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Author Topic: Common mode current on AC wiring?  (Read 4243 times)
N4MQ
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Posts: 145




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« Reply #30 on: September 07, 2017, 04:39:43 AM »

Have you thought about installing the detector in a faraday shield  that is grounded to prove the signal is on the power line and its not detecting the rf in the wiring of the detector?   It would be easy to wrap it in foil, and ground it for a TEST to see how the signal is being picked up.

The problem is either on the power line or direct pickup due to sensitive design, knowing which would be helpful.  In the Army I was taught how to quickly kill a snake, by cutting off its tail ....all you needed to know was which end was the tail.

Enjoy, Woody
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G3RZP
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Posts: 8140




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« Reply #31 on: September 08, 2017, 01:09:36 AM »

the OP asked

Quote
- Is there some level of such RF current that is significant (e.g. some level that can be ignored besides 0Ma)?

FYI and FWIW, European EMC standards - which are mostly derived from IEC (International Electrotechnical Committee) standards - require immunity to 30mA modulated 80% by 1kHz in the HF bands on leads such as power and signal.
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KC1BMD
Member

Posts: 610




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« Reply #32 on: September 08, 2017, 02:01:43 PM »

Have you thought about installing the detector in a faraday shield  that is grounded to prove the signal is on the power line and its not detecting the rf in the wiring of the detector?   It would be easy to wrap it in foil, and ground it for a TEST to see how the signal is being picked up.

The problem is either on the power line or direct pickup due to sensitive design, knowing which would be helpful.  In the Army I was taught how to quickly kill a snake, by cutting off its tail ....all you needed to know was which end was the tail.

Enjoy, Woody

Yes. In my starting post I wrote under "Problem:
     "...get tripped, only when plugged in, not when unplugged on battery."
and,
     "...wrapping in foil does not help."
I figured these two tests indicated it was coming in on the AC wiring.
« Last Edit: September 08, 2017, 02:05:32 PM by KC1BMD » Logged
N4MQ
Member

Posts: 145




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« Reply #33 on: September 08, 2017, 08:35:41 PM »

I was suggesting that the shield be grounded, if it was not it would just act like rabbit ears. Woody
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WB4SPT
Member

Posts: 496




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« Reply #34 on: September 09, 2017, 06:28:43 AM »

the OP asked

Quote
- Is there some level of such RF current that is significant (e.g. some level that can be ignored besides 0Ma)?

FYI and FWIW, European EMC standards - which are mostly derived from IEC (International Electrotechnical Committee) standards - require immunity to 30mA modulated 80% by 1kHz in the HF bands on leads such as power and signal.

Yep.    2 days ago, I had a design of mine tested to iec61000-4-6 to 10V conducted;  .15 to 80MHz;  For my stuff, 10V or 67mA input RF current CM.  Not a peep from a classic opamp design, but it took  a bit of on board ferrite and bypass to get there.  

To the OP;  stop the madness, go with battery CO units,  I did at my own home.  Even networked, the battery lasts much longer than a year.
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KC1BMD
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Posts: 610




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« Reply #35 on: September 09, 2017, 06:45:26 AM »

I was suggesting that the shield be grounded, if it was not it would just act like rabbit ears. Woody

It seems I completely missed your suggestion: "faraday shield  that is grounded". Note to self: must read more carefully. I thought about it but didn't try it. Not sure where I would ground it being on the second floor. If I grounded it to the AC safety ground (3rd prong of outlet) could that make for a bigger set of rabbit ears? Also, not that wrapping in tin foil is a perfect classic Faraday Cage but in the classic example, at least, I don't think it needs to be grounded to shield the contents, correct?
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KC1BMD
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Posts: 610




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« Reply #36 on: September 09, 2017, 06:46:35 AM »

... To the OP;  stop the madness, go with battery CO units,  I did at my own home.  Even networked, the battery lasts much longer than a year.

I will certainly consider going that route if the replacement detectors start misbehaving.
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WB4SPT
Member

Posts: 496




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« Reply #37 on: September 09, 2017, 09:25:43 AM »

I was suggesting that the shield be grounded, if it was not it would just act like rabbit ears. Woody

Also, not that wrapping in tin foil is a perfect classic Faraday Cage but in the classic example, at least, I don't think it needs to be grounded to shield the contents, correct?

Correct.   Shields do not require "grounding".  However, cable shields do need to be attached to metallic housings to keep the overall shield construct continuous.  Sometimes folks call the attachment of shields and drain wires "grounding".  But, for shield purposes, attachment to earth has no purpose. 
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