TECHNICAL MANUAL

TM-RFI-001 Rev A

RADIO FREQUENCY INTERFERENCE (RFI) AND QRM MITIGATION

Complete Home Station Noise Reduction System

DOCUMENT NUMBER: TM-RFI-001 Rev A
DATE: 2 May 2026
SUPERSEDES: None (initial issue)
APPLICABLE TO: All ham shack installations; residential environment; co-location with computers, peripherals, LED lighting, and household appliances.

TABLE OF CONTENTS

CHAPTER 1 — FUNDAMENTALS OF RFI AND QRM

1.1 Definitions

RFI (Radio Frequency Interference): Any RF signal that degrades the quality of a desired radio communication. RFI may originate from intentional transmitters (legal or illegal) or from unintentional emitters (electronic devices).

QRM: Amateur radio shorthand for manmade interference. Specifically denotes non-atmospheric interference on a radio channel. Used to distinguish from QRN (atmospheric noise, static) and QSB (fading).

Noise floor: The lowest signal level readable on a receiver. Determined by thermal noise (unavoidable) plus all received manmade noise (RFI). Lower noise floor = greater sensitivity = better weak-signal capability.

S-meter scale (ITU standard): Each S-unit = 6 dB. S9 = −73 dBm at HF. S0 = −121 dBm (below noise floor of most receivers) S1 = −115 dBm S5 = −91 dBm S9 = −73 dBm S9+10 = −63 dBm S9+20 = −53 dBm S9+60 = −13 dBm (very strong — possible damage threshold)

A noise floor of S7 (−79 dBm) means: any signal below S7 is buried in noise. Reducing noise floor to S1 (−115 dBm) improves sensitivity by 6 S-units = 36 dB. This is the difference between hearing a DX station and not hearing it.

1.2 Types of RF Noise

Broadband noise: Continuous spectral content across a wide frequency range. Source: switching power supplies, motor brushes, SMPS inverters. Signature on SDR: raised noise floor across entire panadapter display.

Narrowband interference: Strong signal on one or a few specific frequencies. Source: oscillators (CPU clocks, PWM controllers), LO leakage, harmonics. Signature: one or more discrete “birdies” (narrow spikes) on panadapter.

Periodic impulsive noise: Repeated bursts at a regular interval. Source: microcontroller timers, PWM dimmer switching, motor commutation. Signature: S-meter twitches rhythmically; audio has periodic “buzz” or “crackle.”

Conducted interference: Noise travels via electrical conductors (mains wiring, signal cables, coax braid). Enters shack via AC power line or coax feed line.

Radiated interference: Noise radiates as electromagnetic waves through space. Enters shack via antenna, or couples directly to equipment through proximity.

1.3 Noise Figure, Noise Temperature, and MDS

Thermal noise power at temperature T in bandwidth B: P_noise = k × T × B = 1.38×10⁻²³ × 290 × B = 4.0×10⁻²¹ × B (watts) = −174 dBm/Hz + 10×log10(B)

At B = 2400 Hz (SSB bandwidth): P_noise = −174 + 33.8 = −140.2 dBm At B = 500 Hz (CW): P_noise = −147 dBm

Minimum Detectable Signal (MDS): P_noise + receiver NF Good HF receiver (NF = 10 dB), SSB BW: MDS = −130 dBm = S0−9 dB SDR with LNA (NF = 3 dB), SSB BW: MDS = −137 dBm

Effect of RFI on MDS: If RFI raises the noise floor by 20 dB (S4 to S8), then MDS worsens by 20 dB. You have lost all sensitivity advantage of a good receiver/antenna combination.

Conclusion: Noise mitigation is as important as transmit power and antenna gain. Reducing noise by 10 dB is equivalent to doubling transmit power of the other station.

CHAPTER 2 — RFI SOURCES IN THE MODERN HOME

2.1 Survey of Common Home RFI Sources

The following categories cover the vast majority of residential RFI:

Category 1: Switching Power Supplies (SMPS) Every wall wart, laptop charger, phone charger, and LED power supply contains a switching regulator operating at 50 kHz–3 MHz. The switching creates harmonics extending into the HF amateur bands.

Category 2: LED Lighting Modern LED bulbs use switching drivers (same as SMPS). Additional noise from PWM dimming. Some LED drivers intentionally spread-spectrum their switching frequency to pass EMC testing — this creates broadband hash rather than discrete harmonics.

Particularly troublesome: LED strip lights with cheaply made constant-current drivers. Cheap LED “corn bulb” replacements: notorious for severe HF interference.

Category 3: Computers and Peripherals - USB 3.0 generates a strong noise signature at 2.5 GHz and harmonics down to HF - HDMI cables radiate 165 MHz carrier and harmonics (affects VHF bands) - Ethernet switches: broadband digital noise - CPU core voltage regulators: switch at 200–500 kHz × number of phases - GPU power regulators: especially during high GPU load

Category 4: Smart Devices and Wireless - Smart meters: power line carrier (30–500 kHz) + ZigBee (2.4 GHz) - WiFi routers: 2.4 GHz and 5 GHz broadband noise - Baby monitors (older 49 MHz models): directly in ham bands - Baby monitors (DECT 6.0): 1.9 GHz but local desensitization - Plasma TVs (older): severe HF noise; modern OLED/LCD much better - Touchscreen dimmer switches: often generate significant QRM

Category 5: Appliances and Mechanical - Refrigerator compressor: impulsive noise on startup and stop; AC induction motor - Vacuum cleaner: universal motor (brushes = arcing = broadband noise) - Dimmer switches (TRIAC): generate harmonics of power line frequency - Electric blankets / heating pads: similar to TRIAC dimmers - Fluorescent lights (magnetic ballast): 50/60 Hz and harmonics - CFL bulbs: switching power supply noise + ballast harmonics

2.2 Identifying Your Specific Noise Sources

Every home is different. Systematic identification is required before mitigation.

STEP 1: Baseline measurement. Note noise floor on each ham band with all shack equipment on, nothing else running. Use SDR noise floor monitor (Chapter 9) or S-meter plus careful note-taking.

STEP 2: Circuit breaker test. Turn off all circuit breakers except shack power. Re-measure. Reduction = mains-conducted noise from rest of house. Progressive re-energization of circuits identifies which circuit(s) contribute most.

STEP 3: Device elimination. With individual circuit identified, walk through the circuit unplugging devices one at a time. When noise floor drops: that device is the culprit.

STEP 4: Near-field probe confirmation. Use H-field sniffer (Chapter 8) near confirmed device. Verify S-meter peaks near the device.

STEP 5: Coupling path identification. Once source identified, determine HOW the noise reaches your antenna: - Conducted via mains: mains filter at shack entry will help - Conducted via coax braid: feed line choke at antenna will help - Radiated direct to antenna: source must be physically suppressed

CHAPTER 3 — CONDUCTED vs. RADIATED INTERFERENCE

3.1 Conducted Path

Noise travels through electrical conductors. Main conducted paths:

AC mains wiring: The mains power network is a giant antenna. Switching supply noise generated by any device on the building’s wiring appears on all outlets. A SMPS in one room conducts noise to your shack’s power outlets.

Mitigations: - AC mains EMI filter at shack power entry (Chapter 5) - Ferrite cores on offending device’s power cord - Separate circuit for shack (dedicated 20A circuit from panel) - Replace offending device with low-noise alternative

Coaxial feed line braid: See Chapter 4 (common-mode current) for the detailed treatment. This is often the dominant path for HF noise.

Signal cables (audio, USB, Ethernet): Noise coupling via signal reference ground. Use shielded cables. Install ferrite clamps on both ends.

3.2 Radiated Path

Noise travels through space as electromagnetic radiation. Directly couples to antenna.

Near-field coupling: Distance < λ/2π ≈ λ/6. Fields fall off rapidly (1/r² or 1/r³). At 7 MHz, near-field region extends to λ/6 = 7.1m. Sources within 7m couple strongly.

Far-field: Distance > λ. Fields fall off as 1/r. Neighbor’s equipment can couple at far field if strong enough.

Mitigations for radiated path: - Physically separate antenna from noise sources (height, distance) - Use directional antenna to null toward noise source - Active noise cancellation (Chapter 7) - Shielding the noise source itself (if possible/legal)

3.3 Which Path Dominates?

Test: Disconnect antenna. Does noise floor drop? - YES: noise primarily enters via antenna/feed line (radiated or common-mode conducted) - NO: noise enters via mains or ground (conducted via wiring)

Test: Connect dummy load instead of antenna. Does noise floor drop significantly? - YES: noise primarily via antenna - NO: noise is generated internally in the receiver or enters via power supply

CHAPTER 4 — COMMON-MODE CURRENT AND FEED LINE NOISE

4.1 Why Coax Braid Carries Noise

A coaxial cable has two electrical modes: 1. Differential mode: Current flows forward on center conductor; return on inner surface of braid. Signal energy stays INSIDE the coax. 2. Common-mode: Current flows in the SAME direction on both conductors (like a single wire). This current flows on the OUTSIDE surface of the braid.

Common-mode current arises from: - Ground potential difference: The antenna feedpoint ground and the shack ground are at different RF potentials. This drives current on the outside of the braid. - Station RF: During transmit, current returns to station through the braid exterior instead of through the antenna radials/ground. - External noise coupling: The entire coax exterior acts as a noise receiving antenna. Every noise source with a ground path can drive current on the braid.

4.2 Effect on Reception

The coax braid exterior is typically 10–30 meters long (antenna mast height + shack run). At 7 MHz, λ = 42m; 20m of coax braid = λ/2 — a resonant antenna!

Noise current on the braid creates a received noise signal indistinguishable from a signal arriving at the antenna. The NEC model (nec_models/dipole_cm_current.nec) demonstrates how 30% common-mode current on a 3m stub distorts the dipole pattern and fills in nulls — dramatically increasing noise pick-up.

Quantitative effect: A well-measured example: - Before choke at antenna: noise floor S7 on 40m - After choke at antenna (FT-240-31 × 2, 8 turns): noise floor S3 - Improvement: 4 S-units = 24 dB

This 24 dB improvement represents the noise that was being injected via the coax braid.

4.3 Choking the Common Mode

A common-mode choke (see Chapter 6) presents high impedance to the common-mode current path while being transparent to the differential signal inside the coax.

Required choke impedance for good suppression: Suppression (dB) = 20 × log10(Z_CM / Z_load) For 20 dB: Z_CM / Z_load = 10, so Z_CM = 500Ω (into 50Ω load) For 30 dB: Z_CM = 1580Ω For 40 dB: Z_CM = 5000Ω

A 2× FT-240-31, 8-turn choke provides Z_CM > 10,000Ω at 7 MHz (see Chapter 6). This gives theoretical suppression > 46 dB — excellent.

4.4 Choke Placement Strategy

Primary choke: At the antenna feed point. This is the highest-leverage placement. - Prevents house noise from traveling up the braid - Prevents transmitter RF from returning on the braid - Forces the antenna feed impedance to be purely differential (balanced)

Secondary choke: At the shack entry point (where coax comes through the wall). - Second line of defense for any residual common-mode current - Particularly important if the outside coax run is in conduit with other wiring

Tertiary (optional): On the shack side, at the back of the transceiver. - Protects the transceiver from RFI that enters after the shack entry choke - Useful if the problem is computer noise coupling to coax via RF ground loops

4.5 Balanced Antennas

A truly balanced antenna (center-fed dipole with tuned feedline) inherently has less common-mode current than an unbalanced antenna (vertical, end-fed):

Dipole: Feed point is balanced. With a choke balun at the feed point, near-zero common-mode current. This is the quietest antenna configuration.

Vertical / end-fed: Inherently unbalanced. Common-mode current is normal operation — the braid is the radial system. MUCH more noise pickup. For these antennas, a choke at the shack entry is especially important.

EFHW (end-fed half-wave): Despite high-impedance match transformer, common-mode is significant. Install 1:49 UNUN + choke at shack entry.

CHAPTER 5 — AC MAINS EMI FILTER

5.1 Why Filter at the Mains Entry

The AC mains wiring in a house acts as a distributed antenna — any RF energy on the wiring radiates and couples to nearby equipment. By filtering the AC power at the point where it enters the shack, you: 1. Block conducted noise from reaching shack equipment 2. Block transmitter harmonics from entering the mains and radiating back into your antenna 3. Provide a clean reference ground for the shack

5.2 Filter Design

See schematic TM-RFI-SCH-001 for full circuit detail.

Differential-mode (DM) filter: X2 capacitors across Line-Neutral and series inductors on each conductor. Attenuates noise between the two conductors.

Common-mode (CM) filter: Y2 capacitors from Line and Neutral to Earth Ground, plus a bifilar common-mode choke (CM inductor). Attenuates noise common to both conductors relative to earth.

MOV transient suppressor: Absorbs lightning-induced spikes. Required in any outdoor antenna installation environment.

5.3 Installation

WARNING: Mains voltage installation must comply with NEC (USA), CEC (Canada), or applicable local electrical codes. If unsure, consult a licensed electrician.

1. Install filter in a suitable enclosure (printed enclosure TM-RFI-ENC-001 or commercial metal project box).
2. Wire IEC C14 inlet to filter input; filter output to multiple IEC C13 outlets for shack equipment.
3. Earth ground: #12 AWG minimum from filter chassis ground terminal to building grounding electrode conductor at the main panel or to a local ground rod bonded to the main ground.
4. Place shack power distribution box on the bench; plug all shack equipment into it.

5.4 Safety Requirements

• All mains connections: minimum 14 AWG wire with appropriate ratings.
• X2 capacitors: rated specifically for across-the-line use. Do not substitute.
• Y2 capacitors: rated for Y2 service. Do not substitute general-purpose caps.
• Fuse: slow-blow, properly rated for the load (10A for typical shack).
• Earth ground: mandatory. The filter is ineffective and potentially dangerous without a solid earth ground connection.

CHAPTER 6 — COMMON-MODE CHOKES: FERRITE SELECTION AND WINDING

6.1 Ferrite Material Properties

Ferrite cores work by absorbing (dissipating) RF energy in the magnetic material. The key parameter is complex permeability: µ = µ’ − j×µ’‘. The µ’’ (imaginary, loss) component is what we want — high µ’’ = high absorption at target frequency.

Select by frequency: | Mix | Best frequency range | Application | |—–|———————|————-| | 31 | 1–30 MHz (best 1–10 MHz) | HF feed line chokes | | 43 | 1–300 MHz (best 10–50 MHz) | HF + low VHF | | 61 | 10–300 MHz | Upper HF + VHF | | 75 | 0.1–5 MHz | AM broadcast suppression | | 77 | 0.1–5 MHz | Low-frequency applications |

For most ham station use: Mix 31 is the right choice. Stacking two FT-240-31 cores covers 160m through 10m with excellent suppression.

6.2 Core Size Selection

FT-140-31 (1.4” OD): Fits up to RG-58 (5mm OD). Limited turns. FT-240-31 (2.4” OD): Fits RG-8X (7.5mm OD) for 8 turns. Standard choice. FT-240-43: Same size, Mix 43. Use when noise is above 30 MHz.

For RG-213 (10.3mm OD), FT-240 hole (25.4mm) allows 5–6 turns. Acceptable. For RG-8 (10.3mm OD): same as RG-213. Use Palomar FB-73-6401 (larger, 38mm hole) for 8 turns of large coax.

6.3 Winding Procedure for Coax Choke

See schematic TM-RFI-SCH-002 for detailed winding tables.

For FT-240-31 × 2 stacked, RG-8X, 8 turns:

1. Tape the two FT-240-31 cores together with 2–3 layers of Kapton tape.
2. Mark the center of a 2m piece of RG-8X coax with a marker.
3. Place the stacked cores on the coax, centered.
4. Make the first pass: bring both sides of coax through the hole together.
5. Continue looping the coax through the core hole, maintaining neat, tight turns.
6. Count 8 complete passes through the center hole.
7. Secure loose turns with cable ties or electrical tape.
8. Install in choke housing (TM-RFI-ENC-002).
9. Fit PL-259 connectors on both ends.
10. Test impedance with NanoVNA (see Section 6.4).

Verification with NanoVNA: - Connect Port 1 to one end of choke. - Terminate other end in 50Ω (or leave open for impedance measurement). - Measure S11 (reflection): at each frequency, observe impedance. - At 7 MHz: |Z| should exceed 10 kΩ (confirms effective choking).

6.4 Choke Quality Factor Expectations

A well-wound choke should meet these minimums:

If measured values fall short: verify correct core mix (stamped on core or check ferrite color); verify adequate number of turns; check for turn-to-turn shorts.

6.5 Feed Line Choke for Specific Antenna Types

Center-fed dipole: One choke at the feed point. Can use wound coax on form or toroid choke. For simple installation, the wound coax (“ugly balun”) is easiest.

Vertical with radials: Two chokes recommended: one at the base (feed point) and one at the shack entry. The feed point choke prevents common-mode current from flowing down the outside of the coax; the shack entry choke blocks any that remains.

End-fed half-wave: Three chokes: at the UNUN/transformer output, at the shack entry, and optionally one midway on long coax runs.

Inverted-L or T-antenna: At least shack entry choke; feed point choke if transmission line is coax; otherwise use a proper ATU with balanced output.

CHAPTER 7 — ACTIVE NOISE CANCELLATION

7.1 Principle

The active noise canceller uses a reference (auxiliary) antenna positioned near the noise source to sample the interference. The reference signal is phase-shifted and amplitude-adjusted to match the noise component in the main antenna, then subtracted.

This technique is highly effective when: - The noise source is nearby (strong reference signal) - The desired signal arrives from a different direction than the noise - The noise is coherent (stable phase and amplitude) — most SMPS noise is coherent

Limitations: - Only one noise direction can be cancelled at a time - Phase adjustment must be re-tuned when frequency changes significantly - Desired signal is slightly attenuated (~3 dB) in the cancel direction

7.2 Reference Antenna Selection

The reference antenna should pick up maximum noise with minimum desired signal:

Short vertical whip (1–2m): Best for noise arriving from high angles or nearby horizontal sources. Place near the noise source, away from the main antenna direction.

Small loop: Aimed at the noise source. The null can be directed away from the desired signal direction for best signal-to-noise cancellation ratio.

Sense antenna (existing): If you already have a small loop RDF antenna, it can serve as the reference antenna for noise cancelling.

7.3 Adjustment Procedure

See schematic TM-RFI-SCH-003 for circuit details.

1. Connect main antenna to MAIN port (from feed line).
2. Connect reference antenna (short whip, aimed at noise source) to REF port.
3. Connect output to SDR or transceiver antenna input.
4. Tune to noise frequency. Maximize SDR display on noise birdie.
5. Note reference level: Is there signal on the reference port? Enable bypass momentarily to confirm reference picks up the noise.
6. Phase adjustment:
   • Slowly rotate RV1 (Phase 1) through full rotation.
   • Observe S-meter for dip. Stop near minimum.
   • Slowly rotate RV2 (Phase 2) through full rotation.
   • Observe deeper dip. Stop at minimum.
7. Gain adjustment:
   • Rotate RV3 (Gain) for deepest null.
   • The null deepens as gain approaches the correct setting; then rises again.
   • Set gain at deepest point.
8. Iterate: Phase 1 → Phase 2 → Gain, repeat until null bottoms out.
9. Check signal preservation: Tune to a known signal. It should be present (possibly slightly weaker). If signal is cancelled along with noise, the reference antenna is pointed at the signal direction — reposition reference.

Expected cancellation: 20–40 dB of noise reduction. Under ideal conditions, a single null can achieve >50 dB cancellation (theoretically infinite). In practice, noise source motion, multipath, and component drift limit practical null depth.

7.4 Digital Noise Cancellation Software

For SDR software-based noise cancellation:

GQRX DSP options: - Noise Blanker (NB): impulsive noise (clicks, arc discharge) - Noise Reduction (NR): LMS adaptive filter for continuous noise - AGC: prevents noise bursts from desensitizing receiver

SDR++ plugins: - Noise blanker plugin - Audio NR via rtty/voice processing

GNU Radio: Implement LMS adaptive filter flow graph: Reference signal → LMS filter (tap weight adaptation) LMS output subtracted from main signal → cleaned output Filter adapts to minimize residual noise power.

CHAPTER 8 — NEAR-FIELD RFI SNIFFER PROBES

8.1 Overview

Near-field probes allow locating noise sources to within centimeters. They exploit the rapid fall-off of near-field electromagnetic fields (1/r² to 1/r³) to distinguish the primary source from secondary radiators.

See schematic TM-RFI-SCH-004 and enclosure TM-RFI-ENC-004 for construction details.

8.2 H-Field Loop Probe Use

The H-field loop responds to magnetic field (current flow). Use it to find: - Switching inductor cores in SMPS - Transformer windings - PCB traces carrying high-frequency switching current - CPU voltage regulator inductors

Usage: 1. Connect probe to SDR via 20 dB attenuator (prevents overload near strong sources). 2. Tune SDR to noise frequency. 3. Hold probe near suspect device. 4. Move slowly across device surface — S-meter peaks over switching inductors. 5. Rotate probe: null (minimum) when loop plane points toward source. Peak when loop plane is parallel to current direction. 6. Most sensitive position: loop plane parallel to PCB with inductor on PCB.

8.3 E-Field Rod Probe Use

The E-field rod responds to electric field (voltage switching). Use it to find: - High-voltage switching nodes (drain of FET in SMPS) - PWM control signals - Clock outputs on PCBs - Any high-impedance rapidly-switching node

Usage: 1. ALWAYS use 20 dB attenuator — E-field near a SMPS drain can be hundreds of volts! 2. Tune SDR to noise fundamental frequency. 3. Touch probe tip toward PCB surface, 1–2cm distance. 4. Traverse PCB — peaks over switching transistor drain pins, gate drive traces. 5. Work from device exterior inward to identify highest-field location.

8.4 Probe Safety

• Never touch probe tip to live mains-voltage conductors.
• Use probe only on low-voltage (<50V) circuit boards or on the OUTSIDE of appliances.
• For investigating mains-powered devices: probe through plastic case only.
• If device case can be opened: unplug the device first, discharge capacitors, then investigate with device powered via isolation transformer.

CHAPTER 9 — ESP32 AUTOMATED NOISE FLOOR MONITOR

9.1 Overview

The automated noise floor monitor (TM-RFI-FW-001) scans all amateur bands every ~22 seconds and logs the noise floor level. This enables:

• Trending: Is the noise floor rising over time? Indicates developing equipment fault.
• Diurnal patterns: Higher noise during day (appliances on) vs. night (appliances off).
• Appliance correlation: Note timestamp of noise rise; correlate with when devices were turned on.
• Mitigation verification: Log before and after installing a choke or filter. The improvement is precisely documented.

See schematics TM-RFI-SCH-005 for hardware, firmware TM-RFI-FW-001 for software.

9.2 Hardware Setup

Components required: - ESP32 DevKit V1 - Si5351 breakout board (Adafruit or bare chip with 25 MHz crystal) - SA612AN mixer (DIP-8) - SPF5189Z LNA (SOT-89-3, on breakout or hand-soldered) - AD8307 (SOIC-8 or DIP-8) - 10.7 MHz ceramic BPF (Murata CFWLA10M7KA0-B0) - Input LPF components (220 nH × 2, 68/120 pF capacitors) - 3.3V regulator and decoupling - Optional: SD card module for data logging

Antenna: A 20cm non-resonant vertical wire is adequate for noise monitoring (the monitor does not need gain — it measures noise, not weak signals). A small loop (~100mm) is quieter (less susceptible to electric field noise).

9.3 Web Dashboard

Connect to WiFi AP “RFI-Monitor” (password: rfimonitor), then browse to http://192.168.4.1. The dashboard shows:

• Band grid: 11 cards (160m–2m), each showing current noise level in dBm with color coding (green/yellow/orange/red) and a mini bar.
• History chart: Rolling time-series plot, all bands overlaid, 40 data points.
• Alert threshold: Adjustable — set to your typical noise floor baseline. Any band exceeding threshold shows a red alert indicator.
• Export CSV: Downloads current snapshot as CSV for offline analysis.

9.4 Interpreting Results

Normal noise floor ranges (typical suburban/rural ham station): | Band | Quiet (rural) | Typical suburban | High urban noise | |——|————–|—————–|—————–| | 160m | −110 dBm | −90 dBm | −70 dBm | | 80m | −115 dBm | −100 dBm | −85 dBm | | 40m | −120 dBm | −105 dBm | −90 dBm | | 20m | −120 dBm | −110 dBm | −95 dBm | | 10m | −125 dBm | −115 dBm | −105 dBm | | 6m | −125 dBm | −120 dBm | −110 dBm |

If your measured levels are significantly higher than “typical suburban,” significant noise reduction is achievable.

CHAPTER 10 — CYD DISPLAY FOR NOISE MONITORING

10.1 Overview

The CYD (ESP32-2432S028R) touchscreen provides a portable, always-on noise floor display. It connects via Bluetooth to the ESP32 noise monitor and shows:

• Per-band noise level in dBm (11 bands simultaneously)
• Color-coded bars: green (quiet) → yellow → orange → red (noisy/alert)
• Mini sparklines per band (40-point history)
• Alert indicators when any band exceeds threshold
• Touch control of alert threshold

10.2 TFT_eSPI Configuration

The CYD uses a specific pin mapping. Ensure User_Setup.h in TFT_eSPI library:

#define USER_SETUP_ID 206
#define ILI9341_DRIVER
#define TFT_MOSI 13
#define TFT_SCLK 14
#define TFT_CS   15
#define TFT_DC    2
#define TFT_RST  -1
#define TFT_BL   21
#define TOUCH_CS 33

10.3 Display Layout

Top row: Title “RF Noise Monitor” + Bluetooth connection indicator + scan count.

Main area: 2-column × 6-row grid of band cards. Each card shows: - Band name (e.g., “40M”) in cyan - Current noise level in dBm - Color-coded fill bar (proportion of −40 to −120 dBm range) - Mini sparkline (40-point history)

Alert row: Appears when any band exceeds threshold. Lists affected bands.

Status bar: Connection state, threshold value, web UI hint.

Touch controls: - Tap left half of status bar: decrease threshold by 5 dBm - Tap right half of status bar: increase threshold by 5 dBm

CHAPTER 11 — GROUNDING, BONDING, AND SINGLE-POINT GROUND SYSTEMS

11.1 The Single-Point Ground (SPG) Principle

Problem with multiple ground connections: If the shack’s coax shields, equipment chassis, and power ground connect to earth at multiple points, ground loops form. RF current flowing between ground points is indistinguishable from antenna signal.

Solution: All RF ground connections should converge at one point — the Single-Point Ground (SPG). From the SPG, a single large conductor bonds to earth ground.

11.2 SPG Implementation

SPG bus: A copper busbar (25mm × 3mm copper strip or braid), typically mounted at the entry point of feed lines through the shack wall.

Connected to SPG: - All coax shield ground connections - All equipment chassis ground connections - Lightning arrestor grounds - Any bonded conduit entry

From SPG to earth: Single conductor, minimum #6 AWG bare copper (or larger). Shortest possible path to earth electrode. Avoid sharp bends (inductance).

11.3 Feed Line Entry Bonding

The NEC (National Electrical Code) and good amateur practice both require all antenna feed lines to have a grounding/lightning arrestor at the building entry point.

Recommended entry panel: 1. Mount an outdoor-rated bulkhead panel at the point where coax enters the building. 2. Install coaxial lightning arrestors (Polyphaser, ICE, or similar) on each feed line. 3. Bond all lightning arrestor chassis to the SPG busbar. 4. Bond SPG busbar to building ground electrode with #6 AWG or better.

11.4 Equipment Grounding Inside Shack

Star topology: All equipment chassis connect via short conductors to the SPG bus. Do NOT daisy-chain grounds (A → B → C → SPG creates ground loops A-B and B-C).

Practical implementation: - Short (#14 AWG) pigtails from each equipment chassis to SPG bus - SPG bus directly to earth ground via the entry panel

11.5 RF Ground Loops

An RF ground loop forms when two pieces of equipment are grounded via the signal cable connection AND separately via their chassis/power ground paths. The loop area acts as a receiving antenna for magnetic fields (magnetic flux through the loop).

Identify loops: Use H-field probe near connecting cables. If S-meter peaks when probe is near a signal cable (USB, audio, HDMI), there is a ground loop in that cable.

Break the loop: Ferrite clamp on the cable (both ends) adds CM impedance and disrupts the loop. For audio cables: use a transformer-coupled audio isolator (Hum-X or Jensen ISO-MAX). For USB: use USB isolator (ADUM4160).

CHAPTER 12 — SYSTEMATIC RFI HUNT: FIELD PROCEDURES

12.1 Investigation Protocol

Phase 1: Documentation - Record baseline noise floor (all bands, time of day, day of week). - Use noise monitor to generate 24-hour trend (if possible). - Note any temporal correlation with known appliance schedules.

Phase 2: Source Localization 1. Circuit breaker test: identify which branch circuit carries the noise. 2. Device elimination on that circuit: identify which device. 3. Near-field probe confirmation: verify source by proximity. 4. Coupling path identification: conducted (mains) or radiated.

Phase 3: Remediation Select the appropriate mitigation from Chapter 13. Apply mitigation. Re-measure noise floor. Document improvement.

Phase 4: Documentation of Completion Record: device found, coupling path, mitigation applied, improvement in dB. File in station logbook. Future technicians benefit from this history.

12.2 RFI Hunt Field Log Template

DATE/TIME: _______________  OPERATOR: _______________
FREQUENCY: _______ MHz     NOISE LEVEL: _______ dBm (S __)
SUSPECTED SOURCE: ______________________________________
LOCATION IN HOME: ______________________________________

Circuit breaker test results:
  Breaker  |  Noise with ON  |  Noise with OFF
  ─────────────────────────────────────────────
           |                 |
           |                 |
           |                 |

Device elimination on circuit ___:
  Device removed              |  Noise level
  ──────────────────────────────────────────
                              |
                              |
  CULPRIT: __________________ |  _________ dBm

Coupling path: [  ] Mains conducted  [  ] Radiated  [  ] Feed line braid

Mitigation applied: ________________________________________
Noise level after: _________ dBm   Improvement: _____ dB (__ S-units)

Notes: __________________________________________________

12.3 Difficult-to-Locate Sources

Problem: Noise is intermittent and doesn’t correlate with any obvious device.

Approach: - Use noise monitor logging (Chapter 9). Review 24-hour trend. - Note EXACT time of onset. Correlate with thermostat/appliance schedules. - Check neighbor’s property for smart meters, solar inverters, EV chargers. - Cyclic noise with 20–30 min period: refrigerator compressor cycling.

Problem: Noise is present even with all circuits off at the panel.

Approach: - Check with all equipment UNPLUGGED (not just off) — some devices have standby SMPS. - Check if noise changes when transmitter is keyed (possible re-radiation/intermodulation). - Disconnect antenna; if noise disappears, source is on a neighbor’s property or public utility equipment (power line, smart meter, streetlight ballast).

CHAPTER 13 — DEVICE-SPECIFIC REMEDIATION

13.1 Switching Power Supplies (Phone Chargers, Wall Warts)

Best fix: Replace with quality supply (Anker, Belkin, Apple OEM — EMC certified). Look for FCC Part 15 Class B, EN55032 Class B markings.

If replacement not possible: 1. Ferrite clamp (Mix 31) on both AC side and DC output cable. 2. Y-capacitors (2200 pF to chassis/earth) from Line and Neutral: ONLY if chassis is earthed — floating chassis cannot use Y-caps. 3. Place device in grounded metal enclosure (ferrite-lined box is overkill for most).

Expected improvement: 15–30 dB with good ferrite placement.

13.2 LED Lighting

Best fix: Replace lamp with verified low-noise unit. Look for: - EN55015 Class B certified (European EMC directive) - “Flicker-free” LED drivers often have better EMI design - Incandescent or halogen lamps: zero switching noise

If replacement not possible: 1. AC line filter on the lamp’s power circuit (see Chapter 5, scaled down). 2. Ferrite ring (Mix 75 or Mix 31) on AC cord near lamp. 3. LED panel replacement: replace just the driver board with high-quality alternative (Meanwell LDD series or similar EMC-certified driver).

Note on LED dimmers: TRIAC dimmers (phase-cut) generate broadband hash in addition to the LED driver noise. Replace with 0–10V PWM dimmer system with ferrite-filtered output cable, or eliminate dimming entirely.

13.3 Computer Systems

CPU/GPU VRM noise (200–500 kHz × harmonics into HF): 1. Ferrite clamp on all cables exiting computer (power, monitor, USB, audio). 2. Computer case grounded to SPG (add ground wire from case to SPG bus). 3. If noise from monitor: ferrite on video cable both ends; grounded DVI/HDMI cable.

USB 3.0 (2.5 GHz noise): 1. Ferrite cores (Mix 43 for 2.5 GHz) on USB cables — snap-on type. 2. Shielded USB 3.0 cable. 3. Consider using USB 2.0 for peripherals that don’t require high speed.

Ethernet: 1. Use shielded Cat6A cable (STP/SFTP). 2. Ground shield at switch end only (not both ends — ground loop risk). 3. Ferrite clamps on unshielded cable (both ends, Mix 31). 4. Consider fiber optic Ethernet (no RF coupling at all) for critical links.

13.4 Solar Inverters and Charge Controllers

Solar inverters are frequently the most severe HF noise source in modern homes. Grid-tie inverters switch at 10–50 kHz and produce harmonics across HF.

Mitigation options: 1. Ferrite on DC cables from panels to inverter (Mix 31, multiple turns). 2. Ferrite on AC output cables. 3. Commercial EMI filter on AC output (Schaffner FN2410 or FN2070 series). 4. Verify inverter has current EMC certification; some cheaper inverters fail EMC. 5. Shielded conduit for DC wiring runs. 6. Contact installer/manufacturer — many countries have regulations prohibiting equipment that causes interference to licensed radio services.

Important: Filing an FCC complaint (in USA) or OFCOM complaint (UK) may be appropriate if a commercial inverter is found to be non-compliant. Your Technician/ General/Extra license gives you standing as an affected licensed station.

13.5 Smart Meters

Smart meters use power-line carrier (PLC) and/or wireless (ZigBee, 900 MHz). The PLC modulation (30–500 kHz) may be audible on 160m–40m as buzzing or tones.

Options: 1. Mains filter (Chapter 5): blocks PLC from entering shack circuits. 2. Contact utility: some utilities will relocate or replace smart meters near affected licensed stations. 3. Note: removing or bypassing a utility smart meter is illegal and dangerous.

13.6 Plasma TVs (Legacy)

Modern OLED and LCD TVs are much better; plasma TVs were notoriously bad.

If a plasma TV is the source: 1. First try: ferrite clamps on mains power cord (Mix 31, multiple turns wound). 2. Replace TV — plasma technology is obsolete; modern equivalents produce minimal noise.

CHAPTER 14 — TROUBLESHOOTING

14.1 Choke Not Reducing Noise

14.2 Mains Filter Not Reducing Noise

14.3 Active Canceller Not Achieving Null

14.4 Noise Monitor Inaccurate Readings

APPENDIX A — FERRITE CORE QUICK REFERENCE

Source: Fair-Rite Products Corp. AL values: see Fair-Rite catalog (fair-rite.com). For Jim Brown’s (K9YC) measured impedance data: see his application notes AN-01005.

APPENDIX B — COMMON RFI FREQUENCIES AND SIGNATURES

APPENDIX C — NEC MODEL INTERPRETATION GUIDE

C.1 dipole_cm_current.nec

Run in xnec2c:

xnec2c /path/to/dipole_cm_current.nec

Model A (comment out GW 3 and CM EX on stub): Shows ideal dipole azimuth pattern. Model B (include GW 3 and add CM excitation): Shows distorted pattern.

Compare the azimuth patterns at 10° elevation. The filled-in nulls in Model B represent the noise pickup added by common-mode feed line current.

C.2 shielded_loop_rfi.nec

Near-field probe pattern verification. Look at: - Azimuth at elevation=0°: should be figure-8 (bidirectional) - Broadside elevation pattern: should show deep null perpendicular to loop plane - Peak: along loop axis (in-plane direction)

C.3 noise_canceller_array.nec

Run each model separately (comment out other EX cards): - Model 1: Omnidirectional main vertical - Model 2: Short reference whip — slightly different pattern - Model 3: Combined with cancellation — cardioid pattern with null at 0°

The cardioid pattern of Model 3 shows the desired signal preservation (90° direction) while nulling the noise at 0°.

APPENDIX D — PARTS LIST AND SOURCES

D.1 Ferrite Cores

D.2 Mains Filter Components

D.3 Active Noise Canceller Components

D.4 Noise Monitor Hardware

D.5 Estimated Build Cost

End of Technical Manual TM-RFI-001 Rev A

This manual covers construction and operation of receive-side interference mitigation equipment. Always comply with applicable electrical codes when working with mains voltage. The Single-Point Ground system described herein also constitutes best practice for lightning protection; consult a professional for comprehensive lightning protection installation.