Mounting A Terrestrial TV Aerial In The UK

Page loading is staged  -  as soon as possible the calculator's form unlocks so that you can begin entering data, further options becoming enabled as the code to support them completes loading.  The options available also depend on the chosen receiver location as follows:

• Anywhere  -  Google Map displays a direction line for aligning the aerial, while the calculator shows the true azimuth (degrees clockwise from True North), and another azimuth corrected either against Grid Variance (Grid Convergence, Grid Angle) for use with a printed map, or against Magnetic Variance (Magnetic Declination) for use with a compass.  As accurately as possible, enter locations for the receiver and transmitter, click the map button, and if required drag the marker(s) to more precise locations.

  The map has four choices of base layer selectable by 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.

  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.

  If you plan to print the map, please read the Notes.

• British Isles and Eire  -  Signal Profile displays a vertical elevation along the signal path.  UK users can choose a transmitter via one of the various UK lists and the transmitter's location and height will be entered automatically.  Users from Eire will need to determine and enter manually not just the transmitter's location but also the height of its antenna above sea level.

  The signal profile is shown as a straight line (allowing for refraction conventionally by assuming a radius of Earth 4/3 × the real one), with 60% (red) and 100% (orange) Fresnel zones, and intervening clutter (brown), terrain (green), and possibly curvature of the Earth (black).

  If you are within range and have a clear Fresnel zone, then you should get a good signal, but obstructions will progressively weaken it, blocking it when they cross the line of sight.  Obstructions into the orange zone will probably not be serious, but any into the red might require sensitive aerials and/or amplification to get a good enough signal.  If the signal is blocked, then you can only receive it indirectly, perhaps by reflection off other buildings or refraction over or around obstacles.  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.

• UK Nations (including the CI, the IoM, and NI)  -  The calculator has transmitter data for the entire UK, including locations, antennae heights, DSO dates, transmissions, channel numbers, etc.  Further details are given in UK Transmitter Help.

• UK Mainland Nations & the IoM  -  UK OS Map displays the signal path (default) or the locations of the transmitters in the calculator's current list.  Click the buttons beneath the navigator bar to swap between the two modes.

  Coordinate conversion between Lat/Lon and E/N doesn't use OSTN02™, so coordinate readouts should only be considered accurate to around 5m.

  OS's OpenSpace® on-line mapping covers UK mainland down to street level (other UK nations are covered by other agencies and are either only available via OS 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. 

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 would 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.  UK Post Code look up should find codes for the entire UK, but to reduce the chances of usage capping by Google, does so by searching the OpenSpace UK mainland database first, only trying Google on failure.  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 above (though then you will have to wait for everything to reload).

Clicking on a map button fixes latitude and longitude as the method of designating the receiver location.  Dragging a marker to a new location updates the calculator and may also recentre the map(s).  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.

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

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.

Transmitter data is available for the entire UK, including the CI, the IoM, and NI.  Data is primarily from Ofcom, but also Digital UK, and the BBC, see the links at the bottom of the page.  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 from the entire UK List.  For slow browsers on very slow machines, unavoidably this option may take a long time to appear after being chosen  -  this is because it takes such a time to load so many transmitters into the page element  -  so if the browser times the script out offering to cancel it, if you are prepared to let it run on at least two or three times, you will probably find that it will eventually complete and thereafter the page will work normally.
• Alphabetical list of transmitters broadcasting all digital muxes.
• 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 to find those transmitters nearest to the receiver  -  up to five nearest each of relays, transmitters broadcasting all digital muxes, and mains  -  likely to give the strongest signals.  Before using this feature, it's recommended to open the Google Map and exactly position the receiver marker (green).  Thereafter, the marker can be adjusted up to 15m before terrain data will be refetched from the server.

  It's important to understand that the accuracy of such predictive calculations will be limited by the completeness with which many factors (explained further in the General Introduction) are taken into account.  The official Digital UK Postcode Checker, has access to the best data, yet is considered 'pessimistic' because of the strictness of its correction for availability over time, and because the interpretation of its results on a per post code basis leads to post codes with mixed reception being marked out of coverage, even for individual addresses within them that may be within coverage.  By contrast, this one is probably too optimistic because, although it works from the actual location of the receiving aerial, it lacks transmitter radiation patterns and any correction for availability over time.  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.  In particular, signal levels given here should be regarded as likely relative rather than absolute.  For comparison, minimum useable signal levels at the aerial are around 54dBµV/m for analogue receivers, and 47dBµV/m for digital.

  Note that, during the search, the form and the markers on the map are locked, and occasionally and randomly, internet traffic or other problems may prevent completion and unlocking.  In this situation there are several means of escape:

  • A Cancel button is displayed whenever the calculator is locked;
  • The search script has a timeout;
  • Many browsers throw up a dialog box to allow breaking out of a script that has hung;
  • A few older browsers may hog the CPU so much that cancellation has no effect, and then there will be no alternative to reloading the page, to aid which this option cannot be set via a URL parameter, it will be changed to the previous option on reload.
• Additionally, UK Place Name can be used to search for a transmitter of which the exact name is uncertain.  Any transmitters so found will be inserted at the top of the results denoted by (Tx), and a dialog box will offer to select the first one found as the chosen transmitter.  When a transmitter is chosen from the results, UK National List and the chosen transmitter are set.

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 relays will change groups, some becoming mains, others fall into disuse, others come into use.  In general, DSO occurs in two stages a fortnight apart, each between midnight and the following midday.  At Stage 1, existing analogue BBC 2 and any existing low-powered digital BBC A are replaced by BBC A at full power.  At Stage 2, all remaining analogue and low-powered digital transmissions are replaced by digital transmissions at full power.  However, broadcast interactions between neighbouring transmitters are so complex that many will require additional channel &/or power changes before and after DSO, as the system as a whole is adjusted to its intended final state.  Both stages of DSO and these further so-called 'retune events' require that local residents retune their equipment to pick up the changes.

Besides DSO, there is 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 some transmitters late in DSO are broadcasting an additional HD mux (here called PSBHD) until DSO, while those transmitters already DSO-ed had BBC B retro-converted during 2010.

Further, by EU agreement, the UK is releasing UHF channels 61 and 62 from TV broadcasting to other purposes.  As the start of DSO preceded this agreement, some transmitters, mostly from those DSO-ed early, require additional retune events releasing these channels.

Where it is available sufficiently far in advance, information on all the above will be shown as follows.  Transmitters not currently in use are grey, relays off white, mains bright white.  Current transmissions are white, those not current are red, and those mid-DSO are orange.  Notes in the transmissions tables warn of those retune events that were known at the time of last update.