Use a stated free-space, time-gated, waveguide, coaxial or fixture method and record the reference reflector or standard, antenna or port geometry, distance, sample size, incidence, polarization, frequency grid, gating and uncertainty. Reflectivity and transmission loss answer different questions and must not be mixed.
After coupon screening, install the exact backing, adhesive, seams, corners and compression. Repeat the original enclosure, chamber, near-field or coupling measurement with unchanged cables, probes, lids and instrument state. Verify the entire target band, not only the deepest null.
Reject the result when the baseline is not repeatable, edge diffraction dominates the coupon, an open seam remains, or performance drifts after power, thermal, humidity, vibration or maintenance exposure. Retain raw traces, configuration photos, material lot and drawing revision so a later replacement can be compared at the same boundary.