Unit 4 — Calibration and Tuning

TM-ANT-013 — Open Handout TM Chapters: Chapter 5, Chapter 6, Appendix A ELOs: Calibrate the COMPACT TRAP DIPOLE 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-013. Read Chapter 5 — Calibration Procedure, Chapter 6 — Tuning and Adjustment, and Appendix A completely.

Then come back here.

Chapter 5 — Calibration Procedure

5-1. NANOVNA SWR AND IMPEDANCE SWEEP

NOTE: The NEC2 model file for this antenna is included in the antenna directory. Run it with xnec2c, 4nec2, or any NEC2-compatible engine to generate polar plots, impedance data, and gain figures. The NanoVNA measurements in Chapter 5 should be compared against NEC2 predictions — deviations >3 dB or >20% impedance indicate a construction error.

1. Perform SOLT calibration on NanoVNA using the SOL (Short-Open-Load) kit at the antenna end of the feed line.
2. Set NanoVNA sweep range to cover ±10% of target center frequency (example: 40M → 6.5–7.8 MHz).
3. Connect NanoVNA to feedpoint. Navigate to CH0 S11 display. Select Smith Chart and SWR graphs.
4. Record: frequency of minimum SWR (fres), SWR at fres, SWR at band edges, R + jX at fres.
5. Resonance is confirmed when X ≈ 0 and R ≈ 73 Ω (simple dipole) or per design (matched system).
6. Compare measured fres to design frequency. If fres is too high, the element is short — lengthen each arm 1–2 in. If fres is too low, the element is long — trim each arm 1 in. Repeat until fres is within ±0.5% of design frequency.

Chapter 6 — Tuning and Adjustment

6-1. RESONANCE ADJUSTMENT

Trim or extend element arms symmetrically to shift resonance. Each 1-inch change in total length shifts resonance by approximately fMHz/468 × 12 kHz for a 40M dipole. Adjust in 2-inch increments and re-measure SWR between adjustments. Finalize element length when SWR at design frequency is <1.5:1 or as specified.

6-2. IMPEDANCE MATCHING

If impedance at resonance differs from 50 Ω, adjust element height (increases ground effect), add a matching network (L-network, λ/4 transformer, or series capacitor), or use a 4:1 balun for designs with higher feed-point impedance such as folded dipoles (50 Ω).

Appendix A — Calculations and Formulas

Half-wave dipole element length (each arm, feet)
Larm = 234 / fMHz

Velocity factor correction (for insulated wire)
Ladj = Larm × Vf (Vf ≈ 0.95 for #14 AWG PVC)

Radiation resistance (half-wave dipole, free space)
Rr = 73.1 Ω

Gain over isotropic (free space)
G = 2.14 dBi

Height for DX angle (degrees elevation for max gain)
θ ≈ arcsin(1 / (2h/λ)) for h > 0.25λ

Key Formulas Summary

• Larm = 234 / fMHz
• Ladj = Larm × Vf (Vf ≈ 0.95 for #14 AWG PVC)
• Rr = 73.1 Ω
• G = 2.14 dBi

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: 3 dB. 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 COMPACT TRAP DIPOLE 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