All faultfinding roadmaps · FF-04 · Deployed / workshop / training
A mismatch alarm, high reflected power or a suspected antenna / feeder fault.
Have to hand
Suitable cable / antenna analyser or VNA, calibration kit, rated loads / adapters, feed diagram and equipment limits.
Before you start
Isolate the feed from transmitters and DC sources as specified before passive measurements. Never connect an active transmitter directly to an ordinary analyser input. Live power tests require the specified rated coupler, attenuation, load and DC protection.
Follow the equipment manual and local procedure. If a tool, permission, safe test setup or required measurement is unavailable, record the result as unknown and escalate with the available evidence. Change one item at a time.
For offline use, print or save this page as a PDF using your browser’s Print command. Include any linked roadmaps you expect to need.
Checks on this page
- 1. Establish a valid measurement setup
- 2. Interpret VSWR and return loss
- 3. Locate and separate the mismatch
- V. Verify recovery
- E. Escalate
1. Establish a valid measurement setup
- Identify the operating band, system impedance, reference plane and permitted VSWR / return-loss limit. Check that instrument, leads, adapters and loads are suitable.
- Isolate and verify the feed according to the equipment procedure. Calibrate at the connection where the assembly under test begins; include / remove test-lead effects as the instrument procedure requires.
Is the isolated setup ready for a valid measurement?
- Yes
Sweep the operating band and save the trace.
- No / missing equipment or limit
Record the alarm and the missing requirement. Arrange a properly equipped test.
Next: E · Escalate with evidence.
2. Interpret VSWR and return loss
- Sweep the specified operating band and record the worst reading and its frequency at the stated reference plane.
- VSWR closer to 1:1 means less mismatch. Larger positive return loss means less reflection; an S11 log-magnitude trace becomes more negative as the match improves. Do not reverse these display conventions.
Is the result within the specified limit across the operating band?
- No
Localise the mismatch before replacing the antenna or radio.
- Yes
Check insertion loss if the original symptom persists. A lossy cable can hide a poor antenna match at the radio end.
3. Locate and separate the mismatch
- Inspect connectors and use distance-to-fault with correct cable settings to locate discontinuities. Compare against the baseline; DTF locates features and is not a substitute for the specified return-loss sweep.
- With the feed isolated, test sections using suitable known-good components and a correctly rated termination. Record what is included at each reference plane. Recalibrate if the setup requires it.
Was the faulty component identified, corrected and the sweep brought within limits?
- Yes
Confirm the feed’s insertion loss before returning the full system to service.
- No
Escalate with sweep / DTF traces, test setup, calibration point and substitutions tried.
Next: E · Escalate with evidence.
Match is only one part of RF health
A VSWR of 2:1 corresponds to about 9.54 dB positive return loss and 11.1% reflected power. These are equivalent readings, not an acceptance threshold.
For forward and reflected powers measured at the same reference plane: |Γ| = √(Preflected / Pforward); VSWR = (1 + |Γ|) / (1 − |Γ|); return loss = −20 log10(|Γ|). These passive-system expressions need positive forward power and 0 ≤ |Γ| < 1 for a finite VSWR.
A good match does not prove that the antenna radiates efficiently, that cable loss is acceptable, or that the application works.
V. Prove the service is restored
- Restore the approved configuration, connections, covers and weather seals. Remove test equipment and temporary changes using the equipment procedure.
- Repeat the original failing user operation and the relevant measurements. Check both ends where applicable; ping success or a good RF reading alone is insufficient.
- Compare before / after readings with the equipment limits, required service level and baseline under representative traffic.
- Observe for the agreed period, record remaining limits and return the equipment to the responsible operator. A brief pass does not establish long-term availability.
Meets the agreed requirements: record the evidence and corrective action in the fault record. Close the fault through the normal reporting process.
Fails / no correction / still uncertain: keep the fault open. Record the last confirmed working boundary and remaining symptoms; avoid repeating unchanged checks. Escalate with evidence.
E. Escalate with evidence
Send the record through the approved support channel. Exclude credentials, tokens and unnecessary sensitive details.
- System / location, UTC time, impact, exact failing operation, last working state and recent changes.
- Roadmap code and check reached, expected result, actual readings, instrument / command used, units and measurement reference point.
- Relevant interface counters, addressing / route, neighbour state, bounded path tests, resolver results and service logs.
- For RF: measurements at both ends, band / bandwidth / mode, baseline, cable type / length, calibration settings, traces and installation observations.
- Changes and substitutions tried, their results, current configuration, working boundary, missing tools / access and the next support action needed.
Fault record
Use your incident log, or print this space for field notes. This page does not store incident details.
UTC date / time and operator
System / location and incident reference
Symptom and last working state
Check reached and expected result
Before / after readings, units and test setup
Change / substitution and result
Verification operation and observation period
Open / resolved, handover owner and next action
Technical references
The decision sequence combines the references below. Product examples explain the method; use the manual for your deployed equipment to set acceptance limits.
- Anritsu — understanding cable and antenna analysis (PDF)
- Anritsu — distance-to-fault measurements
- Rohde & Schwarz — how to measure cable loss
- Keysight — instrument input protection (PDF)
Reference content: revision 1, 15 September 2026. Generic source-reviewed guidance; use site and equipment-specific procedures for actual work.