Pulse-width scale of Fourier coefficients ($n\sim T/w$)

For the Fourier series of a periodic rectangular pulse, the sinc function is nearly one when $nw/T\ll 1$ and has its first zero at $nw/T=1$. Thus narrowing a pulse increases the range of significant Fourier coefficients. Only integer harmonic indices occur, so an envelope zero need not be an exactly vanishing coefficient.