Substituting Coax and Routing It Past an AC Feeder
An installer is adding a VHF comm antenna to the top of a pressurized fuselage. The approved drawing calls out 18 ft of RG-400 (M17/128) coaxial cable with a TNC connector at the radio end. Stock has only RG-58A/U cable and matching crimp connectors, and the installer argues that both are 50 ohm and have about the same loss at 120 MHz, so the substitute will work. The planned route crosses a 115 V AC feeder at 90 degrees once, but runs parallel to it for about 3 ft, bends through a 1 in. radius around a frame cutout, and passes within 18 in. of an existing GPS antenna. The radio is a 16 W comm.
1. Evaluate the cable substitution. Is the installer's reasoning about loss relevant to the decision?
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The substitution is not acceptable, and the loss argument misses the point.
- The installer is right that both are 50 ohm and that loss at VHF is broadly similar - that is precisely why loss is not the deciding factor.
- The real differences: RG-400 has a PTFE dielectric rated to roughly 200 C and a double shield; RG-58A/U uses polyethylene and a single braid, with a much lower temperature rating and less shielding effectiveness. In a hot, high-RF area the temperature rating alone can disqualify it.
- The drawing is approved data. A substitute cable requires approved data or engineering disposition, not a judgment call on the ramp.
- The build also differs: connectors, strip dimensions, and crimp tooling are cable-specific, so substituting drags a whole new termination process with it.
Correct action: obtain the specified cable, or obtain engineering approval for an equivalent before installing anything.
2. Evaluate the planned routing. What would you change and why?
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Three issues, all fixable before the cable is pulled:
- Parallel run to the AC feeder. About 3 ft of parallel routing couples 400 Hz or 115 V harmonics into the receive path, showing up as a buzz or reduced sensitivity. Reroute for maximum practical separation, and where a crossing is unavoidable, keep it at 90 degrees - the single crossing already planned is fine; the parallel section is not.
- Bend radius. A common rule for coax is a minimum bend radius of about 10 times the cable OD. RG-400 is roughly 0.195 in. OD, so about 2 in. minimum. A 1 in. radius is roughly half that and will deform the dielectric, changing impedance and degrading VSWR. Reroute the frame cutout or use a grommet and a wider sweep.
- Proximity to the GPS antenna. 18 in. from a comm transmitting 16 W is a co-site interference risk - L1 is close enough that the comm can desense or blank the GPS. Check the antenna manufacturer's and OEM's separation criteria; if 18 in. is inside them, relocate the antenna.
Also confirm the route does not penetrate a pressure boundary without an approved fitting.
3. What tests are required before the installation is signed off?
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- Continuity and short check with a DMM before the radio is connected: center to center end to end, shield to shield, and no short between center and shield. Catch a whisker or a folded braid before it becomes a radio.
- Insulation resistance only at the voltage the maintenance manual permits. Do not assume a 500 V megger is acceptable on a coax run just because it is an insulation test.
- VSWR / forward and reflected power check with a calibrated instrument at the operating frequency. Roughly 2:1 is the outer acceptable limit and 1.5:1 is the target; a high reading points to a bad connector, a crushed cable, or a bad antenna ground plane.
- Never key the transmitter without a load connected.
- Confirm final routing has adequate clamps, chafe protection, and separation, then document the cable part number, connector part numbers, and test results.