Feed-Line Loss and Coaxial-Cable Fault Diagnosis

Quick Answer: Diagnose a feed line with inspection, known loads, insertion-loss or return-loss tests, and comparison at both ends—not SWR at one frequency alone.

Why This Subject Matters

Reliable HF diagnosis depends on understanding the measurement, the physical system and the limits of the test method. This chapter develops the engineering first and then turns it into a safe field procedure.

Engineering Foundations

1. Core mechanism

Matched line loss rises with frequency and depends on conductor and dielectric properties. High SWR increases current and voltage variation and generally adds loss beyond the matched specification.

2. What the measurement means

Opens and shorts transform along a line, so either can appear as many different impedances depending on frequency and length. Water, crushed dielectric and poor shields create distributed faults.

3. System consequence

A known 50-ohm load at the far end should remain close to 50 ohms; deviation or ripple helps identify cable and connector problems.

Useful Relationships

Loss (dB) = 10 log10(Pin/Pout)
Power ratio = 10^(−loss_dB/10)
Distance-to-fault uses propagation velocity and measured delay

These relationships are diagnostic tools, not substitutes for instrument accuracy, calibration, component ratings or the complete installed geometry.

Worked Example

A 3 dB feed-line loss delivers about half the input power to a matched load. Under mismatch, the actual loss can be greater than the published matched-line figure.

Diagnostic Workflow

Step 1: Inspect jacket, bends and connectors.
Step 2: Disconnect sensitive equipment.
Step 3: Test a known load directly.
Step 4: Move the known load to the far end of the cable.
Step 5: Sweep the cable across frequency.
Step 6: Measure insertion loss when equipment is available.
Step 7: Replace suspect jumpers one at a time.

Stop-transmitting rule: Stop immediately for arcing, smoke, hot connectors, damaged insulation, unstable supports, an RF burn, unexpected equipment resets, or any possibility of contact with utility wiring. Remove power before touching, moving, or opening RF components.

How to Interpret the Result

A controlled test should distinguish at least one competing explanation. If a change does not isolate a subsystem, restore the original condition and choose a better test. Preserve analyzer traces, photographs, power and duty-cycle notes, cable identities and environmental conditions.

Common Mistakes and Misconceptions

Testing only DC continuity
Assuming low-frequency success proves HF-wide integrity
Ignoring adapters
Reusing water-contaminated coax without evaluation

Relevant Chameleon Handbook Paths

CHA COAX assemblies Product DNA: Complete System Overview
CHA LEFS Series Product DNA: Complete System Overview
CHA EMCOMM systems Product DNA: Complete System Overview
CHA RXL Product DNA: Complete System Overview
Master Antenna Troubleshooting Decision Tree
Antenna Measurement and Troubleshooting Field Worksheet

When Professional Help Is Appropriate

Use qualified assistance for utility-line proximity, tower or structural work, lightning protection, mains wiring, unexplained RF burns, fire damage, repeated arcing or measurements requiring energized exposed circuits.

Building Reliable Diagnostic Evidence

A useful measurement must answer a defined question. Before connecting an instrument, write the competing explanations. For example: is the fault in the radio, jumper, main feed line, matching device, radiator, return path, support geometry, or surrounding environment? Choose a test that separates at least two of those possibilities. A test that changes several parts at once may improve the symptom while hiding the actual cause.

Control the measurement plane. An impedance measured through a feed line is the impedance transformed by that line, not necessarily the impedance at the antenna terminals. Record every jumper, adapter, switch, choke and tuner present during the test. When possible, move the calibrated reference plane to the feed point or first prove the feed line with a known load. Recalibrate after changing adapters or frequency range when the instrument requires it.

Control the operating conditions as well. Record frequency, mode, transmitter power, duty cycle, elapsed transmit time, supply voltage under load, antenna geometry, cable routing, soil or counterpoise condition, wind, precipitation and nearby objects. Low-power analyzer measurements and full-power operation stress the system differently; a clean analyzer trace cannot rule out heating, saturation or voltage breakdown.

Use Known Standards

A noninductive dummy load, known-good short jumper and verified instrument provide reference points. Test the standard directly first, then through the suspect subsystem. If the standard no longer looks correct after adding the subsystem, the added cable, connector, switch or adapter deserves attention. A “known-good” part is only useful when its frequency and power ratings cover the test.

Repeatability and Uncertainty

Repeat the test without intentionally changing anything. If the result moves, the system or method is unstable. Instrument accuracy, connector repeatability, calibration quality and environmental coupling limit how many digits are meaningful. Preserve the raw trace and describe the setup; do not reduce an entire sweep to one minimum-SWR number.

Four Questions for Interpreting Any Result

Is it frequency-dependent? Smooth resonance shifts usually suggest geometry or loading; periodic ripple often suggests multiple reflections or line effects.
Is it power-dependent? A fault appearing only at operating power suggests heat, arcing, saturation, protection behavior or contact resistance.
Is it movement- or weather-dependent? Wind, cable motion, wet insulation and changing ground coupling point toward mechanical or environmental causes.
Is it time-dependent? A gradual change during a carrier or digital transmission suggests thermal accumulation, supply sag or control instability.

Documenting the Conclusion

Separate observation from inference. “Reactance changed from +35 ohms to +5 ohms after shortening the radiator” is an observation. “The radiator was electrically too long” is an interpretation supported by that observation. Record the failed hypothesis as well as the successful one so the next operator does not repeat the same unproductive test.

A repair is complete only after the original trigger has been reproduced safely without the symptom, a new baseline has been saved, and the installation has passed mechanical and safety inspection. If the system cannot be returned to the original test condition, label the result provisional.

Related Handbook Pages

Antenna Measurement Reference Planes
Master Antenna Troubleshooting Decision Tree
Feedline Loss and Overall System Efficiency
Understanding Common-Mode Current
RF Safety and Stop-Transmitting Conditions