It's known (and it's the basis of many designs of microwave devices) that electromagnetic wave propagation in a medium occurs in a good way (i.e. without reflection and higher order modes excited), if there are no discontinuities, or at least they are spatially slow compared to the wavelength.
There are lots of examples of this:
1) Horn Antenna: Why is a horn antenna (which as a slow and sweet enlargement of its transverse section) better than an abrupt interruption of its supplying rectangular waveguide? Because the last will have an abrupt discontinuity, the antenna won't be matched well to free space impedance and so its radiation will be mostly reflected back to the waveguide.
2) Transitions between transmission lines: for instance this is a slow transition from a microstrip line to a couple of symmetric strips. Why can't this transition be fast (less than a wavelength)? There would be a huge mismatch.
3)Polarizers: I've seen that many antennas have not a single polarizer layer, but many layers in which there is a slow rotation of the strips in order not to abruptly rotate the electric field to get the desired polarization. A single layer is often referred not to work at multiple frequencies, but only inside a narrow band.
My question is: why a slow transition may determine an almost perfect match, without reflection? A slow transition still is a succession of discontinuities. Each of them will cause reflection, and the starting and ending points of the transition will be mismatched as if the transition were fast. I can't imagine why slow discontinuities are so well seen by electromagnetic waves.