= Transmission-line stop band
{title2=$|\operatorname{tr}M/2|>1$}
For a real-frequency lossless reciprocal periodic line, a cell <transfer matrix> has <determinant> one. If half its trace lies outside $[-1,1]$, the Bloch multipliers are real reciprocal numbers rather than unit-modulus propagation factors. The physical interior field decays and carries no net power to infinity. A real reference line is then totally reflected by a semi-infinite structure; a finite structure can still tunnel.
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