Curtain array
Curtain arrays were developed during the 1920s and 1930s when there was a lot of experimentation with long distance shortwave broadcasting. The underlying concept is to achieve gain over the simple dipole, possibly by folding one or more dipoles into a smaller physical space, or to arrange multiple dipoles such that their radiation patterns reinforce each other, thus concentrating more signal into a given target area.
The first curtain array to achieve popularity was the Sterba curtain, patented by Ernest J. Sterba in 1929[1] and this was used by Bell Labs and others during the 1930's and 1940's. The Sterba curtain is however a narrowband design and is only steerable by mechanical means. However, as far back as the mid-1930s, Radio Netherlands was using a rotatable HRS antenna for global coverage. Since the 1950's the HRS design has become more or less the standard for long distance high power shortwave broadcasting (> 1000 km).
More often than not Distributed or Branch Feed curtains are used, not Sterba arrays. The Distributed or Branch Feed curtains are considered to be classical HRS type antennas. There are 4 mathematical model types of ITU HRS type HF antennas, so these non-Sterba feed designs do indeed offer greater flexibility for transmission system designers.