Table of Contents
CHAPTER 1 — GENERAL INFORMATION
1-1. SCOPE
This manual covers use of a physically measured half-wave dipole antenna as a frequency standard and antenna analyzer calibration reference. At resonance, the dipole exhibits a known impedance (~70 Ω) and a predictable resonant frequency based on physical length. This provides a quick sanity check for NanoVNA calibration and antenna analyzers.
CHAPTER 2 — THEORY OF OPERATION
2-1. HALF-WAVE DIPOLE RESONANCE
Where L is the total tip-to-tip length. At resonance, feedpoint impedance is approximately 70–73 Ω in free space; ground effects reduce this to 20–50 Ω near ground. Height above ground affects both resonant frequency (lowered slightly) and feedpoint impedance.
| Total Length (m) | Total Length (ft) | Resonant Freq (MHz) | Band |
|---|---|---|---|
| 20.07 | 65.8 | 7.1 | 40m CW |
| 10.03 | 32.9 | 14.2 | 20m SSB |
| 6.69 | 21.9 | 21.3 | 15m |
| 4.78 | 15.7 | 29.8 | 10m |
| 2.00 | 6.56 | 71.3 | 4m/VHF |
2-2. NANOVANA MEASUREMENT
The NanoVNA measures complex impedance Z = R + jX. At resonance: X = 0 (or near zero). The resonant frequency is observed as the zero-crossing of the reactance curve, or as the minimum of |S11|.
CHAPTER 3 — MATERIALS AND CONSTRUCTION
3-1. DIPOLE CONSTRUCTION
| Item | Specification |
|---|---|
| Wire | #14–#18 AWG bare copper or aluminum, measured to ±5 mm |
| Center insulator | Ceramic or UHMW polyethylene |
| End insulators | Ceramic strain insulators |
| Feed coax | RG-58 or RG-174, 1/2 wavelength preferred (reduces common mode) |
| Ferrite choke | 5 turns #14 AWG through T240-43 or 3 FB-31-5621 beads |
CHAPTER 4 — ASSEMBLY PROCEDURES
- Calculate total dipole length for target frequency: L(m) = 142.5 / f(MHz).
- Cut wire to calculated length, measuring total tip-to-tip at final assembled length. Include connector pigtail in total length.
- Connect center of dipole to coax center; both halves are half of total length.
- Install ferrite choke balun at feedpoint to reduce common mode current.
- String dipole horizontally at height ≥λ/4 above ground for near-free-space behavior.
- Record exact measured length in calibration log.
CHAPTER 5 — CALIBRATION PROCEDURE
- Connect dipole feed coax to NanoVNA PORT 1 (through ferrite choke).
- Set NanoVNA center frequency to designed fr, span ±10%.
- Display |S11| and reactance (X) vs. frequency.
- Find frequency where X = 0 (reactance zero crossing). This is fresonant.
- Compare fresonant to calculated: error% = (fmeas − fcalc) / fcalc × 100.
- If fmeas < fcalc: dipole is electrically longer than expected (nearby objects, velocity factor, or ground proximity effects).
- If NanoVNA reads fmeas correctly compared to GPS/WWV reference, the NanoVNA frequency axis is calibrated.
CHAPTER 6 — TUNING AND ADJUSTMENT
To trim dipole to exact target frequency: shorten from tips in 5 cm increments if frequency is too low; add wire clips if too high. Re-measure after each adjustment. Height, proximity to trees/buildings, and ground conductivity all affect resonant frequency.
CHAPTER 7 — VERIFICATION
- Compare dipole fresonant against WWV frequency to verify NanoVNA frequency accuracy.
- Verify feedpoint R at resonance: R = 50–90 Ω depending on height. Values outside 20–120 Ω suggest calibration error or connection issue.
- Log: wire length, height, fcalc, fmeas, R at resonance, date.
APPENDIX A — CALCULATIONS AND FORMULAS
APPENDIX B — EXAMPLE RESULTS
| Target f | L total (m) | Height (m) | fcalc (MHz) | fmeas (MHz) | R (Ω) |
|---|---|---|---|---|---|
| 7.100 MHz | 20.07 | 6.1 | 7.100 | 7.032 | 48 |
| 14.200 MHz | 10.03 | 5.0 | 14.200 | 14.155 | 62 |
| 21.300 MHz | 6.69 | 4.5 | 21.300 | 21.24 | 61 |