RFI Sources

You sit down at your radio on a weekend morning hoping to work some DX, and immediately notice that 40 meters sounds terrible — a constant broadband roar from S4 to S6 sitting across the entire band. You try 80 meters, then 20 meters. The hash is there on every band. This is radio frequency interference (RFI), and it is one of the most persistent and frustrating challenges facing amateur radio operators in the twenty-first century.

The proliferation of electronic devices in modern homes and neighborhoods has dramatically increased the RF noise floor experienced by many amateur stations. Switch-mode power supplies are now in virtually every device plugged into an electrical outlet. Solar energy systems with high-power inverters are appearing on rooftops across the country. LED lighting has replaced incandescent bulbs. Each of these technologies, if poorly designed or installed, can radiate RF interference that affects reception from the AM broadcast band through the top of the HF spectrum and beyond.

This lesson teaches you how to recognize, categorize, locate, and remediate the most common RFI sources. Armed with this knowledge, you can systematically identify what is creating your noise problem and apply the right solution.

Categories of RFI

Radio frequency interference comes in many forms, and classifying a noise source helps you understand both its likely origin and the best approach to addressing it. Here are the most useful ways to categorize RFI:

Conducted vs Radiated Interference

Conducted interference travels through electrical wiring — the device generates RF currents that flow back through its power leads, through the house wiring, and couple into any antenna connected to that wiring system (directly or through capacitive or inductive coupling). A switching power supply that generates RF noise on its input leads will spread that noise throughout the building's electrical system, making it appear on any wire that acts as an antenna — including your coaxial cable shield if your station is not properly bonded.

Radiated interference travels through the air as electromagnetic waves. The interfering device's internal circuitry, power leads, or output leads act as unintentional antennas and radiate RF energy. Your receive antenna picks up this radiated energy directly. Radiated interference decreases with distance in the same way any other RF signal does — it falls off with the square of distance, or 6 dB per doubling of distance in the far field.

Narrowband vs Broadband Interference

Narrowband interference occupies a small portion of spectrum — it might appear as a carrier with sidebands, a buzzing tone, or a signal that is only a few kilohertz wide. Examples include a switching supply that creates a fundamental tone at a specific switching frequency and harmonics at multiples of that frequency.

Broadband interference spreads across a wide swath of spectrum — it might span from a few hundred kilohertz to 30 MHz or higher. Power line arcing and switch-mode power supplies with poor filtering often produce this kind of interference. On a receiver, it appears as a general rise in the noise floor across many bands simultaneously.

Intentional vs Unintentional Radiators

Regulatory bodies classify RF-emitting devices based on whether they are designed to radiate RF energy. Intentional radiators (transmitters, Wi-Fi routers, Bluetooth devices) are licensed or authorized to emit on specific frequencies. Unintentional radiators (computers, power supplies, appliances) are not intended to emit RF but do so as a byproduct of their operation. All the RFI sources discussed in this lesson are unintentional radiators, subject to FCC Part 15 limits in the United States.

Common RFI sources at a typical suburban home. Each source has a characteristic spectral signature that helps identify it.

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Switch-Mode Power Supplies

Switch-mode power supplies (SMPS) are now the dominant technology for converting line voltage AC to low-voltage DC. You will find them in virtually every modern electronic device: smartphone chargers, laptop power bricks, LED lamp drivers, desktop computers, flat-screen televisions, network routers, and gaming consoles. Their universal adoption is driven by their efficiency — a good SMPS converts 85–95% of input power to output DC, compared to only 50–70% for a traditional linear transformer-based supply. But the switching operation that makes them efficient is also what makes them generators of broadband RF noise.

A switch-mode supply works by switching a transistor (usually a MOSFET) on and off at a high frequency — typically 50 kHz to 500 kHz. This rapid switching creates sharp current transitions, and sharp current transitions in a conductor generate RF energy. The fundamental switching frequency and all of its harmonics appear as potential interference, and harmonics of a 50 kHz fundamental extend through the entire HF spectrum (the 100th harmonic of 50 kHz is 5 MHz; the 200th harmonic is 10 MHz; the 600th harmonic is 30 MHz).

The key variable in how much interference a given SMPS generates is the quality of its output and input filtering. A well-designed supply includes EMI filtering on both the input (common-mode chokes, X and Y capacitors) and the output (LC filters). Poorly designed supplies — typically inexpensive units, particularly those manufactured outside established regulatory markets with minimal testing — may have almost no filtering and radiate significant RF noise.

The characteristic signature of SMPS noise on a receiver:

• Broadband hash that typically extends from below 1 MHz to above 30 MHz
• Often strongest on the lower HF bands (160m, 80m, 40m) and decreasing higher in frequency
• May have a "carrier" or discrete tone at the switching frequency and its harmonics
• Constant when the device is on — does not change with weather, time of day, or temperature once the supply has warmed up
• Disappears immediately when the device is unplugged

The most effective way to identify an SMPS as your noise source is the on/off test: unplug every device in your home one at a time while monitoring the S-meter on a quiet frequency. When the noise drops, you have found the culprit. Keep in mind that the SMPS interference often travels through the household electrical wiring and couples to your station and antenna through multiple paths, so simply moving a device further from your antenna may not help — the noise can travel electrically through the wiring.

Remedies for SMPS noise include:

• Replace the offending unit with a higher-quality supply that carries FCC Part 15 compliance documentation
• Add common-mode ferrite chokes on the power leads of the offending device (see the ferrite chokes lesson)
• Install an inline EMI filter on the device's power cord
• If the device cannot be replaced, move it further from your antenna and ensure it is on a separate circuit from your station

Solar Inverters and Charge Controllers

Solar photovoltaic systems have become widespread across the United States, and they represent a growing and increasingly severe source of HF interference for amateur radio operators. A solar system consists of panels, a charge controller or maximum power point tracker (MPPT), a battery bank (for off-grid or hybrid systems), and a grid-tie inverter that converts the DC from the panels to AC for use in the home or export to the utility grid.

The inverter is the primary RFI source. String inverters convert DC from multiple series-connected panels directly to AC at line voltage. The conversion process uses high-power switching transistors (IGBTs or MOSFETs) operating at switching frequencies from 10 kHz to 100 kHz. At power levels of 2,000–10,000 watts, even small amounts of RF noise — fractions of one percent of the switching power — represent significant absolute RF levels that can overwhelm HF reception within a considerable radius of the installation.

Reports in amateur radio literature describe total HF degradation from nearby solar installations — operators who had clean bands for years suddenly finding every frequency from 3 to 30 MHz unusable after a neighbor installs a grid-tie solar system. The noise from inverters tends to be especially broadband, covering the entire HF spectrum simultaneously, and can radiate for distances of 50–200 meters.

MPPT charge controllers used in off-grid systems are smaller in power than grid-tie inverters but use similar switching technology and can also generate significant HF noise. The characteristic signature is similar to a large SMPS: broadband hash across the HF spectrum, constant during operation.

Addressing solar inverter RFI:

• Install ferrite common-mode chokes on the DC cables between panels and inverter
• Install an inline RF filter on the inverter's AC output leads before they connect to household wiring
• Bond the inverter chassis to the house safety ground if it is not already
• Contact the inverter manufacturer — reputable manufacturers have application notes on reducing EMI, and some have emission-reducing firmware updates
• File an FCC complaint if the noise exceeds Part 15 limits — FCC enforcement against solar inverter manufacturers has occurred
• When possible, work with the neighbor installing the system to specify a lower-interference inverter brand

LED and CFL Lighting

The transition from incandescent to more efficient lighting technologies has created a new category of HF interference. Both compact fluorescent lamps (CFLs) and many LED retrofit lamps use switching power supplies to drive their light sources, and the same switching noise that makes SMPS units problematic also applies here.

Compact fluorescent lamps use a high-frequency electronic ballast (typically 20–60 kHz) to drive the gas discharge tube. The ballast is a switching supply, and without adequate EMI filtering it generates harmonics across the HF spectrum. CFL interference tends to peak below 10 MHz, with harmonics that are often detectable on 80m and 40m, and sometimes on 20m.

LED retrofit lamps vary enormously in their RFI characteristics. High-quality LED lamps from reputable manufacturers use well-filtered SMPS drivers with proper EMI suppression components and comply with FCC Part 15 Class B limits. Budget LED lamps — particularly those purchased from discount sources or unverified online retailers — often have minimal filtering and can radiate significant HF noise. The interference pattern is similar to SMPS: broadband hash from LF through HF, typically strongest below 10 MHz.

Outdoor LED floods, landscape lighting systems, and particularly LED strip lighting with cheap inline SMPS drivers are common culprits. These tend to use inexpensive unregulated SMPS drivers that prioritize cost over EMI performance. String lighting and decorative lighting (especially around holidays) driven by cheap switching supplies can also be problematic.

The most practical approach to LED RFI:

• Replace budget LED lamps with units from established manufacturers that specify FCC Part 15 Class B compliance
• Professional-grade LED drivers are significantly better filtered — the extra cost buys quieter operation
• Use the on/off test room by room, lamp by lamp, to identify which specific units are causing problems
• For hardwired fixtures that you cannot replace, ferrite chokes on the lamp's power leads can sometimes help

Characteristic power line noise on a spectrum analyzer: broad hash across HF with 60 Hz harmonic structure. The noise is caused by arcing at damaged or contaminated insulators or hardware on utility poles.

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Power Line Noise

Power line noise is one of the oldest sources of HF interference, and it remains extremely common. Unlike the device-based sources described above, power line noise comes from the utility distribution infrastructure itself — the poles, wires, transformers, and hardware that bring electricity to your home and neighborhood.

Causes of Power Line Noise

Power line noise is generated by electrical arcing — small electrical discharges that occur at points where the electrical connection is imperfect or where insulating materials have degraded. Common sources include:

• Arcing at damaged or contaminated insulators: Ceramic or glass insulators on utility poles can crack, chip, or accumulate a layer of conductive contamination (salt spray, industrial deposits, bird droppings). When wet, this contamination creates a leakage path that arcs at line voltage, producing intense broadband RF noise.
• Loose hardware: Bolts, clamps, and pole-top hardware that have loosened over years of vibration and thermal cycling can create intermittent metallic contact that arcs.
• Corroded ground clamps: The safety ground connections on utility poles and transformer pedestals must be electrically clean. Corroded connections arc at the ground current they carry.
• Failing transformers: Internal arcing in pad-mounted or pole-mounted distribution transformers that are beginning to fail can produce extremely strong HF noise.
• Guy wire contact points: Utility poles with guy wires have hardware where the wire attaches to the pole and to the ground anchor. These connections can develop arcing over time.

Characteristic Signature of Power Line Noise

Power line noise has a very distinctive character that makes it recognizable once you know what to listen for:

• Harmonic structure: Because the arcing occurs in synchrony with the 60 Hz AC power cycle, the noise has a 60 Hz (and 120 Hz) harmonic structure. On a spectrum analyzer, you see a forest of peaks at 120 Hz intervals across the HF spectrum.
• Sound character: On an AM receiver, power line noise has a characteristic harsh buzzing sound. On SSB, it sounds like a clicking or crackling that repeats at the power frequency rate.
• Weather dependence: Insulator contamination arcing is dramatically worse in wet weather (fog, rain, heavy dew). If your noise is significantly worse on humid or rainy days, suspect power line insulator contamination. Dry weather may show minimal noise from the same pole that creates severe interference when wet.
• Seasonal variation: Arcing at loose hardware can be worse in winter (thermal contraction) or after storms that have stressed the infrastructure.

Locating Power Line Noise

Power line noise can travel considerable distances along the power lines before radiating from the actual source. The strategy for locating it:

1. Set your receiver to AM mode on a frequency severely affected by the noise (typically 3–7 MHz)
2. Use a portable AM broadcast radio as a simple direction-finding tool — its built-in ferrite bar antenna is directional (null perpendicular to the antenna axis)
3. Walk along the utility line in the direction the noise appears strongest, rotating the radio to find the null direction
4. The noise level will increase as you approach the source pole and decrease as you pass it
5. Note the pole number (stamped into the pole or on a tag) for reporting

Reporting Power Line Noise

In the United States, utility companies are legally required to fix power line noise that exceeds FCC regulations. Contact the utility company using their power quality or power line noise reporting number. When you call:

• Give the specific pole number or address of the noise source
• Describe the frequencies affected and the noise level (S-units or dBm)
• Note whether the noise is worse in wet weather (helpful diagnostic information for the utility's repair crew)
• Document the interference with audio recordings and, if you have one, screenshots from an SDR waterfall display
• Follow up if the issue is not resolved — the first call may be handled by a general customer service representative who does not understand the problem

Larger utility companies have dedicated power quality or EMC departments that handle amateur radio interference complaints. The ARRL provides guidance and sample complaint letters for dealing with utilities, and some ARRL Technical Information Specialists have experience navigating these processes.

Electric Fence Controllers

Electric fence controllers are used throughout rural areas to confine livestock. They work by applying periodic high-voltage pulses to the fence wire — typically a pulse lasting about 0.3 milliseconds, repeating at 1–2 pulses per second. The sharp leading edge of each high-voltage pulse generates a broadband RF impulse. The fence wire then acts as an antenna and radiates this impulse efficiently across the HF spectrum.

The characteristic signature of electric fence interference is one of the most distinctive of all RFI sources:

• Very regular clicking at 1–2 Hz on all HF frequencies — exactly once or twice per second
• On a spectrum analyzer, you see a broadband noise floor that briefly rises by 10–20 dB at each pulse interval
• Completely consistent — does not change with weather or time of day (unless the fence energizer switches to test mode)
• Present on every frequency from LF through VHF simultaneously

Because the fence wire can be hundreds or thousands of feet long and is effectively a horizontal antenna at the height of a few feet, an electric fence can radiate interference over a wide area. Interference from a fence 1–2 km away is not unusual on 160m and 80m.

Remedies for electric fence interference:

• Install a dedicated fence noise filter or RF choke at the controller's output terminal, between the energizer and the fence wire
• Ensure the fence wire is not running parallel to your antenna feed line or antenna — parallel runs couple RF efficiently
• Modern fence controllers marketed as "low-radiation" or "EMC-compliant" are significantly better than older designs — the newer pulse shaping reduces the harmonic content of the pulse spectrum
• For your own fence, consult the manufacturer about noise suppression options
• For a neighbor's fence, a diplomatic approach explaining the problem and the available low-noise solutions is usually the most effective first step

Plasma TVs, Arc Welders, Light Dimmers, and Computers

Plasma TVs

Plasma television sets were a major source of HF interference for many years. The gas-discharge cells in a plasma panel, while operating at much lower power than a neon sign, use the same fundamental principle of ionized gas conducting electricity — a process that generates RF noise. A plasma TV could raise the HF noise floor by 20 dB or more in an adjacent room, making HF operation from a home with a plasma TV essentially impossible without significant shielding. Plasma TVs are now largely out of production, but older units remain in service and continue to cause problems. If you encounter a neighbor with a plasma TV and severe HF noise, that is likely the cause.

Arc Welders

Arc welders used in home garages and small shops generate extremely broadband, impulsive RF noise during operation. The welding arc is a high-current electrical discharge — essentially an extended arc similar to what causes power line noise, but intentional and continuous. Arc welding can wipe out HF reception across all bands for the duration of the welding session. MIG and TIG welders using high-frequency arc starting are particularly problematic. This is generally a temporary interference source, but if welding occurs in a neighboring workshop during your operating time, it is a significant issue. There is limited recourse beyond operating at different times or installing a very effective bandpass filter with exceptional out-of-band rejection.

TRIAC-Based Light Dimmers

Phase-cutting light dimmers — the traditional type that uses a TRIAC (a bidirectional thyristor) to chop the AC waveform — create RF interference from the sharp current transitions at the chopping point. Standard TRIAC dimmers are cheap and widely used, but they generate conducted and radiated noise primarily below 1 MHz and at low HF frequencies (160m, 80m). Replacing TRIAC dimmers with modern phase-cutting dimmers that include integrated EMI filtering, or with PWM dimmers designed for LED loads, typically eliminates this interference source.

Computers and Peripherals

Modern computers and their peripherals generate several types of interference:

• USB 3.0 interference: The USB 3.0 (and USB 3.1/3.2) standard uses data encoding that generates noise across a broad frequency range centered around 400–500 MHz, but harmonic content can extend into HF bands. USB 3.0 ports on laptops have been found to cause significant noise on 70 cm (432 MHz) and similar bands.
• HDMI cables: High-speed HDMI cables carrying 4K video content generate high-frequency signals in the GHz range, but the cables act as antennas and can radiate harmonics into lower frequency bands.
• Ethernet cables: Gigabit Ethernet runs at frequencies up to 500 MHz, and unshielded Cat5e or Cat6 cables can radiate at HF frequencies as their switching harmonics extend downward. Shielded CAT cable (S/FTP) and ferrite chokes on cable runs near antennas reduce this significantly.

Locating RFI

A systematic approach to RFI location is more effective than random investigation. The following process handles most residential RFI situations.

Step 1: Characterize the Noise

Before searching, gather as much information about the noise as possible:

• Which frequencies are affected? Is it all HF, or specific bands?
• Is the noise constant, periodic (regular clicking), or variable?
• Does it change with time of day or weather?
• When did it start? Did anything change in your home or neighborhood around that time?

Step 2: The On/Off Test

For noise that might come from within your home: at a time when the noise is present, go to your circuit breaker panel and turn off circuit breakers one at a time. Monitor your radio between each switch. When the noise disappears, you have identified the circuit. Then restore that circuit and turn off individual outlets on that circuit (using power strips) to identify the specific device.

Step 3: Direction Finding

For noise that persists after all circuits in your home are off, the source is external. Use a portable receiver with a directional antenna for direction finding:

• A small loop antenna or a ferrite bar AM receiver has a figure-8 directional pattern with sharp nulls
• Walk around your property and neighborhood, noting the direction of strongest signal
• Triangulate using readings from two or more positions to narrow down the source location
• An SDR waterfall display is helpful: the spectral signature you see in the waterfall can help distinguish power line noise (harmonic structure) from SMPS hash (broadband) from fence clicking (periodic impulses)

Step 4: Frequency Analysis

The spectral signature of the noise often identifies the source type. Use an SDR waterfall display or spectrum analyzer to look at the noise:

• Periodic impulses at 1–2 Hz = electric fence
• Harmonic structure at 120 Hz intervals = power line arcing
• Broadband hash, constant = SMPS or switching supply
• Broadband hash that appeared after solar installation = solar inverter
• Noise only when lights are on = LED or CFL lamp driver

Regulatory Framework

Amateur radio stations in the United States operate under an FCC license (Part 97). The devices that cause RFI are regulated under different parts of the FCC rules, and understanding the regulatory framework helps you know your rights and how to escalate a complaint effectively.

FCC Part 15 — Unintentional Radiators

FCC Part 15 sets limits on RF emissions from unintentional radiators — electronic devices not designed to transmit radio signals. Part 15 Class B limits apply to residential-use devices (computers, televisions, peripherals). Part 15 Class A limits (slightly higher, less restrictive) apply to commercial equipment. Devices that violate Part 15 limits are operating illegally, and the FCC has authority to require corrective action.

In practice, FCC enforcement of Part 15 against individual device types or small manufacturers is resource-limited. The FCC's complaint process allows you to file online, but expect a slow response to individual complaints unless the interference is widespread or involves a known problematic product. The most effective enforcement pathway for device-based RFI (SMPS, LED drivers, etc.) is often to file a complaint that is then investigated alongside multiple similar complaints about the same product or manufacturer.

FCC Part 18 — Industrial, Scientific, and Medical Equipment

ISM equipment (industrial heaters, medical imaging equipment, scientific instruments) is regulated under Part 18, which allows higher emission levels than Part 15 but requires these devices to accept interference from licensed radio services. Amateur radio is a licensed primary service, and Part 18 devices that cause harmful interference to amateur stations may be subject to FCC enforcement.

Power Line Noise

Power line noise is regulated under both FCC jurisdiction and the jurisdiction of the Public Utilities Commission (PUC) in each state. FCC has authority when the interference comes from a Part 15-covered device on utility infrastructure; the PUC has authority over the utility company's maintenance obligations. Reporting to both agencies, and to the utility company directly, gives the best chance of a timely resolution.

Amateur Radio as a Primary Service

In the United States, amateur radio holds primary status in its allocated frequency bands. This means licensed amateur stations have a regulatory right to be free from harmful interference in those bands. This standing is the basis for FCC complaints about RFI. When filing an FCC complaint, clearly identify yourself as a licensed amateur radio operator, list your call sign, specify the bands being affected, and document the interference level and its impact on your station's operation.

In practice, the most effective resolution path for neighbor-generated RFI is usually direct and diplomatic communication. Most people generating RFI are doing so unknowingly and have no idea their solar inverter or LED lights are causing problems. A polite visit explaining the situation, perhaps with printed information about the issue and the available solutions, resolves many cases without regulatory involvement. Reserve the regulatory route for cases where diplomacy fails or where the interference comes from commercial installations where the operator cannot or will not cooperate.