DC-Grounded Coax at HF: Why “Ground” Doesn’t Tame RF
and Why Coax Can Absolutely Get Hot
A persistent ham-radio myth goes like this: “If the coax is DC grounded, then it’s grounded for HF too... so it can’t radiate, and it can’t get hot.”
That’s not how RF works.
ARRL’s Kristen McIntyre, K6WX has addressed this confusion head-on in her talk often referred to as “Ground is a Myth!”... separating DC bonding from RF behavior, and why “ground” is frequently misunderstood on HF.
DC ground and HF behavior are different things
At DC, “ground” is usually a simple idea: a reference node and a safety return path.
At HF, “ground” is not a magic sink where RF disappears. RF currents still must complete a loop, and the loop they choose depends on impedance (including inductance and capacitance)... not on whether something measures 0 Ω on a multimeter.
So when someone says “my coax is grounded,” the right follow-up is:
Grounded how... at DC only, or as a genuinely low-impedance path at the operating frequency?
If your fix is “add more ground,” but your symptoms are hot coax, RF in the shack, unpredictable tuning, or RFI... you’re almost always dealing with common-mode current, not a lack of DC bonding.
Why DC-grounding a coax shield doesn’t “ground the HF”
Coax does not need “earth” to carry the desired HF signal
In normal coax operation (differential mode), equal-and-opposite RF currents flow on the outside surface of the center conductor and the inside surface of the shield. The fields largely cancel outside the cable... which is why coax can be quiet and non-radiating when it’s behaving properly.
Whether the shield is DC bonded to a ground rod, station chassis, or an entrance panel does not change coax geometry or characteristic impedance. The intended HF energy is already “contained” by the coax structure.
A DC short is not necessarily an RF short
Two conductors can be DC shorted and still be effectively RF open (or vice versa), because transmission lines transform impedances with length and frequency.
A classic example: around odd multiples of a quarter wavelength, a short at one end can be transformed into a very high impedance at the other end... and the reverse is also true. So “it’s grounded at DC” does not mean “it’s grounded at 7 MHz.”
Ground wires become inductors at HF
Even a “short” ground strap has inductance, and inductive reactance rises with frequency:
XL = 2π f L
At HF, small inductances can become tens to hundreds of ohms... meaning a perfectly good DC bond can be a fairly poor RF return path.
Coax supports three current paths... and the third is the troublemaker
A coax line can carry multiple current modes at the same time:
- Differential-mode current inside the coax (the wanted signal)
- Differential-mode return current on the inside of the shield (still part of the wanted signal system)
- Common-mode current on the outside of the shield (unwanted)
That outside-of-shield current appears when the antenna/feed system is unbalanced, asymmetrically coupled to its surroundings, or when the feedline becomes part of the return path because the antenna system doesn’t provide a clean one.
And here’s the kicker: “bonding the coax at the shack entrance” is not a universal cure. If the outside of the coax is already acting like part of the antenna, your new “ground strap” can simply become another part of the unwanted antenna.
Yes, coax can absolutely get hot
The claim “the outside of coax can’t get hot” confuses ideal differential-mode behavior with real-world common-mode behavior.
Shield current creates loss... and loss becomes heat
If the coax shield carries significant common-mode current, it’s acting like a conductor in free space carrying RF. Real conductors have resistance (especially at RF, where current is confined to the surface), connectors have contact resistance, braid transitions have loss, corrosion increases resistance, and tight bends can concentrate current.
That dissipation shows up as:
- Warm coax (sometimes “why is this cable warm?” becomes “why is this cable melting?”)
- RF voltage in the shack (RF bites on chassis, mic, key, USB, audio gear)
- Indoor radiation (RFI into everything that looks like wiring)
Chokes and ferrites can heat... sometimes catastrophically
A common-mode choke works by presenting impedance to common-mode current... but ferrite loss behaves like resistance. If common-mode current is high, the choke can dissipate real power and heat up fast.
A hot choke is not “proof your choke is working”... it’s proof your system is driving significant common-mode into it.
What DC grounding is genuinely good for
Even though DC grounding doesn’t “control HF” by itself, it is still valuable for:
- Safety bonding (keeping exposed metal at the same potential)
- Static bleed on antennas and feed systems that otherwise float at DC
- Surge strategy when combined with a proper entrance panel and arrestors
Static is not theoretical. In wind, dry air, snow, or nearby storm conditions, antennas can accumulate charge that stresses tuners, transformers, and radios. A controlled DC discharge path improves reliability and repeatability.
DC grounding without loading HF: use a bleeder approach
The goal is simple:
- Provide a controlled DC path to discharge static
- Keep the path high impedance at HF, so it does not become a meaningful RF load
- Optionally add surge clamping for fast, high-voltage events
Done correctly, you can “DC-ground” a feed system for static and protection... while leaving HF behavior essentially untouched.
Product example (RF.Guru): Open-Wire & Coax Bleeder for DC grounding and static discharge... designed to provide a controlled DC path to ground without becoming a meaningful RF load across HF.
A bleeder is not a substitute for a proper lightning entrance strategy... it’s the static/DC piece of the puzzle.
The real takeaway: fix common-mode... don’t worship “ground”
If you’re trying to stop feedline radiation, hot coax, RF in the shack, or unpredictable tuning, you usually get better results by addressing balance and common-mode control... not by adding more DC “grounds.”
Practical moves that actually target the RF problem:
- Use a 1:1 current balun / common-mode choke where it belongs (often at the feedpoint... sometimes additional stages are needed depending on routing and environment).
- Route coax away from the antenna’s near field and avoid running it parallel to radiators for long distances.
- Keep the feed system physically symmetrical where symmetry matters (especially near the feedpoint).
- Measure common-mode current if you want to stop guessing (a clamp-on RF current approach turns “internet arguments” into a number).
- Use DC grounding/bleeders for static and a proper entrance/grounding system for safety and surge control... but don’t expect DC bonding alone to fix HF.
Mini-FAQ
- Can a DC-grounded coax radiate at HF? — Yes... if common-mode current flows on the outside of the shield, the coax becomes part of the antenna system.
- Why doesn’t a ground rod “sink” the RF? — RF still needs a loop, and the loop it takes is set by impedance at frequency... not by DC continuity.
- Why does a short ground strap often fail at HF? — Because it has inductance... and its reactance rises with frequency, turning a DC “short” into a sizeable RF impedance.
- Is a hot choke a good sign? — It’s a sign you have significant common-mode current. The choke is dissipating real power... so the root cause still needs addressing.
- What’s the clean way to DC-ground for static without detuning HF? — Use a purpose-built bleeder approach: a controlled DC path that stays high impedance at HF.
- What usually fixes “hot coax” fastest? — Proper common-mode control (feedpoint choking and sane coax routing) rather than adding more random ground connections.
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