The EMI Finder

This article describes a UHF receiving system used to easily locate a very wide variety of sources of electromagnetic interference (EMI). The system consists of a sensitive narrow-bandwidth 318 MHz receiver, a directional handheld Yagi antenna and a headset (see Figs 1-3). Some of the typical broadband emitters of radio frequency interference (RFI) that you can pinpoint are powerline insulators, computers and industrial equipment. The following are the principle technical features of the receiving system, which contribute to its effectiveness: • High gain, directional five-element hand-held Yagi antenna for precise position location of noise emitters (see Fig 2). • Simple construction. • Surface acoustic wave RF filter (SAWF) for good front-end selectivity. • High-gain, low-noise figure RF preamplifier (LNA) for high sensitivity. • Regenerative detector stabilized with a surface acoustic wave resonator (SAWR) produces a very narrow detection bandwidth hence a low noise floor. • High detection sensitivity, –136 dBm minimum discernable signal. • Low-loss direct coupling technique between the LNA and regenerative detector. • Low-voltage, low-current operation for long battery life. • With only two active RF components, the circuit is simple and the cost low. • All surface-mount devices and components for best RF performance, easy construction and low cost. • Receiver design technique useful at other frequencies including the 70-cm amateur band.