Co-channel interference or CCI is crosstalk from two different radio transmitters using the same frequency. There can be several causes of co-channel radio interference; three examples are listed here.
'Adverse weather conditions: During periods of abormally high-pressure weather, VHF signals which would normally exit through the atmosphere can instead be reflected by the troposphere. This tropospheric ducting will cause the signal to travel much further than intended; often causing interference to local transmitters in the areas affected by the increased range of the distant transmitter.
- Poor frequency planning: Poor planning of frequencies by broadcasters can cause CCI, although this is rare. A very localised example is Listowel in the south-west of Ireland. The RTÉNL UHF television transmitter systems in Listowel and Knockmoyle (near Tralee) are on the same frequencies but with opposite polarisation. However in some outskirts of Listowel town, both transmitters can be picked up causing heavy CCI. This problem forces residents in these areas to use alternative transmitters to receive RTÉ programming.
- Overly-crowded radio spectrum: In many populated areas, there just isn't much room in the radio spectrum. Stations will be jam-packed in, sometimes to the point that one can hear loud and clear two, three, or more stations on the same frequency, at once. In the USA, the FCC propagation models used to space stations on the same frequency are not always accurate in prediction of signals and interference. An example of this situation is in some parts of Fayetteville, Arkansas the local 99.5 FM KAKS is displaced by KXBL 99.5 FM in Tulsa, particularly on the west side of significant hills. Another example would be of Cleveland's WKKY 104.7 having interference from Toledo's WIOT 104.7 FM on the Ontario shore of Lake Erie, as well as Woodstock's CIHR (on rare occasions), which is also on 104.7 FM, due to the signals travelling very far across Lake Erie.
Co-channel interference may be controlled by various radio resource management schemes.