[RFI] Common Mode Noise?

[Dave (NK7Z)]

Scott,

I have a set of Coax pliers I use with a plastic insert so I don't scuff 
up the connector!  :)

73, and thanks,
Dave (NK7Z)
https://www.nk7z.net
ARRL Volunteer Examiner
ARRL Technical Specialist, RFI
ARRL Asst. Director, NW Division, Technical Resources


On 1/21/25 12:36, K9MA 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 at contesting.com> wrote:
>>
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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 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
>>>
>>>
>>>
>>>
>>>
>>>
>
>