On California’s Mt. Wilson, dual RFS broadband panel arrays provide sculpted digital/analog signals for four broadcasters.
With the launch of digital terrestrial television (DTT) has come a generally more demanding broadcast environment. Not only has the number of channels on-air increased dramatically, but also the corresponding issues of spectrum loading and interference have placed additional burdens on operators to ensure they do not breach their licenses while achieving coverage objectives.
The customization of an antenna radiation pattern to suit a specific physical location and intended coverage area is, therefore, an attractive prospect for broadcasters. Broadband panel arrays are well-suited to such pattern sculpting requirements. This is due to the use of parallel feed systems that permit amplitude and phase adjustment, plus physical adjustment, of individual panels within an array. This ability to customize both the horizontal and vertical radiation patterns of panel arrays provides broadcasters with planning flexibility and optimized system performance.
Tailoring the pattern
A traditional panel array is uniform. The normalized vertical radiation pattern (VRP) is the same at all azimuth angles, and the normalized horizontal radiation pattern (HRP) is the same at all elevation angles. Radiation pattern control is achieved by varying the power and phase of the signal fed to each panel and the physical configuration of the array. In a uniform array, the magnitude and phase relationship between levels on each face, and between faces on each level, is constant.
A non-uniform approach to array design is an option in order to tailor the pattern more precisely for a specific application. By changing the aperture between faces of an array, or by having different vertical phase relationships, different VRPs at different azimuth angles can be achieved.
However, the effect of varying the VRP is to introduce transition regions into the pattern. These need to be carefully monitored because interactions in such transition regions match neither adjacent face, and the possibility exists that the VRP in these regions could be unacceptable.
Figure 1. Vertical radiation pattern on a 12-level antenna. Click here to see an enlarged diagram.
This issue is illustrated in Figure 1, which shows typical VRPs in a complex array that uses both aperture and phase relationship variation within the array. The figure shows the VRP on a 12-level face, an adjacent eight-level face and a location halfway between them. Clearly, the VRP between the two faces is significantly departed from either of the adjacent faces.
Consequently, designing an array using these techniques requires spherical integration of the array, where the field is calculated at all angles of azimuth and elevation in a 3D simulation. This assessment needs to encompass the VRP to the required depression angle at all azimuth angles, as these complex configurations can introduce pattern nulls or reductions in undesirable locations.
Figure 2 demonstrates the precise control of HRP in the same array. As the depression angle decreases, field strengths increase in the west, north and east. However, the peak field strengths are restricted to the south, while maintaining a more uniform field strength closer to the transmission site.
Figure 2. Horizontal radiation pattern variation with elevation angle. Click here to see an enlarged diagram.
This level of antenna pattern customization can effectively reduce interference, which has become a critical issue as analog services are overlaid with DTT. This interference control is achieved by limiting overspill outside the coverage area, and by ensuring the specified coverage area field strengths are sufficiently high to minimize the likelihood of outside interference causing reception degradation. It not only ensures good conditions for reception, but also facilitates spectrum allocation and frequency re-use.
Other benefits include minimized compromise on coverage in main areas, generally lower reflections in the transmission environment and maximized array gain. This gain maximization can facilitate cost reductions in both set-up and ongoing running costs, as lower base power options can be considered.
Operational bandwidth and power handling
The operational bandwidth of a broadband antenna is the overlay of the impedance and pattern bandwidths. Panel arrays, due to fundamental dipole characteristics of the panels and their parallel feed systems, generally provide the broadest impedance bandwidth, which is the frequency bandwidth over which the system has an acceptable VSWR. In many cases, this bandwidth can cover the full band (e.g. UHF band, full 470MHz to 860MHz).
The array impedance is a function of the individual panel impedance, panel disposition and the configuration of the feed system. Techniques such as the inversion of panels, phase rotation and perturbation, and reflection cancellation within the power divider networks can be used to optimize array impedance and greatly reduce reflections within the array.
Pattern bandwidth is the frequency range over which the pattern characteristics vary within acceptable limits. A broadband antenna will generally have a relatively low level of pattern variations over the bandwidth.
However, design techniques used to optimize impedance bandwidth can degrade pattern bandwidth. For example, phase rotation of an array, while providing good patterns at the design frequency, will not function over a wide frequency range because the design techniques are frequency specific. This technique can limit pattern bandwidth to about 20 percent in a UHF panel array. A non-phase rotated array can achieve pattern bandwidths approaching 100 percent in the case of a UHF panel array.
Panel arrays can be configured to handle a wide range of power levels due to their parallel feed systems, which are designed to achieve the desired power handling and power distribution ratios. The number of inputs to the panel array will typically vary from one to four; these branch out until the individual panel is reached, using line sizes from around 7/8-inch to 8-3/16-inch. This allows average power levels, starting at a few watts to 100kW per input at UHF frequencies.
With the introduction of DTT services, the voltage ratings of arrays also have become an important consideration. If DTT services are combined, system voltage ratings can become critical before average power ratings. DTT services have a higher peak-to-average voltage ratio than analog services (7.5dB to 10dB, depending on the modulation scheme). Therefore, an array could fail due to voltage breakdown (arcing) before exceeding its average power rating (overheating).
Panels for performance
In an increasingly demanding broadcast environment, panel arrays offer solutions to broadcasters not available in any other antenna type. With a proven record of reliability, a well-maintained panel array could have a life expectancy of more than 20 years, with many having been in service for longer. Due to their method of construction and the design options available, panel arrays allow a high level of control in meeting site dependant requirements.
The ability to vary radiation characteristics within an array with respect to azimuth and elevation angle makes it possible for an optimum panel array to be designed for each unique set of requirements. These requirements may be such that the panel array is highly complex, using a wide range of design techniques to achieve optimum performance. This optimum antenna performance integrated into the complete broadcast system design has the potential to reduce initial infrastructure costs as well as ongoing operational costs.
Ashley Bicknell is a communications engineer at Radio Frequency Systems.
