You may see that this structure is sized with a distance between the antenna and the plate equal to \$\lambda/4\$. My professor has told me the reasons of this choice are that:
1) The antenna irradiates waves both at right and left. But we want all the wave at left. Therefore we put a very good conductor plate at left, in order to reflect the wave towards right.
2) The distance between the antenna and the previous plate is sized to be \$\lambda/4\$ because in this way it acts as a quarter wave transformer with a short circuit load (represented by the plate), and so the short circuit is seen as an open circuit by the antenna.
So, it is a narrow band system, designed to work ideally for only a single frequency. If for instance we excite the antenna with a frequency such that the distance antenna - plate becomes \$\lambda/2\$, the antenna will see a short circuit.
Now my question is: why is it a problem if the source (antenna) sees a short circuit? From a circuital point of view, I'd say that it is a voltage source in parallel with a short circuit, which is dangerous because it means infinite current which may damage the circuit. But this is not a circuit. So which is the disadvantage/danger of this condition?