Unit 4 — Calibration and Tuning

TM-ANT-046 — Open Handout TM Chapters: Chapter 5, Chapter 6, Appendix A ELOs: Calibrate the MOXON RECTANGLE ANTENNA to resonance; tune SWR to within acceptance criteria; apply frequency calculations Estimated time: 45 minutes (actual lab work required)

Step 1: Read the TM

Open TM-ANT-046. Read Chapter 5 — Calibration Procedure, Chapter 6 — Tuning and Adjustment, and Appendix A completely.

Then come back here.

Chapter 5 — Calibration Procedure

1. SOLT calibrate NanoVNA at feed point of antenna (on bench, 3 ft above ground for initial check).
2. Set sweep: center frequency ±10%.
3. Adjust gamma match: slide shorting bar toward center to increase impedance; away from center to decrease. Adjust trimmer capacitor to null reactance.
4. Target: SWR <1.5:1, R = 45–55 Ω, X = −5 to +5 Ω at design frequency.
5. Once matched, verify SWR across full target band. Record: SWR at center, SWR at band edges, F/B ratio (compare field-strength meter readings front vs. back).
6. At final installed height (away from nearby objects), re-verify SWR — ground proximity affects impedance.

Chapter 6 — Tuning and Adjustment

Gamma match adjustment: Moving the shorting bar changes the equivalent inductance in the match network; moving the trimmer changes reactance compensation. Typical procedure: set trimmer to midrange; slide shorting bar for minimum SWR; fine-adjust trimmer for minimum SWR; iterate. Director lengths can be adjusted ±2% to trade off gain vs. F/B ratio per NEC2 simulation.

Appendix A — Calculations and Formulas

Wavelength in free space (m)
λ = 300 / fMHz

Gain vs. boom length (estimate)
G(dBd) ≈ 10 log10(2.5 × Lboom/λ) for L>0.3λ

Boom correction factor for element length (metallic boom)
ΔL = −0.0085 × dboom × delem / λ² (approximate)

Key Formulas Summary

• λ = 300 / fMHz
• ΔL = −0.0085 × dboom × delem / λ² (approximate)

Calibration vs. Tuning — Distinction

Calibration (Chapter 5): initial setup measurements to establish current antenna state — resonant frequency, SWR at design frequency, impedance at feedpoint. No adjustments yet; you are characterizing what you built.

Tuning (Chapter 6): active adjustments based on calibration data. Trim the element, adjust the matching network, or change height/orientation until Chapter 7 acceptance criteria are met.

Acceptance Criterion

Chapter 5 specifies a criterion: SWR <1.5:1. Confirm the exact value in the TM.

Write the exact criterion from the TM here before you start:

Lab Checklist

• [ ] Antenna assembled per Chapter 4 and installed at operating height
• [ ] NanoVNA or SWR bridge connected per Chapter 5
• [ ] Chapter 5 calibration measurements taken and recorded
• [ ] Resonant frequency identified (minimum SWR point)
• [ ] SWR at design frequency recorded
• [ ] Chapter 6 tuning performed if needed
• [ ] Acceptance criterion met

Calibration Log

Practice Problems

P4-1. The design frequency of the MOXON RECTANGLE ANTENNA is stated in Chapter 1. Using the formula from Appendix A, compute the theoretical element length for that frequency. Show all work.

P4-2. Your NanoVNA shows minimum SWR at a frequency 3% above the design frequency. Which direction do you adjust — lengthen or shorten? By what percentage?

P4-3. SWR at resonance is 1.8:1, but the acceptance criterion is SWR ≤ 2.0:1. Does the antenna pass? What does SWR 1.8:1 mean in terms of reflected power?

P4-4. Compute reflected power percentage for SWR = 2.0:1. Formula: reflected power (%) = ((SWR − 1)/(SWR + 1))² × 100

Answer Key — Practice Problems

P4-1. See Appendix A. For a dipole: L(ft) = 468/f(MHz). For a quarter-wave vertical: L(ft) = 234/f(MHz). Apply the formula specific to this antenna type.

P4-2. Resonance is above design frequency → antenna is electrically short → lengthen the element. Adjustment magnitude: approx. 3% longer.

P4-3. SWR 1.8:1 passes the 2.0:1 criterion. Reflected power: ((1.8−1)/(1.8+1))² × 100 = (0.8/2.8)² × 100 = 8.2% reflected.

P4-4. SWR 2.0:1: ((2−1)/(2+1))² × 100 = (1/3)² × 100 = 11.1% reflected. Most of the power still gets through.

Checkpoint

Before proceeding: - [ ] Calibration measurements taken and logged - [ ] Antenna tuned to within the acceptance criterion - [ ] You can compute element length adjustment from frequency error

→ Proceed to Unit 5