The transverse electromagnetic (TEM) cells are used for electromagnetic applications such as interference/compatibility (EMI/EMC) experiments of the small RF devices in a laboratory environment. The standard electromagnetic fields generated in the shielded environment of a TEM cell can also be used for calibration of the RF field probes. In this section, a 50 Ω open TEM cell is designed and fabricated to generate standard electromagnetic fields from DC up to 3.5GHz. To overcome the impedance mismatch and improve the voltage standing wave ratio (VSWR) along the TEM cell, a new piecewise linear tapering method is proposed for the inner conductor of the cell. We use some idea to improve reflection and transmission coefficient, especially transition coefficient. For example, slotted walls, slotted septum, resistors and long narrows ferrites. Our results are completely compatible with commercial instrument in the world and even in some parameters such as transmission coefficient are better than the others.