What you are seeing here is a simultaneous plot of the frequency contour (which encloses the right-half plane) and the Nyquist plot, which is the transfer function G(s) evaluated along the frequency contour. The movie starts with the positive imaginary axis; as w goes from 0 to infinity, G(j*w) goes from magnitude 1.5 and phase 0 to magnitude 1 and phase 0 (encompassing 360 degrees of phase change along its path). The frequency loop at infinity is not shown; limit of G(j*w) as w->infinity (you can see where the red line ends and the green line begins), and so the entire loop at infinity is mapped to the point at 1. Because G(s) has only real coefficients, G(-j*w) = -G(j*w), and so the green line is a mirror image of the red one (reflected across the real axis). For continuity, the green line is drawn from a frequency of -infinity up to zero.

Because G(s) is a continuous function, a closed contour in the frequency domain will always map into a closed contour in the Nyquist diagram.

This movie is also available in Quicktime format for Macintosh.

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8/27/96 LJO