Co-Channel Interference: Why Freeview Breaks Up in Good Weather

Co-channel interference happens when a distant transmitter using the same frequency as your local one arrives strongly enough for your receiver to see both signals at once. It cannot decode either cleanly, so the picture pixelates or the channel disappears. The classic UK trigger is settled high pressure, which creates atmospheric ducting that carries distant and continental signals hundreds of miles further than normal. It typically affects some multiplexes and not others, hits whole neighbourhoods at once, and clears when the weather changes — there is nothing wrong with your equipment, and retuning during an episode usually makes things worse.

Why the same frequency is used twice

There is not enough UHF spectrum for every UK transmitter to have unique frequencies. After the 700MHz clearance handed the top of the band to mobile operators, roughly 470–700MHz is left to carry every multiplex from every transmitter in the country.

The solution is frequency reuse: the same channel is allocated to transmitters far enough apart that neither reaches the other’s service area. The plan is carefully engineered, with the reuse distance calculated from transmitter power, aerial patterns, terrain and the statistics of normal atmospheric propagation.

The word doing the work there is normal. The plan holds for the overwhelming majority of the time. It is designed to be exceeded for a small percentage of hours per year — and those hours are what you notice.

Tropospheric ducting: the mechanism

Under settled high pressure — still air, clear skies, warm days, the classic British anticyclone — the atmosphere forms a temperature inversion: a layer where air gets warmer with height instead of cooler. That layer bends UHF radio waves back towards the ground instead of letting them escape into space, and they then bounce between the inversion and the surface, travelling far beyond the horizon. This is a tropospheric duct.

Sea surfaces are especially good at it, because the air above cool water forms very stable inversions. Which is why the worst-affected UK areas are the coasts, particularly:

The east coast — the North Sea provides a highway for Dutch, Belgian, German and Danish transmitters. Areas served by Sudbury, Tacolneston and Belmont know this well.
The south coast — French transmitters across the Channel, affecting reception from Dover and Rowridge.
The Irish Sea, in both directions.
Low-lying inland areas with long, flat paths to distant domestic transmitters.

Under a strong summer duct, a transmitter 400km away can arrive at a strength that would normally require it to be 40km away.

How to recognise it

Co-channel interference has a signature that, once you know it, is hard to confuse with anything else:

It happens in good weather. Calm, warm, settled, often the nicest week of the year. Everyone expects reception problems in storms; almost nobody expects them in sunshine, which is why this fault gets misdiagnosed so often.
It affects some multiplexes and not others. Because each multiplex sits on a different frequency, and only the frequencies that clash are affected. So BBC One goes but ITV is fine, or all the channels on one MUX vanish together while the rest of the EPG is untouched. Our guide to which channels sit on which multiplex lets you check whether your missing channels share a MUX — if they do, that’s a strong indicator.
It’s often worse in the evening and overnight, as inversions strengthen after sunset, and eases mid-morning as the sun breaks up the layer.
It affects your neighbours too. If the whole street has the same problem in the same week, it is not your aerial.
It lasts days, not minutes, and ends when the weather breaks.
It comes back every summer, in roughly the same conditions.

Contrast that with rain-related faults and wind-related faults, which behave in the opposite way entirely.

What not to do

Do not retune. This is the single most damaging response, and the most common. During a ducting episode your TV may fail to find the affected multiplexes, and an automatic retune will then delete those channels from the guide. Worse, in a strong duct a receiver can occasionally lock onto the interfering transmitter and store its services, leaving you with channels that don’t work and a channel list that needs rebuilding. If you have already done this, our guide to missing channels after a retune explains the recovery.

Do not fit an amplifier. Amplifying makes the interference louder in exact proportion to the wanted signal. The ratio between them is what matters, and an amplifier cannot change a ratio — see boosters vs amplifiers.

Do not replace the aerial in a panic during an episode. A new aerial in the same position with the same directional characteristics will do the same thing. If an aerial change is the answer, it’s a specific change, chosen deliberately — see below.

What actually helps

For an occasional summer episode in an otherwise good installation, the right answer is genuinely to wait. But where a location is affected badly and repeatedly, real improvements exist. All of them work by improving the ratio between the wanted signal and the unwanted one:

Better directivity and front-to-back ratio. Interference usually arrives from a different direction to your transmitter — for east-coast ducting, from behind or to the side. An aerial with a high front-to-back ratio, such as a mesh- or grid-reflector design, rejects signals arriving from the rear far better than a basic Yagi. This is the most effective single change in most cases. Our aerial types guide covers the designs.

Precise alignment, including deliberately offset alignment. An aerial’s radiation pattern has nulls — directions in which it receives almost nothing. A skilled installer can sometimes align so that a null points at the interference while keeping the wanted transmitter comfortably within the main lobe. This costs a little wanted signal to remove a lot of unwanted signal, and it needs a meter and patience rather than luck.

Polarisation discrimination. If your transmitter is vertically polarised and the interferer is horizontal (or vice versa), correctly matching your aerial to your transmitter buys you meaningful rejection of the other one for free. This is one of several reasons a mis-polarised aerial is worth correcting.

Lowering the aerial. Counter-intuitive, but sound: ducted signals arrive at shallow angles and are often stronger higher up. Dropping an aerial can improve the ratio even though it slightly reduces the wanted signal. Related reasoning is in how high should a TV aerial be.

Switching transmitter. Where a second transmitter covers your address from a different direction, moving to it may sidestep the clash entirely — at the cost of possibly losing regional news for your area, and sometimes the commercial multiplexes if the alternative is a relay. Worth checking on our transmitter directory or postcode checker.

Accepting a different platform. If a location suffers badly every summer and none of the above gets it to reliable, satellite or IP delivery avoids the problem completely, because neither shares frequencies with anything terrestrial. Freeview vs Freesat covers the trade-offs.

Co-channel from other sources

Ducting is the main cause but not the only one:

Terrain reflections. A strong reflection of a distant co-channel transmitter off a hillside or a large building can produce a permanent, weather-independent version of the same problem. The fix is directional: reject the direction the reflection comes from.
Self-interference (multipath). Your own transmitter’s signal arriving twice — direct plus reflected — with enough delay between them to confuse the demodulator. Symptoms look similar; the fix is the same class of aerial change.
Relay overlap. In fringe areas between two transmitters, both may be receivable and neither dominant. This is an alignment and aerial-choice problem rather than a weather one, and it’s permanent until addressed.

Reporting it

Persistent interference that isn’t weather-related may be a genuine fault or an unlicensed transmission, and there are people whose job it is to find out. Restore TV handles mobile-network interference with Freeview; Ofcom handles licensed and unlicensed interference investigation more broadly; the RTIS service run for broadcasters gives reception advice by postcode. If a whole neighbourhood is affected outside a high-pressure spell, those are the routes rather than a new aerial.

If your reception fails every summer in the good weather and you’d like it dealt with properly rather than annually, send us your postcode with a note of which channels go and when. We route the enquiry to the vetted installer covering your area. Be aware that the honest answer for a mild, occasional case is “this is atmospheric and it will pass” — a good installer will say so rather than sell you an aerial you don’t need.