DC-Grounded Coax at HF: Why “Ground” Does Not Tame RF
DC-Grounded Coax at HF: Why “Ground” Does Not Tame RF
Bonding a coax shield can be essential for safety and lightning protection. It does not guarantee that the same point is a low-impedance RF reference or that current on the shield exterior has disappeared.
RF.Guru working definition: Common-mode current is the non-cancelling phasor-sum current in a specified set of conductors, evaluated at a defined cross-section and using a declared current-direction convention. In the intended differential transmission-line mode, the outgoing and return currents are equal and opposite, so their phasor sum is zero. When they do not cancel, the remaining current must close through another reference or return path—such as the outside of a coax shield, a mast, equipment chassis, station wiring, nearby structures, earth, the operator, or distributed coupling through the environment.
This broader working definition is especially useful in practical antenna systems. On transmit, non-cancelling current on the outside of the coax can make the feedline and connected structures part of the radiating antenna system unless that path is intentional, clearly defined and properly controlled—for example by providing the required return path and placing a suitable common-mode choke at the correct boundary.
Kristen McIntyre, K6WX, called her presentation Ground Is a Myth! because “ground” changes meaning with the problem. That is the right place to start: protective earth, lightning bonding, a DC reference and an RF return path are related, but they are not interchangeable.
Watch Kristen McIntyre, K6WX, present “Ground Is a Myth!” at the 2024 ARRL National Convention. The useful narrative is not “remove ground.” It is “name the function and trace the current path.”
Safety boundary: never remove or interrupt protective-earth, equipotential-bonding, lightning-protection or code-required conductors to cure an RF symptom. RF control must be designed around the required safety system.
DC Continuity Does Not Define HF Impedance
A shield can read nearly zero ohms to a station bond while the same route presents appreciable impedance at HF. Conductors have distributed inductance and capacitance, and their impedance changes with frequency, geometry, length and nearby metal. A bond that is excellent for fault current can still sit at a substantial RF voltage relative to another point.
Transmission-line transformation matters too. A short circuit at one end of a lossless line can appear as a high impedance near an odd quarter wavelength at the other end. A real station has loss, multiple conductors and coupling, but the lesson remains: DC topology alone does not predict RF current.
The Wanted Coax Mode Does Not Need Earth
In the intended coaxial transmission-line mode, forward current flows on the centre conductor and equal, opposite return current flows on the inner surface of the shield. The electric and magnetic fields are largely confined to the dielectric between them. That mode is defined by the coax geometry and termination; it does not need the soil to carry the wanted signal.
The outer surface of the shield is another conductor surface. Current there belongs to an exterior mode whose return path can include the antenna, mast, radials, station wiring, protective bonding, operator and surrounding structures. When that current is significant, the feed line participates in the installed antenna system.
A Shack-Entrance Bond Is Not a Universal Choke
Bonding coaxial shields at the building entrance is part of a proper safety and surge strategy. It can also change the exterior-current network. It does not automatically present high common-mode impedance at the feedpoint or force exterior current to zero along the cable.
If the outside of the coax is already a return conductor, a new strap can become another branch of that RF network. The result can be better, worse or simply different depending on its impedance and coupling. This is why random “more ground” experiments often move an RFI symptom without solving it.
Can Coax Get Hot?
Yes, but the heat source must be identified. Differential current produces conductor and dielectric loss in every coax. A poor connector, corroded braid transition, undersized cable, high SWR or excessive average power can raise local temperature. Exterior common-mode current can add loss and concentrate current at connectors, chokes, bends or discontinuities.
Warm cable alone does not prove common mode. Compare temperature along the line, inspect terminations, calculate differential line loss for the actual load and measure exterior current. A thermal image becomes useful only when it is tied to the electrical mode and accepted power.
A Hot Ferrite Is a Power Measurement Waiting to Happen
A common-mode choke presents complex impedance to exterior current. Its resistive component dissipates power. Heating therefore depends on the common-mode current spectrum, complex impedance, ferrite material, core volume, winding voltage, duty cycle, cooling and ambient temperature.
A temperature rise can mean that the choke is absorbing unwanted common-mode power, but “hot means working” is not a specification. It may also mean the choke is undersized, placed at the wrong current maximum, driven outside its material range or being stressed by a missing intended return path. Measure current on both sides and compare against the component’s thermal and voltage limits.
What DC Bonding Is Genuinely For
- Protective bonding. Keep exposed conductive parts within the required touch-voltage boundary during faults.
- Lightning and surge strategy. Route surge current through a coordinated entrance, bonding and protection system designed to applicable standards.
- Static discharge. Provide a controlled path for charge that may accumulate on an otherwise floating antenna or feed system.
- Reference control. Establish a predictable low-frequency or DC relationship between equipment where the circuit requires it.
A static bleeder can provide a DC path while presenting a high impedance across the operating band. That is a component-level function, not a lightning arrestor. Its resistance, voltage rating, creepage, surge energy, weather protection and failure mode must suit the installation.
Diagnose the RF Path Instead of Guessing
- Draw every conductor. Include the coax exterior, mast, rotor and control cables, radials, station bonds, mains protective earth and nearby metal.
- Measure exterior current. Use a calibrated clamp-on RF current probe or a repeatable comparison probe at several cable positions.
- Change one boundary. Add a known choke, alter cable routing or define a counterpoise, then repeat A/B/A measurements.
- Separate receive and transmit symptoms. A path that imports local noise can differ from the path that carries high transmit current.
- Check heat under real duty. Record accepted power, waveform, mismatch, time and ambient temperature.
- Leave safety bonding intact. Coordinate any permanent change with applicable electrical and lightning-protection requirements.
Primary and Authoritative Technical Sources
- Kristen McIntyre, K6WX, “Ground Is a Myth!”—the ARRL National Convention presentation that prompted this article’s distinction between different meanings of ground.
- ARRL QST, John Portune W6NBC and John Lamano KJ6ER, feed-line common-mode-current article—exterior coax current and the limit of an entrance grounding strap as an RF cure.
- ARRL, “Lightning Protection for the Amateur Radio Station,” Part 3—coax routing, entrance protection and bonding as a coordinated system.
- IEC 62305-1:2024, Protection Against Lightning—General Principles—the safety boundary for structure, installation and person protection.
Practical Conclusion
“Grounded” is incomplete unless the sentence says why, where and at what frequency. A DC bond may be essential and still fail to control current on the outside of a coax shield at 7 MHz.
Keep the safety and lightning system intact. Then fix the RF problem with a defined return path, suitable common-mode impedance, sensible routing and measurements that show where the current actually flows.
Mini-FAQ
- Can DC-grounded coax still carry RF current on its outside? Yes. The DC bond and the exterior RF mode are different questions; the RF current follows the complete impedance network at frequency.
- Does bonding the shield at the entrance stop common mode? Not necessarily. The bond is important for protection, but it is not automatically a high-impedance common-mode boundary at the antenna.
- Can common-mode current heat coax? It can add loss and local heating, but differential loss, mismatch and connector faults must also be checked.
- Is a hot choke proof that it works? No. It proves a temperature rise. Current, complex impedance, duty cycle and component limits are needed to interpret it.
- Can a resistor bleed static without loading HF? A properly rated network can provide a DC path while remaining high impedance at HF, but it is not a substitute for lightning protection.
- Should I disconnect protective earth to reduce RF? No. Keep required safety bonding intact and solve the RF path with choking, routing, symmetry and a defined return conductor.