Unit 4 — Calibration and Tuning

TM-ANT-035 — Open Handout TM Chapters: Chapter 5, Chapter 6, Appendix A ELOs: Calibrate the HELICAL ANTENNA — 2M SATELLITE 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-035. Read Chapter 5 — Calibration Procedure, Chapter 6 — Tuning and Adjustment, and Appendix A completely.

Then come back here.

Chapter 5 — Calibration Procedure

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. SOLT calibrate NanoVNA at feedpoint (antenna connector).
  2. Set sweep range: ±10% of design center frequency.
  3. Connect NanoVNA to antenna. Record SWR minimum and frequency, R + jX at resonance.
  4. Compare measured impedance to NEC2 predicted values. Investigate discrepancies >20%.
  5. Verify SWR ≤2.0:1 at operating frequency. If not, adjust matching per Chapter 6.
  6. For gain verification: compare received signal strength (using TinySA as receiver) from a known beacon vs. reference antenna.

Chapter 6 — Tuning and Adjustment

Element lengths can be adjusted by ±2% of design length to shift resonance and optimize impedance. For matching network adjustment: follow gamma-match, delta-match, or transformer-tap procedure per the specific implementation. Refer to NEC2 model for sensitivity analysis — the model can be re-run at ±2% element length to determine which element most affects resonance and F/B ratio.

Appendix A — Calculations and Formulas

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

Effective gain formula (over isotropic)
GdBi = 10 log10(4π × Aeff / λ²) where Aeff = effective aperture (m²)

Special parameters
3-turn helix, 4.5 in diameter, 4.5 in pitch, ~8 dBic gain, RHCP, 50 Ω direct feed via microstrip transformer

Key Formulas Summary

  • λ (m) = 300 / fMHz
  • GdBi = 10 log10(4π × Aeff / λ²) where Aeff = effective aperture (m²)

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 ≤2.0:1. 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

Parameter Measured Specification Pass/Fail
Resonant frequency (MHz) (design freq ± tolerance)
SWR at design frequency (from Ch. 5)
Feedpoint impedance (Ω) (from Ch. 2)

Practice Problems

P4-1. The design frequency of the HELICAL ANTENNA — 2M SATELLITE 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