Thickness reduction of unidirectional spiral antennas

This paper presents different solutions allowing the thickness reduction of unidirectional spiral antennas without compromising their broadband properties. First, the effects of adding a ground plane to the radiating element are analysed. Then, the microstrip spiral antenna is loaded by chip resistors, the antenna exhibits a good circular polarization over a 3:1 bandwidth with a thickness of 0.02λ
_max
. In a following part, the spiral antenna is placed 0.013λ
_max
above a lossy ground plane, a good impedance matching and circular polarization are achieved over a 9:1 bandwidth. Finally, the spiral antenna is implemented on an EBG substrate, the thickness is 0.1 λ
_max
, the bandwidth is limited to 70% but this solution leads to a high efficiency antenna.