A resistor is soldered between the center conductor and shield of a female BNC connector which is plugged into the end of a length of coax cable. Plug the other end of the coax into the antenna input of an SDR receiver. Here a standard-value 47-ohm 1/4 watt resistor is used to match the 50-ohm impedance of the coax cable and SDR antenna input (below). The resistor acts as a pick-up loop antenna which is used to 'sniff' for RF noise sources while monitoring the SDR receiver display.|
RF sniffer pick-up loop antenna.
| Tune the SDR to the desired frequency and watch the FFT display for noise as you sniff around with the resistor pick-up loop. The exact source of RF noise such as the inter-connecting cable of a digital device, the power cable of a device, or other wiring can be pinpointed using this method. Upon locating an offending noisy cable or wiring, probe along its length with the resistor pick-up loop to determine which end is radiating the strongest signal(s). That's where to first install a common-mode choke or ferrite bead to initially reduce the noise, then slide the choke along the cable to find the 'sweet spot' for maximum noise reduction.|
An SDR tuned to 5000 kHz (5 MHz) shows noise picked up by the RF sniffer when it's placed next to the computer LED monitor.
| Obviously if the RF noise is determined to be radiating from the viewing area of an active display such as a nearby computer or TV monitor (above photo), then common-mode chokes on any of the cables won't be very effective. The best options are to either turn off the offending device, relocate the radio receiver, or install an antenna at a remote location preferably outdoors some 20-feet away from any noise sources with a noise-reduced lead-in to the receiver. Visit the ¡BAMLog! Antenna section for some low-noise medium wave antenna suggestions.|
| 73 and Good RF Sniffing!|