RF cavity analysis with spreadsheet software

High power RF systems are commonly used in fusion research. At Princeton, they are required to deliver power in the megawatt range at frequencies of 30 to 80 MHz. As both frequency and power output are increased, circuits composed of conventional inductors and capacitors become difficult to realize. Under these conditions, cavities composed of sections of transmission line are employed as tuned circuits. A simple method is presented, whereby transmission line impedance transformations are calculated utilizing spreadsheet software such as Microsoft Excel. From these calculations, RF cavity models may be constructed, yielding information such as plate impedance, currents and voltages at various points in the cavity, and bandwidth. This method was used in the design to convert both 80, and 40-80 MHz FMIT sources to 30 MHz, for use on TFTR.