The waves from a radio station can reach a home receiver by two paths. One is a straight-line path from transmitter to home, a distance of $30.0 \mathrm{km} .$ The second is by reflection from the ionosphere (a layer of ionized air molecules high in the atmosphere). Assume this reflection takes place at a point midway between receiver and transmitter, the wavelength broadcast by the radio station is $350 \mathrm{m},$ and no phase change occurs on reflection. Find the minimum height of the ionospheric layer that could produce destructive interference between the direct and reflected beams.