A 20-foot vertical is far shorter than a quarter wavelength on 80 meters. Helical loading can make a short radiator usable, but it also adds inductive loss and resistance and narrows the operating bandwidth. This project is a first-pass engineering study for a 20-foot, base-fed vertical with a 3.800 MHz target. Its dimensions and performance still need to be modeled and tested.
Why load a 20-foot vertical?
At 3.800 MHz, a free-space wavelength is about 259 feet, so a quarter wavelength is roughly 64.7 feet. A 20-foot radiator is about 0.077 wavelength. A helical winding can increase the radiator’s electrical length without using a full-height mast, but the loading stores energy and raises current in the winding. That can increase loss and make the usable bandwidth narrower than it would be with a full-size radiator.
The goal here is a space-saving experiment, not a claim that a shortened vertical will equal a full-size quarter-wave antenna. The final result will depend on the winding, conductor, support, matching network, radial field, and nearby objects.
First-pass winding geometry
The working example uses 58 turns around a 2-inch-diameter former, with 3.7 inches of axial pitch from turn center to turn center. The geometric length of the wire in a helix can be estimated from:
L = N × √[(πD)² + P²]where N is the number of turns, D is the winding diameter, and P is the pitch. Keep all lengths in the same units.
For that example, the winding spans about 58 × 3.7 inches, or 214.6 inches (17.9 feet) along the mast. The wire itself follows the helix: 58 × √[(π × 2 inches)² + (3.7 inches)²], which is about 423 inches (35.2 feet), before allowing extra wire for connections and adjustment.
This is geometry arithmetic only. It does not show that 58 turns will resonate at 3.800 MHz. The winding diameter, insulation, distributed capacitance, mast, ground system, and surroundings all affect the antenna. Turns and spacing must be selected through electromagnetic modeling or iterative analyzer measurements.
A radial field to evaluate
One proposed starting layout is 32 equally spaced radials, each 33 feet long. That is about 1,056 feet of radial wire, with 11.25 degrees between adjacent radials. The wires would connect to a common radial plate at the base and lie on or just below the surface.
| Radials | Length each | Total wire | Status |
|---|---|---|---|
| 16 | 33 ft | 528 ft | Possible starting layout |
| 24 | 33 ft | 792 ft | Possible starting layout |
| 32 | 33 ft | 1,056 ft | Current example layout |
| 32 | 65 ft | 2,080 ft | Larger-footprint alternative |
These are layout options, not resonant-length prescriptions. On-ground radials are part of the RF return system, and their practical value comes from coverage and reduced ground loss; they do not all need to be quarter-wave resonant. The available footprint and installation conditions will determine what can be built.
Feed, matching, and measurement
The concept is base-fed with 50-ohm coax. Before choosing an L-network or other matching circuit, I need to measure the feed-point impedance with the final radiator and radial system in place. A 1:1 current choke at the feed line is part of the proposed feed arrangement. A low SWR alone would not establish good radiation efficiency.
The source illustration’s example component values, 50–100 kHz bandwidth, and 1 kW rating have not been verified, so I have not treated them as design limits or promised performance. The matching components must be selected and rated after impedance and voltage/current conditions are understood. Testing should begin at low power, with tuning checked again after installation and weather changes.
Mechanical plan and safety
The current concept calls for a 20-foot fiberglass support with engineered guying and a stable base. The support and hardware still need a structural design for the installation site; ordinary unsupported PVC is not a suitable assumption for a 20-foot structure. The radiating wire would be secured with UV-resistant supports, and the base enclosure and exposed connections would need weather protection and strain relief.
- Choose a location with adequate clearance from overhead power lines and other hazards.
- Install and raise the mast with suitable support and helpers; maintain clearance from utility lines.
- Evaluate RF exposure and keep people clear while the antenna is energized.
- Account for high RF voltage and loading losses in a shortened vertical and its matching network.
- Weatherproof exposed connections and recheck the antenna after installation.
What remains open
A fully wound 20-foot radiator is compact, but may have greater conductor and loading losses. Another version would use a straight lower section with helical loading higher up. Which arrangement works better depends on the final dimensions and measurements; neither version is a proven 3.800 MHz design yet.
Before build-ready plans can be written, the project still needs a selected support diameter, a known radial footprint, a target power level, and a preferred winding arrangement. The winding must then be modeled or tuned on the finished structure, and the installed antenna must be measured. Construction photos and final plans can follow that work.
Based on “80-Meter Helically Wound Vertical Antenna,” a preliminary construction and engineering guide. The calculations in this draft follow the document’s corrected written example; no independent performance testing has yet been reported.
John Miller, K6MM’s related 160-meter design uses a capacitance hat. A capacitance hat may also be needed for this 80-meter concept, subject to modeling and testing. Read the related 160-meter design.