The Gigahertz Transverse Electromagnetic (GTEM) cell was developed about 20 years ago and the applications using GTEM cells for electromagnetic compatibility (EMC) measurements are continuously increasing. The research described in this Thesis proposes a new method for characterizing emissions from electrical appliances using a GTEM cell. The research adapts the method used in the Transverse Electromagnetic (TEM) cell where the measurements are done by using a two-port system which includes a phase measurement. This is a restriction in GTEM cells because only one port exists and normally only the magnitude reading can be performed. This requires the development and application of new techniques, which allow the description of emitters in terms of equivalent electric and magnetic dipoles measured using a GTEM cell with phase measurement. A full field 3D Transmission Line Model (TLM) model of a GTEM cell is generated. Further simulations based on this model were performed to assess the behaviour of the emitters and when the GTEM cell is under actual working conditions. The model is also used to validate results obtained from measurements. The assumption that the dipole moments from a general emitter are in phase is also studied and evidence is presented to assess the validity of this assumption. The impact of the phase variations within an EUT towards the total radiated power estimated according to IEC 61000-4-20 is discussed and a technique to measure phase using a GTEM cell is introduced.