BBC Freedom Of Information - transmitter radiation patterns
It has always been a great disappointment that the full radiation patterns for transmitters are not published by the transmitter companies.
A few weeks ago, I used the Freedom of Information Act to request this information from the BBC.
And, very kindly, they have provided it all!
This will mean, when I have processed the data, to be able to provide improved prediction maps and other facilities on the site.
I'll be posting more about what this data means in the next week. However, if you want to look at the raw information yourself, it can be downloaded here as a ZIP file (10.2MB).
- There is one file for each multiplex on each transmitter (3713 patterns in total), although not all sites are guaranteed to be built, and other sites might be required to be added later. Note that the two-digit number in the file name is the frequency channel.
- The attached spreadsheet is an index which lists each transmitter, multiplex, and filename.
- The antenna pattern files all end in a .plt file extension, but these are all plain text files that can be read by any text editor. However, they are in unix format, not MSDOS format, so they use only line feed instead of line feed and carriage return at the end of each line. If your text editor does not support this, there are plenty of free unix-to-MSDOS conversion utilities that you can download.
- There are two types of PLT files 2-dimensional and 3-dimensional.
- The 2-dimensional patterns contain a line near the start that says, for example, "HORIZONTAL 360" meaning that this file contains a horizontal radiation pattern (ie a normal top-down view of the antenna pattern not to be confused with the antenna polarisation) with 360 measurement points. This is followed by 360 measurements (1 degree apart) in dB below maximum erp. Not all 2D files have a 1 degree resolution some may be 5 degrees or 10 degrees.
- The 3-dimensional patterns (recognisable by their much larger file size) are more complex. These contain a line for example like "PATTERN 360 101". This means the antenna pattern has 360 horizontal measurements around (1 degree resolution) and for each of those there are 101 sets of results at different angles of elevation from the horizontal (ie. the vertical radiation pattern at each of the 360 degrees). The next line of the file lists those 101 angles which might typically run from -5 degrees (above the horizontal) to +20 degrees (below the horizontal) in 0.25 degree steps. For each of these 360 x 101 points is a measurement in dB below maximum erp. The number of horizontal points is not always 360 (but mostly is) and may be 5 degree or 10 degree resolution instead, and the number of vertical elevations recorded will vary a lot, and will not always be at the same resolution (eg. another possibility might be 76 different elevations between 0 and 15 degrees below the horizontal in 0.2 degree steps). The steps are not guaranteed to be evenly spaced in all cases, so you must read the top line of elevation angles in order to interpret the data correctly.
- A small number of antennas (27) do not yet have a plt file assigned to them. For these, in the spreadsheet, there is a column which gives a simple 10 degree resolution pattern as a 72 character string (36 two-digit values for the angles between 0 and 350 degrees).
- This is similar to the above (520 patterns in total) except that the antenna pattern files all end in a .rpd file extension.
- Within the RPD file is information about the transmitter name and frequency channel, and the filename consists of the transmitter number (5 digits) followed by the 2 digit frequency channel. The included spreadsheet includes a transmitter number to name conversion for these files.
- The format of the antenna pattern data is similar to the PLT files, giving the number of points around the pattern, and the number of elevations for which patterns exist, and the range of those elevations. Eg 72 11 0.00-10.00 means 72 points around (5 degree resolution, starting at zero degrees), and 11 elevations between 0 and 10 degrees (below horizontal), ie 1 degree apart. Unlike PLT files, the RPD elevation angles are always equally spaced.
With regard to the appearance of a transmitting aerial seeming to be a single vertical pole, what you are seeing is merely a non-conductive protective 'sleeve' covering the active elements of the actual aerial system. Often, the active elements are formed by a series of trapezoid slot aerials which are spaced around a cylinder, the signal forming a resonance pattern that is either vertical or horizontal - but you can't physically see that unless the protective cover is removed. The polarity of the radiated signal is determoned by that resonance pattern and by making the trapezoidal slots have either a vertical or a horizontal pattern affects how you need to fit your receiving aerial.
Main transmitters generally radiate omnidirectionally, that is to say all around them. The more local 'lite' transmitters often use aerial systems designed to only radiate in specific directions. Some of these use aerial systems that are on one ot two side of the mast so producing a directional readiation pattern. Often done to control potential interference with other transmissions.
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No matter what colour it appears to be, it started out as white, usually a matt finish, and probably has become covered in atmospheric 'grime'. The covers are always as electrically non-conducting as is possible, so rather more like an insulator, so the signal can be radiated from the actual active aerial elements within. The 'reflectance' is as near zero as possible to allow the RF signals to radiate out of the covering, hence the careful choice of materials. They do not need to be optically transparent at all as light as we see it has a wavelength between 450nm and 850nm roughly whereas TV signals in the Bands 4 and 5 used for terrestrial television and 4G mobile signals is in the range from around 0.75m at 400 MHz down to about 0.375m at 800 MHz (300MHz = 1m wavelength, SEE Frequency - Wavelength Chart for more values). The prime purpose of the covering is weather protection.
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