From Microwave Passive Direction Finding

3.2 SPIRAL ANTENNA

The spiral antenna, which first appears in the literature in the 1956 1961 period (2), is one of the most useful and popular antennas for microwave direction finding since it exhibits wide RF bandwidth and a relatively constant bandwidth beamwidth, is circularly polarized, and can be easily spatially deployed. It is important to understand these general features and the types and modes of the spiral antenna for a full understanding of the usefulness of this type of antenna.

The spiral antenna obtains wide frequency coverage by following the principles of slow-wave or periodic type structures. Simply stated the concept relates to designing an antenna that contains, within its design structure, a radiating dipole pair that is essentially frequency self-selecting by the application of an exciting signal within the frequency capabilities of the antenna. The antenna can perhaps best be understood by imagining sets of dipole antennas of specific lengths connected in series, starting with the highest resonant frequency antenna and ending with the lowest. They are fed by a two-wire transmission line at the high-frequency end, at feed points A and B as shown (for a planar spiral) in Figure 3-2. Any signal with a 180 degree out-of-phase current applied by the transmission line to the antenna will seek out the dipole pair antenna that resonates, or brings the currents in phase, at the signal frequency passing from the highest frequency antenna pair toward the lowest frequency antenna pair . By resonance we mean the...

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