Merv's Brain Dump

Chapter 1 — Introduction and Scope

This manual covers RFI (radio frequency interference) mitigation for amateur stations: active noise cancelling (phased auxiliary antenna nulling), common-mode chokes (see also TM-GEAR-007), a near-field RFI sniffer probe, and an ESP32-based noise monitor with CYD waterfall display for continuous noise floor tracking. The active noise canceller can suppress local noise sources by 20–40 dB, dramatically improving weak-signal copy.

Chapter 2 — Theory of Operation

2-1 Active Noise Canceller Principle

A reference (auxiliary) antenna is aimed at the local noise source. Its output (mostly noise) is phase-shifted and amplitude-adjusted to match the noise component in the main antenna signal. When subtracted from the main signal, the noise cancels while the desired signal (from a distant direction) is preserved:

V_main = V_signal + V_noise_main
V_ref  = V_noise_ref  (negligible V_signal in reference direction)
V_out  = V_main − A × e^(jφ) × V_ref ≈ V_signal

The phase (φ) and amplitude (A) are adjusted manually or automatically until the noise is minimized. Practical cancellation: 20–40 dB, limited by noise source geometry and antenna placement.

2-2 Near-Field Sniffer Probe

A small shielded loop (30–50 mm diameter) coupled to a SMA connector is held near suspected RFI sources. The loop responds to the magnetic component of the near-field. Connected to an SDR or TinySA, the probe identifies the source and its harmonic spectrum, enabling targeted mitigation (ferrite beads, cable rerouting, shielding).

Chapter 3 — Equipment and Materials

Chapter 4 — Construction

4-1 Active Noise Canceller

1. Build the ERA-3SM preamplifier: MMIC in SOT-143 package, bias via 130Ω resistor from +9V, input/output 100 pF DC-block caps.
2. Build the phase shifter: an all-pass filter network using a varactor diode (BB515 or SMV1248) whose capacitance is set by a 0–10V control voltage. This provides 0–360° phase range across the HF band.
3. Build the subtractor: TL072 dual op-amp configured as a differential amplifier. Gain set by 10 kΩ / 10 kΩ resistors (gain = 1). Apply main signal to non-inverting input; reference signal (after phase shift and attenuation) to inverting input.

4-2 Near-Field Sniffer Probe

1. Wind a 3-turn loop of RG-174 coax, 35 mm diameter. Connect the shield at both ends to the outer conductor of the SMA connector; connect the center conductor at one end only to the SMA center pin. (The shield gap should be at the midpoint of the loop to form a Faraday-shielded loop; solder a jumper to close the shield everywhere except the deliberate gap.)

Chapter 5 — Operating Procedures

5-1 Noise Canceller Adjustment

1. Position the reference antenna to maximize noise pickup relative to signal: point it at the noise source (if known) or orient it to maximize S-meter noise level.
2. Enable the canceller. Adjust phase (φ) slowly through 0–360° while monitoring the noise level (S-meter or audio). A strong null will appear at the correct phase setting.
3. At the null, adjust amplitude (attenuation) for the deepest null. Alternate between phase and amplitude adjustments; a few iterations converge to the optimal setting.
4. Save settings for later use. Note: the optimal settings change if the noise source moves or changes character. Re-adjust if noise returns.

5-2 RFI Source Location with Sniffer Probe

1. Connect sniffer probe to TinySA. Set span to 1–30 MHz with 10 kHz RBW. Look for peaks that correlate with the interference.
2. Move probe near suspected sources (switching power supplies, LED drivers, computer power bricks, solar charge controllers) until the signal peaks.
3. Identify the interference frequency and its harmonics. If the fundamental is a known utility frequency (switching PSU at 65 kHz, LED driver at 120 Hz), that identifies the source.

Chapter 6 — Calibration

1. Noise canceller: verify that the reference signal is at least 10 dB above the main antenna noise level. If the reference is too weak, the canceller cannot achieve a deep null. Add preamp gain or move the reference antenna closer to the noise source.
2. Noise monitor: calibrate the noise floor baseline by running 24 hours of background measurements at night when local noise is minimal. Save the baseline to SD card; any future increase indicates a new noise source.

Chapter 7 — Verification and Acceptance

1. Noise canceller: achieves at least 15 dB noise reduction on a known local interference source (verified by S-meter before and after).
2. Sniffer probe: detects a 5 mW 14 MHz test signal from a 1 cm loop placed 50 mm from the probe (sensitivity check).
3. Noise monitor: generates a waterfall display with no dropouts for 24 hours of continuous operation.
4. Log: date, noise source identified and mitigated, dB reduction achieved, mitigation method (CMC, canceller, shielding), operator.

Appendix A — Common RFI Sources

Appendix B — CMC Placement Priority Order

1. Antenna feedpoint (always first)
2. Shack entry panel (coax entry)
3. All AC power cords within the shack
4. USB and serial cables to peripherals
5. Audio cables between transceiver and computer