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In a recent amateur radio build, I deduced (most likely incorrectly) that a longish (maybe 20 cm) length of power cable carrying DC voltage to an IF board could benefit from some shielding. Careful not to introduce ground loops, I decided that grounding on one end was just the ticket.

It didn't make any difference to the noise issue I had at the time, but now that I'm past the tinker-and-see phase I've had a chance to think more rationally about what I actually did.

Some research indicates that it doesn't work like you think it should.

So, I'm asking the question here: apart from creating gimmick capacitors, is there ever a good reason to ground just one end of a coax?

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    \$\begingroup\$ In my opinion: Grounding shields for coax (signaling), power cables (multiconductor) and signaling cables (multiconductor) are all different things, requiring different strategies. Also, one must consider the whole system design and external interactions. This can be a complicated issue, that depends highly on your application. But in general, coax cables are expected to be grounded at both ends (after all, most of the time, the conductor is exposed on the coaxial connector). \$\endgroup\$ Aug 15 at 15:00
  • \$\begingroup\$ A cable shield should always connect at both ends to the local chassis. If you see any improvement from disconnecting one end, then you like abuse the shield as some kind of low frequency signal return conductor, which is bad design. \$\endgroup\$
    – tobalt
    Aug 15 at 15:02
  • \$\begingroup\$ Yes, I think I see that now. At the time, my thought process was: I'm experiencing noise in my IF stage; perhaps it's getting in through my power supply through RF interference and long power wires; I'll shield the power positive, but I'll be careful not to let current flow in the shield. \$\endgroup\$
    – Buck8pe
    Aug 15 at 15:07
  • \$\begingroup\$ current in the shield is no problem. But it must not be signal return currents that flow in the shield \$\endgroup\$
    – tobalt
    Aug 15 at 15:14

2 Answers 2

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Current through the center conductor of a coaxial cable needs to return to its source (possibly via capacitance, i.e. displacement current). If it does not return via the coax outer conductor, it must return via some other path. If the currents in the inner and outer conductors of a coax are equal in amplitude but opposite in direction, then their magnetic fields outside the coax will cancel. By reciprocity, this means that an external source of magnetic interference will not affect the current in such a coax.

On the other hand, if the return current flows through some path far away from the center conductor, the total circuit will enclose a significant area. Any time varying magnetic fields within that area will induce noise in the circuit.

For these reasons, a coax is usually configured to provide a return path for the signal through the outer conductor. As such the outer conductor needs to be connected on both ends.

That is for a coax. However there are shielded cables and times when a shield should not be grounded on both ends. Such might be the case, for example, when shielded twisted pair is used. In this case, the signal and its return current flow through a pair of wires, and none of the signal (should) flow through the shielding. Grounding on one end might be beneficial in such a case, but whether it is so depends upon the actual noise sources that need to be defended against.

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    \$\begingroup\$ "There are times when a shield should not be grounded on both ends." - Then expand these please..Without that, this is basically a non-answer. \$\endgroup\$
    – tobalt
    Aug 15 at 16:22
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    \$\begingroup\$ Henry Ott has plenty of examples where he recommends grounding the shield at one end only. When the frequencies are low, and the return path is also inside the shield (e.g. twisted pair), then you may only want to ground one end. Although I took Mr Ott's 4 day class a long time ago, I don't know it well enough to defend an answer. There are books written on this stuff, you can't expect it to be explained in a Q&A forum. +1 for the attempt. \$\endgroup\$
    – Mattman944
    Aug 15 at 16:54
  • \$\begingroup\$ It's too late to edit my comment on the OP's question, and since you mentioned coax "usually configured to provide a return path", I think that it's fair to say that the coax is typically bonded at both ends (connected to chassis & grounded) in such a manner where its return currents are likely, with the designer's best effort, to flow through the shield. There are, of course, exceptions. "It depends." \$\endgroup\$ Aug 15 at 17:26
  • \$\begingroup\$ I suspect that in practice most folks would do a shield as I do. Try it in all possible configurations (there are only 4) and see which works best.... \$\endgroup\$
    – Kyle B
    Aug 15 at 21:19
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    \$\begingroup\$ @TimWilliams a shield grounded at one end can protect a twisted pair from low frequency electric fields present if the cable comes into proximity with a mains power line. \$\endgroup\$ Aug 16 at 12:05
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I suppose resonant stubs are an extension of your gimmick capacitor example, so I won't go further than merely mentioning the example.

Leaving the shield open on one side, negates its effect as far as magnetic shielding goes.

The shield carries two currents at high frequency: inside (signal) and outside (shield or CM).

At low frequencies (where the skin depth in the shield is comparable to its thickness), these currents bleed together over distance, and you get the expected DC result: the shield is just a resistor.

At high frequencies, the currents stay separated (shielded), by some degree depending on frequency and length. Like 10s of dB.

So we restrict analysis to high frequencies, where things are interesting.

When we have a signal going into coax, the return current flows inside the shield.

If the far end of the coax is grounded, then the return current flows onward to the destination. Signal quality is good, everyone is happy. Shield currents may be annoying, but to the extent they don't couple to the signal (so: depends on attenuation vs. frequency), they can be ignored. Which we know is worse at low frequencies, so this can introduce mains-frequency ground loop for example. The shield current can also act as an antenna (the inductive link between two boxes, at least one of which is isolated from ground plane / earth), or a end-shorted dipole (slot antenna) when the ends are joined by ground plane. Which can exacerbate EMC problems by providing a gain peak at harmonic frequencies, making the shield less effective there. (This is where ferrite beads shine the most: they introduce resistance to the CM mode, dampening these peaks.)

If the far end of the coax is open, then the return current has to flow back over the shield, up to the source, and back again along whatever ground plane lies between the two ends. The loop is completely unshielded to incident magnetic fields, signal quality is trash because the shield acts as a stub length in series with the signal, and any ground-loop voltage that might be present between connections, is superimposed directly on the signal (at all frequencies, not just low frequencies).

By most measures, the latter is strictly worse.

The one advantage that you get, is electrostatic shielding, which might be of value at frequencies below the stub resonance length. And, electric fields are the dominant concern at high impedances. So, all together: I think the advice arises from the bad old days of vacuum tube equipment, especially audio amplifiers where the bandwidth isn't enough to worry about stub lengths*, and stub lengths are small (within chassis).

*But the tubes might not know that. Typical receiving tubes have significant gain over 100MHz (if still rather little gain-bandwidth). "Grid stopper" resistors are a common sight, to dampen possible tuned-grid oscillation modes in these circuits.

So, it is best to ignore such advice, and ground responsibly.

A reminder that coax can be used differentially. The shield-to-circuit-GND connection can be "lifted" on some impedance (also serving to dampen shield resonances, as a ferrite bead does), as long as the signal receiver has adequate CMR and CMRR (range, rejection ratio) to handle expected noise levels. This completely solves GND loop problems up to the CMR. And RFI (and CMRR) can still be maintained by bypassing shield to circuit GND; the crossover frequency should pull in somewhere before CMRR gets too poor (since op-amps are limited in CMRR vs. F).

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  • \$\begingroup\$ I feel I should accept an answer and it's a pity you can't split the gong, but I'll give it to Math on the basis that he/she got there first. Appreciate the effort, I've learned plenty from the help I've received. \$\endgroup\$
    – Buck8pe
    Aug 18 at 10:55

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