A Dual-Counterpoise Quarter-Wave Vertical Above Seawater
A Dual-Counterpoise Quarter-Wave Vertical Above Seawater
A quarter-wave vertical, four raised radials and one additional conductor floating at the seawater surface: a practical coastal proposal with two kinds of return-path structure, but only one coupled antenna system.
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.
At the coast, I would rather give a vertical a deliberate return structure than ask the coax, dock or rigging to find one for me. The arrangement I am proposing is concrete: four raised radials establish the starting counterpoise, and one additional floating conductor couples the feedpoint return side to the seawater. The reason to combine them is control—not a promise that five wires automatically beat every other ground system.
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.
The Four-Plus-One Proposal
For a controlled single-band coastal installation, the proposed layout is:
- One vertical radiator, initially near a quarter wavelength and adjusted as part of the installed system.
- Four raised radials, initially near a quarter wavelength, spread approximately evenly around the feedpoint and kept clear of people, conductive supports and the expected high-water level.
- One additional water-side conductor, supported at or near the saltwater surface and connected to the same RF return terminal as the raised radials. Its electrical length and coupling must be established in that position; it is not automatically another resonant quarter-wave radial.
The vertical connects to one feed terminal; the radial branches connect to the other, normally the coax-shield terminal in a directly fed monopole. Any feedline choke belongs on the station side of those intended branches, not between the radiator and its counterpoise. This describes the RF arrangement, not permission to connect it to an existing vessel's bonding system or deploy a wire in navigable water; those safety boundaries are addressed below.
I would establish the raised-radial antenna first, then add the floating branch with the transmitter off and compare at low power. That keeps a deliberate return structure in place while the water connection is investigated. The engineering benefit is that the additional seawater path need not be the only planned return path. Whether it also improves efficiency or field strength is a separate result—not something the layout alone proves.
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.
The four raised radials and floating conductor are what I mean here by a dual-counterpoise arrangement: two kinds of return-path structure, not two independent RF grounds. They form one coupled network whose branch currents depend on frequency, conductor length and height, water contact, hull material, feedline route and nearby metalwork. Adding the fifth branch can change current in all the others; their contributions cannot be assigned separate gains and added together.
Define the boundary before tuning. Mark the radiator, every radial, water-coupled conductor, mast, hull, coax shield, choke, matching network and electrical bond. Any current leaving that drawing through an unmodelled cable or structure can alter the impedance and pattern.
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.
Seawater is not an automatic feed terminal. The far-field ground boundary and the local feed-current return are related but different problems. A nearby sea surface can improve propagation while an undefined feedpoint still drives current onto coax, rigging or vessel wiring.
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 the practical starting geometry in this proposal, not a universal minimum or optimum. A larger ground screen may be better when radials must lie close to lossy surfaces. In his discussion of vertical ground systems over saltwater, Rudy Severns, N6LF, describes successful use of as few as two opposing elevated radials directly over seawater and explains how tide and radial height affect tuning. That supports treating radial geometry and the actual water boundary together, not insisting on one count for every site.
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:
λ/4 = c / (4f) ≈ 74.95 / fMHz metres
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:
| Construction | Dominant coupling | Important variables |
|---|---|---|
| Bare metal in seawater | Direct conductive contact plus distributed capacitance | Exposed area, contact condition, salinity, oxide and marine growth, metal compatibility, unintended DC paths and mechanical wear |
| Insulated wire at the surface | Distributed capacitive coupling through the insulation | Dielectric thickness and loss, wetted area, frequency, immersion, insulation integrity and capacitance to water |
| Wire suspended above water | Capacitive coupling through air and any support material | Height, wave and tide movement, wire length, support permittivity and proximity to other conductors |
An insulated conductor does not become a low-impedance seawater connection merely because it touches the surface. Estimate or measure its capacitance and include XC = 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. Its practical attraction here is keeping the conductor near the moving surface while making inspection and recovery easier. A floating wire does not fix the vertical-to-water spacing when the vertical itself is mounted on a fixed support, so tides and waves can still change the system. 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, including its applicable amendments, covers specified 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.
Putting the Proposal into Practice
- Classify the site. Distinguish beach portable, dock, metal or composite vessel, shore-powered vessel and navigable water.
- Draw every current path. Include RF, AC protective earth, DC negative, cathodic protection, lightning conductors, hull and marina connections.
- Select a baseline counterpoise. Choose radial number, elevation and symmetry from a model and mechanical constraints—not a fixed recipe.
- Model the installed boundary. Include seawater, finite shoreline, conductor height, hull, mast and the intended water-coupling branch.
- Measure at low power. Save complex impedance and radial/exterior-coax current with tide, salinity, temperature, cable route and geometry documented.
- Add the water conductor as an A/B variable. Compare bare, insulated or absent states only when electrical and corrosion safety permit.
- Control common mode. Qualify any choke across frequency, power and temperature, then verify the installed current reduction.
- Verify field behaviour. Repeat signal and noise measurements in several bearings against a stable reference antenna.
- Increase power in steps. Check joints, matching components, choke temperature, arcing, exposure boundaries and onboard interference.
- Reinspect after immersion and weather. Look for insulation damage, water ingress, corrosion, loosened hardware and changed tuning.
Why I Would Choose This Arrangement
For a temporary coastal station with space for raised wires and a safely controlled water-side area, the four-plus-one arrangement is worth considering because it separates a practical starting point from an optional extension. Establish the elevated-radial vertical, then decide whether the floating branch earns its place. You do not have to immerse every radial or rely solely on an uncertain water contact.
If the extra conductor produces no useful benefit, makes the antenna less stable, or creates an access or corrosion problem, leave it out. The raised-radial antenna remains a legitimate choice. If it provides a useful return path with better repeatable field behaviour, keep it and tune the complete combination. The point is to use seawater deliberately without surrendering control of the antenna's current paths.
Two kinds of counterpoise, one antenna: the raised radials supply the planned starting return structure; the floating conductor adds a water-coupled branch. Their interaction determines the result. Keep the local RF return, the wider seawater propagation environment and the vessel's safety systems distinct.
Primary engineering and safety references
- ITU-R P.527-6 — Electrical characteristics of the surface of the Earth
- ITU-R M.2335-0 — Sea-path reference parameters and ground-wave modelling
- ARRL / Rudy Severns, N6LF — Some Thoughts on Vertical Ground Systems Over Saltwater
- ISO 13297:2020 and amendments — Small craft AC and DC electrical systems
- IEC 60092-507:2014 — Electrical installations in small vessels
- ISO/TR 10134:2020 — Lightning-protection practices for small craft
- ABYC — Current small-craft electrical, cathodic-protection and lightning standards
- ABYC — Cathodic bonding and the E-2 protection boundary
- ITU-T K.52 — Guidance on compliance with human-exposure limits
- ICNIRP — Radiofrequency exposure guidelines
Mini-FAQ
- What does the four-plus-one proposal actually add? It combines a raised-radial starting counterpoise with an additional water-coupled branch, so the floating conductor need not be the only planned return path. The branches interact; no automatic gain or efficiency improvement follows from their number.
- Are two or four radials always enough above seawater? No. Radial count, length, height and symmetry must be chosen for the installed current distribution, loss, pattern and unwanted feedline paths.
- Does seawater guarantee more gain? No. Its high conductivity can reduce loss and improve vertically polarised propagation, but gain and elevation pattern depend on height, shoreline geometry, sea-path bearing and the complete antenna.
- Should a water-side conductor be one quarter wavelength? Not automatically. Direct contact, distributed capacitance, insulation, immersion and nearby metal change its electrical length and impedance.
- Is an insulated floating wire the same as a seawater ground? No. It is a capacitive electrode whose impedance depends on wetted area, dielectric properties, frequency and installation.
- Does good SWR prove the counterpoise is efficient? No. Loss can improve the apparent match. Check complex impedance, branch current, component heating and field behaviour separately.
- Is a feedpoint choke always sufficient? No. A choke only adds impedance to one exterior-current path. Its effect must be measured, and it cannot replace an intended counterpoise.
- Can the RF counterpoise be connected to vessel bonding? Only after the vessel’s protective-earth, DC-negative, cathodic-protection, galvanic-isolation and lightning systems have been evaluated under the applicable marine standards.
- Does the water radial provide lightning protection? No. Marine lightning protection is a coordinated system of air terminals, down conductors, bonding, grounding terminals and surge protection.