The use of coplanar waveguide (CPW)-fed ultra-wideband antennas in applications of multi-system integration has been demonstrated in this paper. Spiral slot antennas and feeding structures were fabricated on the same plane of the substrate so that the circuit process and the position alignment could be simplified. A CPW-fed spiral slot antenna possessing the ultra-wideband characteristic is also suitable for integration with a monolithic microwave integrated circuit (MMIC) module. Variations of the measured initial resonant frequency due to substrate thickness, number of turns and slot width are discussed in this paper. In addition, two topologies of the CPW-fed spiral slot antenna were devised and measured to demonstrate the capability of lowering the initial resonant frequency by adding a circularly microstrip stub at the end of the feedline and placing a short pin to terminate the spiral slot and feedline. According to the measured results, the CPW-fed spiral slot antenna covered most of the commercial wireless communication and satellite communication systems in radio frequency (RF), microwave and millimeter-wave applications.