Plasma chemistry has applications in many emerging fields, such as nitrogen fixation, synthetic fuel synthesis, and gas treatment. Most of the published research on microwave plasma has focused on gas pressures below one atmosphere, recently some experimental results have been published for gas pressure at one atmosphere. However, higher pressures have yet to be explored, presenting an opportunity to make significant breakthroughs. In this project, the structures to couple electromagnetic waves to a plasma column have been studied using electromagnetic simulations, for operating pressures at or above one atmosphere. Simulations referred to conducted experimental measurements have been carried out which showed that the electron density of the plasma could be determined using simulations.