Trisecting an angle with the Conchoid of Nicomedes

This shows how the Conchoid can be used to trisect an angle.
Drag point H and watch how the length of IH changes and how the ratio of angle EDF to EDH changes. Under what condition does that ratio become 3? Click the 'Show Conchoid' checkbox. What do you think the three things that define this Conchoid are? (location of pole, which line is the directrix, and what is the 'distance')?