Whitepaper

A Practical Guide to Antennas for EMC Compliance

Published by Ametek CTS

This AMETEK CTS technical note is a comprehensive reference guide for engineers and technicians working with antennas in EMC compliance testing. It begins with electromagnetic wave fundamentals — how electric and magnetic fields are mutually dependent, how they propagate, and how the Poynting vector describes the direction and magnitude of energy flow. Free space impedance (377 Ω) and power density relationships are introduced as the foundation for everything that follows. The antenna section covers the full set of key parameters: reflection coefficient and VSWR (both quantifying impedance matching quality), gain versus directivity (gain includes efficiency losses; directivity does not), effective aperture (linking gain to signal capture area), radiation pattern and beamwidth (including the half-power beamwidth trade-off between directivity and power requirements for immunity testing), and polarization (linear, circular, and elliptical — with the EMC-specific note that standards require both horizontal and vertical measurements, and that cross-polarization isolation matters in practice). The radiated and received power section derives the Friis transmission equation, free-space path loss (noting the 6 dB loss for every doubling of distance or frequency), and both the receive antenna factor (AF) and transmit antenna factor (TAF) — the critical conversion factors between measured voltage and field strength. Two worked examples illustrate real-world application: immunity testing with the CBL6111D BiLog antenna per IEC 61000-4-3 (30 MHz–1 GHz), and emission testing with the HLA6121 active loop antenna per CISPR 11 (150 kHz–30 MHz). A practical calculation example shows how to determine required transmit power for a 10 V/m field at 3 m and 10 m distances. An annex provides all logarithmic conversion formulas for power (dBm), voltage (dBμV), current (dBμA), and field strength (dBμV/m, dBμA/m) used throughout RF and EMC work.