What is a sweep test?

A sweep test (RF line sweep) measures how well a wireless site's transmission line carries RF energy. The technician injects a known signal at one end and measures the reflected energy across the relevant frequency band, producing three core results: return loss (how much energy is reflected back), cable loss (insertion loss along the line), and distance-to-fault (DTF: where any reflections are physically located along the line).

Return loss and VSWR (Voltage Standing Wave Ratio) measure the same physical phenomenon (energy reflected back due to impedance mismatch), but expressed differently. Higher positive return loss is better; 20 dB return loss corresponds to S11 of -20 dB. VSWR is a ratio (1.0 is perfect; 1.5 is good; 2.0 is marginal). Most carriers spec return loss; legacy sites still see VSWR thresholds.

Carrier acceptance thresholds vary by band and program. Common new-build targets are 20 dB return loss across the full operating band, with marginal acceptance to 18 dB on legacy or retrofit work. Anchor site programs and high-band 5G NR work often spec tighter: 22 dB and above. The exact threshold lives in the RFDS or scope document.

DTF (distance-to-fault) is the sweep test mode that locates where along the line a reflection is happening. DTF estimates distance along the cable from the calibration plane. This is what crews use to find and fix line problems without re-sweeping the entire site.

A sweep test measures linear RF performance: return loss, cable loss, DTF. A PIM test measures non-linear behavior, interference generated when high-power signals mix at non-linear passive components. A line can pass a sweep test (good return loss, clean DTF) and still fail PIM if it has a contaminated connector. Both tests run at acceptance.

Crews use a calibrated cable-and-antenna analyzer suitable for the required tests. The tester is calibrated immediately before the sweep using a known open/short/load standard. Crews keep cal kits on the truck and re-cal between sites.