What is RFI?
RFI (Radio Frequency Interference) happens when RF energy gets where it shouldn't. This can go two ways:
- Your transmitter interferes with other devices
- Other devices interfere with your receiver
Common RFI Symptoms
Transmit RFI (Your Station Causing Problems)
- Computer resets when you transmit
- LED lights flicker
- TV or stereo makes noise during transmit
- Hot microphone or RF burns
- SWR changes when you touch things
Receive RFI (Other Things Causing Problems)
- Buzzing or humming on received signals
- Hash across the band
- Noise that goes away when you unplug something
- S-meter readings with no signals
Finding the Source
For Transmit RFI
- Reduce power and see if the problem decreases
- Check all bands - problem may be worse on some
- Walk around the house while transmitting (carefully) to locate affected devices
For Receive RFI
- Turn off circuits at the breaker panel one by one
- Use a portable receiver to hunt the noise
- Check if noise is present on all bands or just some
- Note if the noise has a pattern (on/off, cycling)
Common RFI Sources
Your Station
- Common mode currents on feedline
- Poor station grounding
- Antenna too close to the house
- RF getting into microphone cable
In Your Home
- LED light bulbs (especially cheap ones)
- Switching power supplies
- Computer equipment
- Solar panel inverters
- Battery chargers
- Touch lamps
- Dimmer switches
- Plasma TVs
Outside Your Home
- Power line noise (arcing insulators)
- Grow lights
- Electric fences
- Neighbor's devices
Solutions for Transmit RFI
Choke Common Mode Current
The #1 solution. Add ferrite chokes to:- Feedline at antenna
- Feedline at shack entry
- Power cables
- Computer cables
- Microphone cable
Improve Station Grounding
- Single point ground for all equipment
- Short, heavy ground straps
- Good earth ground connection
Increase Antenna Distance
Get the antenna further from the house if possible.Use a Tuner Wisely
A tuner helps your radio, not your feedline. If your feedline has high SWR, you may have more RFI.Filter the Affected Device
Add ferrite chokes to the cables of devices that are affected. Clip-on ferrites work well.Solutions for Receive RFI
Find and Eliminate
If you can identify the source, fix it at the source. Replace noisy LED bulbs, move chargers, etc.Filter AC Power
- Use AC line filters
- Isolate your station on its own circuit
Common Mode Chokes
Chokes on your feedline help receive noise too.Noise Cancellers
MFJ and others make noise cancelling units that can null out specific noise sources.Directional Antennas
Some antennas can be aimed to null out the noise direction.Specific Solutions
LED Light Bulb Noise
Replace cheap LEDs with quality brands (Philips, GE). Some hams have success with incandescent bulbs in problem areas.Computer Noise
- Ferrites on all cables
- USB cables are often culprits
- Ethernet cables can radiate
- Consider powerline ethernet adapters (they're terrible for RFI)
Solar Panel Inverter
Contact the inverter manufacturer. Many will provide filters. FCC regulations require them to fix interference.Power Line Noise
Report to your utility company. They're required to fix it. Document with recordings and time stamps.Prevention
When building your station:
- Plan for single-point grounding
- Include chokes from the start
- Run feedlines away from power wires
- Use quality feedline and connectors
- Choose quality power supplies
Resources
- ARRL RFI page: arrl.org/radio-frequency-interference-rfi
- ARRL Technical Information Service can help with RFI
- K9YC's RFI tutorials are excellent technical references