Quarter-Wave Verticals Above Seawater: Counterpoise and Safety

A quarter-wave monopole is not defined by the whip alone. Current leaves one feed terminal through the radiator and must return to the other through elevated radials, a ground screen, a metal hull, a capacitive seawater electrode, the feedline exterior or some combination of these paths. The engineering task is to make the intended paths dominant, efficient, repeatable and safe.

Start with the Complete Current Network

At the feedpoint, the sum of the currents in the radial, hull, bonding and exterior-feedline branches must complete the source current. Calling one group “the counterpoise” is convenient, but the RF system includes every conductor and displacement-current path coupled strongly enough to carry current.

An intended arrangement might use several elevated radials plus an optional conductor at the water surface. That does not create two independent RF grounds. It creates one coupled network whose branch currents depend on frequency, conductor length and height, water contact, hull material, feedline route and nearby metalwork.

What Seawater Changes

Seawater is much more conductive than typical dry ground. ITU-R P.527-6 treats saline-water complex permittivity as a function of frequency, temperature and salinity. Some ITU propagation studies use 5 S/m conductivity and relative permittivity 80 as reference values for average-salinity sea paths. Those are model inputs, not universal measurements for every harbour, estuary or season.

A highly conductive surface can reduce ground loss and improve reflection of vertically polarised fields. A sufficiently long sea path in the wanted direction can support lower-loss surface-wave propagation and may improve low-elevation performance compared with a poor land path.

It does not guarantee a particular gain or takeoff angle. The answer also depends on antenna height, finite shoreline geometry, sea-path length and bearing, waves, salinity, temperature, nearby vessels, harbour structures and the land beyond the water. A vertical on a beach can therefore have a direction-dependent pattern even when the radiator is geometrically vertical.

How Many Elevated Radials?

There is no universal minimum. A small number of tuned elevated radials can produce low loss when their currents are controlled, the geometry is sufficiently symmetrical and unwanted paths are suppressed. More radials can reduce sensitivity to individual wire detuning and local asymmetry, but count alone does not prove efficiency.

Two radials may be useful in a carefully modelled directional or mechanically constrained installation. Four equally spaced radials are a practical test geometry. A larger ground screen may be better when radials must lie close to lossy surfaces. Rudy Severns, N6LF, documented both seawater installations and extensive radial experiments; his results show that performance depends on radial number, length, elevation and the complete environment rather than one rule for every vertical.

Elevated radials carry substantial RF current and radiate. Their open ends can develop high RF voltage. Unequal height, droop, loading or proximity to a hull changes branch current and can skew the azimuth pattern. Measure or model current in each radial instead of assuming equal division.

Length Is an Electrical Result

The free-space quarter-wave scale is:

This is a geometric starting point, not a cut length. End effect, conductor diameter, insulation, radial droop, feedpoint hardware, height, water loading and mutual coupling change the installed resonance. The familiar shorter wire formulas include empirical assumptions that need not apply to marine geometry.

Tune the complete radiator-and-radial network, not each wire in isolation. A match at one port does not prove low counterpoise loss, equal radial current or the intended pattern.

A Water-Surface Conductor Is an Optional Branch

A conductor supported on or near the water can add a deliberate RF-current path, but its behaviour depends strongly on construction:

An insulated conductor does not become a low-impedance seawater connection merely because it touches the surface. Estimate or measure its capacitance and include X_C = 1/(2πfC) in the model. A bare conductor makes the vessel’s metal system and the seawater electrolyte part of a possible galvanic or stray-current network. Neither construction should be assumed to resonate at a free-space quarter wavelength.

Floating is not electrically superior to shallow immersion in every installation. It can make tide tracking, recovery and inspection easier, while immersion may change coupling and mechanical loading. The choice should follow measurement plus navigation, entanglement, wildlife, corrosion and storm-risk assessment.

Feedline Common Mode Must Be Measured

In the intended coaxial mode, centre-conductor current is balanced by opposite current on the shield’s inner surface. Current on the shield exterior is a separate branch involving the antenna, mast, hull, station wiring and surroundings. It can alter feed impedance, pattern, onboard interference and touch voltage.

A common-mode choke adds frequency-dependent complex impedance to that exterior path. It does not create a missing counterpoise, force radial currents to be equal or guarantee zero cable radiation. Ferrite material, winding geometry, cable, parasitic capacitance, power, duty cycle and temperature determine its installed behaviour.

Measure exterior current with a characterised clamp-on RF current probe at repeatable points. Record frequency, transmitter power and probe transfer impedance. Repeat after changing the radial system, water conductor, cable route or choke while keeping all other variables fixed.

Impedance, Efficiency and Pattern Are Different Results

SWR describes the port match at a declared reference plane. Loss in water coupling, earth, hull joints, matching components or unintended current paths can move the resistance toward 50 Ω while reducing radiated power. Conversely, reducing loss can reveal a less convenient impedance even when efficiency improves.

Use separate evidence for separate questions:

• Complex impedance: a calibrated VNA measurement at, or accurately de-embedded to, the feedpoint.
• Branch current: repeatable current-probe measurements on radials, bonds and the feedline exterior.
• Loss and heating: network measurements, component temperature and conductor-joint inspection at realistic duty cycle.
• Pattern: a converged full-system model and repeated field measurements in several bearings.
• Operational value: rapid A/B observations against a stable reference antenna with receiver settings and propagation time controlled.

A receive-noise change alone is ambiguous. It can come from pattern, common-mode pickup, local coupling, receiver state or wanted-signal loss. Record wanted signal and noise separately.

Corrosion: Name the Mechanism

“Electrolysis” is too vague for fault finding. Three different mechanisms matter:

• Ordinary corrosion attacks a material through its environment.
• Galvanic corrosion occurs when dissimilar metals are electrically connected in an electrolyte.
• Stray-current corrosion is driven by unintended current—especially DC—entering or leaving submerged metal.

An RF radial, coax shield or tuner bond can accidentally bridge parts of a vessel’s cathodic-protection, DC-negative, shore-power or galvanic-isolation arrangement. Do not connect an antenna counterpoise indiscriminately to through-hulls, engine blocks, protective earth, sacrificial anodes or marina structures.

Use marine-rated conductors, sealed strain-relieved terminations and compatible metals. Inspect any immersed or splash-zone part. On a permanent vessel installation, have a qualified marine electrician or corrosion specialist assess the proposed RF bonds against the vessel’s actual cathodic-protection and electrical design.

ABYC separates cathodic protection, galvanic isolators, AC/DC electrical systems and lightning protection into different standards. ISO 13297 covers AC and DC installations on small craft, while IEC 60092-507 covers specified small-vessel electrical installations. The applicable standard depends on vessel, jurisdiction and electrical system.

Protective Earth and Lightning Are Separate Systems

An RF counterpoise is not a substitute for AC protective earth, DC fault-current conductors, equipotential bonding, cathodic protection or a lightning downconductor. Do not interrupt protective earth with an RF choke. Do not bypass a certified galvanic isolator or isolation transformer with a coax shield or ad hoc RF strap.

A quarter-wave radial or water-coupled wire is also not a complete lightning-protection system. ISO/TR 10134 describes established lightning-protection practices for small craft. A compliant design coordinates air terminals, low-impedance down conductors, bonding, immersed grounding terminals, surge protection and separation from wiring. Use a marine lightning specialist where protection is required.

RF and Physical Safety at the Water

Elevated radial ends, matching networks and poorly controlled metalwork can present RF burn and contact-current hazards. Water-surface conductors add entanglement and access risks for swimmers, crew, wildlife, propellers and mooring gear. Do not transmit while people can enter the controlled area or contact antenna conductors.

Perform a site-specific exposure assessment using the limits applicable in the operating jurisdiction. ITU-T K.52 provides a current assessment procedure based on accessibility, antenna properties and emitter power; ICNIRP publishes exposure limits from 100 kHz to 300 GHz. Near-field electric and magnetic fields and contact current must be considered separately where applicable.

Secure, mark and recover temporary wires. Obtain permission from the vessel owner, harbour or site authority. De-energise and discharge matching components before service. Avoid transmitting during thunderstorms, but do not treat operational disconnection as a lightning-protection design.

A Controlled Installation Workflow

1. Classify the site. Distinguish beach portable, dock, metal or composite vessel, shore-powered vessel and navigable water.
2. Draw every current path. Include RF, AC protective earth, DC negative, cathodic protection, lightning conductors, hull and marina connections.
3. Select a baseline counterpoise. Choose radial number, elevation and symmetry from a model and mechanical constraints—not a fixed recipe.
4. Model the installed boundary. Include seawater, finite shoreline, conductor height, hull, mast and the intended water-coupling branch.
5. Measure at low power. Save complex impedance and radial/exterior-coax current with tide, salinity, temperature, cable route and geometry documented.
6. Add the water conductor as an A/B variable. Compare bare, insulated or absent states only when electrical and corrosion safety permit.
7. Control common mode. Qualify any choke across frequency, power and temperature, then verify the installed current reduction.
8. Verify field behaviour. Repeat signal and noise measurements in several bearings against a stable reference antenna.
9. Increase power in steps. Check joints, matching components, choke temperature, arcing, exposure boundaries and onboard interference.
10. Reinspect after immersion and weather. Look for insulation damage, water ingress, corrosion, loosened hardware and changed tuning.

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