Double$-$slit interference: Two radio antennas are $120$ m apart on a north$-$south line, and they radiate in phase at a frequency of $3\mathrm{.}4$ MHz$.$ All radio measurements are made far from the antennas. If the east$-$west reference line passes midway between the two antennas, what is the smallest angle from the antennas, measured north of east, at which constructive interference of two radio waves occurs? $($c $=3\mathrm{.}00108$ m$/$s$)$