Circularly polarized antennas have received much attention for finding applications in satellite positioning, radio frequency identification (RFID) and sensor systems with the development of modern wireless communication. To be resistant to the depolarization effects, bad weather conditions and multi path propagation, it is imperative for the antennas to have CP characteristics, which is insensitive to the respective orientations on the useful frequency operation when applied to launch and receive electromagnetic waves. Therefore, there is a large demand for advanced CP antennas. However, many features (i.e. wideband, omnidirectional) which are available in linearly polarized antennas are difficult for CP to be achieved. CP generation requires two orthogonal components of the electric field, each with equal magnitude and 90º phase different, however, for wider angles it is difficult to maintain the condition for the CP in a wideband. The present researches are compact CP antennas and omnidirectional CP with electrical small size. The antenna evaluated from split-ring loop structure. The mechanisms of generating CP antenna are novel techniques. During the past researches, I got some design and measurement techniques on CP.