If you tried shooting down a harvester with basic ships, dont worry. You cannot shoot them down with less than 3 jets and even so you'll lose at least one. Not worth the effort. Just wait until it lands or disappears.
If the plane is just too fast, build more bases with interceptors. Make your planes approach it from many directions, like when you go hunting. Dont try chasing them.
Also, try not equipping your ships with heavy weapons and armor to make them fly faster.
