Vertical Antenna
Part of Radio
A vertical antenna radiates and receives equally in all horizontal directions, making it the natural choice for mobile operation, emergency communication, and locations where directional antennas are impractical.
Why This Matters
Not every antenna site allows a horizontal dipole hung between two high supports. A vertical antenna requires only one support point at its base, can be erected quickly in the field, and — crucially — radiates at low angles toward the horizon, making it well-suited for long-distance sky-wave communication. The omnidirectional horizontal pattern means no pointing is required: you are always radiating toward every possible contact simultaneously.
For a rebuilding community, the vertical antenna represents a practical workaround for constrained sites. A church steeple, a hilltop, a ship’s mast, a free-standing tower — all can support a vertical antenna that covers an entire region with one installation. Mobile communication, where direction of contact cannot be predicted, relies almost entirely on vertical antennas.
The physics of vertical antenna operation also introduce the concept of the ground plane, which fundamentally affects low-angle radiation. Understanding and implementing a proper ground plane or counterpoise transforms a mediocre vertical into an efficient long-distance antenna. This is the most commonly neglected aspect of vertical antenna construction and the most commonly the reason vertical antennas underperform expectations.
Vertical Antenna Basics
A quarter-wave vertical is the classic design: a conductor standing vertically, with its base at or near ground level, fed from the bottom. Its length is λ/4, where λ is the wavelength of the target frequency. For 7 MHz: λ/4 = 300/(4×7) ≈ 10.7 meters. For 3.5 MHz: λ/4 ≈ 21.4 meters. For 14 MHz: λ/4 ≈ 5.4 meters.
At resonance, the feed-point impedance of a quarter-wave vertical over a perfect ground plane is approximately 35–36 ohms. This is close enough to 50-ohm coaxial cable that the SWR is only about 1.5:1 — acceptable without a matching network. Over a real ground or imperfect ground plane, the impedance rises (losses increase the apparent resistance), often to 50–75 ohms.
A half-wave vertical eliminates the need for a ground plane but its feed impedance is approximately 73 ohms (same as a half-wave dipole) and feeding it requires matching if using 50-ohm coax. The half-wave vertical can be center-fed (like a dipole stood on end) or end-fed with a matching network. For large structures like transmitting towers, half-wave and 5/8-wave verticals are common because of their low-angle radiation advantage.
The 5/8-wave vertical (length = 0.625λ) has a radiation pattern with a deeper low-angle main lobe than a quarter-wave vertical, making it preferred for long-distance sky-wave communication. However, its feed impedance is high (approximately 200 ohms), requiring a matching coil or transformer.
Ground Plane and Counterpoise Systems
The vertical antenna works with the Earth itself as the return conductor — the vertical element is only half of the complete antenna system. If the Earth is a perfect conductor, the quarter-wave vertical over it behaves exactly like a half-wave dipole. Real earth is resistive, and current flowing through the soil dissipates as heat, reducing radiation efficiency.
A radial ground plane replaces the uncertain and resistive real earth with a set of conducting wires laid on or buried in the ground, radiating outward from the antenna base. Each radial is λ/4 long in the ideal case, though shorter radials still help. The minimum effective system has 4 radials; the standard for a fixed station is 32–120 radials; more radials always improve efficiency.
To install a buried radial system: from the antenna base, run wires outward in all directions, burying them 5–15 cm deep or simply laying them on the ground surface. Number of radials vs. length trades off: 32 radials of λ/4 outperforms 4 radials of the same length, but 16 radials of λ/2 outperforms either in absolute efficiency.
An elevated ground plane uses 4 radials mounted at or above the antenna base, angled downward at 30–45 degrees. This design is used for “ground plane” vertical antennas mounted on rooftops, vehicle roofs, and portable applications. The downward-angled radials increase the feed impedance to approximately 50 ohms (convenient for direct coax feed) and eliminate the need for buried radials. Height above ground should be at least λ/4 for best performance.
Practical Construction Methods
Simple wire vertical: suspend a wire of the correct length vertically from a high point — a tree branch, a mast, a tall pole. At the base, connect to the coax or feedline through a matching network if needed. This works and is easy to deploy in the field. The challenge is supporting the wire vertically without having a tall structure at the feed point.
Self-supporting aluminum tube vertical: assemble sections of aluminum tubing (telescoping if possible) into the required height. Use electrical conduit, tubing from salvaged equipment, or purpose-drawn aluminum. Each joint must make good electrical contact — clean surfaces and stainless steel hose clamps. At the base, mount on an insulating platform (PVC pipe or dry hardwood) and connect to the coax through a weatherproof fitting.
Loaded vertical: when the full quarter-wave height is impractical, a loading coil placed in the antenna element adds electrical length without physical length. A coil at the base (base loading) or at the midpoint (center loading) or near the top (top loading) can resonate an antenna much shorter than λ/4. Top loading is most efficient because it concentrates the high-current portion of the antenna at maximum height. A loading coil reduces radiation resistance and increases Q (narrower bandwidth), but the penalty for moderate shortening (50–70% of full length) is acceptable.
Mobile verticals: mount on a vehicle roof, hatch, or trailer ball. The vehicle body provides the ground plane. A ball-mount with spring tension allows the antenna to flex without breaking when hitting low branches. A loading coil built into the antenna base brings the resonant frequency into the target band; adjust by varying the coil tapping point.
Phased Arrays and Directional Verticals
A single vertical antenna is omnidirectional — it radiates equally in all horizontal directions. For applications requiring gain toward one direction (long-haul communication along a known path), two or more verticals can be combined in a phased array.
Two verticals spaced λ/4 apart, with the signals fed 90 degrees out of phase, produce a cardioid pattern — enhanced in one horizontal direction, suppressed in the opposite. This “two-element cardioid” provides about 3–4 dBd gain in the forward direction and a significant reduction (null) toward the back. Simply reversing the phase relationship reverses the forward direction, allowing you to switch between two 180-degree-apart directions.
Four verticals arranged in a square, with a 90-degree phase sequence (0°, 90°, 180°, 270°), produce a near-circular beam. Systems like this are used for medium-wave broadcasting and regional coverage.
The four-square array of four quarter-wave verticals in a λ/4 square, fed with a specific phasing network, is a popular amateur radio array providing approximately 5–6 dBd gain in one quadrant direction with good front-to-back ratio. It is switchable among four directions by changing which element is fed first in the phase sequence.
Common Installation Problems
Antenna too close to metal structures: any metal structure within λ/4 of the antenna base will absorb and re-radiate the signal, distorting the radiation pattern and reducing efficiency. This includes guy wires, metal fences, and building frames. Non-conducting guy wires (synthetic rope with ceramic strain insulators at intervals) are necessary for tall vertical installations where guys are needed for support.
Inadequate ground plane: the most common problem. Symptoms include high SWR, low measured antenna current, RF current on the coax outer shield, and RF “bites” (RFI on audio equipment in the shack). Fix by adding more radials.
Moisture ingress: the feed point is a vulnerable location. Water entering the coax connector causes intermittent high SWR and eventual coax failure. Apply self-amalgamating silicone tape over all weatherproof connectors, and allow a drip loop in the coax so water cannot run into the connector along the cable.
Resonant frequency drift: verticals change resonant frequency with weather (moisture alters effective capacitance of the surroundings), season (soil moisture and conductivity), and temperature (metals expand and change length). Trim the vertical element length for resonance at the temperature and season of primary use, and monitor SWR regularly.