Suppress Ethernet interference to ham/CB radio
While Ethernet itself is subject to interference from ham radio transmission at high transmit power (noted by packet loss or dropped connections during radio transmission), the more common problem is that Ethernet cables can act as antennas and emit signals that interfere with ham radio or CB radio reception, from HF to VHF frequencies. The usual culprit is common mode emissions from the Ethernet cable, which can be mitigated by using the appropriate type “mix” of ferrite beads or chokes on the Ethernet cable close to each end of the Ethernet cable. If the devices have shielded Ethernet ports, using shielded CAT6a Ethernet cables can also help reduce interference. Note that shielded cables might also make interference worse if the shield is not grounded at both device ends - experimentation is necessary - try with one cable first, with other interfering devices unplugged from the network. Most IoT hubs and VoIP hubs have non-metallic cases, unshielded Ethernet ports and connect by default at 100 Mbps. A tell-tale sign of 100 Mbps Ethernet interference is that the interference is present when the hub is powered on and connected to the network, but disappears when the Ethernet cable is unplugged from the hub, even if the hub is still powered on. The RF interference characteristic of 100 Mbps Ethernet is approximately 61 kHz spaced tones across the HF bands that are constantly present regardless of data traffic. To save costs, the devices may often omit internal ferrite chokes on the Ethernet port, so adding external ferrite chokes on the Ethernet cable can help reduce interference.
Measuring improvements by experimentation
An AM receiver tuned to the RF frequency of interest is the minimum requirement, but an SSB receiver is usually more sensitive for detection. Due to the AGC action of the receiver, making small improvements to the interference may not be detectable by ear. In general, a more effective measurement is a spectrum analyzer view typical to a software defined radio (SDR) receiver, which can show the interference power level in dB and the improvement from mitigation efforts with “peak hold” to detect the overall change before / after mitigation. Broadband hash from power supplies - whether USB or other “wall wart” or inline power supplies (unrelated to Ethernet) is often not detectable by ear, and is better seen by signal strength meter or spectrum analyzer view.
Ethernet interference mitigation ideas
A cost-free solution can be to force the Ethernet ports the hubs are connected to to operate at 10 Mbps instead of 100 Mbps. This may focus the unwanted common-mode RF emissions to lower frequencies. Where switch and endpoint support it, test fixed 10 Mbps full-duplex on both ends of the link.
If forcing hubs to 10 Mbps is not suitable, whether due to failed link negotiation, insufficient bandwidth, or persistent interference, add ferrite chokes to the Ethernet cables as the next step. The ferrite mix is frequency range dependent. If a wide range of frequency bands experience interference, use multiple chokes with the appropriate distinct mixes to cover the different frequency ranges. An off-the-shelf solution is the DX Engineering ISO-PLUS Ethernet RF Filters DXE-ISO-PLUS-2, which are designed for 10/100/1000 Mbps Ethernet and cover a wide frequency range with 20 - 30 dB of interference suppression across the HF radio bands. For frequencies below about 25 MHz, a common mix is the Type 31 mix. For frequencies above about 25 MHz, a common mix is the Type 43 mix. Seek a size like FT240-31 or FT240-43, which can accommodate multiple loops of multiple Ethernet cables for increased suppression. Consider kits like from Palomar Engineers which include multiple chokes with different mixes for a range of frequencies.