VHF COM Weak Range After Antenna Gasket Replacement
During a scheduled inspection on a Cessna 172S, the VHF COM1 antenna was removed to replace a deteriorated gasket and the surrounding fuselage skin was touched up with primer and topcoat. The antenna was refitted with new hardware and sealed. Before the aircraft is released, the AME notes that COM1 transmits and receives, but the tower reports readable but weak transmissions at 25 nm; the previous log showed reliable communication at 60 nm. On the ramp, a portable SWR meter connected at the radio end of the 50-ohm RG-58A/U coax shows 2.9:1 at 118.00 MHz and 1.8:1 at 136.975 MHz. The coax run is about 5.5 m, with one bulkhead connector behind the panel and a right-angle connector at the antenna. The radio is a 10-watt unit. The aircraft is needed for a charter flight in two hours. The AME has a multimeter, a bonding tester, new connectors, and access to the manufacturer's maintenance manual.
1. What does the SWR pattern suggest, and which faults would you eliminate first? Explain your reasoning.
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The SWR is high at both ends of the band, but worse at the low-frequency end (2.9:1 at 118.00 MHz versus 1.8:1 at 136.975 MHz). A completely open or shorted centre conductor would normally produce a very high SWR across the band, so this looks more like an impedance mismatch whose electrical length changes with frequency. That points first to the antenna installation and its ground/reference plane rather than to the radio itself.
Priority checks:
- Antenna base bonding to the fuselage skin. Primer and topcoat under the antenna base can insulate the antenna from the airframe, effectively removing part of the counterpoise and changing the feed-point impedance.
- The right-angle connector at the antenna. It is mechanically stressed, may have a partially open shield or a deformed centre pin, and can act as a frequency-dependent transformer.
- Coax condition and routing. Look for crushed cable, a pulled shield, corrosion in the bulkhead connector, or a gasket that has lifted the antenna base enough to change the gap capacitance.
- Antenna part number and condition. Confirm it is the correct type and that the new gasket is the approved item, not a thicker generic seal.
The 10-watt radio and the short 5.5 m coax run do not explain a 2.9:1 mismatch. Do not begin by adjusting the radio or replacing it; the fault is after the radio.
2. Outline the checks and corrective actions you would complete before return to service, and state what you would record.
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Work from the antenna back to the radio:
- Inspect the antenna base, gasket, hardware, and skin. Remove paint and primer from the bonding contact area to bare metal, fit the approved gasket, and torque the fasteners to the manual value.
- Measure bonding resistance from the antenna base to nearby primary structure and compare with the manufacturer's limit. A high reading would confirm a poor ground reference.
- Inspect and, if suspect, replace the right-angle connector. Check the centre pin, shield crimp or solder, and strain relief. Check the bulkhead connector and the coax for crush or water ingress.
- If available, perform a coax continuity/insertion-loss check or time-domain reflectometer check. A shield with a partial open will not always show on a simple continuity test.
- Reconnect and recheck SWR at 118.00, 127.00, and 136.975 MHz. Compare with the maintenance manual or accepted installation limits. Do not release on the basis that 'it receives'.
- If the SWR remains high after bonding and connector repair, replace the coax or antenna as applicable and retest.
Record the original complaint, SWR readings before and after, bonding resistance, connectors or parts replaced, the manual reference used, and the final functional check. If the system does not meet the approved performance standard, do not sign it out; the two-hour charter pressure is not a technical reason to release an unsatisfactory communication system.
3. The operator argues that the radio is 'working' and asks you to sign the aircraft out for the charter. How do you respond?
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A radio that produces audio is not necessarily airworthy or fit for the intended operation. The reported usable range has fallen from about 60 nm to about 25 nm, and the SWR indicates a real feed-line or antenna mismatch. That can cause further damage to the transmitter and can degrade both reception and transmission in conditions where the crew most needs communications.
I would explain that the release decision is based on the approved maintenance data and the required communication performance, not on the presence of sidetone or a successful ramp check. I would either complete the corrective actions and verify performance, or record the aircraft as not released for the flight. If the operator has an approved deferral or minimum equipment list provision, I would check whether this specific defect is eligible and whether the required operational limitations apply; I would not invent a deferral for a known mismatch.
The sign-off should state what was found, what was corrected, the final test results, and the reference data. If the defect remains, the record should make the reason for non-release clear.