The radiant activity and structure of the helion and antihelion sporadic meteor sources have been examined at monthly intervals over a period of 103 months between 1986 and 1995, using data obtained with the Grahamstown meteor radar. To facilitate the analysis, the contribution of three strong showers to the activity of the source regions has been removed. Marked seasonal trends are found, with the behaviour of each source following a similar pattern each year. Rms deviations in interannual activity for a given month lie between upper limits of ~10 and ~30 per cent of the mean for that month. It is found that centres of activity, not generally associated with recognized showers, appear within the source regions at positions characteristic of a particular month. Known shower radiants can be identified in the helion source, while there is a possible connection between some prominent features in the antihelion source and showers whose main activity does not occur until later in the year. The activity in both regions rises to a maximum in mid-year, after which that of the helion source decreases significantly so that the antihelion region dominates for most of the latter half of the year. The results, together with independent orbital data, suggest that these sources are associated with material from short-period comets which can be separated according to age into a uniform background and a structured component made up of past and present streams. As a by-product of the analysis it is shown that a meteor is, on average, most likely to be detected by an all-sky radar at mid-latitudes if its radiant lies at the celestial pole. The resulting polar artefact is well demonstrated in the Grahamstown data.