Hi Dust,
It can be easily designed using EAGLE CAD user script named 'spiral-coil.ulp'. The script draws 180 degree arcs that radius gradually increased from the center toward the outside. And, you are able to draw two set of spiral coil pattern after rotating the first one in 180 degree. Imagine a couple of mosquito coils in a box. If you connect each coils together at the center then you have a long heater pattern with magnetism cancelled.
It can be easily designed using EAGLE CAD user script named 'spiral-coil.ulp'. The script draws 180 degree arcs that radius gradually increased from the center toward the outside. And, you are able to draw two set of spiral coil pattern after rotating the first one in 180 degree. Imagine a couple of mosquito coils in a box. If you connect each coils together at the center then you have a long heater pattern with magnetism cancelled.