Unit 4 — Calibration and Tuning

TM-ANT-041 — Open Handout TM Chapters: Chapter 5, Chapter 6, Appendix A ELOs: Calibrate the KMR-400 COMPACT MAGNETIC LOOP 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-041. 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 antenna end of feed line (or at feedpoint balun output).
2. Set sweep: fundamental resonance ±20%.
3. Verify fres where X = 0. Record R and SWR.
4. Expected: R = 100–150 Ω at resonance before matching. After 2:1 balun: R = 50–75 Ω at 50 Ω coax.
5. Trim loop perimeter in 6-inch increments to shift resonance to design frequency. Do not cut — fold wire back on itself until correct length found, then cut cleanly.
6. Check harmonic resonances at 2f0, 3f0; verify SWR acceptable for multiband use.

Chapter 6 — Tuning and Adjustment

Trim loop perimeter to shift resonance. Each 1-ft change in total perimeter shifts resonance approximately 10–20 kHz at 40M. An ATU at the feed line allows operation across a 2:1 frequency range from a single cut length. For delta loops, adjusting the feed point position (base corner vs. apex) changes the feed impedance but not the resonant frequency.

Appendix A — Calculations and Formulas

Full-wave loop perimeter (feet)
P = 1005 / fMHz

Radiation resistance (full-wave loop, free space)
Rr ≈ 100–150 Ω (shape-dependent)

Gain over dipole at same height
ΔG ≈ 1.5–2 dB

Key Formulas Summary

• P = 1005 / fMHz

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: 2:1. 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 KMR-400 COMPACT MAGNETIC LOOP 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