Unit 4 — Calibration and Tuning

TM-ANT-018 — Open Handout TM Chapters: Chapter 5, Chapter 6, Appendix A ELOs: Calibrate the ELEVATED RADIALS VERTICAL 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-018. 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 ELEVATED RADIALS VERTICAL 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