I have an Internet modem/router device connected to a 20 foot LMR400 coaxial cable which uses N-to-N type connectors to connect to a 10 foot LMR400 cable, which is then connected into an omnidirectional antenna for cellular Internet reception. This is all on top of a very large metallic structure (a big metallic tower basically)

I've been told that I shouldn't let the N-type connectors touch any of the metal on the structure, as that creates a "ground loop" and additional paths to ground. I'm supposed to put rubber on the connectors or at least wrap them in electrical tape.

But I don't quite understand the electrical/physical reasons as to why the connectors should not touch any metal. Why does this cause a problem? I've read something about "reference" and "potential" being messed up by multiple paths to ground... could someone explain what people mean by that?

  • \$\begingroup\$ That only applies if there is a ground fault or SMPS noise filter current<5ns . \$\endgroup\$ – Sunnyskyguy EE75 May 8 at 1:28

Coaxial cable "works" because the two currents (center conductor and shield), which flow in opposite directions, are equal in magnitude and share a common axis. This balance is what causes the magnetic field that is produced by those currents to cancel out perfectly outside of the cable. By reciprocity, it is also what makes it immune to the influence of external fields.

As soon as you allow any of the shield current to follow another path, this balance is broken, and the immunity from interacting with external H fields is lost.

  • 1
    \$\begingroup\$ But at any frequency into the MHz and beyond, the fraction of the return current flowing through the shield vs any other paths will be so close to 1 as to be indistinguishable without extremely fancy test equipment and techniques. Section 2.5 of Joffe has a good explanation for why this is: books.google.com/… \$\endgroup\$ – pericynthion May 9 at 1:10

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