UKGlossary
The words this site uses without stopping to explain them.
- Multiplex (mux)
One transmission carrying several television channels at once. Freeview has six, and everything inside one arrives or fails together — which is why channels disappear in groups. Which channel is on which.
- PSB and COM
The three public-service multiplexes (PSB1–3) and the three commercial ones (COM4–6). PSBs are carried by almost every transmitter; COMs only by the main stations, so a relay carrying three multiplexes is complete rather than faulty.
- DVB-T and DVB-T2
The two transmission systems in use. PSB3 — the HD multiplex — is the only DVB-T2 one; the other five are DVB-T. DVB-T2 does not need a stronger signal, which is widely believed and not what the planning figures say. The numbers.
- Channel and frequency
A UK UHF channel number is a slot 8 MHz wide: C21 is 474 MHz and each step up adds 8. Post-clearance television runs from C21 to C48. [V]
- Offset
A small deliberate frequency shift within a channel, used to reduce interference between distant transmitters sharing it. Not a different channel.
- ERP
Effective radiated power — how much a transmitter puts out in a given direction, once the aerial's own focusing is counted. It is not the power going into the aerial, and it is the figure this site prints.
- Radiation pattern
How a transmitter's power is distributed around the compass. Ofcom publish these for FM and DAB and almost never for television, so radio coverage here is directional and television coverage is not.
- Polarisation
Whether the signal is transmitted horizontally or vertically — which sets whether the aerial's elements go across or up and down. Getting it wrong costs a great deal of signal, and some masts publish both.
- Aerial group
The part of the band a transmitter uses, and therefore which grouped aerial works there. Why it changed in 2020.
- Relay
A small transmitter filling in somewhere a main station cannot reach, usually carrying the three public-service multiplexes only. Most of the register is relays.
- Margin
How far above the threshold a predicted signal sits, in decibels. Nought is exactly at the edge; the areas this site calls served sit a median 14 dB above it.
- Decibel (dB)
A ratio on a logarithmic scale: 3 dB is double the power, 10 dB is ten times, 20 dB is a hundred times. Small numbers cover large differences, which is why signal levels are quoted this way.
- Field strength
How strong the signal is in the air at your location, in dB(µV/m) — before your aerial has done anything with it. The threshold for fixed rooftop Freeview reception is 45 dB(µV/m). [D]
- Best server
The transmitter predicted to be strongest at a given place. Not the same question as which transmitters reach it — usually several do, and the map here can show either.
- Output Area
The smallest census area, around 130 households. Coverage here is predicted at the centre of one, so an answer is about your neighbourhood rather than your roof.
- Fresnel zone
The cigar-shaped volume around the line between transmitter and aerial that needs to stay clear for the signal to behave as though nothing is in the way. Roughly 60% of it clear is the usual rule.
- Lift
Settled high pressure bending signals further than usual. It brings in distant transmitters, which is why reception can get worse rather than better: they arrive on the same channels as yours.
- Ensemble
DAB's equivalent of a multiplex — one transmission carrying several stations, which arrive or fail together. Every ensemble and its line-up.
- RDS
The data an FM transmission carries alongside the audio: the station name your radio displays, and the identifiers it uses to retune between transmitters.
- Evidence class
The [V] [D] [I] [A] badges beside claims on this site: verified, documented, inferred, assumed. What each one means.
Something has stopped working · How the coverage figures are made