EMRFD Message Archive 11992

Message Date From Subject
11992 2015-12-13 13:44:35 n1vc Grounding the Shield of one or both ends of coax
Slowly but surely my SA project is shaping up. But there is one thing I am still a little confused about.. Let's take for example the POS-200 VCO and the first mixer board combination: There is RF in IF out, VCO CNTRL, VCO sample, etc. I am using 1590BB boxes and RG-174 to BNC's. I believe I read that you should only ground one side of the shield not both. I don't remember the reason why. Secondly, if the former is true that only one side is grounded, which side should it be? Should it be the point where it connects to the PCB or the side that connects to the BNC solder terminal? Please help me get this straight in my mind once and for all.

Vince N1VC

11993 2015-12-13 14:27:40 John Marshall Re: Grounding the Shield of one or both ends of coax
Grounding only one end of the shield is done sometimes in audio circuits to avoid multiple ground path problems. In circuits where the coax is being used as a transmission line, to provide shielding and a defined characteristic impedance, both ends are grounded. Think of it this way - current flows in a circuit so whatever is going down the center conductor of the coax needs a return path to its source. If one end of the shield is open, the return path will be through the chassis, the power supply, or someplace else you don't want it to be.

John, KU4AF
Pittsboro, NC

11994 2015-12-13 15:01:38 Jim Strohm Re: Grounding the Shield of one or both ends of coax
If you ground both ends of coax connecting two signal source / destinations, you introduce the possibility of a ground loop.  If you ground only one end, you introduce the possibility of a signal leak.

How large that leak might be is probably related to the size of the gap in the coax (this is a simplified analysis) but for a 1mm gap on the unshielded end, you're probably good through HF. 

Obviously, when the gap approaches a quarter-wave of the frequency of the signals on the coax, things change and you'd likely be better off using semi-rigid coax between shielded circuit boxes. Even at HF, RG-174 can be leaky at the signal-to-noise ratios that many folks on this group concern themselves with. 

As always, experimentation with the actual circuitry trumps all empirical statements.  ;)


PS -- To all the tinfoil hat folks, I've never done extensive leakage or shielding experiments, but if you are using copper PCB circuit enclosures you should be able to use stained-glass copper foil tape to shield your coax runs if you solder the edges of the tape to itself, and then to the PCB enclosures at each end.

Just don't ask me what the impedance is between the coax shield and the foil wrap, vis-a-vis the signal leakage current  :)  For this kind of application I use semi-rigid coax stripped from old celphone amps to keep things copacetic.  I've got that semi-rigid crawling out of my junkbox like poisonous snakes in a bad horror movie.

11995 2015-12-13 15:32:18 jmlcs2000 Re: Grounding the Shield of one or both ends of coax
Hi Vince, I have personally successfully built three SA's, one of which which is Wes' QST SA and one which is a modified version of his QST SA. In all cases I grounded the shield of the coax cable at both ends. In addition, I ran separate runs of power to each module's box from the input terminal of the positive and negative voltages so that power was not applied in series to several module's boxes with the same power run......the only time I had a feedback loop problem was as a result of poor grounding of a module's box. Hope this will be of some assitance, Best of Luck 73, John K5IRK
11996 2015-12-13 17:46:15 vasilyivanenko Re: Grounding the Shield of one or both ends of coax

Good discussion ! This topic gets brought up on blogs, groups and email chains at least annually.
Many variables (angular frequency , parasitic reactances & ground impedance for example )  + math and physics  -- well explained here:

Once I omitted the braid connection on the load end of some coax connecting 2 UHF RF stages to stop some high frequency noise and saw the difference on the SA and 'scope during experiments. 

11997 2015-12-13 18:28:08 Ashhar Farhan Re: Grounding the Shield of one or both ends of coax
you have to ground both ends. always.

ground loop problems will go away if the coax is long enough!
the clean thing to do is to add gaunella transformers at both ends
the dirty thing to do is to slip some ferrite beads over the coax.
in all these cases, the transformations of the transmission line will kick-in and RF isolate the two modules.

11998 2015-12-13 19:03:40 Will Re: Grounding the Shield of one or both ends of coax

Could you not wind turns on a toroid at one end or even in he middle to stop currents flowing on the outside of the co-ax.  Would stop HF but not DC or audio.

I have seen  co-ax with two separate layers of screening. the outer one only earthed at one end the inner screen earthed at both.