Mounting A Terrestrial TV Aerial In The UK

Group	Channel Range	Cap colour		Schematic showing aerial group overlap
A	21 - 37	Red
B	35 - 53	Yellow
C/D	48 - 68	Green
E	35 - 68	Brown
K	21 - 48	Grey
W	21 - 68	Black or none
Polarity	Alignment Of Elements
H	Horizontal		—
V	Vertical		|

Note that where a transmitter uses a semi-wideband group (E or K), Ofcom often suggest wideband (W) as an alternative, resulting in E/W or K/W.  However, for performance reasons, I suspect that in such cases most professional installers would recommend the narrowest group aerial that can accommodate the signals required, and not a wideband.

If some of the terms or techniques mentioned here are unfamiliar, you are advised to read the General Introduction  -  see also the list of useful links below.

If you are unsure of the format for inputting data, hovering the pointer over an input will bring up a short message describing the format expected.  In general:

• Latitudes, longitudes, grid and magnetic variances can be entered as decimal degrees or DD:MM:SS.  In both cases, southern and western values can be entered as negative or by appending S or W, northern and eastern as positive or by appending N or E.
• All other data such as grid references and post codes should be entered in their normal format.  Irish grid references should be flagged with a leading I.  For example, that for Divis transmitter should be entered as IJ287750, although as it happens this transmitter is in the area of overlap between the two systems, so the nearest UK equivalent of NW412309 will also work.

UK Place Name look up covers UK mainland only, except that the transmitter version also searches the UK transmitter list, but UK Post Code look up should find codes for the entire UK.  Other post codes and places, for example Eire, can be looked up using the World Place Name option, but the database is very US-centric, so append an appropriate country name, for example ,Eire.

When you are satisfied with your settings, you can save them by pressing Submit and then bookmarking, marking as a favourite, the resulting URL, the parameters in which will recreate your original settings in the calculator.  You may wish to do this once you have positioned a map exactly on your aerial site as described below (though you will then have to wait for everything to reload).

Clicking on a map button fixes latitude and longitude as the method of designating the aerial location, and creates a map with a marker and azimuth (direction) line for aligning it.  Dragging a marker to a new location recentres the map(s) and updates the calculator.  Likewise where necessary changes in the calculator will be reflected in the map(s).  After creating a map, changing settings other than receiver latitude & longitude, or choosing a different transmitter, is not recommended and may not be possible.

The Google map has four choices of base layer selectable by clicking the buttons in the top right hand corner.  Satellite pictures take significantly longer to load, so it's quickest to use Map to get as close as possible to the intended final location and zoom level before switching to Satellite or Hybrid.  If you plan to print the map, please read the Notes.

Satellite images are often subject to perspective distortion, occasionally to an extent significant in the context of aligning a TV aerial, thus making their results potentially less reliable than the initial impression given by their appealing simplicity.  Work entirely from ground level  -  position the marker at the point on the ground beneath the proposed aerial siting, and choose as aiming point a landmark the base of which is crossed by the line.

For UK, there is also an option to create an OS map, which can be switched between showing the signal path (default) or the locations of transmitters.  OS's OpenSpace® on-line mapping covers UK mainland down to street level (other areas of the UK are covered by other agencies and are either only available at low-scale or not at all).  If, despite the coordinate vetting, you manage to create a map of somewhere not actually covered, the map may just show nothing, or stick on an area on the north east coast, in which case Ctrl-Click the map button to remove it, and just use the Google one.  Coordinate conversion between Lat/Lon and E/N doesn't use OSTN02™, so coordinate readouts should only be considered accurate to around 5m.

For UK mainland only, there is also an option to display the signal path from your chosen transmitter, which is displayed as a straight line (allowing for refraction conventionally by assuming a radius of Earth 4/3 × the real one) accompanied by 60% (red) and 100% (orange) Fresnel zones, and the intervening terrain in green; the curvature of Earth may also be shown in black.  If you have a clear line of sight with no obstructions in the Fresnel zone, then you should get a good signal, but obstructions in the Fresnel zone will progressively weaken it, until crossing the line of sight blocks it.  If only the orange zone is obstructed, that will probably be ok.  Obstructions into the red might mean that you will require sensitive aerials and/or amplification to get a good enough signal.  If the signal is blocked, then you can only get a signal indirectly, for example by reflection from other buildings or refraction over the top of hillsides with rocky outcrops.  In marginal cases, you might be advised to consult a reputable CAI installer with good local knowledge, or to consider Freesat.  If the relevant maps have been created, clicking in the top third of the display will centre the Google map on an area of the signal path, the bottom third the OS map, and the middle third both.

Transmitter data is available for the entire UK, including CI, IoM, and NI.  Data is primarily from Ofcom, but also Digital UK, and the BBC, see the links below.  Note that I am aware of conflicts of information between sources for about 25% of the transmitters, and while I have endeavoured in good faith to resolve these correctly, it's unlikely that I have succeeded in every case, and while I apologise in advance for any errors there may be here, I accept no responsibility for their consequences.

For receiver locations within the UK, transmitters can be selected:

• Alphabetically
• By ITV region
• By transmitter group (where a transmitter changes group at DSO, it may appear in more than one)
• In distance order out to 100km, approximately 62 miles, default selection will be nearest main transmitter

• By searching a terrain database to find those transmitters nearest to the receiver  -  up to five nearest relays and the five nearest main transmitters  -  likely to give the strongest signals.  Note that meanwhile the form and the markers on the map are locked.  If internet traffic or other problems prevent completion many browsers allow breaking out of a script anyway, but for those that don't there's a Cancel button, although there will still be some that hog the CPU so much that clicking it has no effect, and then there will be no option except to reload, to aid which this option cannot be set via a URL parameter, it will be changed to the previous option on page reload.

  As the terrain data has a limited daily quota, please ensure greatest efficiency of use as follows:

  • Use the Google Map to position the receiver marker (green) exactly before choosing this option.  Thereafter, the marker can be adjusted up to 15m before data will be refetched from the server.
  • Note down the results to avoid the need for rerunning the option.

  It's also important to understand the limitations of such calculations.  As explained in the General Introduction, the actual signal at a given location depends on many factors, and the accuracy of predictive calculations will depend on the number of such factors taken into account.  The official Post Code Checker works on a postcode basis rather than the aerial location, and is considered pessimistic, while this one works from the actual location, but is probably too optimistic.  In particular, signal levels given here should be regarded as likely relative rather than absolute.  Thus, although the predictions of either checker are a useful pointer, neither should be taken as fact, and wherever possible should be checked against other such information, preferably against actual measurement.  As a guide, minimum useable signal levels at the aerial are around 54dBµV/m for analogue receivers, and 47dBµV/m for digital.

The UK is currently undergoing a nationwide conversion from analogue transmissions to digital, known as DSO.  UK transmitters are arranged in groups with main transmitters covering general geographical areas, and relays (sub-transmitters) filling holes in the coverage of the main transmitters.  As DSO progresses, at least 150 transmitters will change groups, some relays become mains, a few fall into disuse, and a few come into use, and these changes will be mirrored in the calculator's selectors, transmissions from the currently selected transmitter, and map markers.  Transmitters not currently in use are grey, relays off white, mains bright white.  Current transmissions, or where there is yet no data in areas of late DSO, are white, those not current are red, and those mid-DSO are orange.

Besides DSO, there is now also a simultaneous upgrade of one digital mux  -  BBC B, aka PSB 3 and Mux B  -  to HD (DVB-T2), which cannot be decoded by SD (DVB-T) tuners.  In most cases, this will occur at DSO, but five transmitters due for late DSO will broadcast an additional HD mux until DSO, while those transmitters already DSO-ed will have BBC B retro-converted during 2010.  Information on such cases is included in the transmissions table for affected transmitters.