QUOTE (from Scott, K9MA): There will always be received current on the
shield of the coax, but grounding the shield properly, as Jim suggests,
will keep it out of your receiver. ............
However, a poorly grounded coax shield can lead to all sorts of RF in the
shack.
****************************************************************************************************
****************************************************************************************************
If "grounding" (earthing) reduces or alleviates "RF in the Shack" a portion
of the antenna is missing. The RF fields are seeking a return. Fix the
antenna FIRST. "Grounding" is far too often thrown around as a solution to
everything bad with RF. It seldom solves anything!!
Besides, what if your "ground" wire is 1/4-lambda long? A 1/4-lambda
"earthed" conductor will reflect as an open circuit. So, now you have a
frequency-dependent connection to earth. So what good is that? Fix the
antenna, FIRST.
Dave - WØLEV
On Tue, Jan 21, 2025 at 8:45 PM K9MA <k9ma@sdellington.us> wrote:
> That test, connecting only the center conductor of the coax, won't tell
> you anything. Almost certainly, the noise level will greatly increase,
> because now the whole feedline is acting as an antenna. There will
> always be received current on the shield of the coax, but grounding the
> shield properly, as Jim suggests, will keep it out of your receiver.
>
> Note that I'm not talking about current from your transmitter, for which
> common mode chokes will help, but that's an entirely different matter.
>
> However, a poorly grounded coax shield can lead to all sorts of RF in
> the shack. If that problem suddenly appears, the first thing to do is to
> tighten all the coax connectors. Always use a pliers!
>
> 73,
> Scott K9MA
>
>
> On 1/21/2025 12:03 PM, Bob WA2SQQ wrote:
> > Thanks Jim
> > I’ve done what you suggested. My reason for asking was more about a
> friend
> > of mine who swears that it’s the easiest test. The test I described was
> > intended to be done at the back of the radio. I’m currently in FL so I’m
> > going to try it when I get home later this week.
> >
> >
> >
> >
> > *Best 73 de WA2SQQElmwood Park, NJ"The only dumb questions are those we
> > don't ask"*
> >
> >
> > On Tue, Jan 21, 2025 at 12:21 PM<rfi-request@contesting.com> wrote:
> >
> >> Send RFI mailing list submissions to
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> >>
> >> Today's Topics:
> >>
> >> 1. Question about common mode noise (Bob WA2SQQ)
> >> 2. Re: Question about common mode noise (Jim Brown)
> >>
> >>
> >> ----------------------------------------------------------------------
> >>
> >> Message: 1
> >> Date: Mon, 20 Jan 2025 22:09:39 -0500
> >> From: Bob WA2SQQ<wa2sqq@gmail.com>
> >> To: rfi<rfi@contesting.com>
> >> Subject: [RFI] Question about common mode noise
> >> Message-ID:
> >> <
> >> CAK47ybnPWbsaXQGgc0KiGB+mW6OaZDMzHgiBQcqP92cxr2tCyA@mail.gmail.com>
> >> Content-Type: text/plain; charset="UTF-8"
> >>
> >> I?m
> >> I?m seeing what appears to be wide band possibly common mode noise on
> 160
> >> thru 40m.
> >> A friend of mine suggested
> >> 1 Dosconnect the antenna to verify no noise and no S meter indication.
> >> 2 Connect the antenna and tune to a portion of each band with no signals
> >> 3 Partially disconnect the antenna leaving only the center of the pl259
> >> connected.
> >> 4 Compare the noise with shield of coax connected and disconnected.
> >>
> >> He swears that if the noise increases when the shield is connected,
> that?s
> >> a positive indication you have an abundance of common mode noise.
> >>
> >> I?m not so sure this is a valid test. Comments?
> >>
> >>
> >>
> >>
> >>
> >> *Best 73 de WA2SQQElmwood Park, NJ"The only dumb questions are those we
> >> don't ask"*
> >>
> >>
> >> ------------------------------
> >>
> >> Message: 2
> >> Date: Mon, 20 Jan 2025 21:43:46 -0800
> >> From: Jim Brown<jim@audiosystemsgroup.com>
> >> To:rfi@contesting.com
> >> Subject: Re: [RFI] Question about common mode noise
> >> Message-ID:
> >> <91e3ff79-0e07-449d-8ea7-ca9f1056bec9@audiosystemsgroup.com>
> >> Content-Type: text/plain; charset=UTF-8; format=flowed
> >>
> >> On 1/20/2025 7:09 PM, Bob WA2SQQ wrote:
> >>> He swears that if the noise increases when the shield is connected,
> >> that?s
> >>> a positive indication you have an abundance of common mode noise.
> >>>
> >>> I?m not so sure this is a valid test. Comments?
> >> It's certainly an indication of failure to do proper grounding and
> >> bonding. Coax shields should go to a lightning arrestor where they enter
> >> the building, ideally on the exterior wall of the shack. The arrestor
> >> MUST be bonded to grounding for the building, which, BY LAW, must be
> >> bonded to all other grounds in the building, including entry for power,
> >> telephone, CATV, satellite dish, and any other antennas.
> >>
> >> On the operating desk, all parts of the station must have their chassis
> >> bonded together by short, fat copper, and that must be bonded to the
> >> grounding for the building at the closest point. And all cable shields
> >> should be bonded to the chassis AT THE POINT where they enter equipment.
> >>
> >> If you've done all this, any common mode current on the shield is going
> >> to go into that ground bus to all of the driven rods, not into your
> >> radio or other equipment in your shack.
> >>
> >> Study N0AX's excellent ARRL book on the topic, to which I contributed
> >> extensively, and/or the pdf of slides for talks I've given at Pacifcon,
> >> Visalia, and to several ham clubs.
> >>
> >> 73, Jim K9YC
> >>
> >>
> >>
> >>
> >>
> >>
>
>
> --
> Scott K9MA
>
> k9ma@sdellington.us
> _______________________________________________
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> http://lists.contesting.com/mailman/listinfo/rfi
>
--
*Dave - WØLEV*
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