Merv's Brain Dump

Chapter 1 — Introduction and Scope

This manual covers a compact portable RF power meter covering all fifteen amateur radio bands from 160 meters (1.8 MHz) through 20 cm (1.3 GHz). The meter uses two directional coupler sections optimized separately for HF (1.8–30 MHz, toroid directional coupler) and VHF/UHF (50–1300 MHz, PCB microstrip coupler). Power ranges are switchable: 1W, 10W, 100W, 1000W. Accuracy: ±5% typical, ±10% worst case. Insertion loss: <0.1 dB. Size: 160×100×50 mm.

Chapter 2 — Theory of Operation

2-1 Directional Coupler Principle

A toroid transformer (T1) with 1 primary turn (the center conductor of the transmission line) and N secondary turns couples a fraction of the forward and reflected waves. The coupling factor C (dB) = −20 log10(N) for a toroid coupler. Typical values:

2-2 Schottky Diode Detector

1N5711 Schottky diodes detect the coupled RF. Forward voltage ~0.25V at low signal levels (square-law region: V_out ∝ P_in) and ~0.4V at high levels (linear region: V_out ∝ V_in). The ADS1115 16-bit ADC samples V_fwd and V_ref; firmware converts to power using a calibration table stored in NVS.

2-3 SWR Computation

|Γ| = sqrt(P_ref / P_fwd)
SWR = (1 + |Γ|) / (1 − |Γ|)

Chapter 3 — Equipment and Materials

Chapter 4 — Construction

4-1 HF Toroid Winding

1. Thread the main transmission line center conductor through the toroid core once (1 primary turn).
2. Wind the secondary winding on the same core: N2 turns (select per Table 2-1 for desired power range). Use #26 AWG Teflon-insulated wire.
3. Wind FWD and REF termination resistors (R1, R2 = 51Ω 1%) directly at the secondary winding ends. Lead length ≤10 mm each.

4-2 PCB Microstrip VHF/UHF Coupler

The microstrip directional coupler for 50–1300 MHz is a PCB structure: two parallel 50Ω microstrip traces (2.9 mm wide on 1.6 mm FR4) separated by a 0.1 mm gap and coupled over a λ/4 length at the band center. Design frequency: 700 MHz (λ/4 at 700 MHz in FR4 = 28 mm). See PCB layout guide for Gerber file dimensions.

Chapter 5 — Operating Procedures

1. Connect transmitter to IN (N-type), antenna (or dummy load) to OUT. Select HF or VHF/UHF section. Select power range (start at highest range and reduce until needle is >30% FSD).
2. Key transmitter. CYD displays forward power (W), reflected power (W), and SWR in real time. Update rate: 10 Hz.
3. For digital modes (FT8, WSPR): set display to AVERAGE mode (5-second rolling average) to see mean power through the duty cycle.
4. At end of session: record peak power, SWR, and band in operating log.

Chapter 6 — Calibration

1. Connect a calibrated RF power source (TinySA in generator mode, or signal generator with known output) to the IN port. Load OUT with a calibrated 50Ω dummy load.
2. Apply −10 dBm, 0 dBm, +10 dBm, +20 dBm at 14 MHz (HF calibration). Record ADC voltage vs. known power.
3. Fit a calibration curve (linear or polynomial) to the V_det vs. P_in data. Store coefficients in ESP32 NVS. Accuracy after calibration: ±5% over the calibrated range.
4. Repeat calibration procedure at 144 MHz for the VHF/UHF section.

Chapter 7 — Verification and Acceptance

1. Apply known 5W at 14.175 MHz. Display must show 5.0 ± 0.3W.
2. Apply known 5W at 145.0 MHz. Display must show 5.0 ± 0.3W.
3. Short OUT connector (SWR = ∞). Verify CYD shows SWR ≥99:1 and forward/reflected power are approximately equal.
4. Insertion loss verification: compare power reading with coupler in-line vs. direct connection. Difference must be <0.2 dB.
5. Log: date, calibration source, power levels checked, measured vs. expected values, operator.

Appendix A — Power / SWR Quick-Reference

Appendix B — Toroid Core Selection Guide