Terrestrial broadcast transmissions can be affected by any large structure, and the responsibility to mitigate issues often falls to the developer.
Ofcom produced guidelines in 2009 titled ‘Tall structures and their impact on broadcast and other wireless services’. These guidelines cover a range of technical issues concerning interference to wireless communication systems.
This article however, focuses solely on TV, although you can learn more about the impact of large structures on wireless services by reading Pt 1 of our Ofcom guideline review.
Summary of Key Points set out in the Ofcom Guidance Regarding TV Interference
- TV services could be affected by a large structure.
- It is recommended that potential impact is considered at the design and planning stages of a project.
- Problems are most likely for structures that are significantly taller than those around it or structures on high terrain.
- As a rule of thumb, if a new development is of a similar height to the surrounding structures it is unlikely to affect signals in the area.
- Issues arise when a structure is on or close to the path between transmitter and receiver.
Signal Blocking and Reflection
- The two mechanisms that can adversely affect a wireless service are Signal Blocking and Reflection.
- Signal blocking occurs when a receiver is in the shadow of a large structure, which has the effect of reducing the strength of the received signal.
- Reflection occurs when the transmitted signal bounces off the structure and is subsequently received at a different location. The outcome is that the signal will be received twice – once via the direct path from the transmitter and once via the reflected path caused by the structure. The problem is that the reflected path is longer and therefore the reflected signal arrives later than the direct one, which can cause interference.
- Reflection issues are of more concern to analogue transmissions than digital ones. Wind turbines, fro example, can cause complex reflections because they will fluctuate.
- The reflection zone – the area where reflected signals may cause problems – can be thought of as circular. However, since TV aerials are directional, the reflection zone is shaped more like a keyhole, that is a circle around the structure with a widening ‘slot’ shape in the shadow of the structure.
- Interference in the circular portion of the keyhole typically does not occur beyond 500 metres.
- Interference in the ‘slot’ portion of the keyhole typically does not occur beyond 5 kilometres.
Analogue, Digital and Satellite TV
- Analogue television services are quite seriously affected by signal reflections. This can lead to something called ‘ghosting’ where pale shadows appear next to the main picture on the TV screen.
- Digital TV is much better at coping with reflection issues.
- Digital TV does not suffer from ghosting, but if there are reflection issues a digital receiver will need a stronger signal than if there are no reflections. This means areas with a weak signal may be more vulnerable to interference from reflections.
- Satellite TV is not generally affected by new structures, unless the development blocks the line of sight between a dish and a satellite.
- Interference to satellite TV is usually only disrupted if a tall structure is located very close to an existing building.
- The elevation angle of a satellite dish in the UK receiving from the Astra satellites is between roughly 17 degrees (in Scotland) and 26 degrees (in the very south of England).
- Where possible impacts are identified or suspected, a desktop assessment and/or reception survey can be undertaken.
How We Can Help
- TV Written Schemes – often required to discharge planning conditions relating to TV interference.
- Desk based assessment of potential TV interference .
- TV surveys.
Ofcom, 2009, Tall structures and their impact on broadcast and other wireless services.