Many clusters of galaxies contain radio sources, and many radio sources are in galaxy clusters.
In addition to sources in clusters, we should also include those sources in smaller structures that will later be incorporated into clusters. This includes small groups and clusters not rich enough to be included in the Abell catalog. Thus, a typical cluster will have been host to several radio sources in its lifetime. The subclusters that went into making a rich cluster also contained radio sources. The majority of radio sources were either in clusters or in units that were later incorporated into clusters, so that most of the radio sources that have existed thorughout the history of the universe have since been incorporated into clusters, and those presently not in clusters will be incorporated into clusters at some future time.