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ADAPTIVE AERIALS FOR UHF REBROADCAST LINKS
From an original article by MD Windram, L Brunt and EJ Wilson

PART ONE - INTRODUCTION

The large number of UHF television transmitters operating in Bands IV and V has brought about an increased risk of co-channel interference (CCI) in the rebroadcast receiver (RBR) links that are used to provide programme feeds for the vast majority of transmitters. Particularly formidable problems faced those planning a high-grade RBR link over the 135 km sea-path between Stockland Hill, Devon and Alderney, the nearest of the Channel Islands. To provide a solution IBA engineers developed an adaptive aerial array which automatically and continuously adjusts its polar diagram and in which the constraint is that of minimising the level of CCI relative to the wanted signal.

This article discusses the basic requirements and design of complex adaptive arrays and the implementation of a system constructed as a 2x2 array of 8x2 dipoles. The array provides a gain of about 24 dB and has a continuously-adjustable rejection performance of about 45dB or some 20dB better than can normally be achieved using even a complex fixed array. The array has to reject unwanted signals only 7° from the bearing of the wanted signal. Some suggestions are made for simpler four-element adaptive arrays which would be suitable for difficult sites, including island sites, where there are fewer potential sources of interference.

The total number of UHF transmitters in operation in the United Kingdom is now around 4000, crowded into a total of 44 available channels in the UHF band.

Fig. 1: In 1974. following investigation of a number of alternative proposals for providing a high-quality 625-line colour programme link between the UK and the Channel Islands, it was concluded that the most satisfactory solution would be to re-engineer the existing 405-line link receiving station on Alderney, providing a new aerial designed to receive the UHF broadcast transmissions from the 250 kW transmitting station at Stockland Hill, near Honiton, South Devon and then to provide a programme feed to Jersey by means of a special microwave link. While signal strength of the Stockland Hill transmitters was considered just adequate (provided space diversity aerials were used) it was recognised that there were many potential sources of co-channel interference.

Although the distribution of transmitters is being carefully planned to avoid interference problems within the appropriate programme service areas, effects such as anomalous propagation cause considerable problems to broadcasters who need to pick up signals outside these service areas as part of the broadcasting network. The UHF link from Stockland Hill, Devon, to Alderney, which forms part of the broadcast network for the Channel Islands, as shown in Fig.1, is of particular interest. This is perhaps the most difficult UHF link in the broadcasting network and many alternative routes were considered before this was finally chosen. It is for this link that the first RBR adaptive aerial system was developed. There are, however, potentially several other UHF links in the United Kingdom which, although not exhibiting quite such severe propagation characteristics, are nevertheless still a problem and for which a simplified form of adaptive aerial system would be an ideal solution.

Fig. 2: IBA propagation studies showed that harmful interference levels from a number of existing or planned transmitters would exist for more than the acceptable 1% of the time. Some but not all transmitters would use ‘offset’ carrier frequencies, and some interference might be expected from stations such as Kippure (Ireland) along paths close to the wanted signals from Stockland Hill.

Requirements and Design of Adaptive Array

The UHF path from Stockland Hill to Alderney is an over-the-horizon sea path of some 135km in length, and, characteristic of such paths, the received signal is very variable in strength with a range ~60dB and generally very weak. For this reason, the signal is susceptible to co-channel interference (CCI), from both existing and proposed transmitters as shown in Fig. 2. Kippure for example is at a particularly narrow angle off the wanted signal of 70°. Figure 3 shows the bearing and field strengths of the interference. The inner ends of the lines represent the median field strengths and the outer ends represent the field strengths exceeded for 1% of the time. Note for example that the field strength of Crystal Palace, London is for 1% of the time more than 6dB greater than the median field strength of Stockland Hill.
Fig. 3: A polar plot of predicted field strengths against bearings of potential sources of CCI. The inner (weaker) signals represent the median (50%) values and the outer ends the field strengths likely to be exceeded for 1% of the time (e.g. during pronounced tropospheric propagation conditions). Considerable aerial directivity with nulls of the order of 45 dB is clearly required if CCI is to be maintained within acceptable protection ratios for 99% of the time.

To obtain a broadcast quality signal, it has been shown, both by theoretical propagation predictions and by practical measurements, that the reception pattern of an aerial on Alderney needs on occasions to have null depths of the order of 45dB in the directions of the interfering sources. It is not possible to use conventional aerials for this degree of rejection for the following reasons:

  1. Initial mechanical and electrical assembly tolerances of conventional aerials tend to limit the designed null depths to 25-30 dB.
  2. The pointing accuracy required for very deep nulls is extremely high, setting impossibly high constraints on mounting tolerances.
  3. The apparent direction of the interference can vary with time. Such variations may be caused by combinations of propagation mechanisms, such as tropospheric scatter etc.

For these reasons, the IBA decided to investigate the properties of adaptive arrays. The advantages of such an array include:

  1. automatic adjustment of the aerial pattern to give minimum interference.
  2. no need for prior knowledge of the bearing of the interference; the array is therefore able to handle interference from sources not previously predicted.
  3. ability to handle multiple sources of interference up to a limit which can be defined.
  4. ability to track any apparent changing of direction of a source that might result from propagation effects.
  5. there is no longer a severe mounting and aerial tolerancing problem.

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