File:AI Mk.IV Radar Display E(MOS)1448.jpg
Across the top of both displays is a large triangular shape caused by ground returns. The broadcast from the Mk. IV covered the entire forward hemisphere of the aircraft, so the signal struck the ground at some point. Directly under the aircraft was the closest point, which causes the long return at the base of the triangle. The reflections from points further in front of the aircraft reflect back less energy, both because they are further away as well as the signal increasingly being reflected forward, away from the aircraft. This causes the rapid reduction in signal strength at the top of the triangle. The base of the triangle is at a range equal to the altitude of the aircraft, which limited the useful range of the display.
The display often shows a similar shape at the other end of the displays, the "bleed through" from the transmitter. This could be suppressed with a bias control, and has been almost entirely removed on this display. It is just visible on the far left of the vertical display.