Structural danger of printed-circuit transmission lines when they are used for millimeter-wave devices

We discuss in depth the danger relating to a structural problem of printed-circuit transmission lines. This danger is caused by the leakage effect that occurs on uniform length of printed-circuit transmission lines on which the dominant mode is purely bound at ordinary microwave frequencies but becomes leaky at millimeter-wave frequencies. Such a leakage effect at millimeter-wave frequencies, though it is not widely recognized and can therefore be unexpected as well as undesired, are rightly important for a designers of millimeter-wave integrated-circuit devices