Co-Channel TV Interference

TV interference (often referred to as TVI) can be the result of high pressure, unusual weather or atmospheric conditions. Such conditions can also give rise to what is known as "Co-channel TV interference". The effect of this results in a poor TV picture with lines, horizontal bars (see picture - left), patterning or wavering effects. these are often most noticeable on dark areas of the picture. In extreme cases it may even be possible to read foreign language text on screen captions from the interfering signal.

Co-channel interference often happens when areas of high atmospheric pressure are present or when a static area of high pressure is dislodged by an approaching weather front causing "ionospheric or troposheric ducting" and temperature inversions. During this time, the lower parts of the ionosphere temporarily reflect radio waves back down to earth over extended distances giving much greater transmitter coverage than normal. Unfortunately, there is no cure for co-channel television interference, other than to wait for the prevailing weather conditions that cause it to pass.

High atmospheric pressure often allows reception of UHF TV stations many hundreds of miles away, often including French TV, Dutch and German TV channels which are normally totally out of range. In some cases very strong interfering signals can be experienced during enhanced VHF and UHF tropospheric conditions and the interference can last for a day or two and occasionally much longer.

Digital terrestrial TV services such as Freeview can be affected by co-channel interference too, causing the digital TV picture to freeze with a low signal quality reading and signal dropout. Co-channel interference does not normally affect Sky TV or any satellite TV service, but it can cause interference on the UHF channel used to distribute Sky TV or satellite around your home.

Interestingly, whilst the signal from a TV transmitter might be enhanced at the fringe area of coverage during high pressure enhanced radio conditions, areas closer to the transmitter may actually experience reduced signal levels, giving a poor TV picture.