Roy Lewallen wrote:
Next, do the same for a transmission line. Show how two coherent
traveling waves can be produced which will propagate together in the
same direction but out of phase with each other, resulting in a net zero
field at all points beyond some summing point.

Roy, it is done all the time represented by the
S-Parameter equation for the reflected wave
toward the source. Assuming an impedance discontinuity
in a transmission line:

b1 = s11(a1) + s12(a2) = 0

If b1=0, then s11(a1) and s12(a2) are indeed two coherent
traveling waves propagating together in the same direction
but out of phase with each other. The net reflected wave
field is zero.
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73, Cecil http://www.w5dxp.com