The improvement to whip is 3 stepped:
(1) Most important is cutting it to ¼ wavelength (67mm for 1090 Mhz) and provides the largest chunk of improvement. The % improvement depends on how much the original whip was deviating from 67mm. Larger the deviation, larger will be improvement.
(2) Placing whip over a metallic can, box or sheet to enlarge it’s insufficient groundplane provided by it’s tiny magnetic base.
(3) Although enlarging groundplane by above method improves reception by another 10% to 20%, but since the shield of coax is not solidly connected to ground plane, and groundplane is not optimally dimensioned, the real benefit is not achieved. When you make a regular, properly dimensioned Spider or Cantenna, you get the full optimization.
For me the main problem is surrounding high rise buildings in various directions, including my own building. Also in direction of Ottawa & Montreal, down town Toronto exists at about 20kms from my place, and high rise buildings restrict my range to 150 nm in that direction. Under this condition, even a good high gain antenna like Flightaware’s cannot overcome this problem.
See my coverage with Cantenna. In most of un-obstructed directions I get 200+ nautical miles, and in the best direction (south east) upto 250+ nautical miles., but in the direction of surrounding high rise buildings, my range drops to 50 nm or completely blocked