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PCBD-Dec2014

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58 The PCB Design Magazine • December 2014 The title may be confusing for many tech- nologists accustomed to dealing with electri- cal issues in traditional PCBs, but if you design PCBs that operate at microwave frequencies, it makes perfect sense. With microwave PCB de- sign, it is not uncommon to have a conductor run come to a stop, followed by a space, fol- lowed by another conductor run, with the RF energy propagating through the discontinuity without the slightest problem. . Not Your Traditional Design In traditional PCB design world, this con- ductor–space–conductor configuration is called an "open," and it is considered a reject that yields a dead circuit. But microwave PCB design is much different. How does RF energy propa- gate through conductor discontinuities in mi- crowave PCBs? When considering microwave technology, it is all about waves. Specifically, it is the elec- tromagnetic (EM) wave that propagates on the PCB, and the wave properties are manipulated by the PCB design to get the desired circuit per- formance. To picture a simple example of an EM plane wave on a PCB, you can think of the wave in a cross-sectional view that looks like a sine wave. This sine wave will have different lo- cations with high and low energy and ¼ of the sine wave is one of the maximum power points. A ½ sine wave is where the wave returns to zero and has no energy. These fractions of the sine wave are appropriately called ¼ wavelength and ½ wavelength, respectively. LIGHTNING SPEED LAMINATES column by John Coonrod rogers CorPoraTIon Making a Connection with Conductor Discontinuities

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