Unit 4 — Calibration and Tuning

TM-ANT-061 — Open Handout TM Chapters: Chapter 5, Chapter 6, Appendix A ELOs: Calibrate the T-ANTENNA (INVERTED-T) 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-061. 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 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. SOLT calibrate NanoVNA at the end of the feed line (antenna side).
2. Set sweep: center frequency = design band center; span = ±20% of center.
3. Connect CH0 port to antenna feedpoint. Enable Smith Chart and SWR displays.
4. Record: fres (X = 0), SWR at fres, R at fres, SWR bandwidth (2:1 SWR points).
5. Resonance target: X = 0 ±5 Ω, R = 36–52 Ω (ground losses shift R above 36.6 Ω).
6. If fres too high: element is short, extend by 1–2 in. If fres too low: element too long, trim 1 in. Repeat.

Chapter 6 — Tuning and Adjustment

6-1. ELEMENT LENGTH ADJUSTMENT

Trim or extend the vertical element to set resonance. Each 1-inch change shifts fres by approximately 20–30 kHz at 40M. Adjust in 2-inch increments. If using a loading coil, adjust coil tap position to shift resonance; moving tap toward the feed end increases inductance and lowers resonance.

6-2. RADIAL SYSTEM OPTIMIZATION

Measure R at resonance before and after adding radials. A decrease in R indicates reduced ground loss (desired). Continue adding radials until further additions change R by <1 Ω. The point of diminishing returns is typically 16–32 radials for buried systems, 4–8 for elevated resonant systems.

Appendix A — Calculations and Formulas

Quarter-wave element height (feet)
L = 234 / fMHz

Radiation resistance (quarter-wave over perfect ground)
Rr = 36.6 Ω

Efficiency (η)
η = Rr / (Rr + Rg + Rcoil)

Loading coil Q (air-wound)
QL = XL / Rcoil where XL = 2πf·L

Key Formulas Summary

• L = 234 / fMHz
• Rr = 36.6 Ω
• η = Rr / (Rr + Rg + Rcoil)
• QL = XL / Rcoil where XL = 2πf·L

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 T-ANTENNA (INVERTED-T) 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