Build a 2-Meter Ground Plane Antenna
The quarter-wave ground plane is the purest, most instructive first antenna build in amateur radio — a single vertical element above a set of radials that together form a complete antenna system with a predictable 50 Ω feedpoint impedance and a clean omnidirectional vertical radiation pattern. At 2m the entire antenna fits in one hand, builds in under an hour from aluminum rod and an SO-239 connector, and costs under $10. Every concept in antenna theory — resonance, ground plane behavior, feedpoint impedance, radial angle effects, and SWR tuning — is visible and measurable in this simple structure. This guide builds a 2m ground plane from 3/16-inch aluminum rod for both the vertical element and the four radials, tuned for the full 144–148 MHz 2m band.
The Ground Plane Concept
A quarter-wave vertical antenna works by using a conductive ground plane (or its equivalent) as a mirror — the ground plane reflects the vertical element's radiation to create the same pattern as a full half-wave dipole:
Radial Angle and Impedance — The Key Adjustment
The most useful practical feature of the ground plane antenna is that the feedpoint impedance is adjustable by changing the angle of the radials below horizontal. This lets the builder set the impedance to 50 Ω without any matching network:
Vertical Element Length
The vertical element is a quarter-wave at the design frequency. The standard calculation uses a velocity factor slightly below 1.0 for a metal rod in free space:
Ground Plane vs J-Pole vs Yagi — Choosing the Right Antenna
The ground plane is not the highest-performance option but has unique advantages that make it the right choice in specific situations:
- Best use for ground plane: learning antenna construction, temporary stations, emergency/portable use where a flat radial plate is available (e.g., vehicle roof), and applications where the antenna must sit directly on a metal surface (the vehicle body becomes the ground plane).
- vs J-pole: the J-pole has ~2 dBd more gain, requires no ground plane, and is simpler to tune. For a fixed station, the J-pole is nearly always the better choice. The ground plane wins only when the mounting surface is conductive (vehicle roof) or when the builder wants the simplest possible antenna for learning.
- vs Yagi: the Yagi has 10+ dBd more gain but is directional. For omnidirectional operation (FM repeaters, APRS, simplex), any vertical — ground plane, J-pole, or Slim Jim — is preferred over a Yagi.
- Mobile use: a quarter-wave vertical with the vehicle roof as ground plane is the standard mobile 2m antenna — the most common configuration in ham radio. The magnetic-mount mobile antenna is simply a quarter-wave vertical on a magnetic base that uses the vehicle roof as its ground plane.
| Element | Starting length | Final length (target) | Notes |
|---|---|---|---|
| Vertical element | 20.0 inches | ~19.2 inches | Trim to shift resonance to 146 MHz; quarter-wave |
| Radial 1 | 19.5 inches | ~19.2 inches | Same length as vertical element; droop 45° below horizontal |
| Radial 2 | 19.5 inches | ~19.2 inches | At 90° from Radial 1 |
| Radial 3 | 19.5 inches | ~19.2 inches | At 180° from Radial 1 (opposite Radial 1) |
| Radial 4 | 19.5 inches | ~19.2 inches | At 270° from Radial 1 (opposite Radial 2) |
| Radial droop angle | — | 40–50° below horizontal | Bend to adjust feedpoint impedance toward 50 Ω |
| SO-239 connector | — | At feedpoint hub center | Center pin → vertical element; shell → 4 radials via hub plate |
VHF Ground Plane Calculator
Materials for a 2m ground plane from aluminum rod and SO-239
Hub Plate Construction
The hub plate is the heart of the ground plane antenna — it mounts the SO-239, connects the four radials to the coax shield, and provides the structural support for the entire antenna. A simple square plate of 1/8-inch aluminum flat stock is the most practical homebrew approach:
Alternative Hub Approaches
Several simpler hub alternatives avoid the need to fabricate an aluminum plate:
- PL-259 male plug as hub: drill four evenly spaced holes around the PL-259 barrel and thread the radial rods through the holes. The PL-259 screws directly onto an SO-239 panel mount — no separate hub plate needed. Compact and surprisingly robust. A common approach for portable ground planes.
- Chassis-mount SO-239 with solder lugs: use a 4-hole flanged SO-239 and solder the radial rods directly to the mounting flange holes. Quick and permanent, but requires a soldering iron capable of heating the aluminum rod-to-brass flange joint adequately. The joint may be electrically marginal if the solder does not flow fully around the rod.
- Commercial SO-239 radial plate: several vendors sell pre-drilled SO-239 hub plates specifically for ground plane antennas at $5–15. These are fully equivalent to a homebrew plate and save fabrication time. Search for "SO-239 ground plane radial plate" to find available options.
- Brass T-fitting: a 1/4-inch brass pipe tee fitting with the center port for the vertical element and the side ports for two radials, with two additional holes drilled in the body for the remaining two radials. Connects via an adapter to an SO-239. Mechanically strong and requires no metalworking beyond drilling.
Building the 2m Quarter-Wave Ground Plane
This is the most mechanical and least electrical of all the VHF builds in this series — the performance is determined almost entirely by element lengths and radial angles, both of which are directly measurable and adjustable. There is no soldering involved if using the mechanical hub plate approach.
Cut All Aluminum Elements
From the 3/16-inch aluminum rod, cut five pieces: one vertical element at 20 inches and four radials at 19.5 inches each. A total of 98 inches (8.2 feet) of rod is used — a 10-foot piece provides adequate material with a short offcut:
Fabricate the Hub Plate
Cut a 3 × 3 inch square from 1/8-inch aluminum flat stock. Mark and drill the following holes:
Set Initial Radial Angle and Mount the Antenna
Before connecting the feedline, set the radials to 45° below horizontal as the starting tuning position. With the hub plate held level, each radial should point outward and downward at approximately 45° from horizontal — like the arms of an inverted umbrella:
Measure SWR and Adjust Radial Angle
Connect the NanoVNA to the SO-239. Hold the assembly at arm's length, oriented vertically, and sweep 130–165 MHz. Two adjustments control the antenna performance:
Permanently Mount and Weatherproof
Once SWR is confirmed, mount the hub plate to its permanent support. If mounting on a metal mast, note that the metal mast may slightly affect the SWR — re-measure after mounting on the permanent support and make any final small adjustments to radial angle:
Verify Final Performance On-Air
With the antenna at its operating height, verify both SWR and actual on-air performance. Connect the NanoVNA at the shack end of the coax and sweep 140–155 MHz for the final confirmation:
Why 5/8-Wave Outperforms Quarter-Wave
The 5/8-wave vertical is the most popular mobile and fixed station VHF/UHF antenna because its current distribution concentrates radiation at a lower angle than the quarter-wave, producing approximately 3 dBd of gain — the same as the Flower Pot and competitive with the Copper Cactus:
When to Build Quarter-Wave vs 5/8-Wave
Both antennas are valid choices — the decision depends on what the builder values:
- Quarter-wave (this guide): simplest construction, no matching network, impedance adjusted by radial angle alone, broadest bandwidth. Best for a first build, emergency/portable use, or any application where simplicity and reliability matter more than gain.
- 5/8-wave: 3 dBd gain improvement over quarter-wave, lower radiation angle. Requires a matching network (coil or capacitor hat) that introduces one additional tunable component. Best for a permanent fixed station upgrade that outperforms the quarter-wave in repeater range and APRS coverage.
- Practical advice: build the quarter-wave first as described in this guide. Verify it works, understand the tuning process, and get comfortable with NanoVNA measurements. Then build a 5/8-wave version with a simple series coil matching network as a follow-on project. The second build will be faster, better understood, and benefit from the reference point provided by the quarter-wave.
- Cost difference: minimal — the 5/8-wave uses more rod but less coax and the matching coil adds $1–2 in materials. The 5/8-wave costs approximately $11–13 total, vs $9 for the quarter-wave.
| Symptom | Most likely cause | Diagnosis | Fix |
|---|---|---|---|
| SWR high everywhere — no dip in 130–165 MHz sweep | Vertical element not connected to SO-239 center pin, or radials not connected to shell | Measure DC continuity from coax center to vertical element and from coax shield to any radial | Re-secure vertical element connection to center pin; verify all four radial tabs have solid electrical contact with hub plate |
| SWR minimum present but above 2.5:1 everywhere | Radial angle producing wrong impedance; or poor radial connection | Check radial angle — should be 40–50° below horizontal; check radial connection resistance | Increase radial droop to 50–55°; clean and re-apply Noalox at all aluminum contact points; re-tighten radial nuts |
| SWR minimum correct but 5–10 MHz from target frequency | Vertical element significantly wrong length | Measure vertical element length — compare to 19.2-inch target | If too long: trim in 0.5-inch steps; if too short: cannot easily extend — cut new element |
| SWR rises significantly when mast-mounted vs hand-held test | Metal mast coupling to antenna — lowers resonance 1–2 MHz | Compare SWR mounted vs held free — difference confirms mast coupling | Trim 0.25 inch from vertical element to compensate for mast detuning; or use non-conductive mast material |
| Radial angles unequal — pattern distorted | Radials bent to different angles during installation | Sight from above and below — all radials should appear at equal angles; measure with protractor | Bend radials individually to match the angle of the one nearest target SWR; verify equality before finalizing |
| SWR rises after several months outdoors | Aluminum oxide at radial connection points — high-resistance contacts | Remove and re-clean connections — if SWR drops after cleaning, oxide is confirmed | Clean with wire brush; re-apply Noalox; re-tighten all hardware; apply lacquer to exposed aluminum surfaces |
Does a ground plane really need four radials — will two work?
Two radials work, and three or more also work — but four is the standard that produces the best balance between pattern symmetry and construction simplicity. With only two radials directly opposite each other, the antenna pattern is asymmetric — slightly stronger in the direction perpendicular to the radial plane. With three radials, the asymmetry improves significantly. With four radials at 90° intervals, the pattern is essentially symmetric and the feedpoint impedance with radials drooped 45° is close to 50 Ω. Adding more than four radials (six, eight) provides diminishing returns in pattern quality and essentially no measurable gain improvement. Four is the practical optimum for a homebrew ground plane.
Can I use the roof of a building or a vehicle as the ground plane instead of radials?
Yes — this is exactly how commercial vehicle mobile antennas work. A quarter-wave vertical mounted on a large conductive surface (vehicle roof, metal shed roof, metal HVAC equipment housing) uses that surface as its ground plane entirely. The larger the conductive surface, the better it approximates an ideal ground plane. A vehicle roof is about 4 square feet — adequate for 2m but slightly small for ideal performance. For a rooftop installation on a building, the roof metal (if the building has a metal roof) or the metal HVAC equipment can serve as ground plane. Connect the SO-239 shell to the roof metal using a short braided strap, mount the quarter-wave vertical element vertically, and the installation is complete with no separate radials needed.
Why do some ground plane designs use downward-sloping radials instead of horizontal?
The radial angle controls the feedpoint impedance. Horizontal radials (0° droop) produce approximately 35 Ω, which gives a 1.4:1 SWR mismatch to 50 Ω coax. Drooping the radials downward progressively raises the impedance toward 50 Ω (at about 45° droop). Designs intended for direct 50 Ω coax connection therefore specify drooped radials. Some commercial designs droop radials at 45° by default for this reason. A few designs use horizontal radials with a small series capacitor at the feedpoint to compensate the impedance mismatch — this is electrically equivalent but adds a component. The drooped radial approach in this guide eliminates the need for a matching component entirely and is the cleanest homebrewing solution.
How does the ground plane compare to a magnetic mount mobile antenna?
A magnetic mount mobile antenna is essentially a quarter-wave vertical using the vehicle roof as its ground plane — electrically identical to this homebrew ground plane but packaged with a magnet for removable mounting. Commercial mag mounts are optimized, professionally manufactured, and typically include a 17-foot coax with a PL-259 — very convenient for vehicle use. The homebrew ground plane in this guide performs identically to a good mag mount at the same height, and outperforms cheap mag mounts whose short elements compromise efficiency. The homebrew advantage: custom frequency tuning, specific mounting hardware choices, and the educational value of understanding exactly what the antenna does electrically. The commercial mag mount advantage: convenient magnetic mounting, pre-made coax, and no build time.
Is a ground plane antenna suitable for APRS iGate use?
Yes, with a caveat about gain. A ground plane at 0 dBd gain will receive APRS packets from a smaller geographic area than a J-pole (~2 dBd) or Flower Pot (~3 dBd) at the same height. For an iGate in a dense urban area where most APRS traffic originates from within 10–20 miles, the ground plane's coverage is fully adequate. For an iGate at a suburban or rural site trying to maximize coverage radius, the additional gain of a J-pole or Flower Pot produces meaningfully more packets received. The ground plane is a good starting antenna for an APRS iGate — it is quick to build, works reliably, and can always be upgraded to a collinear later when the coverage limitation becomes apparent from the digipeater statistics.
Can I scale this design for 70cm?
Yes — divide all element dimensions by approximately 3 (the ratio of 432/146). The vertical element becomes 6.4 inches, each radial becomes 6.4 inches, and the hub plate can shrink proportionally. At 70cm the entire antenna fits in the palm of one hand. The same 45° radial droop for 50 Ω impedance applies, and the same NanoVNA tuning procedure works identically. A 70cm ground plane is an excellent companion to the 70cm Yagi — the omnidirectional ground plane covers local FM repeater operation while the Yagi handles satellite downlinks and weak-signal work. Both antennas can share the same mast with a simple spacer separating them at an appropriate electrical distance (at least 18 inches between the hub plates at 70cm).