In h.f. broadcasting, frequencies and aerials are chosen to give the optimum signal strength to listeners within the desired service area. The varying nature of the ionospheric transmission medium makes it desirable to measure the distribution of energy actually achieved over that area.In the experiments described, pulse echo measurements of the back-scattered energy from ground irregularities in the service area were made, the echoes being observed on a range/amplitude display. In tests with two aerial systems, the echo patterns clearly showed the differing energy distribution. The patterns agreed well with those calculated for the two aerial systems from parabolic ionospheric-layer theory and the radar equation.Further tests at night under varying ionospheric conditions showed that the propagation modes to 5 000km range were correctly evaluated by back-scatter sounding at times when ionospheric predictions, corrected by local vertical-incidence soundings, gave erroneous results.