During the 1930s the broadcast industry recognized the problem of multipath fading, that at night high angle waves reflected from the ionosphere interfered with the ground waves, causing an annular region of poor reception at a certain distance from the antenna. It was found that the diamond shape of the Blaw-Knox tower had an unfavorable current distribution which increased the power emitted at high angles. By the 1940s the AM broadcast industry had abandoned the Blaw-Knox design for the narrow, uniform cross section lattice mast used today, which had a better radiation pattern. It was found that reducing the height of the monopole mast from 225 electrical degrees to 190 degrees could eliminate the high angle radio waves that caused fading. Sectional masts were also developed in this era.