Unit 4 — Calibration and Tuning

TM-ANT-050 — Open Handout TM Chapters: Chapter 5, Chapter 6, Appendix A ELOs: Calibrate the OFF-CENTER FED DIPOLE (OCFD) 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-050. 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 (150–200 Ω at feed point → 50 Ω via 4:1 balun).

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 OFF-CENTER FED DIPOLE (OCFD) 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