Spiral antennas are particularly known for their ability to produce very broadband, almost perfectly circularly-polarized radiation over their full coverage region. Because of this polarization diversity and broad spatial and frequency coverage, many different applications exist, ranging from military surveillance, ECM, and ECCM uses, to numerous commercial and private uses, including the consolidation of multiple low gain communications antennas on moving vehicles.
Generally, spiral antenna are made of wire. For the typical wire spiral antenna, the performance advantages mentioned above come at the price of size and complexity. While the radiating elements of a wire spiral may be planar, the feed network and balun structure generally are not, and combine to add weight, depth, and significant complexity to the system. Furthermore, because a planar spiral antenna radiates bi-directionally, an absorbing cavity is generally used to eliminate the radiation in one direction, adding even more depth to the antenna. While some designs exist that integrate the feed and balun into the cavity and reduce the complexity somewhat, the cavity is still at least a quarter-wavelength deep at the lowest frequency of operation, adding significant thickness to the overall antenna structure.
The above-mentioned limitations in the prior art make conformal mounting in the skin of a vehicle difficult for prior art spiral antennas. Conformal mounting generally results in poor pattern coverage at angles far off the axis of the spiral due to the metallic skin of the vehicle. Furthermore, the size and weight of prior art spiral antennas, including cavity backing and balun structures, makes conformal mounting prohibitively difficult.
Thus there is a need for an improved simple, broadband, spiral antenna. There is a further need for a spiral antenna which can easily be incorporated into the skin of a moving vehicle in a streamlined/aerodynamic manner, without hindering the radiation pattern performance of the antenna. There is also a need for a unidirectional spiral antenna with an integrated balun and feed which is simple, thin and light. There is a still further need for a spiral antenna having a balanced feed and properly terminated arms which can match any input impedance.
A slot spiral antenna with a planar integrated balun and feed. The slot spiral is produced using standard printed circuit techniques and comprises a dielectric substrate having a conductive layer which is etched to form the radiating slot spiral. An integrated microstrip feed is included to provide a balanced feed to the slot spiral. Impedance matching is performed between the microstrip feed and the slotline of the slot spiral to maximize energy transfer. A shallow reflecting cavity is included to limit the spiral radiation to one direction. The described antenna apparatus provides a simple, broadband spiral antenna suitable for incorporating into the skin of a moving vehicle.
broadband unidirectional slot spiral antenna