Ground Plane Antennas
The ground plane antenna is a vertical monopole with an explicit, self-contained set of radials that serves as the electrical ground plane — elevated off the real earth. While the vertical antenna described in Lesson M14G uses buried or surface radials to interact with the actual soil, the ground plane antenna brings its radial system up with it, making it independent of soil conditions. This is the standard design for HF portable antennas, VHF and UHF base station antennas, and almost every commercially made vertical base antenna you can buy.
A ground plane antenna with horizontal radials (left, ~36 ohm feedpoint) and with drooped radials at 45 degrees (right, ~50 ohm feedpoint). Four radials are typical but three can be used.
View LargerWhat a Ground Plane Antenna Is
A ground plane antenna is a quarter-wave vertical element mounted at the center of a set of horizontal (or drooped) radial wires, all at the same height above the real ground. The radials do not touch the soil — they are elevated. The center conductor of the coaxial feedline connects to the base of the vertical element, and the shield of the coaxial cable connects to the radials. The radials form the "ground plane" — the electrical reference from which the vertical element radiates.
The critical insight is that the radials do not need to be in contact with the soil to function. They serve as a low-impedance RF return path from the surrounding space back to the feedpoint, recreating the mirror-image effect of a true conducting plane. Because the radials are elevated, the RF currents flow through the wire rather than through soil, so soil conductivity has no effect on efficiency. A ground plane antenna works equally well over sand, rock, concrete, water, or even when hoisted on a mast far above the ground.
This soil independence is the major practical advantage of the ground plane over the buried-radial vertical. A ground plane antenna can be deployed rapidly, moved to different locations, mounted on vehicles, placed on rooftops, or elevated on a mast — all with consistent, soil-independent performance.
The Effect of Radial Angle on Impedance
The feedpoint impedance of a ground plane antenna depends significantly on the angle at which the radials are mounted relative to horizontal. This is one of the most useful and practical design parameters in antenna work:
- Horizontal radials (0 degrees below horizontal): The feedpoint impedance is approximately 36 ohms — the same as a vertical monopole over a perfect infinite ground plane. The radials are acting as a near-perfect mirror.
- Radials drooped at 45 degrees below horizontal: The feedpoint impedance rises to approximately 50 ohms — a good match to standard 50-ohm coaxial cable with no additional matching network.
- Radials drooped more steeply (toward vertical): The impedance rises further, approaching 70–75 ohms when the radials are nearly vertical (close to the configuration of a folded dipole or a collinear).
The physical reason for this impedance change is that as the radials droop from horizontal, they become less like an infinite ground plane (which gives 36 ohms) and more like the lower half of a dipole (which gives 73 ohms). The 45-degree droop is intermediate and produces approximately 50 ohms — the sweet spot for direct coaxial connection without additional matching.
This impedance variation with radial angle is the key design tool for ground plane antennas. By choosing the radial angle, you can select the feedpoint impedance to match your feedline. For a 50-ohm feedline, droop the radials to approximately 45 degrees. For a 75-ohm feedline (used with CATV-type coax), keep the radials close to horizontal. For other impedances, the radial angle can be adjusted accordingly.
Drooped Radials: Getting to 50 Ohms
A drooped radial ground plane with four radials at 45 degrees below horizontal is the most common design for practical ham radio use. The four radials typically each have the same length as the vertical element — a quarter wavelength — and extend symmetrically at 45 degrees downward and outward from the feedpoint.
The four-radial 45-degree design achieves approximately 50-ohm impedance, allowing direct connection to RG-58, RG-213, LMR-400, or any other 50-ohm coaxial cable. The SWR at resonance is typically 1.1:1 to 1.3:1 — excellent. This makes the ground plane with drooped radials one of the simplest fully self-contained antennas to install and use — just connect the coax and operate.
The number of radials is typically 3 or 4 for commercial and practical designs. Three radials spaced at 120 degrees provide a reasonable balance between omnidirectionality and economy of materials. Four radials at 90 degrees provide slightly better pattern symmetry. More than 4 radials continue to improve the ground plane's effectiveness but with diminishing returns.
Quarter-wave length at 146 MHz = 234/146 = 1.60 feet = 0.488 meters = 48.8 cm
Vertical element: 48.8 cm (aluminum or copper tubing, or stiff wire)
Radials: 4 × 48.8 cm, drooped at 45 degrees below horizontal
Feedpoint: mount SO-239 chassis socket at the junction; vertical element connects to center pin, each radial connects to the chassis body.
Expected feedpoint impedance: approximately 50 ohms. Connect RG-8X or LMR-240 coax for a complete, efficient 2-meter antenna that works in any location without soil contact.
Number of Radials
An elevated ground plane does not need the large number of radials that a ground-mounted vertical needs, because the radials are carrying the return current directly through a conductor rather than through lossy soil. Even 2 radials provide a usable ground plane, though 3 or 4 are strongly preferred for pattern symmetry and efficiency.
| Number of radials | Pattern symmetry | Feedpoint impedance (45° droop) | Notes |
|---|---|---|---|
| 2 (collinear) | Elliptical (figure-8 tendency) | ~50 Ω | Usable but not ideal; pattern asymmetric |
| 3 (120° spacing) | Good, slight triangular asymmetry | ~50 Ω | Common for portable designs; light weight |
| 4 (90° spacing) | Excellent, nearly circular | ~50 Ω | Standard for most commercial designs |
| 6–8 | Very good; near-perfect circular | ~45–50 Ω | Marginal improvement over 4; rarely used |
Radiation Pattern
The radiation pattern of a ground plane antenna depends on the radial angle. With horizontal radials, the pattern closely resembles a vertical monopole over perfect ground — maximum radiation near the horizon, falling off at higher elevation angles. With drooped radials, the pattern tilts slightly upward compared to the horizontal-radial version, because the drooped radials themselves radiate slightly, adding a downward-going component that partially cancels the upward vertical radiation and fills in the pattern at moderate elevation angles.
For practical purposes, the drooped-radial ground plane radiates at low angles very effectively — nearly as well as a vertical with an extensive buried radial system. The slight differences in elevation pattern between horizontal and drooped radials are not significant for most operating situations. Both configurations produce the omnidirectional azimuth pattern that is the hallmark of all vertical antennas.
Ground plane antennas mounted high on a mast benefit from the elevation of the feedpoint. The ground plane's own radials create the local mirror; the height above actual ground reduces the interaction between the antenna pattern and the real ground. At VHF and UHF, even a few meters of elevation is many wavelengths, and the antenna performance is essentially the same as free-space performance.
Ground Plane Antennas at VHF and UHF
At VHF (144 MHz) and UHF (432 MHz, 1296 MHz), ground plane antennas are extremely common because the required element dimensions are small and manageable. A 2-meter (146 MHz) ground plane has a vertical element less than 20 inches (50 cm) long, and a 70-cm (432 MHz) ground plane is under 7 inches (17 cm). These compact sizes make ground plane antennas practical for rooftop mounting, mast mounting, mobile use, and portable operation.
Nearly all commercial VHF/UHF omnidirectional base antennas are derivatives of the ground plane design — either simple ground planes, or "collinear" designs that stack multiple half-wave elements in phase to produce gain at low elevation angles. A collinear antenna producing 5–6 dBd of gain is essentially a two-element or four-element phased array of ground-plane dipoles, all fed in phase through a built-in phasing network.
VHF and UHF ground planes are easily home-built. A BNC or SO-239 connector at the center, with four quarter-wave wires soldered to the shell, makes an effective antenna for any VHF or UHF band. The critical dimension is the element length — use the 234/f(MHz) formula, with f in MHz, to find the length in feet, or use the metric formula 71.3/f(MHz). At these frequencies, a digital caliper and careful soldering are more important than elaborate construction.
HF Portable Ground Plane Antennas
At HF frequencies, a quarter-wave ground plane antenna can be practical for portable operation if you accept the significant length of the required elements. For 40 meters (7 MHz), a quarter-wave vertical element is about 33 feet (10 meters). A compact mast or telescoping fiberglass pole can support this. The four radials, each 33 feet long, can be deployed on the ground around the mast base — even without soil contact (laying them on the grass surface is fine).
Many portable HF operators carry 40-meter and 20-meter ground plane antennas for field day and SOTA (Summits on the Air) operations. The antenna is contained in a small bag — just the vertical element (a telescoping pole or a roll of wire), four radials on a reel, and a mounting bracket. The complete antenna can be deployed in 15–20 minutes and provides excellent performance, especially from elevated terrain where interaction with the ground is minimized.
For multi-band portable operation, a trap vertical (covered in Lesson M14J) or an end-fed half-wave antenna (EFHW) with its own matching unit is often more practical than a separate ground plane for each band. However, for a single-band portable antenna, the ground plane is hard to beat for simplicity, performance, and speed of deployment.
Comparing Ground Plane Configurations
| Configuration | Radial angle | Feedpoint Z | Pattern advantage | Best use |
|---|---|---|---|---|
| Horizontal radials | 0° (flat) | ~36 Ω | Lowest radiation angle (best DX) | When 75-ohm coax or matching network available |
| 45° drooped radials | 45° below horizontal | ~50 Ω | Slightly higher angle than horizontal | Best for direct 50-ohm coax connection; most popular |
| Steeply drooped (60–75°) | 60–75° below horizontal | ~65–75 Ω | Fills in higher elevation angles more | Less common; suits 75-ohm coax or special designs |
- A ground plane antenna is an elevated vertical monopole with its own self-contained radials — it does not rely on soil conductivity.
- Horizontal radials give ~36 ohm feedpoint impedance. Radials drooped to 45° give ~50 ohms — a direct match to standard 50-ohm coax.
- Three or four radials are standard for practical ground plane antennas. More radials improve pattern symmetry but offer diminishing returns over 4.
- The pattern is omnidirectional in azimuth with low-angle radiation — same as a ground-mounted vertical with a good radial system.
- At VHF and UHF, the ground plane is the standard omnidirectional antenna due to its small size, simple construction, and good performance.
Frequently Asked Questions
Can I use three radials instead of four on my ground plane?
Yes — three radials at 120° spacing is a common and practical design. The pattern symmetry is slightly less perfect than four radials (a mild triangular asymmetry instead of perfect circular), but for most practical purposes the difference is negligible. Many portable operators prefer three radials to reduce weight and complexity. Commercial antennas with three or four radials are both widely available. The main concern is that all radials must be the same length and at the same angle — asymmetry in radial length is more detrimental than using three instead of four.
Why do commercial 2-meter vertical antennas look like a solid whip with no visible radials?
Commercial omni antennas often use a coaxial sleeve or base-loaded design where the radials are internal to the antenna housing or are replaced by a quarter-wave sleeve choke around the coaxial feedline. The sleeve is connected to the shield of the coax at a point one quarter wavelength from the feedpoint, creating an internal ground plane. This design is clean and mechanically robust. It is the same electrical concept as explicit radials, just hidden inside the antenna body. When you buy a commercial VHF/UHF collinear, there is often a ground plane junction hidden inside the base section.
How far above the ground should I mount a ground plane antenna?
For VHF and UHF, any height helps — the goal is line-of-sight to stations you want to work. At 2 meters, mounting the antenna at 30 feet (10 meters) provides significantly more coverage than 10 feet. At HF, the radials of the ground plane create the local mirror, so the antenna's relationship to real ground matters less than for a ground-mounted vertical. Higher is almost always better for HF horizon-to-horizon coverage, but even a few meters of elevation makes the ground plane independent of the local soil. The practical answer: get it as high as your mast, tower, or rooftop permits.
Test Your Knowledge
Answer the questions below to check your understanding. Every answer can be found in the lesson above.