Previously, the broadband spiral class of antennas primarily consisted of the wire (or "printed")spiral and spirals with approximately complementary geometry. While both are very broadband radiators, they are still somewhat band-limited due to their associated balun structures, and bulky as a result of their absorbing cavities. A new type of spiral antenna utilizing slot radiating elements is presented that eliminates these drawbacks, relying instead on several new approaches to guarantee its frequency-independent operation over extremely broad bandwidths. The design and integration of a planar infinite balun/feed structure and very shallow reflecting cavity is presented and discussed, as is a new termination technique. Gain, axial ratio, and pattern measurements for both a 6" (15.24 cm)diameter spiral, with flow= 750 M Hz and 6.25:1 bandwidth, and an 18" (45.72lcm) diameter spiral, with flow= 250 M Hz and 15:1 bandwidth, are also given and reviewed.