Merv's Brain Dump

Chapter 1 — Introduction and Scope

This manual covers three directional coupler designs for sampling forward and reflected RF power: the toroidal Bruene coupler (HF 1.8–30 MHz), the transmission-line directional coupler (1–600 MHz), and the resistive bridge coupler (wideband, any frequency, low power only). All produce separate forward (VFWD) and reflected (VREF) DC outputs that drive SWR meters, power meters, or an ADC.

Chapter 2 — Theory of Operation

2-1 Bruene Toroidal Coupler

A ferrite toroid transformer (T1) samples the current in the transmission line (1-turn primary = center conductor). A separate voltage sampling network monitors the line voltage. Combining current and voltage in the correct phase relationship produces the forward and reflected wave voltages separately. Directivity is the ratio of the response to the desired direction vs. the undesired: ≥40 dB with high-precision resistors.

2-2 Transmission-Line Coupler

Two parallel transmission lines run close together for λ/4 at the design frequency. Electromagnetic coupling transfers a small fraction of power to the secondary line. Coupling factor C (−dB) depends on line spacing and length: C = −20 log10(k) where k is the voltage coupling coefficient. Typical: −20 to −30 dB. Used in the SWR meter and RF power meter designs.

2-3 Resistive Bridge Coupler

A 4-resistor Wheatstone bridge measures the complex reflection coefficient directly. One arm is the antenna under test; the opposite arm is the reference (50Ω). At balance (antenna = 50Ω), the bridge output is zero. Imbalance is proportional to (Z_ant − 50) / (Z_ant + 50) = Γ. Limited to low power (<1W); resistors must be non-inductive and matched.

Chapter 3 — Equipment and Materials

Chapter 4 — Construction

4-1 Bruene Coupler Winding

1. Wind 20 turns of #28 AWG Teflon wire on a BN-43-202 core. This is the voltage sampling secondary; the center conductor of the transmission line threads through the core once as the 1-turn primary.
2. Connect resistors R1 and R2 (51Ω 1%) from each end of the secondary to the common center conductor (ground bus). The midpoints of R1 and R2 are the detector nodes (VFWD and VREF before the diodes).
3. Mount the detector diodes (1N5711 cathode toward the detector node) and filter capacitors on a small PCB within 20 mm of the toroid.
4. Thread the SO-239 center conductor through the toroid. Keep the center conductor straight through the toroid bore (not coiled) to maintain transmission line continuity.

Chapter 5 — Operating Procedures

Directional couplers are passive devices; no operational procedure is required beyond installation. Install the coupler inline between the transmitter and the antenna (or load under test). Connect VFWD and VREF outputs to the meter or ADC inputs.

• Never exceed the power rating of the coupler (determined by the center conductor rating and connector type).
• Do not operate with the VFWD or VREF ports open (floating) — load them with ≥50 kΩ (meter input) to prevent undefined voltage states.

Chapter 6 — Calibration

1. Connect a calibrated 50Ω load to the output port. Apply 10W at 14.175 MHz. VFWD must read a consistent voltage; VREF must read <1% of VFWD (indicating <−40 dB directivity).
2. Connect a short circuit to the output port. VREF must equal VFWD (SWR = ∞; 100% reflection). Directivity check: VFWD at short circuit should match VFWD at 50Ω load (forward response should not change with load — it measures only forward power).
3. Measure directivity: directivity (dB) = 20 log10(VFWD_50 / VREF_50). Target: ≥30 dB.

Chapter 7 — Verification and Acceptance

1. Directivity ≥30 dB at all operating frequencies.
2. Insertion loss <0.2 dB (measure S21 through the coupler with NanoVNA).
3. SWR at input port <1.05:1 with a 50Ω load on the output (the coupler must not introduce a mismatch).
4. VFWD response: flat within ±1 dB across the specified frequency range (the coupler must sample equally at all frequencies).
5. Log: date, coupler type, directivity at 7 and 14 MHz, insertion loss, input SWR, operator.

Appendix A — Coupling Factor vs. Secondary Turns

Bruene toroidal coupler, BN-43-202:
  Coupling factor C (dB) = -20 log10(N2)
  N2 = 10 turns: C = -20 dB
  N2 = 20 turns: C = -26 dB  (standard design)
  N2 = 30 turns: C = -30 dB

More turns = weaker coupling = smaller VFWD/VREF output voltage.
For a 1W signal into 50Ω (V_line = sqrt(50) = 7.07V pk):
  At N2=20: V_coupled = 7.07 / 20 = 0.35V pk → after diode: ~0.15V DC

Appendix B — Return Loss and SWR Conversions