After a strange series of reports were received regarding malfunctioning key fobs and disabled cellphone use all within an isolated area, city officials in Evanston, Illinois turned to the ARRL lab for help in investigating the cause of such odd interference.
Originally, officials turned to the FCC with their concerns of intentional radio frequency interference (RFI) but because key fob malfunctions are considered the responsibility of the automaker the issue was not of immediate concern. However, due to the fact that cell phones, a licensed radio service, were also experiencing interference and disruption, the concerns for public safety were high. To ensure that the RFI was not intentional or indicative of malicious intent, the city turned to the ARRL lab for further assistance.
“This situation demonstrates the electromagnetic compatibility problems that are evolving in an atmosphere of noncompliant, unintentional RF-emitting devices.”
Once on site, experts used a noise signature receiver and an antenna to survey the affected area at a frequency range of 315 MHz and 433 MHz (the common frequency range for key fobs) and isolate the interfering signal. The source of interference was determined to be recently replaced neon sign with switching-mode power supply. This problematic power supply generated a substantial signal, interfering with devices within up to 40 feet of the sign. Understanding that the interference was unintentional, the store owner was asked to disable his sign during non-business hours but has not been required to replace it at this time.