On 12/22/2021 1:46 PM, nlsa@nlsa.com wrote:
My question should be, "What does Ethernet EMI look like at VHF?" It might be
different at HF.
YES! The first step in ANY investigation of noise should be with the
widest practical waterfall display at frequencies of interest.
http://k9yc.com/KillingReceiveNoise.pdf
At HF, wired Ethernet is radiated as a common mode signal and heard as
multiple birdies at certain harmonics of the digital waveform. Study
this link for a discussion of the issue, and a list of frequencies where
many of the land in the HF bands and 6M.
http://k9yc.com/RFI-Ham.pdf
At VHF, the mechanism is different -- leakage of those components of the
digital signal above the frequency where the tightly twisted pairs are
no longer tight enough to resist leakage/crosstalk. I hear it as
broadband noise in a 2M talkie close to the cable. I've not looked at
spectrum.
Both mechanisms can be effectively suppressed by the use of shielded CAT
cable provided that the shield is properly terminated at both ends.
Obviously, this requires connectors with a shield contact AND proper
termination of that shield contact within the box to the shielding
enclosure or circuit board's reference layer(s) at the point of entry.
73, Jim K9YC
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