Short answer: the rods on a TV aerial must be set the same way as the transmitter’s signal, and that is either horizontal, which is nearly every main station, or vertical, which is nearly every relay. Get it wrong and a perfectly pointed aerial delivers a fraction of the signal. Separately, an aerial is built for a group of channels, and since every UK Freeview transmitter now sits between channels 21 and 48, older aerials built for the top of the old band are the ones most likely to be wrong.
Polarisation: which way the rods go
A transmitter radiates its signal with the electric field oriented either horizontally or vertically. An aerial receives most efficiently when its elements are oriented the same way, and very poorly when they are at right angles. The difference is not subtle; it is the difference between a working picture and none.
Ofcom’s transmitter data gives the polarisation of every site, and the pattern is stark:
| Sites | Horizontal | Vertical | Both |
|---|---|---|---|
| Transmitting at 10 kW or more (50 sites) | 48 | 1 | 1 |
| Transmitting under 1 kW (1,042 sites) | 75 | 939 | 28 |
Overall, 127 sites are horizontal, 971 vertical, and 32 transmit both, Rowridge on the Isle of Wight being the largest. The one high-power vertical site is Aldeburgh. Main station, horizontal. Relay, vertical. Exceptions exist, which is why Aerial Aligner shows the polarisation for the site you have chosen rather than leaving you to apply the rule.
Why relays are the other way up
A relay normally re-broadcasts a main station’s programmes into an area the main station cannot reach: a valley, a town behind a hill, a stretch of coast. Its coverage overlaps the main station’s fringe. Using the opposite polarisation means an aerial pointed at the relay rejects the main station’s signal, and an aerial on the main station rejects the relay, so the two do not interfere in the overlap. It is a planning tool, and it is why switching from a relay to a main station almost always means turning the rods.
Reading the street
Walk down any road and look up. Horizontal rods mean the street is on a main station; vertical rods mean a relay. A mix means either that the two coverage areas overlap there, or that some installers made different choices. It is a useful check on the app’s ranking but not a substitute for it: streets are quite often pointed at a three-multiplex relay when a main station with the full line-up is reachable.
Groups: which channels the aerial was built for
Freeview is broadcast on UHF channels, and an aerial’s elements are sized for a band of them. Older UK aerials were built for a group, marked with a letter and often a coloured plug, covering part of the band. Conventionally, Group A is the low channels, Group B the middle, Group C/D the top, with Group E and K covering wider spans and W, wideband, covering the lot. The exact range for a given aerial is on its label or the maker’s data sheet, and that is the number to trust rather than the letter.
The reason groups matter now is the 700 MHz clearance. Television was moved out of the top of the UHF band to make room for mobile services, and every UK Freeview transmitter now uses channels between 21 and 48. That is a fact you can check against the Ofcom data for every one of the 1,130 sites. Two consequences:
- A Group C/D aerial, built for the top of the old band, is now built for channels that carry no television. It may still receive the lower channels, poorly. If a house has one and reception is marginal, it is the prime suspect.
- A Group A aerial is fine on a transmitter whose channels all sit in the low band. From the data, that includes Crystal Palace, Belmont, Caldbeck, Caradon Hill, Rowridge and Stockland Hill among the main stations. It is not fine on Sutton Coldfield, whose channels run from 36 to 48.
A wideband aerial covers everything and is what you would fit today unless there is a reason not to. Our guide on finding your transmitter has a table of the main stations and the channels each one uses.
Putting the two together
When you point an aerial, or diagnose one, three things have to agree with the transmitter: the direction, the polarisation and the channel range. Aerial Aligner gives you the first two for every site and lists the channels for the third. If all three are right and the signal is still poor, the problem is in the line of sight or in the cable, and our no-signal checklist is the next stop.
Sources
Transmitter polarisations, powers and channels are from Ofcom’s published transmitter parameters, as bundled in Aerial Aligner; the counts in this guide were computed from that dataset. Aerial group letters are an industry convention rather than an Ofcom designation, and the channel range printed on an aerial or in its maker’s data is authoritative for that aerial.
Dataset counts computed on 11 September 2026 from the Ofcom transmitter data bundled in Aerial Aligner 2.0. Plan CD is not affiliated with Ofcom, Freeview, any broadcaster or any aerial manufacturer.