A Dual-Counterpoise Quarter-Wave Vertical Above Seawater
A quarter-wave vertical is only half of an antenna. The vertical radiator is the visible half. The missing half is the counterpoise: radials, a ground screen, a vehicle body, a metal roof, or a conductor that couples the feedpoint to seawater.
Without a good counterpoise, the antenna may still tune, but the return current will find another path. That path can be the outside of the coax shield, nearby metal, wet wood, dock structure, or lossy soil. The result can be unstable SWR, increased common-mode current, more receive noise, and lost RF power.
For a coastal or marine installation, a very practical arrangement is:
- One quarter-wave vertical radiator
- Four raised quarter-wave radials
- One additional radial floating on the saltwater surface
This gives the antenna system a useful dual-counterpoise behavior. The raised radials provide a predictable resonant RF return path. The floating radial adds another return conductor while coupling the feedpoint region to the highly conductive seawater surface.
Why Seawater Matters
Seawater is far better for vertical antennas than ordinary soil. Average-salinity seawater is often treated in propagation work as having very high conductivity compared with typical land soil. That matters because vertical antennas depend strongly on the quality of the ground or counterpoise environment around them.
High conductivity near and beyond the antenna reduces ground-related loss and supports strong low-angle radiation. This is especially valuable for HF DX, where a low takeoff angle often matters more than the exact feedpoint SWR.
That is why vertical antennas near beaches, docks, salt marshes, reefs, and boats can perform surprisingly well. The seawater does not turn the antenna into magic, but it can provide a far better RF environment than dry, rocky, sandy, or poorly conducting soil.
What the Four Raised Radials Do
The four raised radials are the controlled part of the counterpoise system. They are normally cut near a quarter wavelength for the operating band and connected to the coax shield side of the feedpoint.
Their main job is to give the vertical current a predictable return path. A quarter-wave vertical needs this. If the return path is missing or poorly defined, the system may still show a usable impedance, but that impedance may partly be caused by loss and unwanted current paths.
Raised radials also help stabilize the feed impedance and SWR. This makes the antenna less dependent on random objects such as wet sand, dock structure, tide level, nearby metalwork, or the outer surface of the coax shield.
Four radials spaced roughly 90 degrees apart also help keep the pattern more balanced. One or two elevated radials can work, especially over seawater, but four radials usually make the system less fussy and less sensitive to local asymmetry.
Finally, a proper raised radial system helps reduce unwanted common-mode current on the coax. A feedpoint choke is still recommended, but the choke should not be forced to compensate for a missing counterpoise.
What the Floating Saltwater Radial Does
The floating radial is the seawater-coupling part of the system. It can be an insulated wire, braid, or conductive strap supported by small floats so it stays on or very near the saltwater surface. It is connected to the same feedpoint ground point as the raised radials.
It has two functions at the same time.
First, it is an RF radial. It carries return current like the raised radials. If it is near a quarter wavelength long, it can act as an additional radial conductor for the operating band.
Second, it couples the feedpoint region to the seawater surface. If the conductor is bare, there may be direct conductive coupling. If it is insulated, the coupling is mainly capacitive, but at HF this can still be very effective because the conductor is in intimate contact with a large conductive surface.
This is the dual-use idea: the extra radial is both a normal radial conductor and a seawater-coupling element.
Why Float the Radial Instead of Sinking It?
A conductor deep in the water is not automatically better. At HF, RF current in seawater is concentrated close to the surface. Keeping the radial near the surface is usually more useful than trying to sink it deeply.
Floating the radial also solves several practical problems:
- It stays near the RF-active seawater surface.
- It rises and falls with tide and waves.
- It avoids large tuning shifts caused by changing water depth over a fixed immersed conductor.
- It is easier to inspect, remove, and protect from corrosion.
- It can be insulated to reduce galvanic corrosion while still providing RF coupling.
For temporary operation, an insulated wire floating on foam, small buoys, or floating rope can work well. For long-term service, corrosion, marine growth, mechanical wear, storm exposure, and navigation safety become serious design issues.
Why Not Use Only the Floating Radial?
A single floating saltwater radial can work, and in some portable situations it may work surprisingly well. But by itself, it is not always a clean or predictable counterpoise.
One radial makes the system asymmetric. The coax shield, mast, rig, or dock structure may become part of the return path. That can distort the pattern and increase common-mode current.
The four raised radials prevent the floating radial from becoming the only return path. They make the antenna predictable first. The seawater radial then improves coupling and adds another useful RF return conductor.
Think of it this way:
- Four raised radials: stable antenna counterpoise
- One floating saltwater radial: seawater coupling and extra RF return path
The combination is often more practical than relying on seawater alone, and more controlled than letting the antenna decide for itself where the return current should flow.
Suggested Layout
At the feedpoint, connect the raised radials, the floating radial, and the coax shield together. Install a good common-mode choke on the coax close to the feedpoint. The choke should be on the feedline side of the system, not inserted between the vertical and its counterpoise.
A simple layout looks like this:
raised radial
|
|
raised radial ---- feedpoint ---- raised radial
|
|
raised radial
\
\ floating saltwater radial
\ on saltwater surface
For a single-band antenna, use these starting points:
- Vertical radiator: start near one quarter wavelength.
- Raised radials: use four wires, each near one quarter wavelength, spread as evenly as practical.
- Floating radial: use one wire near one quarter wavelength, floating on the saltwater surface and connected to feedpoint ground.
Approximate starting length:
- Length in feet ≈ 234 / frequency in MHz
- Length in meters ≈ 71.5 / frequency in MHz
These are starting values, not final dimensions. Real antennas are affected by wire diameter, height, radial angle, nearby objects, feedpoint hardware, and the saltwater coupling itself.
Tuning the System
Tune the antenna with the four raised radials first. This gives you a known, controlled antenna system before the seawater coupling radial is added.
Then add the floating saltwater radial and measure again. The impedance may shift because the loss and return-current distribution have changed. That shift is not a failure. It often means the antenna system has become less dependent on lossy or accidental return paths.
Final tuning can be done by adjusting the vertical length, the radial droop angle, or both. If the feed impedance drops below what the transmitter wants to see directly, a small matching network may be a better solution than deliberately adding loss.
How to Judge Performance
Do not judge this antenna only by SWR. SWR tells you how well the transmitter sees the feedpoint impedance. It does not tell you how much power is being radiated.
Better indicators are:
- Field strength at a known distance
- Real on-air reports compared with a reference antenna
- Receive noise behavior
- Stability as tide and weather change
- Measured common-mode current on the coax shield
A small RF current meter on the coax can be very useful. If the outside of the coax is carrying significant RF current, the counterpoise system is not doing all the work it should be doing.
Best Use Cases
This dual-counterpoise vertical is especially useful for:
- Coastal portable HF operation
- DXpeditions on beaches, reefs, islands, and salt marshes
- Temporary dock or pier installations
- Emergency coastal communication setups
- Marine-adjacent amateur radio stations
It is less attractive where the floating radial can become a hazard to swimmers, boats, propellers, fishing gear, or wildlife. In public or navigable areas, mechanical safety and visibility matter as much as RF performance.
Practical Cautions
Keep the raised radials above expected high tide. If the elevated radials are too close to the water, tide movement can change the tuning and loss behavior.
Use insulated wire for the floating radial if corrosion is a concern. Bare copper in seawater will not stay healthy for long. Saltwater is electrically useful, but mechanically and chemically aggressive.
Make the floating radial visible and secured. It should not drift into a propeller, mooring, swimmer, public walkway, or wildlife area.
Avoid fastening radials directly against treated wood or unknown dock material. Wet or treated timber can be conductive and lossy enough to affect tuning and, in high-power cases, may even heat.
Use a feedpoint choke, but do not treat the choke as a substitute for a counterpoise. The choke prevents the coax from becoming part of the antenna. The radials provide the antenna with a deliberate return path.
Conclusion
A quarter-wave vertical above seawater can be an excellent HF antenna, but it still needs a good counterpoise. Seawater improves the RF environment, but it does not remove the need for a controlled return-current path at the feedpoint.
Four raised radials give the antenna a stable, efficient, resonant counterpoise. One floating saltwater radial adds a second benefit: it acts as an additional radial while coupling the antenna system to the highly conductive seawater surface.
That is the strength of the dual-counterpoise approach. The raised radials provide control and stability. The floating radial provides seawater coupling and enhancement. Together, they form a more predictable system than relying on seawater alone, and a more practical system than trying to sink or float every radial in salt water.
Mini-FAQ
- Can a quarter-wave vertical work with only one floating saltwater radial? Yes, especially in temporary coastal setups, but it is less symmetrical and more likely to use the coax or nearby metal as part of the antenna system.
- Why use four raised radials if seawater is already conductive? The raised radials define the feedpoint return path. Seawater improves the RF environment, but it should not be the only thing controlling where the current flows.
- Should the floating radial be bare or insulated? For temporary experiments, bare wire can work, but insulated wire is usually more practical because it reduces corrosion while still coupling well at HF.
- Does better seawater coupling always improve SWR? Not necessarily. Reducing loss can change the feed impedance and sometimes make SWR look worse, even though the antenna is radiating better.
- Do I still need a common-mode choke? Yes. A good choke near the feedpoint helps keep the outside of the coax from becoming an unintended radial or radiator.
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