As part of the Ofcom consultation process, predicted coverage comparison maps were produced showing the difference between the indoor signal coverage from the original Winchester DAB transmission, and the newly proposed coverage model.
Below are the coverage maps as produced by Ofcom:
Map key:
Blue = retained coverage
Red = areas served only by currently licensed coverage
Green = areas served only by the proposed coverage plan
Coverage shown is for predicted indoor coverage (63dBμV/m at 10m above ground level)
It can be seen (in red) that the northern part of the coverage area suffers a slight reduction in indoor coverage compared to the original coverage from the shared BBC antenna at Crabwood Farm. This reduction was mostly due to the lower antenna height of the new transmission site, preventing the signal from reaching further afield. However, it can also be seen that there is a significant improvement in indoor signal coverage to the south of Winchester (in green).
Bearing in mind the difference in BBC transmitter power compared with small-scale DAB, the original BBC antennae provided a strong northbound signal with less southbound coverage. This, in part, was due to the requirement to protect southbound signals interfering with France, (an aspect that we also had to consider in our design), also also because the BBC antennae form part of a larger national coverage model.
Given the BBC DAB services are also broadcast from Rowridge on the Isle of Wight, in a northbound direction covering Southampton, from Toot Hill in Romsey, and from Hannington, (maps shown below from ukfree.tv), the Winchester antennae helps reinforce the BBC signal into Winchester as part of their SFN (Single Frequency Network).
Bearing in mind budget constraints and timescales preventing us from using a bespoke antenna design, (which also had a longer lead-time and would be harder to replace if a fault occurred), we selected an off the shelf antenna array from Radio Structures, with 5dB forward gain, but less gain to the rear. This offered a suitable solution to meeting a very close northbound coverage pattern to the original DAB service, with a slight added advantage of some signal coverage to the south, without affecting France or encroaching on the core Southampton area, which will have it’s own small-scale DAB network later this year.
We felt this was a good compromise in coverage, costs and timescales to meet our business objectives, given a higher forward gain antenna array would have had a narrower beamwidth which would have impacted coverage to the east and west of Winchester.
Please note the following caveats, as detailed in the Ofcom consultation document:
a) The maps are based on computer predictions rather than actual measurements, so are indicative only.
b) ‘Receivable’ is based on the defined signal level required by a receiver that meets the minimum receiver specification (available here: Minimum specifications for DAB and DAB+ personal and domestic digital radio receivers: Digital radio action plan report – GOV.UK (www.gov.uk), although that may not be adequate for receivers built to a poorer sensitivity, or be sufficient in every location.
c) The maps do not take account of any interference from other DAB digital radio services. The likelihood of any such interference will increase as more DAB services are launched, but Ofcom will seek to reduce the impact as far as is reasonably practicable.
d) The maps do not show where reception outside homes (e.g. along roads) may be possible.