Radio Frequency Interference (RFI) Examples

Examples of RFI observed in spectrograms of the upper HF band along with audio recorded by Jove receivers at 20.1 MHz.

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Reference Material - tales of RFI woe and success

Comment toward FCC ET Docket No. 16-191, TAC Noise Floor Technical Inquiry (SUG, 2016)

Radio Frequency Interference (ARRL)
Sounds of RFI (ARRL)
Sounds and Spectrograms of Digital Modes (G4UCJ)

In Compliance Magazine - Annual Reference Guide 2016
The Mitigation of Radio Noise and Interference from On-Site Sources (Naval Postgraduate School, 2009)
Radio Frequency Interference Handbook (NASA, 1971)

Comparing Conducted Emissions from LED Lamps, Wyatt (EDN, 2021).pdf
A Tale of Two Air Conditioners, Lee (QST, 2016).pdf
MFJ-5008 Ultrasonic Receiver, Wilson (QST, 2016).pdf
Cooling a Hot RFI Problem, Traughber (QST, 2015).pdf
More on Furnace RFI, Majka (QST, 2015).pdf
HVAC EMI Generation, Parker (QST, 2013).pdf
The Great RFI Hunt, Wilson (QST, 2013).pdf
Light Bulbs and RFI - A Closer Look, Gruber (QST, 2013).pdf
A Ham Radio Detective Story - Finding an Odd RFI Source, Wolfgang (QST, 2012).pdf
Hands-On Radio Experiment 112 - RFI Hunt, Silver (QST, 2012).pdf
An RFI Story with a Happy Ending, Kriss (QST, 2009).pdf
A Home-made Ultrasonic Power Line Arc Detector, Hanson (QST, 2006).pdf

Galactic Background Noise

Caused by electrons crying out in protest as they're bounced around by the complex galactic magnetic field. This is the ever-present background noise against which all HF-band radio astronomy observations are made.


Power Distribution Systems (Line Noise)

Power line noise can be caused by many things in a power distribution system such as loose hardware, leaky lightning arrestors, and tree branches. The diagonal lines result from the beat frequency between the spectrograph sweep rate and the 120 Hz peaks of the AC power lines.


Cable TV Systems

In this case, caused by a loose power supply connection on a pole-mounted line amplifier.


CB Radio / Shortwave Broadcast / Amamteur Radio Transmitters

In the upper HF band, terrestrial communications signals are more of a problem during the daytime, which is when the ionosphere supports long distance propagation. These signals show up as horizontal bands and can be strong enough to cause the receiver to overload.


Over-the-Horizon Radar Systems

Several nations, most notably the US, Russia, China, and Australia operate very powerful (megawatt-class) radar transmitters that can peer far over the horizon to detect incoming enemy nuclear missiles. These transmitters sweep slowly from below the HF band up to around 30 MHz or above. In the US, each AN/TPS-71 radar transmitter station also oprates a co-located ionosonde transmitter that sweeps slightly faster and stops just below 20 MHz. These diagonal traces are informally called sweepers.


Short Range HF Radar Systems

Small radar sets used by Homeland Security transmit over the entire upper HF band. Their signals are about 150 kHz wide and vary randomly in sweep rate, frequency, and timing. A few such emissions are marked with red arrows in the spectrogram below; however, note that there are far more that could be marked.


Flat Screen Television Set

Caused by something in the TV, probably a cheaply-filtered switch-mode power supply circuit. Spectrogram below is from a 32" Sony, model KDL-32BX330 manufactured 6/2012.


LED Lights

Caused by...?



LED Lights (Again)

Probably caused by the little block-shaped wall-wart USB power supply.



More LED Lights

Harbor Freight. Need we say more?



Heat Pumps

Caused by a variable speed motor controller.


Washing Machine

Observed to occur when a Maytag Neptune washer was in operation.



Electric Golf Carts

Observed as a golf cart drove past the antenna array. Power electronics such as the variable speed motor controller?


Uninterruptable Power Supply (UPS)

The UPS in question is a Tripp-Lite Model SMART1500LCDT. It is not nearly as bad as it was before the addition of AC line filtering and ferrite chokes on both sides of the UPS.

Spectrogram below is WITHOUT the UPS.

Spectrogram below is with UPS on AC power with a load. RFI hash is seen from 15 to 17 MHz. This is after heavy filtering and application of ferrite chokes. Before adding the filtering the RFI hash was all over the spectrogram.


Railroad Engines

Diesel engines drive generators that drive electric motors that move the train; the power system and/or the power control system emits RFI. In this example, a CSX railroad track lies roughly 100 feet from the antenna array. A train supplying coal to GRU's Deerhaven Generating Station (an electric power plant) passes every couple of days.


Spectrum Analyzer

The spectrogram below shows a broad-band noise problem that is active for about four seconds in a 15-second cycle. It seems to be caused by the receiver clock source, a Chinese-made 35 MHz to 4.4 GHz network/spectrum analyzer (, ordered from Ebay. The PCB has "Spectrum Analyzer - BG7TBL 2015-01-07 - 06109A-Y167-10" marked on it.



This spectrogram recorded at LGM Radio Alachua on Jan 3, 2012. Mysterious. CIA mind-control rays?


Last update 18 Jul 2021.
Page maintained by Dave Typinski, davetyp at typnet dot net