[RFI] Common Mode Noise?

[K9MA]

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 at contesting.com> wrote:
>
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>> Today's Topics:
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>>     1. Question about common mode noise (Bob WA2SQQ)
>>     2. Re: Question about common mode noise (Jim Brown)
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>>
>> ----------------------------------------------------------------------
>>
>> Message: 1
>> Date: Mon, 20 Jan 2025 22:09:39 -0500
>> From: Bob WA2SQQ<wa2sqq at gmail.com>
>> To: rfi<rfi at contesting.com>
>> Subject: [RFI] Question about common mode noise
>> Message-ID:
>>          <
>> CAK47ybnPWbsaXQGgc0KiGB+mW6OaZDMzHgiBQcqP92cxr2tCyA at 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 at audiosystemsgroup.com>
>> To:rfi at contesting.com
>> Subject: Re: [RFI] Question about common mode noise
>> Message-ID:
>>          <91e3ff79-0e07-449d-8ea7-ca9f1056bec9 at 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 at sdellington.us