In layman’s terms a DDRR is just a short vertical monopole (vertical post) attached to a transmission line tuned by a reactance (ring plus vacuum capacitor). The reactance of the capacitor is transformed by the transmission line and will under specific conditions (length, value of the capacitor, etc.) make the vertical post resonant. Slight variations of the capacitor will lengthen or make the transmission line “shorter” and allow tuning of the antenna on a certain range.
The DDRR looks like a piece of rubbish and it is, it just does not perform like it looks. It performs EXCELLENTLY!! DDRR means Directional Discontinuity Ring Radiator