Unshielded LAN cables are often a source of unintentional RF emissions. These emissions may arise from common mode signals, such as switching power supply noise, and are often introduced by Ethernet switches. Most LAN patch cables (category 5 and category 6) are unshielded. Any antennas brought in close proximity to the unshielded cable will detect these emissions. Fortunately, these emissions can be mitigated using shielded LAN cables.
An H-field sniffer probe (Figure 3) connected to the RF sensor was used to detect and localize sources of EMI. When the probe was placed near the unshielded LAN cable, the RF sensor picked up the emissions seen below (Figure 1). This ‘forest’ of spurs near 90 MHz was entirely due to the cable. When the cable was replaced with a CAT6 shielded cable these emissions became almost undetectable (Figure 2). Shielded LAN cable may be purchased at any electronics store.
Clip on RF chokes are less effective. We ran an experiment and applied a 10mm RF choke at the RF sensor end of the LAN cable. We observed no reduction of EMI. We then applied the same RF choke to the switch side of the LAN cable. Again, we observed no reduction of EMI.