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A broadband spiral antenna (2 to 18 GHz) is modeled using the FDTD method. The inner turn of the antenna is an Archimedean spiral, while the outer turn is an Archimedean spiral with a zigzag shape. The current-density distribution along the spiral arms is first presented in the time domain, interestingly manifesting the impulse current propagation from the feed point to the outer end of the spiral arms. The performance of the broadband spiral antenna is also measured in an anechoic chamber. The simulated and measured results showed the sensitivity of the axial ratio and radiation pattern to the current distribution. The simulated linear gain, radiation patterns, and axial ratios agree well with the measured data over all of the frequency range from 2 to 18 GHz.