Unit 4 — Calibration and Tuning

TM-ANT-049 — Open Handout TM Chapters: Chapter 5, Chapter 6, Appendix A ELOs: Calibrate the OCTAL 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-049. 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 OCTAL 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