Why "Radials" and Ground Straps Don't Work on a Boat in Ham Radio

Why Land-Based Rules Don't Apply

In amateur radio, it's common knowledge that ground radials and bonding to ground rods help antenna efficiency—on land. But when it comes to maritime HF installations, especially on fiberglass or isolated-metal hull boats, those familiar rules break down. Trying to copy land-based practices can result in poor performance, dangerous RFI, and unnecessary frustration.

The Common Misunderstanding: Radials and Bonding

A widespread misconception is that laying out quarter-wave radials on deck or bonding the ATU (Antenna Tuning Unit) to railings, lifelines, or other onboard metal will improve your antenna system. In reality, this often causes more harm than good.

Deck-mounted “radials” usually aren't elevated or isolated well enough to act as effective tuned elements. They instead become capacitive couplers that unpredictably feed RF back into the rig, causing hot microphones, erratic tuner behavior, and in some cases, even RF burns.

Similarly, bonding random metal parts of the boat (like railings or stays) might seem like “creating a ground,” but these parts are often electrically floating or poorly connected, and introducing RF currents into them can lead to common-mode currents, degraded patterns, and significant onboard interference.

Why Boats Are Different

Boats, particularly non-metallic ones, lack a natural ground plane. Floating on water—even conductive saltwater—doesn't provide the same return path as buried radials in soil. And even on metal hulls, corrosion protection (galvanic isolation) or paint often impairs conductivity at RF.

Rather than thinking in terms of "grounding," marine HF systems should focus on providing the tuner with a sufficiently high-impedance return path to work against.

Forget Radials—Use a High-Impedance Counterpoise

Instead of trying to build a quarter-wave radial system (which only works for non-tuned ¼-wave verticals on a single band), a better approach for use with an ATU is to provide a high-impedance load or counterpoise.

Good options include:

  • Insulated wires of non-resonant length: Typically between 0.05 and 0.25 λ (wavelength), these provide enough impedance to allow the tuner to operate properly across a wide frequency range.
  • RVS mesh laid inside the hull above deck. 
  • A small stainless steel (RVS 316 (marine)) plate, around 50 cm², mounted on or near the hull (above deck) and connected to the ATU or tuner ground. While limited in surface area, it presents a high impedance and can be an effective counterpoise, especially on higher bands or when a long wire is impractical.

⚠️ Avoid bare copper for counterpoise use on boats. Although electrically excellent, copper corrodes rapidly in marine environments, especially when exposed to saltwater. Use tinned copper, marine-grade stainless steel (RVS 316), or marine aluminum foil/strap instead. These offer much better long-term durability and reduce corrosion-induced failure.

Location matters: The counterpoise is not just a connection to ground, it's an open wire (or floating object like an RVS plate) just like the radiator — it stabilises the RF current return path per Kirchhoff’s laws, and provides critical capacitive coupling to the surroundings. For this reason, it should be located in free space near the antenna, not hidden below deck or pressed against the outside of the hull. Keep it elevated (if that's possible), isolated, and in the open air as much as the antenna itself.

These create a capacitive coupling to the surrounding environment and provide a functional RF return path, helping current paths stabilize and improving tuner efficiency.

⚠️ Never Ground Only the Antenna

If you ground only the antenna or ATU to seawater but leave the rest of the system (radio, tuner, power supply) electrically floating, you create dangerous and inefficient conditions:

  • High RF voltages appear between components.
  • The coax shield becomes the unintended return path.
  • Common-mode currents flow on the shield, entering the cabin and affecting other electronics.
  • Symptoms include RFI, audio feedback, hot microphones, and erratic tuner behavior.

This configuration is common on boats but technically incorrect and potentially hazardous.

Two Safe Options for Marine Installations

Fully Floating System

  • Antenna, tuner, and radio system are all electrically isolated from seawater.

    1. Use insulated counterpoise wires or foil.
    2. Install a proper current choke at the feedpoint.

Fully Grounded System

  • Entire RF system (radio, tuner, ATU, power supply) is bonded to a common seawater reference (e.g., bronze ground plate).

    1. All coax shields and equipment grounds connect to this common ground.
    2. Also requires a current choke to block unwanted common-mode return.

⚠️ Mixing floating and grounded approaches is a recipe for RF trouble. Do not ground just one part of the system.

⚠️ A Note on American-Style Seawater Grounded Boats

On American-built sailboats, a sintered bronze plate or through-hull ground plate may already be installed and bonded to onboard systems for lightning protection. In these cases, the boat is not electrically floating. If you choose to use this plate as an RF ground, it is essential to ensure that:

  • The tuner, power supply, and transceiver chassis are all referenced to this plate as a single RF potential. (reference)
  • A dedicated low-impedance connection (e.g., tinned copper strap or braid) must run from the ATU to this seawater plate to ensure equipotential bonding.
  • The counterpoise remains electrically isolated from the seawater plate and is not bonded to it. It must remain a floating (is part of the antenna), high-impedance element for RF.
  • This means two separate conductors leave the ATU: one low-impedance DC ground to the seawater plate, and one high-impedance RF counterpoise wire or RVS element in free space.

This creates a functional seawater-coupled return path but only works if everything is referenced together for DC ground, while still preserving the counterpoise as an AC-floating RF element.

Important distinction: Even when everything is bonded to the same low-impedance ground potential (e.g., via tinned copper strapping to a seawater plate), you still need a high-impedance counterpoise to serve as the reactive return path for the tuner. Examples include:

  • An insulated non-resonant wire (0.05–0.25 λ)
  • A stainless steel mesh above deck
  • A 50 cm² marine-grade RVS plate mounted near the tuner

⚠️ This counterpoise must also be located in free space near the antenna, not against the hull (sea-side) or inside the cabin. Its function relies on capacitive coupling and current stabilization, and this only works properly when the element is exposed and well-isolated.

Without this counterpoise, the tuner may not operate properly or will drive excessive current into the coax shield.

⚠️ Mixing Floating and Grounded Systems: Why It's a Problem

Some sailors attempt to "improve" grounding by connecting only the tuner or ATU to the seawater (via a sintered bronze plate), while leaving the rest of the RF system (radio, power supply, solar controllers, etc.) electrically floating. This is a critical mistake.

Even if a sintered bronze RF ground plate is present, you cannot use it effectively unless the entire RF chain—ATU, transceiver, power supply, and any connected electronics—is bonded to the same ground reference.

If only the ATU is grounded, and the rest floats:

  • RF will seek alternative return paths—usually through the coax shield.
  • This leads to severe common-mode currents, RF burns, and erratic system behavior.
  • You may also inject RF into your nav equipment, solar chargers, or DC wiring.

To safely use a bronze RF ground plate:

  • Bond all grounds (radio chassis, ATU, PSU negative, any DC ground busses) to it with low-resistance connections.
  • Use wide copper braid, copper strap, or heavy-gauge insulated wire.
  • Keep the path between ATU and plate as short and low-inductance as possible.

⚠️ Never mix floating and grounded gear. It's all in—or all floating. There is no safe in-between.

For grounded systems: Avoid copper tape or foil for permanent installations—use tinned copper wiring instead. It offers greater mechanical strength, corrosion resistance, and longevity in marine environments.

Don’t Skip the Choke

No matter how you route your system, a current choke is essential between the tuner and the feedline. It suppresses common-mode currents on the coax shield, which would otherwise re-enter the cabin and cause audio feedback, interference, and distorted radiation patterns.

Chokes are not optional—they’re required on any boat installation, regardless of hull material or antenna type.

Conclusion: Think Like the Sea, Not the Soil

Land-based grounding systems don’t translate to marine environments. On a boat, what matters is:

  • Providing a high-impedance counterpoise
  • Avoiding unnecessary bonding of isolated metal
  • Never grounding only one part of the RF system
  • Suppressing common-mode currents with a proper current choke
  • On boats with a seawater RF ground plate, ensuring all equipment shares that potential
  • Ensuring low-impedance bonding to the seawater reference while still maintaining a separate high-impedance RF counterpoise in free space

Forget the radial field—your boat isn't buried in soil. Instead, design your HF system around how RF behaves at sea. Your tuner will thank you, and so will your crew.

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Written by Joeri Van Dooren, ON6URE – RF, electronics and software engineer, complex platform and antenna designer. Founder of RF.Guru. An expert in active and passive antennas, high-power RF transformers, and custom RF solutions, he has also engineered telecom and broadcast hardware, including set-top boxes, transcoders, and E1/T1 switchboards. His expertise spans high-power RF, embedded systems, digital signal processing, and complex software platforms, driving innovation in both amateur and professional communications industries.