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@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@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
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