An RF power amplifier and an antenna get most of the attention when specifying a test chain, but the passive components connecting them — couplers, attenuators, and terminations — determine whether that chain actually works safely and repeatably. None of these add gain; all of them exist to sample, reduce, or safely dispose of RF power at a known, predictable ratio.
Directional Couplers — Sampling Power Without Disrupting It
A directional coupler is a four-port device that taps off a small, precisely known fraction of the power flowing through the main line, in one direction only. Connected between an amplifier and an antenna, the forward-coupled port shows how much power is actually reaching the antenna; the reverse-coupled port shows how much is reflected back — the basis of VSWR and return-loss measurement.
The two specifications that matter most are coupling factor (how much of the signal is diverted to the coupled port — a 20 dB coupler passes -20 dBm to the coupled port for every 0 dBm at the input) and directivity (how well the coupler distinguishes forward from reflected power — poor directivity lets reflected energy leak into the forward reading and corrupt the measurement). Any unused port on a coupler should always be terminated in the system impedance (typically 50Ω); leaving a port open reflects that energy back into the main line and distorts every other reading on the setup.
Attenuators — Controlled, Predictable Signal Reduction
An attenuator reduces signal level by a fixed, known amount across its rated frequency range, converting the removed power to heat rather than reflecting it. In a test chain, attenuators protect sensitive receiver or analyser inputs from overdrive, and pad down an amplifier's output to a level a downstream instrument can safely handle. The critical rating to check is power handling — an attenuator rated for average power only will fail quickly if placed somewhere in the chain that actually sees the amplifier's full instantaneous power, particularly in pulsed applications.
Terminations — Absorbing Power Cleanly
A termination is effectively a one-port load, built to absorb incident RF power across its rated frequency and power range while presenting a good match (low VSWR) back into the line. Terminations are used on unused coupler ports, as a reference load when characterising an amplifier without radiating, and anywhere a test setup needs a defined 50Ω endpoint rather than an open or shorted connector. Waveguide-interface terminations serve the same role at frequencies where coaxial connectors are no longer practical.
Sizing Rule of Thumb
Every passive component in a chain should be rated for at least the peak power it will actually see — not just the average — with enough margin that normal VSWR mismatches from the antenna or DUT don't push it into a fault condition. Frequency range matters just as much as power: a coupler or attenuator used outside its rated band will still pass a signal, but with degraded, uncalibrated performance that quietly corrupts a measurement without an obvious failure.