In the olden days of black and white, 405-line television, when the frequencies used were "very high" (VHF), the UK two-channel television service was provided using a just 100 transmitters and reception needed a standardaerial.
Later the system was upgraded to "ultra high frequencies" (UHF) for colour and now uses over 1,120 transmitters with more channels on more frequencies.
To enable the best quality reception possible, UHF television aerials are produced in a number of "groups". Each group is designed to operate on a range of channels, blocking all others. This is to prevent interference from "other" transmitters and to decrease losses on those required.
Each group is designated a colour, which is used to mark the aerial. Whilst wideband (black) aerials are designed to receive on all frequencies, their performance can be below a grouped aerial.
The three sections show the three phases. The first is that before digital TV was introduced. This example shows that a group A (red) aerial would have received the original four channels but a group K was required to also receive FIVE. The channel number for FIVE on analogue is shown with an asterisk because it is "out of designated group", group A.
The second section shows the "digital with analogue" phase, where the transmitter has the analogue services from above with the new low power digital services. It can be clearly seen that on this transmitter a group A aerial will not receive any digital services at all, and that even a group K aerial will not receive the multiplex B service on C67. The only choice for digital reception here is a wideband (black) aerial because it is the only coloured band that crosses from left to right.
(You can also see that some of the digital services have plus and minus symbols. This indicates these transmissions use a 166.67kHz offset.)
The final section shows the designated frequencies that will be used for digital television after switchover. Whilst the public service multiplexes BBCA, BBCB, D3+4 are back "in group" (group A), to receive all the multiplexes, including the two potential new ones, the only choice here is wideband.
The performance of grouped aerials for each transmission channel was defined by the old Independent Television Commission (ITC) and is:
The larger the value, the lower the signal level possible.