Table of Contents
CHAPTER 1 — GENERAL INFORMATION
1-1. SCOPE
This manual covers design, construction, calibration, and field operation of the Ground Plane Antenna for amateur radio and communications use. Applicable frequency bands: 2M (144–148 MHz), 70cm (430–440 MHz), 1.25M (222–225 MHz). All calibration procedures use the NanoVNA and TinySA test instruments.
1-2. APPLICABLE REFERENCES
- ARRL Antenna Book — Chapter on Vertical Antennas
- NEC2 model: ground_plane.nec (in antenna directory)
- FCC OET Bulletin 65 — RF Exposure Evaluation
- Radial Systems for Vertical Antennas, Brown, Lewis & Epstein (1937)
1-3. SAFETY PRECAUTIONS
CHAPTER 2 — THEORY OF OPERATION
2-1. RADIATION PHYSICS
Quarter-wave vertical radiator above four ground-plane radials drooped 45° to raise feedpoint impedance to 50 ω. A vertical radiator carries current that is vertically polarized; its electromagnetic wave radiates with E-field vertical. The ground (or radial counterpoise) serves as the electrical mirror-image of the above-ground element: a λ/4 vertical above a perfect ground is equivalent to a λ/2 dipole in free space, with radiation resistance Rr = 36.6 Ω.
2-2. RADIATION PATTERN
A quarter-wave vertical over an extensive ground plane radiates omnidirectionally in azimuth with a low-angle elevation lobe ideal for DX. The elevation angle of maximum radiation (θmax) depends on ground conductivity and number of radials: over a perfect ground θmax ≈ 0°; over average ground with 32+ radials, θmax ≈ 5–15°. The vertically polarized wave follows the earth’s surface better than a horizontal wave at low angles.
2-3. IMPEDANCE AND BANDWIDTH
Feed impedance: 50 Ω. The SWR bandwidth of a simple λ/4 vertical at HF is approximately 5–8% of center frequency for 2:1 SWR, typically covering one amateur band. Loading coils reduce bandwidth in proportion to their Q; top loading preserves more bandwidth than base loading because it maintains higher current along more of the radiator length.
2-4. GROUND SYSTEM DESIGN
Ground loss resistance Rg appears directly in series with the radiation resistance. For maximum efficiency: Rg < Rr. With buried radials, Rg decreases as Nradials increases: 32 radials each 0.25λ gives Rg ≈ 3 Ω. Elevated resonant radials achieve similar performance with only 4–8 radials. For 4 radials at 90° spacing, drooped 45° below horizontal — follow this requirement closely for efficiency.
CHAPTER 3 — MATERIALS AND CONSTRUCTION
3-1. BILL OF MATERIALS
| Qty | Item | Specification |
|---|---|---|
| 1 | Vertical element | Aluminum tubing 1–1.5 in OD or #12 AWG copper wire; length per formula |
| Per design | Radials | #16–#14 AWG copper; per ground system design above |
| 1 | Mast/base insulator | UV-resistant PVC or polycarbonate; must support element tension/weight |
| 1 | SO-239 or N-type connector | Weatherproof; mount at base |
| 1 | Feed line | RG-213 or LMR-400 for permanent install; RG-8X for portable use |
| As needed | Loading coil (if loaded vertical) | T-200 toroidal core or air-wound; Ql > 200 for minimum loss |
3-2. DIMENSION FORMULAS
CHAPTER 4 — ASSEMBLY PROCEDURES
- Install base mounting hardware and base insulator. Verify insulator breakdown voltage rating exceeds 2× operating RF voltage.
- Erect vertical element. For a guyed installation, use non-conductive guy ropes (Dacron); attach guys at top and 2/3 height.
- Install radial system. For buried radials: trench radials 2–4 in deep in lawn. For elevated radials: support at 1.5–2 m above ground, run outward from base.
- Bond all radials at the base via a radial bus ring (copper strap or ring terminal). Bond bus ring to coax braid at feedpoint.
- Connect center conductor of feed line to element base. Connect braid to radial bus ring. Weather-seal with self-amalgamating tape and UV jacket tape.
- Perform initial SWR sweep per Chapter 5 before first RF application.
CHAPTER 5 — CALIBRATION PROCEDURE
5-1. NANOVNA IMPEDANCE SWEEP
- SOLT calibrate NanoVNA at the end of the feed line (antenna side).
- Set sweep: center frequency = design band center; span = ±20% of center.
- Connect CH0 port to antenna feedpoint. Enable Smith Chart and SWR displays.
- Record: fres (X = 0), SWR at fres, R at fres, SWR bandwidth (2:1 SWR points).
- Resonance target: X = 0 ±5 Ω, R = 36–52 Ω (ground losses shift R above 36.6 Ω).
- If fres too high: element is short, extend by 1–2 in. If fres too low: element too long, trim 1 in. Repeat.
CHAPTER 6 — TUNING AND ADJUSTMENT
6-1. ELEMENT LENGTH ADJUSTMENT
Trim or extend the vertical element to set resonance. Each 1-inch change shifts fres by approximately 20–30 kHz at 40M. Adjust in 2-inch increments. If using a loading coil, adjust coil tap position to shift resonance; moving tap toward the feed end increases inductance and lowers resonance.
6-2. RADIAL SYSTEM OPTIMIZATION
Measure R at resonance before and after adding radials. A decrease in R indicates reduced ground loss (desired). Continue adding radials until further additions change R by <1 Ω. The point of diminishing returns is typically 16–32 radials for buried systems, 4–8 for elevated resonant systems.
CHAPTER 7 — VERIFICATION
| Parameter | Requirement | Pass/Fail |
|---|---|---|
| SWR at resonance | < 1.5:1 | ____ |
| Resonant frequency | Within ±1% of design | ____ |
| Feed impedance (R) | 35–55 Ω | ____ |
| Feed reactance (X) | <±10 Ω | ____ |
| Gain (NEC2) | 2 dBi omnidirectional | ____ |
| Efficiency | 90–97% | ____ |
- Confirm SWR meets specification on all design bands.
- Verify resonance frequency within ±1% of design center.
- Confirm radial bond resistance <0.1 Ω with ohmmeter from feedpoint braid to each radial tip.
- Log: date, ground condition, radial count, SWR, R+jX at each band, transmitter output power used for test.
APPENDIX A — CALCULATIONS AND FORMULAS
APPENDIX B — EXAMPLE RESULTS
| Band | fres (MHz) | SWR | R (Ω) | X (Ω) | 2:1 BW (kHz) |
|---|---|---|---|---|---|
| 40M | 7.150 | 1.3:1 | 38 | +2 | 180 |
| 20M | 14.175 | 1.4:1 | 41 | −3 | 350 |