In the late 1990s, it became imperative that the UK had a DTT network in place in a short time-scale. The main build program for the transmitter roll-out was during 1998 and 1999, with heavy financial penalties against the transmission providers for late delivery. The initial plan called for 81 sites, each with transmitter systems for six multiplexes. Some multiplexes were planned to share existing analog antennas; others required entirely new antennas.
An ntl mast “Sandy Heath” in southeast England with both the analog cantilevered antenna and new digital radial fire antennas installed.
To design and provide a realizable solution, the antenna engineers needed to consider many practical aspects, such as antenna gain, wind loading, bandwidth, power handling, RF safety, impedance and cost. In particular, new antennas are limited by the aperture available on the structure, and these must usually be lower than their analog counterparts, which take prime position — often cantilevered from the top of the mast. One notable exception to this was a new antenna in northwest England that was cantilevered above the existing analog system.
DTT antennas generally had to be installed around the support structure, and suitable horizontal patterns were much more difficult to achieve. In order to ensure good performance, all antenna designs were tested on an antenna range prior to manufacture.
Ideally, existing analog antennas would have been used everywhere if that had been possible because this would have provided the best match to analog coverage. Where the analog antennas were usable, often one or more frequencies selected for DTT had restrictions in one or more directions. Therefore, for those specific frequencies, new antennas were designed to achieve the particular horizontal radiation patterns (HRPs) needed. At some sites, three separate new antennas are in use for DTT. The effect is that each multiplex has the best coverage possible, but not necessarily identical.
This resulted in some complex antenna designs involving multiple tiers with many panels per tier. Horizontally polarized broadband panels were the primary building blocks for all new main-station antennas. At low-power relay sites, vertically polarized dual-input 4-lambda cardioids were employed.
In some cases, radial-fire, cogged antennas, which provide an extremely good omnidirectional pattern over a wide frequency band, were employed, especially when the panels were mounted on a large radius lower down the mast.
At some sites, where the available mast loading was severely restricted, skew-fire antennas were used. This is where the antenna panels are arranged to fire tangentially to their mounting radius. While this arrangement does not produce such a good omni pattern, by working closely with the spectrum planners, it was possible to optimize the HRPs to ensure that maximum population coverage was achieved.
A vital factor that had to be checked in both new designs and existing antennas was the safety margin regarding both power and voltage ratings. Analog antenna systems have usually been power limited, but for DTT, it is more often a voltage restriction. There could be up to six DTT multiplexes sharing the antenna, each with 2k QAM carriers as well as the existing analog services. At some point, all these carriers could add up in phase, albeit for an extremely short duration. An assessment was made as to what peak voltage should be allowed for in order to avoid breakdown within the antenna system and without over-engineering the system, which would have made it too costly to build.
Due to the “cliff edge” characteristic of DTT reception at the farther reaches of the service area, the digital broadcasters required that the effective radiated power (ERP) be maintained even during half-antenna operation. To provide this, the antenna systems were designed as two independent halves. During periods of half-antenna operation, the transmitter power is doubled by paralleling-up the passive reserve transmitter with the main transmitter; hence, the 3dB loss of antenna gain is made up in transmitter power, and the ERP is maintained. This has a major impact on the system cost because all the antenna components, including combiners, must be rated to work at four times their normal power levels, as well as requiring a more complex transmitter system. Where existing analog systems were used, certain components were replaced to ensure that the new DTT multiplexes could be transmitted safely.
Because of the high RF field-strengths present in close-proximity to existing operational antennas, it was often necessary to have either reduced power working, or even a complete shutdown, while personnel climb through or work nearby. These interruptions had to be negotiated to occur at a time when they would least affect all the various programs concerned.
With up to five analog TV services, plus all of the national, regional and local radio stations to consider, it was tricky. Often, during events such as the Wimbledon tennis tournament, there was a complete embargo on interruptions, and there was only a limited amount of work that could be undertaken at night, 300 meters up a mast. Coupled with the unpredictable weather and the effect of delays on progress at other sites, it became a project planner’s nightmare to arrange. Nevertheless, all sites were completed by their contractually agreed dates.
Once the new DTT services were being transmitted, coverage was measured using a helicopter. The measurements highlighted that some antenna manufacturers’ panels performed better than others within parts of the UHF band; this was evident by a reduction of intrinsic gain.
At the time when the DTT antennas were measured, there were no detailed specifications in place that contracted the antenna manufacturer to achieve an HRP within a specified tolerance of the helicopter-measured HRP. The specification now has been completed and agreed with both antenna manufacturers and helicopter measurement specialists.
Colin Burnell is antenna systems manager for NTL Broadcast. He acknowledges the assistance of Peter Kemble, Glenn Doel and Bruce Randall, all from NTL Broadcast, in writing this article.
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