In radio propagation, horizontal plane is used to plot an antenna's relative field strength in relation to the ground (which directly affects a station's coverage area) on a polar graph.
If the graph is of an actual or proposed installation, rotation is applied so that the top is 0o true north.
The vertical plane is used to plot an antenna's relative field strength perpendicular to the ground (which directly affects a station's coverage area) on a polar graph.
Normally, the maximum of 1.000 or 0 dB is at the side (unless there is beam tilt), which is labeled 0°, to 90° at the top and −90° at the bottom.
Short wave transmitters want a somewhat higher elevation angle in the vertical plane to encourage skywave propagation, which would refract or reflect radio waves off the ionosphere and back to the ground at a great distance from the transmitter.