Self-complimentary MAW 4-arm Archimedes spiral antenna
Image of the Self-complimentary MAW 4-arm Archimedes spiral antenna.

The basic 4-arm self-complimentary Archimedes spiral already forms part of the Antenna Magus database. By using modulated arm widths, a new class of broadband antennas capable of dual-circular polarisation is produced.

The MAW self-complimentary 4-arm Archimedes spiral antenna radiates a rotationally symmetric, wideband, circularly polarised beam with boresight maximum sum (Mode 1) and reverse (Mode 3) or boresight null (Mode 2) pattern. Both pattern and impedance performance is fairly frequency independent over the operational band.

When the structure is large enough Mode 3 radiates a difference pattern. If, however, the structure is not large enough to support efficient radiation of Mode 3, simple reflection of the current at the ends of the structure produces a return excitation which efficiently radiates like Mode 1, that is a smooth lobe sum pattern, but with the opposite sense of polarisation. Hence, introducing a modulation in the arms to effect a stopband region after the Mode 1 active region and ahead of the Mode 3 region is sufficient to give the required dual-polarisation operation.

Typical gain at the minimum, center and maximum frequency for all available modes