Make Smart Use Of Preamps In RF Testing
A.H. Systems
This A.H. Systems whitepaper explains when and how to correctly use preamplifiers in RF testing setups covering emissions testing, enclosure shielding verification, and open-field RF measurements. The core problem a preamp solves is that incoming signals frequently sit at or below an analyzer's noise floor (−100 to −120 dB), making them undetectable. A preamp typically adds 40 dB of gain to lift the entire signal above the noise floor, effectively expanding dynamic range. Not every setup needs one — high-intensity radiated field (HIRF) testing and some military/automotive applications already have signals strong enough to skip it. The paper then covers practical best practices: match the preamp's frequency range and impedance to the antenna; place it as close to the antenna as possible to minimize cable-induced attenuation; account for the ~5 dB of noise the preamp itself adds by budgeting an extra 6 dB of gain; ensure proper grounding for both antenna and preamp to prevent ESD damage (especially with broadband horns, log-periodics, and biconicals); use input limiters when strong signals risk burning out the preamp's front end; and add bandpass or low-pass filters when out-of-band signals from the device under test could cause overload. The document closes by stressing upfront planning to avoid equipment damage and wasted test time.