This paper presents the study and design of a coplanar waveguide--fed broadband modified folded slot antenna, including new techniques to achieve broad impedance bandwidth when the antenna size is small. Antenna impedance and bandwidth are investigated as functions of slot parameters, substrate dielectric constant, and substrate thickness. Prototypes of broadband antennas are fabricated on RO4003 ($varepsilon$r = 3.38) and are measured. Two types of antennas are proposed with respective bandwidths of 44.4 and 80% with less than 2:1 VSWR.