RFI — Finding and Fixing Interference in Your Ham Shack
Radio Frequency Interference (RFI) is one of the most common and frustrating problems in modern ham radio stations. The proliferation of switching power supplies, LED lighting, solar inverters, and networked devices in homes has dramatically increased the ambient RF noise floor over the past decade. What would have been a quiet S0 noise floor on 40m in 2005 is now often S5–S7 in suburban and urban locations. This guide covers identifying common RFI sources, practical suppression techniques, and when to escalate to the FCC.
Switching power supplies
Switching power supplies are the most common RFI source in modern homes. They operate by rapidly switching a transistor at frequencies from tens of kHz to MHz — the harmonics of these switching frequencies spread across the HF spectrum. Phone chargers, laptop power supplies, TV wall warts, LED driver circuits, and almost any modern power supply uses switching technology. Symptoms include broadband hash across multiple HF bands that disappears when specific devices are unplugged. Test by methodically unplugging devices one at a time while monitoring your radio — the noise will drop or disappear when you unplug the culprit.
LED lighting
LED lighting has replaced incandescent and fluorescent lighting in most homes, and cheap LED drivers are a significant RFI source. The driver circuit that converts AC mains to the DC needed by the LEDs operates at switching frequencies that radiate across the HF spectrum. Budget LED bulbs and strips are particularly problematic. Test by turning lights off room by room while monitoring your radio. Good-quality LED bulbs from name-brand manufacturers (Philips, Sylvania, GE) typically have better EMC filtering than generic brands. LED dimmer circuits are especially noisy and should be avoided near a ham shack if possible.
Solar inverters and battery chargers
Solar panel inverters convert DC from solar panels to AC mains voltage using switching circuitry that can radiate significantly across the HF spectrum. If you have a solar installation, try disconnecting the inverter while monitoring your noise floor — the improvement is sometimes dramatic. Complaints about solar inverter RFI have increased significantly as rooftop solar has become more common. Some inverter manufacturers (Enphase, SolarEdge) have worked to improve EMC compliance; others have not. This is a legitimate FCC Part 15 issue that can be escalated if the manufacturer does not respond.
Plasma TVs, appliances, and miscellaneous
Plasma televisions were a significant RFI source and are now largely obsolete. Refrigerator and air conditioner compressor motors with worn brushes can generate noise. Smart home devices, WiFi routers, and powerline networking (HomePlug/MoCA) all generate noise that can appear in the HF receive path. Powerline networking is particularly problematic because it deliberately injects signals onto your home's AC wiring at HF frequencies — it is inherently incompatible with a quiet HF station and should not be used in a home with an active HF station.
Ferrite chokes
Ferrite chokes are the primary tool for common-mode RFI suppression. Wrapping a cable through a ferrite toroid or clamping a ferrite snap-on around a cable creates a common-mode choke that blocks RF from travelling along the outside of the cable while allowing the intended signal inside to pass normally. Use Mix 31 ferrite for 1–30 MHz (HF) and Mix 43 for 30–300 MHz (VHF). The number of turns through the toroid and the ferrite material determine the choking impedance — more turns and the right material for the frequency give better suppression. Fair-Rite, Amidon, and Palomar Engineers are reliable ferrite sources.
Cable routing and separation
RF noise often couples inductively or capacitively from noise source cables to antenna feedlines running nearby. Routing antenna coax away from power cables, computer cables, and other potential noise sources reduces coupling. Where cables must cross, cross them at right angles to minimise inductive coupling. Keeping the antenna feedline as far as possible from the house wiring and especially from the main power service entry reduces the amount of house-generated noise that couples onto the feedline.
Establish a baseline
Note your noise floor on each band with everything in the shack disconnected from the antenna (use a dummy load). This tells you whether the noise is coming through the antenna or from within the station itself. If the noise disappears with a dummy load, it is coming through the antenna. If it persists, the noise is generated by or coupling into your station equipment.
Narrow the source
With your antenna connected, systematically turn off circuit breakers one at a time (from the breaker panel) while monitoring your noise floor. When turning off a specific breaker causes the noise to drop, you have identified the circuit. Then identify which device on that circuit is the culprit by unplugging devices one at a time with that breaker back on.
Apply suppression
Once you identify the noise source, apply ferrite chokes to the power cord and any data cables connected to the device. If the device is a replaceable consumer product like an LED bulb, try a higher-quality replacement. For switching power supplies, a line filter (toroidal common-mode choke on the AC line) can reduce radiated noise. In some cases, physical relocation of the device away from the antenna is the simplest fix.
Address common-mode on feedlines
If noise is entering through your antenna feedline as common-mode current, install a common-mode choke (choke balun) at the feedpoint where the antenna meets the coax, and another where the coax enters the house. This breaks the path that noise from the house wiring travels up the outside of the coax shield to the antenna feedpoint where it is received as noise.
My neighbour has a device causing my RFI — what can I do?
Start with a friendly conversation. Most people are unaware that their equipment is causing interference and are willing to cooperate. Bring documentation — show them the noise on your receiver when their device is on. If the device is an FCC Part 15 device (which most consumer electronics are), it is required to accept interference but also required to not cause harmful interference to licensed radio services. If informal resolution fails, file a complaint with the FCC or contact the ARRL's Technical Information Service for guidance on formal RFI complaint procedures.
Are SDR receivers better or worse for RFI than traditional radios?
SDR (Software Defined Radio) receivers used with external antennas are subject to the same RFI as any other receiver. However, panadapter-equipped radios and SDR systems make RFI more visible and easier to characterise — you can see the exact frequencies and bandwidth of noise sources on the waterfall. Some dedicated SDR receivers have better dynamic range for close-in large signal handling, which can help discriminate against strong local noise. The antenna and its environment determine the noise floor more than the receiver.
Will a noise canceller help with my RFI problem?
A noise canceller like the MFJ-1026 or ANC-4 can significantly reduce interference from a specific directional noise source by using a separate noise pickup antenna to sample the interference and subtract it from the main antenna signal. It works best when the noise source is directional (coming from a specific location) and when the noise is uncorrelated with any desired signal. It is not a replacement for proper grounding and ferrite suppression — think of it as a last resort for noise that cannot be eliminated at the source.