Ground Planes and Circuit Board Materials for rf development
A ground plane is a continuous conducting sheet, usually copper, which forms the bottom layer of a RF circuit board’s foundation. If you envision signals traveling in An RF board as propagating waves, it’s easy to understand the need for a ground plane. It forms the continuous common reference for propagation and impedance throughout the board.
Those with experience prototyping audio and low speed digital circuits are no doubt familiar with the standard jumper boards with binding posts for power connections. Don’t use these for RF ! Although they contain ground pins or traces, they lack the continuous conducting ground plane sheet which is essential at radio frequencies. And, their multi pin structure contains a host of unwanted stray reactances which can spell disaster at frequencies above a few MHz.
The substrate forms the middle layer in the foundation. It can be made from a variety of dielectric materials, depending on the board’s intended frequency range and application. At the low end of the frequency and price range are fiberboard (G10) and fiber epoxy series (FR4) commercial board materials generally usable up to about 1 GHz, above which dielectric losses increase rapidly. Duroid and other trade name microwave board materials are usable at higher frequencies (and prices).
The top layer of the board contains the copper traces that connect circuit elements. Layout and interconnection of these traces is critical at radio frequencies since every trace is a transmission line and every pad contains some inductance or capacitance – whether intended or otherwise. The importance of proper board layout is one of the key differences between low frequency and RF prototyping.