Numerical analysis of two-arm spiral antennas printed on a finite-size dielectric substrate

A two-arm spiral antenna (SA) printed on a dielectric substrate backed by a conducting plane is analyzed under the condition that the dielectric substrate has a finite side length of L. The analysis is performed using the finite-difference time-domain (FDTD) method. The computation space for the FDTD method is reduced to one-half of the full computation space by virtue of the antenna arm symmetry with respect to the feed point. The numerical evaluations for the radiation characteristics are briefly described, and the SA_∞ (the spiral antenna with a conducting plane of infinite extent) is analyzed. It is found that the input impedance remains constant with side length L⩾0.8 wavelength in this analysis model. It is also found that as L increases, the axial ratio improves with the gain remaining relatively constant (the gain variation is within 1 dB). The SA_finite (the spiral with a conducting plane having the same finite size as the dielectric substrate) is also analyzed. It is revealed that the effects of L on the axial ratio and gain are relatively large compared with those on the former SA_∞