Spiral antenna with frequency-independent coplanar feed for mobile communication systems

The spiral antenna investigated in this contribution is a four-arm spiral antenna, that is fed with a coplanar transmission line. The frequency independent characteristics of the structure are maintained and the feeding structure itself, that does not need any kind of balun, is also frequency independent. The radiation pattern is not that of a conventional two-arm spiral with differential feed but an omnidirectional pattern with a null in the zenith. The measured bandwidth is more than 5:1, therefore it is suitable for a mobile terminal for the coverage of several communication services at one time such as GSM, DCS1800, DECT and UMTS.