Unit 4 — Calibration and Tuning

TM-ANT-021 — Open Handout TM Chapters: Chapter 5, Chapter 6, Appendix A ELOs: Calibrate the END-FED RANDOM WIRE (EFRW) 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-021. 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 end of feed line (shack end) with the antenna deployed at operating height.
  2. Set sweep: target band ±15%.
  3. Connect NanoVNA. Record SWR across band. Resonance appears as dip in SWR (may be very sharp for EFHW).
  4. If SWR dip is at wrong frequency: trim wire end for higher frequency (too long), or add wire for lower frequency (too short). Change ~6 in for ~50 kHz shift at 40M.
  5. Verify SWR on harmonics for multiband use (EFHW is harmonically resonant at 2f, 3f, ...).
  6. Check common-mode current: touch coax braid — if RF is felt, add choke at feedpoint.

Chapter 6 — Tuning and Adjustment

Trim wire for resonance. For multiband EFHW, the 49:1 unun presents high impedance at the design frequency; SWR on harmonics is naturally low if the wire is properly trimmed at the fundamental. If SWR is acceptable at fundamental but poor at harmonics, adjust ATU or add a series capacitor (50–100 pF) in the counterpoise for harmonic correction.

Appendix A — Calculations and Formulas

Wire length for EFHW (feet)
L = 468 / fMHz

49:1 unun impedance transformation
Zwire = 49 × Zcoax = 49 × 50 = 2450 Ω

9:1 unun impedance transformation
Zwire = 9 × 50 = 450 Ω

Key Formulas Summary

  • L = 468 / fMHz
  • Zwire = 49 × Zcoax = 49 × 50 = 2450 Ω
  • Zwire = 9 × 50 = 450 Ω

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: 15%. 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 END-FED RANDOM WIRE (EFRW) 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