Merv's Brain Dump

Chapter 1 — Introduction and Scope

This manual covers construction and operation of a compact portable SWR meter covering all fifteen amateur radio bands from 160m (1.8 MHz) through 20cm (1.3 GHz). Two directional coupler sections — a tandem-match toroid coupler for HF (1.8–30 MHz) and a PCB microstrip coupler for VHF/UHF (50–1300 MHz) — sample forward and reflected power. Schottky diode detectors convert RF to DC; a meter driver circuit drives an analog panel meter (or digital display showing SWR numerically).

Chapter 2 — Theory of Operation

2-1 Tandem-Match Directional Coupler (HF)

Two toroids wound on BN-43-202 or FT-50-43 cores provide −30 dB coupling. The forward arm measures V_fwd (proportional to forward wave voltage); the reflected arm measures V_ref (proportional to reflected wave). Termination resistors (51Ω 1%) on each secondary provide the directionality; directivity is typically >35 dB across 1.8–30 MHz.

2-2 Diode Detector Circuits

1N5711 Schottky diodes detect the coupled RF. In the square-law region (V_in < 50 mV peak): V_dc ∝ P_rf. In the linear region (V_in > 100 mV): V_dc ∝ V_rf. The meter driver compensates for detector law by adjusting the meter scale or applying a lookup table in firmware.

2-3 SWR Calculation

SWR = (V_fwd + V_ref) / (V_fwd − V_ref)   [analog cross-needle meter]
SWR = (1 + sqrt(P_ref/P_fwd)) / (1 − sqrt(P_ref/P_fwd))   [digital meter]

2-4 Range Switching

Resistive dividers on the detector outputs set the effective full-scale power range. Switching from 100W to 10W range adds 10× attenuation before the panel meter, maintaining accuracy across the power range. Six ranges: 5W, 20W, 50W, 200W, 500W, 1500W (switchable by front-panel rotary).

Chapter 3 — Equipment and Materials

Chapter 4 — Construction

4-1 HF Toroid Coupler Winding

1. Wind each BN-43-202 core with 20 turns #28 AWG Teflon wire. Mark start of winding (dot convention: current into dot = positive).
2. For the forward coupler: connect winding start (dot end) to termination R1 (51Ω); winding finish to forward detector D1 (1N5711 cathode). The free R1 end connects to the center conductor bus.
3. For the reflected coupler: reverse winding connections — winding start connects to reflected detector D2; winding finish to R2 (51Ω) to center conductor bus. This reversal makes the circuit directional.
4. Thread the coax center conductor through both toroid cores (1 primary turn each) before mounting in the chassis.

4-2 Meter Driver

An LM324 quad op-amp provides two independent channels (FWD and REF) with adjustable zero and gain. Configuration: non-inverting amplifier with gain set by R_gain (start at gain = 10); output drives panel meter through a 4.7 kΩ series resistor. Adjust R_zero (10-turn pot) for zero reading at no RF input.

4-3 Calibration Circuits

A precision 100Ω trimmer pot in the FWD channel allows full-scale calibration with a known power level. A separate trimmer in the REF channel permits null balancing with a matched load (zero reflected power should give zero meter deflection on the REF meter).

Chapter 5 — Operating Procedures

1. Select HF or VHF section switch. Select power range (start high; reduce until FWD needle is at 50–80% FSD for best accuracy).
2. Connect transmitter to IN (SO-239 or N-type); antenna to OUT.
3. Key transmitter briefly (1–2 seconds). Read FWD and REF needles.
4. Compute SWR: set the FWD meter scale to its reference mark; read SWR directly from the ratio scale, or compute from the formula in §2-3.
5. For antenna tuning: adjust tuner while watching REF needle. Minimum REF deflection = maximum power transfer = lowest SWR.

Chapter 6 — Calibration

1. Connect a calibrated 50Ω dummy load to OUT. Apply known RF power (e.g., 10W at 14.175 MHz from a calibrated transmitter).
2. Adjust FWD calibration trimmer until FWD meter reads exactly full scale for the selected range (e.g., at the 10W mark on the 10W range).
3. Verify REF meter reads zero (or minimum). Adjust REF null trimmer if deflection >2% of FSD.
4. Apply a known 100Ω load (SWR 2.0:1). Verify SWR reads 2.0 ± 0.2.
5. Repeat calibration procedure for VHF section at 145 MHz.

Chapter 7 — Verification and Acceptance

1. SWR accuracy: measured SWR must be within ±0.1 SWR units for SWR <3:1; within ±10% for SWR 3:1–10:1.
2. Power accuracy: forward power reading within ±5% of actual power (verified with calibrated power reference).
3. Directivity check: with 50Ω load on OUT, REF deflection must be <2% of FWD deflection at full power (indicates ≥34 dB directivity).
4. Insertion loss: <0.1 dB at 1.8–30 MHz; <0.2 dB at 50–150 MHz; <0.5 dB at 150–450 MHz.
5. Log: date, calibration load used, forward calibration power, SWR accuracy check results, insertion loss, operator.

Appendix A — Toroid Core Selection

Appendix B — Worked SWR Example

FWD meter reads 8.5 divisions on 10W scale = 8.5W. REF meter reads 0.85 divisions on 10W scale = 0.85W.

|Γ| = sqrt(0.85 / 8.5) = sqrt(0.1) = 0.316
SWR = (1 + 0.316) / (1 − 0.316) = 1.316 / 0.684 = 1.92:1

Acceptable for most antenna systems; fine-tune if SWR >2.0:1 desired.