This article is by Walter W. Muller, a specialist researcher into ancient Arabian history, and I believe originally appeared in Yemen: 3000 Years of Art and Civilisation in Arabia Felix, edited by Werner Daum (ed.), (1987).

I could wish this online version had been formatted to make it more user-friendly, but it's still a very good and authoratitive introduction to the early Arabian kingdoms of Saba, Qataban, Hadhramaut and Himyar.

Outline of the History of Ancient Southern Arabia