a generic set of performance requirements that are likely to be necessary for most wireless antenna installations. Tables 1 and 2 list electrical and mechanical/environmental performance requirements that are representative of typical Radiating Cable Antenna applications.

M/A-COM drew on its over 50 years of experience (beginning with our Adams Russell heritage) in designing and manufacturing high performance RF coaxial cables and antennas to enable the Radiating Cable Antennas to meet or exceed these demanding requirements. The general construction of the antenna is shown in Figure 1. Each component of the antenna, from the center conductor to the radiating apertures, can be individually optimized depending on application-specific requirements. For example, a fixed installation (i.e. a tunnel or in-building application) where weight is not as critical, may use a solid copper center conductor to minimize cable insertion loss, while a vehicular (particularly an aircraft) installation where minimum weight and the ability to bend around tight obstacles are essential, would require a more flexible, lower weight construction as shown. Since M/A-COM maintains an in-house cable design and manufacturing capability, this allows us to provide customized constructions as required. M/A-COM also designs and manufactures connectors so that we can optimize the critical cable-to-connector transition as well as the connector interface’s electrical performance for each assembly.

Cable insertion loss is minimized by proper selection of the cable size and careful attention to the basic cable design and materials used for its construction; the electrical performance of the Radiating Cable Antenna is thus largely dependent on the radiating aperture array parameters. By modifying the size, shape, and spacing of the apertures, the performance can be optimized for a specified frequency range or pattern coverage. Alternatively, if the Radiating Cable Antenna is required to work in close proximity to a ground plane or other obstacles, the aperture array design can be tailored to the specific operating environment. For moderate length antennas with the 0.6" outer diameter cable shown – up to approximately 60' in length – the inherent cable insertion loss is low enough that a uniform aperture pattern may be employed along the entire length of the assembly. For longer antennas in applications where a large cable diameter can not be used, the aperture pattern must be varied to accommodate the cable insertion loss; the radiating sections at the far end of the antenna must produce a higher gain to compensate for the additional loss in these sections. M/A-COM has produced working designs up to approximately 200 feet in length with nearly uniform gain along the entire length of the antenna. An end-to-end extruded FEP jacket protects the apertures and prevents moisture ingression should the radiating cable be installed in humid or moist areas.

M/A-COM employs a structured combination of electromagnetic modeling and physical prototyping to design the Radiating Cable Antenna and to measure and verify performance. Our primary design tool is Ansoft’s HFSS. A typical HFSS model and predicted pattern is shown in Figure 2. Using HFSS, we can quickly vary the antenna parameters to evaluate performance over frequency and also investigate the effects of objects in close proximity to the antenna. The electromagnetic modeling is augmented by physical prototyping. M/A-COM maintains a rapid prototyping capability that enables us to quickly fabricate antenna samples to verify performance. In addition, physically long – and therefore, electrically very large – antennas cannot be computer modeled as full-scale units; our prototyping capability allows us to fabricate and test full length models to ensure requirements are met. Figure 3 shows a short (10') prototype undergoing far-field testing in one of our antenna ranges.

M/A-COM has developed specialized testing methods and test chambers to evaluate the Radiating Cable Antenna system performance. We can measure both far-field array-type antenna patterns as well as near-field performance. Even measuring the antenna insertion loss requires a specialized setup. The antenna must be fully extended (as opposed to a traditional, nonradiating cable assembly which can be measured while coiled up), and clear from interfering objects, since the radiating apertures are affected by their environment. We maintain a large test chamber (approximately 120' x 30') in our manufacturing facility dedicated to these measurements.

M/A-COM maintains in-house control over every aspect of the Radiating Cable Antenna system design, manufacture and test. Each component of the antenna system – from the raw cable materials, construction and fabrication, to the connectors and aperture pattern design– can be individually optimized for application-specific requirements. This in-house capability enables rapid response to produce antenna solutions for unique performance or installation needs.

Radiating Cable Antenna System Performance
M/A-COM’s production Radiation Cable Antenna system units exhibit performance which cl.osely matches the idealized specifications defined in Section II above. Figures 4A through 4D show measured far-field pattern performance at 900, 1850, 2450, and 5850 MHz. While some nulling is evident, the pattern coverage is near ideal. In fact, for the four frequencies shown here, a minimum of 95% of all points over the coverage volume exhibit gain greater than -30 dBi.

Additionally, production units have been subjected to a complete Environmental Qualification program, including all of the environments specified in Table 2. These units are fully qualified for installation in ground-fixed, vehicular, or airborne applications.

Summary
M/A-COM has developed a new line of Radiating Cable Antenna system products. These antennas operate over wide frequency bandwidths, encompassing all of the current cellular, Bluetooth, and WLAN bands. They are suitable for airborne, vehicular, or ground-based installations. The Radiating Cable Antenna system products can be tailored to customer-specific requirements.

M/A-COM
www.macom.com
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