Unframed version suitable for printing

Mounting A Terrestrial TV Aerial In The UK

3  Aerial Alignment Calculator (with internet mapping)

Updated March 2014.

At its simplest, for anywhere in the world this page calculates and displays on a map the direction (bearing or azimuth) to point (align) a terrestrial TV aerial towards a distant transmitter.  However for countries in the British Isles including Eire, particularly for those within the UK, it can do significantly more.  To realise its full potential, first time users are advised to read the General Help.

General Help

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 two choices of base layer selectable by the buttons in the top right hand corner.  Satellite pictures can 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.

    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 simplicity.  Although it may seem counter-intuitive, you should 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, 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.

UK Aerial Help

GroupChannel RangeCap colourSchematic showing aerial group overlap
A21 - 37RedGroup A Cap ColourSchematic showing aerial group overlap
B35 - 53YellowGroup B Cap Colour
C/D48 - 60GreenGroup C/D Cap Colour
E35 - 60BrownGroup E Cap Colour
K21 - 48GreyGroup K Cap Colour
W21 - 60Black or noneGroup W Cap Colour
PolarityAlignment Of Elements

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.

UK Transmitter Help

UK transmitters are arranged in groups with main transmitters, here shown in bright white, covering general geographical areas, and relays (sub-transmitters), here shown in off white, filling holes in the coverage of the main transmitters.  Transmitter data is available here 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 20-25% of the data, 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 that 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.
  • 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 47dBµV/m for digital tuners.

    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.

By the end of 2012, the UK completed a nationwide conversion from analogue transmissions to digital, known as DSO.  Additionally, there was a contemporaneous 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.  Additional HD muxes are also planned.

Further, by EU agreement, the UK has released UHF channels 61 to 68 from TV broadcasting to other purposes, and in the UK these channels will supply 4G mobile services.  As the start of DSO preceded this agreement, some transmitters, mostly from those DSO-ed early, required additional changes to release these channels.  It should be noted also that there is great uncertainty as to whether and by how much 4G services will affect TV reception in those areas where it is broadcast on neighbouring channels 58-60.  There is much talk of homes being provided with filters to be fitted to the aerial downlead, but there remains great technical doubt as to whether filters will really be able to remove 4G adequately from the closest of these TV channels, as well as to what arrangements will be made to distribute these filters and whether and in what circumstances they will be chargeable to homeowners.

As with DSO, any further 'retune events' will require that local residents retune their equipment to pick up changes.  Notes in transmission tables will warn of any known at time of last update.


  • Magnetic variance is computed by the 2010 World Magnetic Model, with estimated error less than 1° until 2015.  Thereafter it will display in red, indicating the model's increasing unreliability over time, in which case you should attempt to get a more recent estimate for your location and enter it manually:
  • Browser compatibility notes (see also the next note about printing):
    • ALL  -  Maps gobble up resources.  If you create both types of map together without letting one complete before starting the other, or, after creating maps, many times change settings in the calculator or drag and zoom forcing continual redrawing, you may find that the Google map becomes blank, or there are jumbled tiles or holes in the OS map, or markers disappear from it, perhaps first from the print preview, then from the map proper.  Such problems may resolve themselves given enough time, but now there is also an option to remove a broken map or signal profile by Ctrl-Clicking the relevant button, so it can be then be clicked again to redraw from scratch.  However, particularly as Google Maps do not document a destroy function, it is questionable whether all the resources used are reclaimed when a map is removed, and, especially if you want to print the maps, you may have to reload the page with your chosen final settings, most easily done by clicking Submit on them, before creating one map at a time, allowing each to load fully before creating the next.
    • FF  -  Users of Firebug may experience freezing on (re)loading the page.  Either disable Firebug, or halt the frozen page loading (click red cross icon) and then do a full reload (shift-click blue circular arrow).  FF2 or earlier, please upgrade at least to FF3.6.28.
    • IE  -  Legacy IE 6-8, and more recent versions in IE7 Standards (Compatibility) Mode may not display the marker and line on the UK OS Map immediately upon its creation, but will show them if the UK OS Map button is clicked a second time once the map has finished loading (Believed to be fixed now).
    • Opera  -  It is regretted that this page no longer supports legacy versions.  Newer Webkit versions have not been specifically tested, but other Webkit browsers such as Chrome and Safari are known to work.
  • Each map is designed to print on a single page of A4 or Letter with top and bottom margins of about 15mm and side margins of about 10mm.  This site has better map printing support than many, but generally printing web content is too often neglected, not least by those who create browsers, and maps particularly so.  In the past I've reported problems to Microsoft, Opera, Google, and OpenLayers, yet problems remained long outstanding, though recent testing has shown a marked improvement in some browsers.
    • ALL  -  Print the Google Satellite Base Layer without the labels.  The latter tend to look fine in Print Preview but prevent the map printing properly.  IE8+ & FF3+ may print them properly, but legacy browsers tend to print only the label & map overlay, without the satellite imagery.
    • FF3  -  Printing individual map pages results in blank maps, you have to print the entire document to get the maps to print properly. Fixed in v3.5.1.
    • IE  -  IE has had an abysmal history of problems printing maps.  Time was that IE6/7 would print both maps correctly but more recent versions only in IE7 Standards (Compatibility) Mode.  However, Google have a policy of only supporting the last three versions of browsers, so IE6/7 no longer print Google maps correctly  the marker background may print black and/or the marker line be missing segments  and as using IE7 Standards Mode was the only way of getting IE8/9 to print correctly, the situation with these two browsers is now worse than it was when they first appeared.  The good news is that after some minor changes to the scripts, IE10+ now print the maps correctly.  In many versions, particularly older ones, if you press Print Preview before the map has completely loaded, the preview may be blank until loading completes, which may take some time, although some versions can be nudged into showing the preview by altering its zoom.
    • Chrome  -  Earlier versions may insert spurious blank pages.
    • Safari  -  Earlier versions may not print the Google Map  please upgrade to the latest version.
    • Opera  -  Legacy version map printing was abysmal; Webkit versions have not been specifically tested, but see the notes above about Chrome and Safari.

Apologies for these inconveniences.

Use decimal degrees or Degrees : Minutes : Seconds?

Where is the receiving aerial?
Where is the transmitter?
How to find azimuth?
Transmitter Settings
GridMag Azim:
Dist (mi):
Dist (km):
Transmitter Type:
Signal path profile


Black LineSignal Path.
Red60% Fresnel Zone - Obstructions into this probably WILL significantly affect the signal.
Orange100% Fresnel Zone - Obstructions into this probably will not significantly affect the signal.
BrownClutter - Natural and artificial surface features such as trees and buildings (resolution ~1km2).
GreenTerrain - Topographic features such as hills and valleys (resolution ~90m2).
BlackCurvature of Earth's surface (may not be visible over short distances).

Many thanks to:

Significant updates to this document
Date Description
19/03/2014 Restored proper printout in IE10+, and by setting IE7 standards to IE8/9.
27/02/2014 Updated with latest Ofcom data, including offsets, & added tooltips to channel details to give alternate mux names, and mux frequencies taking account of offsets.  Corrected: legacy IE markers, world place names not looking up properly, degree symbol in DMS mode.
06/02/2014 Corrected printing problems, updated Notes concerning Internet Explorer problems.
01/02/2014 HTML5, Google, OpenLayers, & OpenSpace map updates, data and form script updates.
11/02/2013 Final update of Ofcom / DigitalUK DSO data.
15/03/2012 Updated with latest Ofcom / DigitalUK data.
06/11/2011 Updated with latest Ofcom / DigitalUK data.  Bugfix to allow spaces in grid references.
11/09/2011 Remade from OS Panorama Open Data the SRTM terrain tiles covering the Shetlands, fixing missing data and sea noise.
05/09/2011 Updated with latest Ofcom / DigitalUK data.  Improved form option locking when maps are drawn.  Refinements to site OpenLayers script and SRTM void data handling.
17/08/2011 Updated with latest Ofcom data.  Improved date handling for DSO start and end days.  Added Signal Profile key, improved Help, and buttons & printing of OpenLayers map.
19/07/2011 Updated with latest Ofcom data.  Updated URL parameter handling to be more memory efficient.
12/01/2011 Updated with latest Ofcom data.  Added knowledge of transmitters broadcasting all digital muxes.
27/10/2010 Updated data for Camperdown, C5 relays, corrected The Wrekin pre-DSO.  Fixed bug loading DMS values from URL.
11/10/2010 To reduce risk of Google usage capping, altered UK Post Code search to try OpenSpace first, then Google.  Released September's major upgrade betas as live.  Cut down OpenLayers script and enabled compression for faster loading.
21/09/2010 Corrected Ofcom data.  Converted to use SRTM terrain and UMD clutter data.  Fixed bugs: no signals found for remote islands; no signal profile for manually entered transmitter details.  Significant rewrite of asynchronous script loading.
08/08/2010 Fixed bug in redrawing signal profile when transmitter changed.
19/07/2010 Updated data files as per latest Ofcom publications.
June 2010 Fixed bugs:  IE data file reading anomalies.  Reading URL parameters when DMS set.  Find operations  Form & marker update and lock/unlock during & after, and not ending for marginal signals.  Aerial Schematic and Signal Path Profile charts not appearing with some browser / operating system combinations.  Improved scales on signal profile.  Support for Chrome and Safari, inc updated browser Notes and covering Opera 10 hang up.
28/04/2010 Added advanced transmitter choice and find options, and corresponding map functionality.  Greatly improved support for IE8 wrt map drawing and printing.  Updated transmitter data including HD / DVB-T2 transmissions.  Updated site scripts for better form handling and asynchronous data loading.  Added this update record.
01/12/2009 Updated transmitter data.  Added transmission display for each transmitter.  Updated site script handling forms.
13/08/2009 Google Map converted to use Google API, rather than OpenLayers API.  Upgraded Ordnance Survey map to use OpenLayers v2.8.  Fixed problem with colours in <Select> elements in the forms in Opera and IE8.  Improved marker dragging and update of calculator.
07/05/2009 First major useable version in its current form, including a signal path profile.