Abstract Radon detectors based on an electrostatic collection of polonium and detection of its alpha decay are a popular choice for the measurement of radon activity. Due to the nature of $^{222}$Rn decays, 88% of radon progeny have a positive charge, thus enabling their collection on an electrode. A simulation software focused on the drift and diffusion of ions in an electric field has been developed, providing a tool to study and characterise such detectors. It has been found that one such device achieves only 44% collection efficiency, caused by the loss of a large fraction of the ions before reaching the collection diode. An improvement of the design is therefore proposed, changing the placement of the sensor resulted in an simulated increase of the collection efficiency up to 93%. Preliminary measurements performed using prototypes of these detectors confirm the results of the simulations.