 Due to the wide pattern of the transmission antenna , some of the signal always hit the ground , reflecting some of it back at the aircraft to cause a ground return . This was so powerful that it was received on all of the antennas , even the upper vertical receiver which would otherwise be hidden from signals below it . As the shortest distance , and thus the strongest signal , was received from reflections directly below the aircraft , this caused a strong blip to appear across all the displays at the range of the fighter 's altitude . The ground further in front of the aircraft also caused returns , but these were increasingly distant ( see slant range ) and only some of the signal was reflected back at the aircraft while an increasing portion was scattered forward and away . Ground returns at further distances were thus smaller , resulting in a roughly triangular series of lines at the top or right side of the displays , known as the " Christmas tree effect " , beyond which it was not possible to see targets . 
