The development of three new EMC antennas, namely the V-conical-lens antenna (VCLA), half of a Transverse Electromagnetic-T (TEM-T) cell (acting as an antenna) and the Q-loop antenna (a quarter of a loop antenna in front of 90 ° comer reflector) is described. These antennas, when calibrated, are designed with a view to employing them in the measurement of on-line Shielding Effectiveness (SE) of conductive composite materials. Test devices incorporating those newly developed antennas for measuring SE against high impedance and low impedance wave are introduced. The theoretical model of the VCLA is developed and design features are presented as a state-of-the-art project with a view to developing this technique in the near future for measuring the plane wave SE of conductive plastics during their production process. A modified TEM-T cell designed to simulate a high impedance field on the material under test (MUT) in its (TEM-T cell's) near field region is presented. The field simulated by this device in the test location is studied theoretically. The device measures the high impedance field SE o f planar sheet-like conductive plastic materials in a situation that attempts to reconstruct the on-line environment likely to prevail in the manufacture o f such plastics. This test device is calibrated by taking into account the background noise, indirect path signal infringement and radiation losses. The newly developed Q-loop antenna is designed to be used to measure the low impedance field SE of conductive plastic materials. An analytical model of the Q-loop antenna is developed using image theory and the theory of pattern multiplication. This model is verified experimentally. Calibration experiments are performed to facilitate applying the antenna in an on-line SE measurement technique. A new class of filled composite material with a two dimensional regular array of conductive flakes (like a Frequency Sensitive Surface (FSS)) in plastic resin is proposed. A theoretical model of the suggested configuration is formulated and used to predict SE values. The SE of such material is also determined experimentally and compared with the theoretical predictions. This SE is compared with the SE o f an available filled composite in which the flakes are randomly distributed. The improvement in the shielding capability of the new class of material is highlighted. Relative radiation patterns of the developed antennas are measured and compared with predictions. Apart from the anomalies which can be attributed to (simplifying) assumptions made in the development of the theoretical analysis, the measured radiation patterns and other antenna parameters are in good agreement with predictions.