Unit 4 — Calibration and Tuning

TM-ANT-043 — Open Handout TM Chapters: Chapter 5, Chapter 6, Appendix A ELOs: Calibrate the LOG PERIODIC ANTENNA — VHF 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-043. 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 LOG PERIODIC ANTENNA — VHF 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