Interesting "source" of RF interference


Ken Sejkora
 

Greetings all,

 

I stumbled upon an interesting/surprising “source” of RF interference in my SDR listening.  I’m using an Airspy HF+ discovery connected through 75-ohm coax to a ~60-foot longwire with a 49:1 unun at the antenna, and ~20 mH common mode chokes on both the antenna and receiver ends of the coax to reduce local noise.  At the computer desk where I listen to the SDR, I have the desktop PC, monitor, speakers, and small desk lamp plugged into a 6-outlet plug strip to facilitate turning everything off with a single switch.

 

Anyway, a couple of weeks ago I plugged a 9-ft two-wire extension cord into the plug strip to power a NiMH charger, as the wall outlet I normally use for charging was being used for some Christmas lights.  Since that outlet got freed up a couple of days ago, I’ve been using that outlet again for charging.  Being relaxed (lazy) over the past few days, I had left the extension cord plugged into the plug strip, even though it wasn’t powering anything.  I happened to be monitoring the 15-meter amateur band at the time, and noticed that the waterfall display exhibited a significant reduction in RF noise levels coinciding to when the extension cord was unplugged.  I tried plugging in and unplugging the cord several times with the same result.  Nothing is plugged into receptacle-end of the extension cord.  It would appear that the cord is acting as somewhat of an “antenna” and feeding RF interference into the AC power for the PC listening station.  I captured a screenshot of what I observed.  The lower portion of the waterfall is when the extension cord is NOT plugged in, and the middle portion from 15:37:30 until 15:38:30 is when the extension cord is plugged into the plug strip, during which time noticeable “bands” of hash interference appear.  The extension cord is unplugged at 15:38:30 for the rest of the waterfall, and the noise is reduced.  Orientation of the extension cord did not appreciably change the level of interference, whether it was stretched perpendicular to the computer desk along the wall, or parallel to the desk into the open room.  Even plugging the extension cord into different wall outlets in the room that are on the same circuit as the PC resulted in injecting RF interference into the signal, but to a lesser extent than plugging into the plug strip.  Similar results were observed on different HF frequencies, but to a lesser extent.

 

Lesson learned – when trying to chase down noise/interference, consider everything.  I never would have guessed that an ‘idle’ extension cord plugged into a grounded plug strip could act as an antenna and feed RF interference into the receiving system.  This has given me a new appreciation of how insidious RF interference can be.

 

Good luck all, and best of 2022 to everyone.  73

 

Ken -- WBØOCV

 


Dave (NK7Z)
 

Hi,

I would put a high Z, ferrite suppression choke on the extension cord at the plug end as a test. If the RFI goes away or is reduced, then as you suggested, the cord is probably a transmitting antenna for the RFI...

It is possible your not adding enough choking impedance to the main antenna feedline, and it is still part of the main antenna system, and picking up the radiation from the extension cord. I might move the feedline around and see if the RFI level is changing as a test of this.

If the extension cord test reduces the RFI level, I might add a few more suppression devices to it, and test again. If the RFI is reduced, then I would conclude the extension cord is radiating the EMI from the charger, and my feedline is picking it up.

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

On 12/31/21 08:25, Ken Sejkora wrote:
Greetings all,
I stumbled upon an interesting/surprising “source” of RF interference in my SDR listening.  I’m using an Airspy HF+ discovery connected through 75-ohm coax to a ~60-foot longwire with a 49:1 unun at the antenna, and ~20 mH common mode chokes on both the antenna and receiver ends of the coax to reduce local noise.  At the computer desk where I listen to the SDR, I have the desktop PC, monitor, speakers, and small desk lamp plugged into a 6-outlet plug strip to facilitate turning everything off with a single switch.
Anyway, a couple of weeks ago I plugged a 9-ft two-wire extension cord into the plug strip to power a NiMH charger, as the wall outlet I normally use for charging was being used for some Christmas lights. Since that outlet got freed up a couple of days ago, I’ve been using that outlet again for charging.  Being relaxed (lazy) over the past few days, I had left the extension cord plugged into the plug strip, even though it wasn’t powering anything.  I happened to be monitoring the 15-meter amateur band at the time, and noticed that the waterfall display exhibited a significant reduction in RF noise levels coinciding to when the extension cord was unplugged.  I tried plugging in and unplugging the cord several times with the same result.  Nothing is plugged into receptacle-end of the extension cord.  It would appear that the cord is acting as somewhat of an “antenna” and feeding RF interference into the AC power for the PC listening station.  I captured a screenshot of what I observed.  The lower portion of the waterfall is when the extension cord is NOT plugged in, and the middle portion from 15:37:30 until 15:38:30 is when the extension cord is plugged into the plug strip, during which time noticeable “bands” of hash interference appear.  The extension cord is unplugged at 15:38:30 for the rest of the waterfall, and the noise is reduced.  Orientation of the extension cord did not appreciably change the level of interference, whether it was stretched perpendicular to the computer desk along the wall, or parallel to the desk into the open room.  Even plugging the extension cord into different wall outlets in the room that are on the same circuit as the PC resulted in injecting RF interference into the signal, but to a lesser extent than plugging into the plug strip.  Similar results were observed on different HF frequencies, but to a lesser extent.
Lesson learned – when trying to chase down noise/interference, consider everything.  I never would have guessed that an ‘idle’ extension cord plugged into a grounded plug strip could act as an antenna and feed RF interference into the receiving system.  This has given me a new appreciation of how insidious RF interference can be.
Good luck all, and best of 2022 to everyone.  73
Ken -- WBØOCV


Ken Sejkora
 

Thanks for the insights, Dave.  Following your advice, I placed some clamp-on ferrites onto the extension cord and moved them to various positions along the length of the cord.  The RFI hash never completely disappeared, but was attenuated the most when the ferrites were at the plug-end of the cord (nearest the power strip), and had little or no effect when placed at the receptacle end of the plug.  I should point out that in none of the cases I described was anything plugged into the extension cord, not during the initial tests or during the later tests.  The NiMH charger was not in use during these tests.  In fact, when I plugged the NiMH charger into the receptacle end of the cord, the RFI hash dropped in signal strength, presumably from the added load.

 

I left the unterminated cord plugged into the power strip, and began coiling it up on itself going toward the power strip.  The RFI hash decrteased substantially as the coil neared the power strip.  I can’t say for sure, but I suspect the reduction may have been due to the formation of a “cable choke” with the looped wires, but it may have been due to the decreased length of the extension cord effectively acting as an “antenna”.

 

Going a bit farther, I made up an AC plug with about 10-feet of single wire attached to one of the prongs.  I wanted to test if the RFI was being introduced on/from the neutral line or the “hot”/live line.  I could easily reverse which line the extended wire was connected to.  Although a small amount of RFI was introduced onto the waterfall, the orientation of the plug made no difference in RFI strength, and it was considerably less than when an equivalent length of dual-line extension cord was plugged in.  It is as though the paired/parallel strand extension cord was acting somewhat as a ‘tuned’ circuit coupling the RFI into or out-of the power strip.  I hooked up a multimeter and the extension cord measured out as about 150 pF if the receptacle-end was not shorted.  The maximum RFI hash seemed to be around 22500 kHz, and with extension cord plugged into the power strip the signal in the vicinity of the hash measured -115 dB dBm compared to a noise floor of -131 dBm, for an overall SNR of about 15 to 16 dBm.  Without the extension cord, the signal strength was about -130 dBm compared to a noise floor of -137 dBm, or SNR of about 7 dBm.  Looking at it a different way, plugging in the extension cord raised the RFI hash signal strength from about -130 dBm to -115 dBm, while increasing the noise floor from -137 dBm to -130 dBm.

 

I can’t say for sure whether the extension cord is coupling some RFI *OUT OF* the plug strip and ‘broadcasting’ out into the ether to be picked up by the SDR, or if the extension cord is picking up the RFI and coupling it back *INTO* the AC power to be passed through the PC-to-SDR interface.  Regardless of the mechanism, it is worth noting that plugging extension cords into an AC outlet can cause some strange things to happed, and are worth considering as a potential source of RFI when using an SDR or other receiver.

 

Thanks again Dave.  Have a great weekend and best of 2022.  73

 

Ken  -- WBØOCV

 

From: Dave (NK7Z)
Sent: Friday, December 31, 2021 11:39 AM
To: main@SDR-Radio.groups.io
Subject: Re: [SDR-Radio] Interesting "source" of RF interference

 

Hi,

 

I would put a high Z, ferrite suppression choke on the extension cord at

the plug end as a test.  If the RFI goes away or is reduced, then as you

suggested, the cord is probably a transmitting antenna for the RFI...

 

It is possible your not adding enough choking impedance to the main

antenna feedline, and it is still part of the main antenna system, and

picking up the radiation from the extension cord.  I might move the

feedline around and see if the RFI level is changing as a test of this.

 

If the extension cord test reduces the RFI level, I might add a few more

suppression devices to it, and test again.  If the RFI is reduced, then

I would conclude the extension cord is radiating the EMI from the

charger, and my feedline is picking it up.

 

73, and thanks,

Dave (NK7Z)

 


Kriss Kliegle KA1GJU
 

Power Strip have any hardware inside for filtering, surge, etc?
What's the extension cord do when plugged into the same outlet the power strip is plugged into?

73 Kriss KA1GJU


Dave (NK7Z)
 

Hi Ken,

Thanks for posting that information. That is the first time I have ever heard of that sort of RFI generation, and I will remember it, next time I am looking for RFI for someone...

You might try laying your coax next to the extension cord. If your RFI increases, it would not hurt to add another ferrite choke to your feedline at the antenna feedpoint.

I run chokes at the feedpoint of my vertical, (underground feedline), and at the rear of my radio. Removal of either does mot make an appreciable difference in RFI levels.

I suspect because I have my feedline underground, and it is not picking up RFI, the chokes are not needed.

I once coupled an O-scope to the AC line through a hi-pass filter with a cutoff at around 1 MHz., the amount of crud on the AC line between 1, and 30 MHz., is frightening.

I hope you get the issue removed, and thank you again for passing on what you learned... I will pass this on to others, so your passing it on here, will help many folks beyond this list! THANK YOU!

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

On 12/31/21 14:23, Ken Sejkora wrote:
Thanks for the insights, Dave.  Following your advice, I placed some clamp-on ferrites onto the extension cord and moved them to various positions along the length of the cord.  The RFI hash never completely disappeared, but was attenuated the most when the ferrites were at the plug-end of the cord (nearest the power strip), and had little or no effect when placed at the receptacle end of the plug.  I should point out that in none of the cases I described was anything plugged into the extension cord, not during the initial tests or during the later tests.  The NiMH charger was not in use during these tests.  In fact, when I plugged the NiMH charger into the receptacle end of the cord, the RFI hash dropped in signal strength, presumably from the added load.
I left the unterminated cord plugged into the power strip, and began coiling it up on itself going toward the power strip.  The RFI hash decrteased substantially as the coil neared the power strip.  I can’t say for sure, but I suspect the reduction may have been due to the formation of a “cable choke” with the looped wires, but it may have been due to the decreased length of the extension cord effectively acting as an “antenna”.
Going a bit farther, I made up an AC plug with about 10-feet of single wire attached to one of the prongs.  I wanted to test if the RFI was being introduced on/from the neutral line or the “hot”/live line.  I could easily reverse which line the extended wire was connected to. Although a small amount of RFI was introduced onto the waterfall, the orientation of the plug made no difference in RFI strength, and it was considerably less than when an equivalent length of dual-line extension cord was plugged in.  It is as though the paired/parallel strand extension cord was acting somewhat as a ‘tuned’ circuit coupling the RFI into or out-of the power strip.  I hooked up a multimeter and the extension cord measured out as about 150 pF if the receptacle-end was not shorted.  The maximum RFI hash seemed to be around 22500 kHz, and with extension cord plugged into the power strip the signal in the vicinity of the hash measured -115 dB dBm compared to a noise floor of -131 dBm, for an overall SNR of about 15 to 16 dBm.  Without the extension cord, the signal strength was about -130 dBm compared to a noise floor of -137 dBm, or SNR of about 7 dBm.  Looking at it a different way, plugging in the extension cord raised the RFI hash signal strength from about -130 dBm to -115 dBm, while increasing the noise floor from -137 dBm to -130 dBm.
I can’t say for sure whether the extension cord is coupling some RFI **OUT OF** the plug strip and ‘broadcasting’ out into the ether to be picked up by the SDR, or if the extension cord is picking up the RFI and coupling it back **INTO** the AC power to be passed through the PC-to-SDR interface.  Regardless of the mechanism, it is worth noting that plugging extension cords into an AC outlet can cause some strange things to happed, and are worth considering as a potential source of RFI when using an SDR or other receiver.
Thanks again Dave.  Have a great weekend and best of 2022.  73
Ken  -- WBØOCV
*From: *Dave (NK7Z) <mailto:dave@...>
*Sent: *Friday, December 31, 2021 11:39 AM
*To: *main@SDR-Radio.groups.io <mailto:main@SDR-Radio.groups.io>
*Subject: *Re: [SDR-Radio] Interesting "source" of RF interference
Hi,
I would put a high Z, ferrite suppression choke on the extension cord at
the plug end as a test.  If the RFI goes away or is reduced, then as you
suggested, the cord is probably a transmitting antenna for the RFI...
It is possible your not adding enough choking impedance to the main
antenna feedline, and it is still part of the main antenna system, and
picking up the radiation from the extension cord.  I might move the
feedline around and see if the RFI level is changing as a test of this.
If the extension cord test reduces the RFI level, I might add a few more
suppression devices to it, and test again.  If the RFI is reduced, then
I would conclude the extension cord is radiating the EMI from the
charger, and my feedline is picking it up.
73, and thanks,
Dave (NK7Z)


vince battle
 

Greetings and Happy New Year,

Where is the best place to get the ferrite chokes? And what kind?

Much appreciate your kind reply.

 

Vincent

 

Sent from Mail for Windows

 

From: Dave (NK7Z)
Sent: Friday, December 31, 2021 3:49 PM
To: main@SDR-Radio.groups.io
Subject: Re: [SDR-Radio] Interesting "source" of RF interference

 

Hi Ken,

Thanks for posting that information.  That is the first time I have ever
heard of that sort of RFI generation, and I will remember it, next time
I am looking for RFI for someone...

You might try laying your coax next to the extension cord.  If your RFI
increases, it would not hurt to add another ferrite choke to your
feedline at the antenna feedpoint.

I run chokes at the feedpoint of my vertical, (underground feedline),
and at the rear of my radio.  Removal of either does mot make an
appreciable difference in RFI levels.

I suspect because I have my feedline underground, and it is not picking
up RFI, the chokes are not needed.

I once coupled an O-scope to the AC line through a hi-pass filter with a
cutoff at around 1 MHz., the amount of crud on the AC line between 1,
and 30 MHz., is frightening.

I hope you get the issue removed, and thank you again for passing on
what you learned...  I will pass this on to others, so your passing it
on here, will help many folks beyond this list!  THANK YOU!

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

On 12/31/21 14:23, Ken Sejkora wrote:
> Thanks for the insights, Dave.  Following your advice, I placed some
> clamp-on ferrites onto the extension cord and moved them to various
> positions along the length of the cord.  The RFI hash never completely
> disappeared, but was attenuated the most when the ferrites were at the
> plug-end of the cord (nearest the power strip), and had little or no
> effect when placed at the receptacle end of the plug.  I should point
> out that in none of the cases I described was anything plugged into the
> extension cord, not during the initial tests or during the later tests.
>   The NiMH charger was not in use during these tests.  In fact, when I
> plugged the NiMH charger into the receptacle end of the cord, the RFI
> hash dropped in signal strength, presumably from the added load.
>
> I left the unterminated cord plugged into the power strip, and began
> coiling it up on itself going toward the power strip.  The RFI hash
> decrteased substantially as the coil neared the power strip.  I can’t
> say for sure, but I suspect the reduction may have been due to the
> formation of a “cable choke” with the looped wires, but it may have been
> due to the decreased length of the extension cord effectively acting as
> an “antenna”.
>
> Going a bit farther, I made up an AC plug with about 10-feet of single
> wire attached to one of the prongs.  I wanted to test if the RFI was
> being introduced on/from the neutral line or the “hot”/live line.  I
> could easily reverse which line the extended wire was connected to. 
> Although a small amount of RFI was introduced onto the waterfall, the
> orientation of the plug made no difference in RFI strength, and it was
> considerably less than when an equivalent length of dual-line extension
> cord was plugged in.  It is as though the paired/parallel strand
> extension cord was acting somewhat as a ‘tuned’ circuit coupling the RFI
> into or out-of the power strip.  I hooked up a multimeter and the
> extension cord measured out as about 150 pF if the receptacle-end was
> not shorted.  The maximum RFI hash seemed to be around 22500 kHz, and
> with extension cord plugged into the power strip the signal in the
> vicinity of the hash measured -115 dB dBm compared to a noise floor of
> -131 dBm, for an overall SNR of about 15 to 16 dBm.  Without the
> extension cord, the signal strength was about -130 dBm compared to a
> noise floor of -137 dBm, or SNR of about 7 dBm.  Looking at it a
> different way, plugging in the extension cord raised the RFI hash signal
> strength from about -130 dBm to -115 dBm, while increasing the noise
> floor from -137 dBm to -130 dBm.
>
> I can’t say for sure whether the extension cord is coupling some RFI
> **OUT OF** the plug strip and ‘broadcasting’ out into the ether to be
> picked up by the SDR, or if the extension cord is picking up the RFI and
> coupling it back **INTO** the AC power to be passed through the
> PC-to-SDR interface.  Regardless of the mechanism, it is worth noting
> that plugging extension cords into an AC outlet can cause some strange
> things to happed, and are worth considering as a potential source of RFI
> when using an SDR or other receiver.
>
> Thanks again Dave.  Have a great weekend and best of 2022.  73
>
> Ken  -- WBØOCV
>
> *From: *Dave (NK7Z) <mailto:dave@...>
> *Sent: *Friday, December 31, 2021 11:39 AM
> *To: *main@SDR-Radio.groups.io <mailto:main@SDR-Radio.groups.io>
> *Subject: *Re: [SDR-Radio] Interesting "source" of RF interference
>
> Hi,
>
> I would put a high Z, ferrite suppression choke on the extension cord at
>
> the plug end as a test.  If the RFI goes away or is reduced, then as you
>
> suggested, the cord is probably a transmitting antenna for the RFI...
>
> It is possible your not adding enough choking impedance to the main
>
> antenna feedline, and it is still part of the main antenna system, and
>
> picking up the radiation from the extension cord.  I might move the
>
> feedline around and see if the RFI level is changing as a test of this.
>
> If the extension cord test reduces the RFI level, I might add a few more
>
> suppression devices to it, and test again.  If the RFI is reduced, then
>
> I would conclude the extension cord is radiating the EMI from the
>
> charger, and my feedline is picking it up.
>
> 73, and thanks,
>
> Dave (NK7Z)
>
>




 


Dave (NK7Z)
 

Hi Vince,

See K9YC's site at:

http://audiosystemsgroup.com/K9YC/K9YC-old.htm

for everything you ever wanted to know about building chokes...

I get my ferrites from Newark, or Digikey... Lowest prices around. IT PAYS TO SHOP AROUND! Some places, charge 10 bucks for an FT-240/31, while the same choke is 5.63 at another place... I ALWAYS use Fair-Rite chokes, almost always mix 31.

Also see the attached chart. If the attach fails I'll send it to you directly.

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

On 12/31/21 15:57, vince battle wrote:
Greetings and Happy New Year,
Where is the best place to get the ferrite chokes? And what kind?
Much appreciate your kind reply.
Vincent
Sent from Mail <https://go.microsoft.com/fwlink/?LinkId=550986> for Windows
*From: *Dave (NK7Z) <mailto:dave@...>
*Sent: *Friday, December 31, 2021 3:49 PM
*To: *main@SDR-Radio.groups.io <mailto:main@SDR-Radio.groups.io>
*Subject: *Re: [SDR-Radio] Interesting "source" of RF interference
Hi Ken,
Thanks for posting that information.  That is the first time I have ever
heard of that sort of RFI generation, and I will remember it, next time
I am looking for RFI for someone...
You might try laying your coax next to the extension cord.  If your RFI
increases, it would not hurt to add another ferrite choke to your
feedline at the antenna feedpoint.
I run chokes at the feedpoint of my vertical, (underground feedline),
and at the rear of my radio.  Removal of either does mot make an
appreciable difference in RFI levels.
I suspect because I have my feedline underground, and it is not picking
up RFI, the chokes are not needed.
I once coupled an O-scope to the AC line through a hi-pass filter with a
cutoff at around 1 MHz., the amount of crud on the AC line between 1,
and 30 MHz., is frightening.
I hope you get the issue removed, and thank you again for passing on
what you learned...  I will pass this on to others, so your passing it
on here, will help many folks beyond this list!  THANK YOU!
73, and thanks,
Dave (NK7Z)
https://www.nk7z.net <https://www.nk7z.net>
ARRL Volunteer Examiner
ARRL Technical Specialist, RFI
ARRL Asst. Director, NW Division, Technical Resources
On 12/31/21 14:23, Ken Sejkora wrote:
> Thanks for the insights, Dave.  Following your advice, I placed some
> clamp-on ferrites onto the extension cord and moved them to various
> positions along the length of the cord.  The RFI hash never completely
> disappeared, but was attenuated the most when the ferrites were at the
> plug-end of the cord (nearest the power strip), and had little or no
> effect when placed at the receptacle end of the plug.  I should point
> out that in none of the cases I described was anything plugged into the
> extension cord, not during the initial tests or during the later tests.
>   The NiMH charger was not in use during these tests.  In fact, when I
> plugged the NiMH charger into the receptacle end of the cord, the RFI
> hash dropped in signal strength, presumably from the added load.
>
> I left the unterminated cord plugged into the power strip, and began
> coiling it up on itself going toward the power strip.  The RFI hash
> decrteased substantially as the coil neared the power strip.  I can’t
> say for sure, but I suspect the reduction may have been due to the
> formation of a “cable choke” with the looped wires, but it may have been
> due to the decreased length of the extension cord effectively acting as
> an “antenna”.
>
> Going a bit farther, I made up an AC plug with about 10-feet of single
> wire attached to one of the prongs.  I wanted to test if the RFI was
> being introduced on/from the neutral line or the “hot”/live line.  I
> could easily reverse which line the extended wire was connected to.
> Although a small amount of RFI was introduced onto the waterfall, the
> orientation of the plug made no difference in RFI strength, and it was
> considerably less than when an equivalent length of dual-line extension
> cord was plugged in.  It is as though the paired/parallel strand
> extension cord was acting somewhat as a ‘tuned’ circuit coupling the RFI
> into or out-of the power strip.  I hooked up a multimeter and the
> extension cord measured out as about 150 pF if the receptacle-end was
> not shorted.  The maximum RFI hash seemed to be around 22500 kHz, and
> with extension cord plugged into the power strip the signal in the
> vicinity of the hash measured -115 dB dBm compared to a noise floor of
> -131 dBm, for an overall SNR of about 15 to 16 dBm.  Without the
> extension cord, the signal strength was about -130 dBm compared to a
> noise floor of -137 dBm, or SNR of about 7 dBm.  Looking at it a
> different way, plugging in the extension cord raised the RFI hash signal
> strength from about -130 dBm to -115 dBm, while increasing the noise
> floor from -137 dBm to -130 dBm.
>
> I can’t say for sure whether the extension cord is coupling some RFI
> **OUT OF** the plug strip and ‘broadcasting’ out into the ether to be
> picked up by the SDR, or if the extension cord is picking up the RFI and
> coupling it back **INTO** the AC power to be passed through the
> PC-to-SDR interface.  Regardless of the mechanism, it is worth noting
> that plugging extension cords into an AC outlet can cause some strange
> things to happed, and are worth considering as a potential source of RFI
> when using an SDR or other receiver.
>
> Thanks again Dave.  Have a great weekend and best of 2022.  73
>
> Ken  -- WBØOCV
>
> *From: *Dave (NK7Z) <mailto:dave@... <mailto:dave@...>>
> *Sent: *Friday, December 31, 2021 11:39 AM
> *To: *main@SDR-Radio.groups.io <mailto:main@SDR-Radio.groups.io
<mailto:main@SDR-Radio.groups.io>>
> *Subject: *Re: [SDR-Radio] Interesting "source" of RF interference
>
> Hi,
>
> I would put a high Z, ferrite suppression choke on the extension cord at
>
> the plug end as a test.  If the RFI goes away or is reduced, then as you
>
> suggested, the cord is probably a transmitting antenna for the RFI...
>
> It is possible your not adding enough choking impedance to the main
>
> antenna feedline, and it is still part of the main antenna system, and
>
> picking up the radiation from the extension cord.  I might move the
>
> feedline around and see if the RFI level is changing as a test of this.
>
> If the extension cord test reduces the RFI level, I might add a few more
>
> suppression devices to it, and test again.  If the RFI is reduced, then
>
> I would conclude the extension cord is radiating the EMI from the
>
> charger, and my feedline is picking it up.
>
> 73, and thanks,
>
> Dave (NK7Z)
>
>


vince battle
 

Thank you Dave, chart came through fine, and thanks for the recommendations

s. Happy New Year (all mankind hopes!)

 

Vincent

 

Sent from Mail for Windows

 

From: Dave (NK7Z)
Sent: Friday, December 31, 2021 4:10 PM
To: main@SDR-Radio.groups.io
Subject: Re: [SDR-Radio] Interesting "source" of RF interference

 

Hi Vince,

See K9YC's site at:

http://audiosystemsgroup.com/K9YC/K9YC-old.htm

for everything you ever wanted to know about building chokes...

I get my ferrites from Newark, or Digikey...  Lowest prices around.  IT
PAYS TO SHOP AROUND!  Some places, charge 10 bucks for an FT-240/31,
while the same choke is 5.63 at another place...  I ALWAYS use Fair-Rite
chokes, almost always mix 31.

Also see the attached chart.  If the attach fails I'll send it to you
directly.

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

On 12/31/21 15:57, vince battle wrote:
> Greetings and Happy New Year,
>
> Where is the best place to get the ferrite chokes? And what kind?
>
> Much appreciate your kind reply.
>
> Vincent
>
> Sent from Mail <https://go.microsoft.com/fwlink/?LinkId=550986> for Windows
>
> *From: *Dave (NK7Z) <mailto:dave@...>
> *Sent: *Friday, December 31, 2021 3:49 PM
> *To: *main@SDR-Radio.groups.io <mailto:main@SDR-Radio.groups.io>
> *Subject: *Re: [SDR-Radio] Interesting "source" of RF interference
>
> Hi Ken,
>
> Thanks for posting that information.  That is the first time I have ever
> heard of that sort of RFI generation, and I will remember it, next time
> I am looking for RFI for someone...
>
> You might try laying your coax next to the extension cord.  If your RFI
> increases, it would not hurt to add another ferrite choke to your
> feedline at the antenna feedpoint.
>
> I run chokes at the feedpoint of my vertical, (underground feedline),
> and at the rear of my radio.  Removal of either does mot make an
> appreciable difference in RFI levels.
>
> I suspect because I have my feedline underground, and it is not picking
> up RFI, the chokes are not needed.
>
> I once coupled an O-scope to the AC line through a hi-pass filter with a
> cutoff at around 1 MHz., the amount of crud on the AC line between 1,
> and 30 MHz., is frightening.
>
> I hope you get the issue removed, and thank you again for passing on
> what you learned...  I will pass this on to others, so your passing it
> on here, will help many folks beyond this list!  THANK YOU!
>
> 73, and thanks,
> Dave (NK7Z)
> https://www.nk7z.net <https://www.nk7z.net>
> ARRL Volunteer Examiner
> ARRL Technical Specialist, RFI
> ARRL Asst. Director, NW Division, Technical Resources
>
> On 12/31/21 14:23, Ken Sejkora wrote:
>  > Thanks for the insights, Dave.  Following your advice, I placed some
>  > clamp-on ferrites onto the extension cord and moved them to various
>  > positions along the length of the cord.  The RFI hash never completely
>  > disappeared, but was attenuated the most when the ferrites were at the
>  > plug-end of the cord (nearest the power strip), and had little or no
>  > effect when placed at the receptacle end of the plug.  I should point
>  > out that in none of the cases I described was anything plugged into the
>  > extension cord, not during the initial tests or during the later tests.
>  >   The NiMH charger was not in use during these tests.  In fact, when I
>  > plugged the NiMH charger into the receptacle end of the cord, the RFI
>  > hash dropped in signal strength, presumably from the added load.
>  >
>  > I left the unterminated cord plugged into the power strip, and began
>  > coiling it up on itself going toward the power strip.  The RFI hash
>  > decrteased substantially as the coil neared the power strip.  I can’t
>  > say for sure, but I suspect the reduction may have been due to the
>  > formation of a “cable choke” with the looped wires, but it may have been
>  > due to the decreased length of the extension cord effectively acting as
>  > an “antenna”.
>  >
>  > Going a bit farther, I made up an AC plug with about 10-feet of single
>  > wire attached to one of the prongs.  I wanted to test if the RFI was
>  > being introduced on/from the neutral line or the “hot”/live line.  I
>  > could easily reverse which line the extended wire was connected to.
>  > Although a small amount of RFI was introduced onto the waterfall, the
>  > orientation of the plug made no difference in RFI strength, and it was
>  > considerably less than when an equivalent length of dual-line extension
>  > cord was plugged in.  It is as though the paired/parallel strand
>  > extension cord was acting somewhat as a ‘tuned’ circuit coupling the RFI
>  > into or out-of the power strip.  I hooked up a multimeter and the
>  > extension cord measured out as about 150 pF if the receptacle-end was
>  > not shorted.  The maximum RFI hash seemed to be around 22500 kHz, and
>  > with extension cord plugged into the power strip the signal in the
>  > vicinity of the hash measured -115 dB dBm compared to a noise floor of
>  > -131 dBm, for an overall SNR of about 15 to 16 dBm.  Without the
>  > extension cord, the signal strength was about -130 dBm compared to a
>  > noise floor of -137 dBm, or SNR of about 7 dBm.  Looking at it a
>  > different way, plugging in the extension cord raised the RFI hash signal
>  > strength from about -130 dBm to -115 dBm, while increasing the noise
>  > floor from -137 dBm to -130 dBm.
>  >
>  > I can’t say for sure whether the extension cord is coupling some RFI
>  > **OUT OF** the plug strip and ‘broadcasting’ out into the ether to be
>  > picked up by the SDR, or if the extension cord is picking up the RFI and
>  > coupling it back **INTO** the AC power to be passed through the
>  > PC-to-SDR interface.  Regardless of the mechanism, it is worth noting
>  > that plugging extension cords into an AC outlet can cause some strange
>  > things to happed, and are worth considering as a potential source of RFI
>  > when using an SDR or other receiver.
>  >
>  > Thanks again Dave.  Have a great weekend and best of 2022.  73
>  >
>  > Ken  -- WBØOCV
>  >
>  > *From: *Dave (NK7Z) <mailto:dave@... <mailto:dave@...>>
>  > *Sent: *Friday, December 31, 2021 11:39 AM
>  > *To: *main@SDR-Radio.groups.io <mailto:main@SDR-Radio.groups.io
> <mailto:main@SDR-Radio.groups.io>>
>  > *Subject: *Re: [SDR-Radio] Interesting "source" of RF interference
>  >
>  > Hi,
>  >
>  > I would put a high Z, ferrite suppression choke on the extension cord at
>  >
>  > the plug end as a test.  If the RFI goes away or is reduced, then as you
>  >
>  > suggested, the cord is probably a transmitting antenna for the RFI...
>  >
>  > It is possible your not adding enough choking impedance to the main
>  >
>  > antenna feedline, and it is still part of the main antenna system, and
>  >
>  > picking up the radiation from the extension cord.  I might move the
>  >
>  > feedline around and see if the RFI level is changing as a test of this.
>  >
>  > If the extension cord test reduces the RFI level, I might add a few more
>  >
>  > suppression devices to it, and test again.  If the RFI is reduced, then
>  >
>  > I would conclude the extension cord is radiating the EMI from the
>  >
>  > charger, and my feedline is picking it up.
>  >
>  > 73, and thanks,
>  >
>  > Dave (NK7Z)
>  >
>  >
>
>
>
>
>




 


Robert Lorenzini
 

I wonder if it could be a bad MOV in the strip?

Bob - wd6dod

On 12/31/2021 2:23 PM, Ken Sejkora wrote:

Thanks for the insights, Dave.  Following your advice, I placed some clamp-on ferrites onto the extension cord and moved them to various positions along the length of the cord.  The RFI hash never completely disappeared, but was attenuated the most when the ferrites were at the plug-end of the cord (nearest the power strip), and had little or no effect when placed at the receptacle end of the plug.  I should point out that in none of the cases I described was anything plugged into the extension cord, not during the initial tests or during the later tests.  The NiMH charger was not in use during these tests.  In fact, when I plugged the NiMH charger into the receptacle end of the cord, the RFI hash dropped in signal strength, presumably from the added load.

 

I left the unterminated cord plugged into the power strip, and began coiling it up on itself going toward the power strip.  The RFI hash decrteased substantially as the coil neared the power strip.  I can’t say for sure, but I suspect the reduction may have been due to the formation of a “cable choke” with the looped wires, but it may have been due to the decreased length of the extension cord effectively acting as an “antenna”.

 

Going a bit farther, I made up an AC plug with about 10-feet of single wire attached to one of the prongs.  I wanted to test if the RFI was being introduced on/from the neutral line or the “hot”/live line.  I could easily reverse which line the extended wire was connected to.  Although a small amount of RFI was introduced onto the waterfall, the orientation of the plug made no difference in RFI strength, and it was considerably less than when an equivalent length of dual-line extension cord was plugged in.  It is as though the paired/parallel strand extension cord was acting somewhat as a ‘tuned’ circuit coupling the RFI into or out-of the power strip.  I hooked up a multimeter and the extension cord measured out as about 150 pF if the receptacle-end was not shorted.  The maximum RFI hash seemed to be around 22500 kHz, and with extension cord plugged into the power strip the signal in the vicinity of the hash measured -115 dB dBm compared to a noise floor of -131 dBm, for an overall SNR of about 15 to 16 dBm.  Without the extension cord, the signal strength was about -130 dBm compared to a noise floor of -137 dBm, or SNR of about 7 dBm.  Looking at it a different way, plugging in the extension cord raised the RFI hash signal strength from about -130 dBm to -115 dBm, while increasing the noise floor from -137 dBm to -130 dBm.

 

I can’t say for sure whether the extension cord is coupling some RFI *OUT OF* the plug strip and ‘broadcasting’ out into the ether to be picked up by the SDR, or if the extension cord is picking up the RFI and coupling it back *INTO* the AC power to be passed through the PC-to-SDR interface.  Regardless of the mechanism, it is worth noting that plugging extension cords into an AC outlet can cause some strange things to happed, and are worth considering as a potential source of RFI when using an SDR or other receiver.

 

Thanks again Dave.  Have a great weekend and best of 2022.  73

 

Ken  -- WBØOCV

 

From: Dave (NK7Z)
Sent: Friday, December 31, 2021 11:39 AM
To: main@SDR-Radio.groups.io
Subject: Re: [SDR-Radio] Interesting "source" of RF interference

 

Hi,

 

I would put a high Z, ferrite suppression choke on the extension cord at

the plug end as a test.  If the RFI goes away or is reduced, then as you

suggested, the cord is probably a transmitting antenna for the RFI...

 

It is possible your not adding enough choking impedance to the main

antenna feedline, and it is still part of the main antenna system, and

picking up the radiation from the extension cord.  I might move the

feedline around and see if the RFI level is changing as a test of this.

 

If the extension cord test reduces the RFI level, I might add a few more

suppression devices to it, and test again.  If the RFI is reduced, then

I would conclude the extension cord is radiating the EMI from the

charger, and my feedline is picking it up.

 

73, and thanks,

Dave (NK7Z)

 



Allan Isaacs
 

I thought much the same Kriss. Some extension cords have a suppressor for a phone lead or similar and some now even have a built-in USB power charging socket.

Or even maybe a pair of capacitors to ground which may be leaking in some way.

Allan G3PIY


From: main@SDR-Radio.groups.io [mailto:main@SDR-Radio.groups.io] On Behalf Of Kriss Kliegle KA1GJU
Sent: 31 December 2021 22:51
To: main@SDR-Radio.groups.io
Subject: Re: [SDR-Radio] Interesting "source" of RF interference

 

Power Strip have any hardware inside for filtering, surge, etc?
What's the extension cord do when plugged into the same outlet the power strip is plugged into?

73 Kriss KA1GJU

 


Ken Sejkora
 

Hi Kriss,

 

Yes, the extension cord did increase RFI when plugged into the wall outlet that the power strip is plugged into, but probably about 5 or 6 dBm lower than when plugged directly into the power strip.  I plugged the extension cord into several other wall outlets in the room that are fed off the same breaker, and there was always a small amount of RFI introduced, but levels were lower the farther away the wall outlet was.  Tomorrow I might try a different extension cord, as O guess it is possible there is some type of strange fault in the cord introducing noise, but I don’t think that’s the case.

 

Thanks again for the question. 

 

73 --  Ken

 

From: Kriss Kliegle KA1GJU
Sent: Friday, December 31, 2021 05:50 PM
To: main@SDR-Radio.groups.io
Subject: Re: [SDR-Radio] Interesting "source" of RF interference

 

Power Strip have any hardware inside for filtering, surge, etc?
What's the extension cord do when plugged into the same outlet the power strip is plugged into?

73 Kriss KA1GJU

 


jdow
 

You'll hate me for this Microsoft answer, 100% accurate and 100% useless. "Where it works."

I guess what that says is work with some trial and accidental success, admitting to no errors.

In general I would start with the choke close to the front end device if it is on a wire to the front end device. Otherwise, try it to find out what works best for your personal nest of wires.

{O.O}   Wish I could give a magic quick solution; but, such does not exist.

(If several copies of this appear, please excuse me. My usual mail service is borked at present. This is a backup.)

On 20211231 15:57:55, vince battle wrote:

Greetings and Happy New Year,

Where is the best place to get the ferrite chokes? And what kind?

Much appreciate your kind reply.

 

Vincent

 

Sent from Mail for Windows

 

From: Dave (NK7Z)
Sent: Friday, December 31, 2021 3:49 PM
To: main@SDR-Radio.groups.io
Subject: Re: [SDR-Radio] Interesting "source" of RF interference

 

Hi Ken,

Thanks for posting that information.  That is the first time I have ever
heard of that sort of RFI generation, and I will remember it, next time
I am looking for RFI for someone...

You might try laying your coax next to the extension cord.  If your RFI
increases, it would not hurt to add another ferrite choke to your
feedline at the antenna feedpoint.

I run chokes at the feedpoint of my vertical, (underground feedline),
and at the rear of my radio.  Removal of either does mot make an
appreciable difference in RFI levels.

I suspect because I have my feedline underground, and it is not picking
up RFI, the chokes are not needed.

I once coupled an O-scope to the AC line through a hi-pass filter with a
cutoff at around 1 MHz., the amount of crud on the AC line between 1,
and 30 MHz., is frightening.

I hope you get the issue removed, and thank you again for passing on
what you learned...  I will pass this on to others, so your passing it
on here, will help many folks beyond this list!  THANK YOU!

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

On 12/31/21 14:23, Ken Sejkora wrote:
> Thanks for the insights, Dave.  Following your advice, I placed some
> clamp-on ferrites onto the extension cord and moved them to various
> positions along the length of the cord.  The RFI hash never completely
> disappeared, but was attenuated the most when the ferrites were at the
> plug-end of the cord (nearest the power strip), and had little or no
> effect when placed at the receptacle end of the plug.  I should point
> out that in none of the cases I described was anything plugged into the
> extension cord, not during the initial tests or during the later tests.
>   The NiMH charger was not in use during these tests.  In fact, when I
> plugged the NiMH charger into the receptacle end of the cord, the RFI
> hash dropped in signal strength, presumably from the added load.
>
> I left the unterminated cord plugged into the power strip, and began
> coiling it up on itself going toward the power strip.  The RFI hash
> decrteased substantially as the coil neared the power strip.  I can’t
> say for sure, but I suspect the reduction may have been due to the
> formation of a “cable choke” with the looped wires, but it may have been
> due to the decreased length of the extension cord effectively acting as
> an “antenna”.
>
> Going a bit farther, I made up an AC plug with about 10-feet of single
> wire attached to one of the prongs.  I wanted to test if the RFI was
> being introduced on/from the neutral line or the “hot”/live line.  I
> could easily reverse which line the extended wire was connected to. 
> Although a small amount of RFI was introduced onto the waterfall, the
> orientation of the plug made no difference in RFI strength, and it was
> considerably less than when an equivalent length of dual-line extension
> cord was plugged in.  It is as though the paired/parallel strand
> extension cord was acting somewhat as a ‘tuned’ circuit coupling the RFI
> into or out-of the power strip.  I hooked up a multimeter and the
> extension cord measured out as about 150 pF if the receptacle-end was
> not shorted.  The maximum RFI hash seemed to be around 22500 kHz, and
> with extension cord plugged into the power strip the signal in the
> vicinity of the hash measured -115 dB dBm compared to a noise floor of
> -131 dBm, for an overall SNR of about 15 to 16 dBm.  Without the
> extension cord, the signal strength was about -130 dBm compared to a
> noise floor of -137 dBm, or SNR of about 7 dBm.  Looking at it a
> different way, plugging in the extension cord raised the RFI hash signal
> strength from about -130 dBm to -115 dBm, while increasing the noise
> floor from -137 dBm to -130 dBm.
>
> I can’t say for sure whether the extension cord is coupling some RFI
> **OUT OF** the plug strip and ‘broadcasting’ out into the ether to be
> picked up by the SDR, or if the extension cord is picking up the RFI and
> coupling it back **INTO** the AC power to be passed through the
> PC-to-SDR interface.  Regardless of the mechanism, it is worth noting
> that plugging extension cords into an AC outlet can cause some strange
> things to happed, and are worth considering as a potential source of RFI
> when using an SDR or other receiver.
>
> Thanks again Dave.  Have a great weekend and best of 2022.  73
>
> Ken  -- WBØOCV
>
> *From: *Dave (NK7Z) <mailto:dave@...>
> *Sent: *Friday, December 31, 2021 11:39 AM
> *To: *main@SDR-Radio.groups.io <mailto:main@SDR-Radio.groups.io>
> *Subject: *Re: [SDR-Radio] Interesting "source" of RF interference
>
> Hi,
>
> I would put a high Z, ferrite suppression choke on the extension cord at
>
> the plug end as a test.  If the RFI goes away or is reduced, then as you
>
> suggested, the cord is probably a transmitting antenna for the RFI...
>
> It is possible your not adding enough choking impedance to the main
>
> antenna feedline, and it is still part of the main antenna system, and
>
> picking up the radiation from the extension cord.  I might move the
>
> feedline around and see if the RFI level is changing as a test of this.
>
> If the extension cord test reduces the RFI level, I might add a few more
>
> suppression devices to it, and test again.  If the RFI is reduced, then
>
> I would conclude the extension cord is radiating the EMI from the
>
> charger, and my feedline is picking it up.
>
> 73, and thanks,
>
> Dave (NK7Z)
>
>




 



Ken Sejkora
 

Thanks jdow.  Although I am indeed interested in identifying and take the steps necessary to eliminate/minimize the source of the interference, the primary objective of my original and follow-up posts was to illustrate that RFI can crop up from what might initially appear to be a very unlikely source.  Who would have thought that simply plugging in an unterminated extension cord would generate 15 dBm RFI at 22 MHz?  That concept would not have occurred to me before yesterday, and in this case it was easy to at least reduce the RFI through a very simple approach – just unplug the stupid cord.  I didn’t need any special shielding, ferrites, chokes, etc. to cure this particular problem.

 

However, now I’d like to go the next step and understand why the cord is introducing RFI, and even more importantly, to let others know of my experience so that they can also learn and hopefully benefit from my experience.

 

Thanks again for your insights, Joanne.  Happy 2022 to all.  73

 

Ken --  WBØOCV

 

From: jdow
Sent: Friday, December 31, 2021 10:11 PM
To: main@SDR-Radio.groups.io
Subject: Re: [SDR-Radio] Interesting "source" of RF interference

 

You'll hate me for this Microsoft answer, 100% accurate and 100% useless. "Where it works."

I guess what that says is work with some trial and accidental success, admitting to no errors.

In general I would start with the choke close to the front end device if it is on a wire to the front end device. Otherwise, try it to find out what works best for your personal nest of wires.

{O.O}   Wish I could give a magic quick solution; but, such does not exist.

 


Dave (NK7Z)
 

Vince,
You are most welcome...

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

On 12/31/21 16:17, vince battle wrote:
Thank you Dave, chart came through fine, and thanks for the recommendations
s. Happy New Year (all mankind hopes!)
Vincent
Sent from Mail <https://go.microsoft.com/fwlink/?LinkId=550986> for Windows
*From: *Dave (NK7Z) <mailto:dave@...>
*Sent: *Friday, December 31, 2021 4:10 PM
*To: *main@SDR-Radio.groups.io <mailto:main@SDR-Radio.groups.io>
*Subject: *Re: [SDR-Radio] Interesting "source" of RF interference
Hi Vince,
See K9YC's site at:
http://audiosystemsgroup.com/K9YC/K9YC-old.htm <http://audiosystemsgroup.com/K9YC/K9YC-old.htm>
for everything you ever wanted to know about building chokes...
I get my ferrites from Newark, or Digikey...  Lowest prices around.  IT
PAYS TO SHOP AROUND!  Some places, charge 10 bucks for an FT-240/31,
while the same choke is 5.63 at another place...  I ALWAYS use Fair-Rite
chokes, almost always mix 31.
Also see the attached chart.  If the attach fails I'll send it to you
directly.
73, and thanks,
Dave (NK7Z)
https://www.nk7z.net <https://www.nk7z.net>
ARRL Volunteer Examiner
ARRL Technical Specialist, RFI
ARRL Asst. Director, NW Division, Technical Resources
On 12/31/21 15:57, vince battle wrote:
> Greetings and Happy New Year,
>
> Where is the best place to get the ferrite chokes? And what kind?
>
> Much appreciate your kind reply.
>
> Vincent
>
> Sent from Mail <https://go.microsoft.com/fwlink/?LinkId=550986
<https://go.microsoft.com/fwlink/?LinkId=550986>> for Windows
>
> *From: *Dave (NK7Z) <mailto:dave@... <mailto:dave@...>>
> *Sent: *Friday, December 31, 2021 3:49 PM
> *To: *main@SDR-Radio.groups.io <mailto:main@SDR-Radio.groups.io
<mailto:main@SDR-Radio.groups.io>>
> *Subject: *Re: [SDR-Radio] Interesting "source" of RF interference
>
> Hi Ken,
>
> Thanks for posting that information.  That is the first time I have ever
> heard of that sort of RFI generation, and I will remember it, next time
> I am looking for RFI for someone...
>
> You might try laying your coax next to the extension cord.  If your RFI
> increases, it would not hurt to add another ferrite choke to your
> feedline at the antenna feedpoint.
>
> I run chokes at the feedpoint of my vertical, (underground feedline),
> and at the rear of my radio.  Removal of either does mot make an
> appreciable difference in RFI levels.
>
> I suspect because I have my feedline underground, and it is not picking
> up RFI, the chokes are not needed.
>
> I once coupled an O-scope to the AC line through a hi-pass filter with a
> cutoff at around 1 MHz., the amount of crud on the AC line between 1,
> and 30 MHz., is frightening.
>
> I hope you get the issue removed, and thank you again for passing on
> what you learned...  I will pass this on to others, so your passing it
> on here, will help many folks beyond this list!  THANK YOU!
>
> 73, and thanks,
> Dave (NK7Z)
> https://www.nk7z.net <https://www.nk7z.net> <https://www.nk7z.net
<https://www.nk7z.net>>
> ARRL Volunteer Examiner
> ARRL Technical Specialist, RFI
> ARRL Asst. Director, NW Division, Technical Resources
>
> On 12/31/21 14:23, Ken Sejkora wrote:
>  > Thanks for the insights, Dave.  Following your advice, I placed some
>  > clamp-on ferrites onto the extension cord and moved them to various
>  > positions along the length of the cord.  The RFI hash never completely
>  > disappeared, but was attenuated the most when the ferrites were at the
>  > plug-end of the cord (nearest the power strip), and had little or no
>  > effect when placed at the receptacle end of the plug.  I should point
>  > out that in none of the cases I described was anything plugged
into the
>  > extension cord, not during the initial tests or during the later
tests.
>  >   The NiMH charger was not in use during these tests.  In fact, when I
>  > plugged the NiMH charger into the receptacle end of the cord, the RFI
>  > hash dropped in signal strength, presumably from the added load.
>  >
>  > I left the unterminated cord plugged into the power strip, and began
>  > coiling it up on itself going toward the power strip.  The RFI hash
>  > decrteased substantially as the coil neared the power strip.  I can’t
>  > say for sure, but I suspect the reduction may have been due to the
>  > formation of a “cable choke” with the looped wires, but it may
have been
>  > due to the decreased length of the extension cord effectively
acting as
>  > an “antenna”.
>  >
>  > Going a bit farther, I made up an AC plug with about 10-feet of single
>  > wire attached to one of the prongs.  I wanted to test if the RFI was
>  > being introduced on/from the neutral line or the “hot”/live line.  I
>  > could easily reverse which line the extended wire was connected to.
>  > Although a small amount of RFI was introduced onto the waterfall, the
>  > orientation of the plug made no difference in RFI strength, and it was
>  > considerably less than when an equivalent length of dual-line
extension
>  > cord was plugged in.  It is as though the paired/parallel strand
>  > extension cord was acting somewhat as a ‘tuned’ circuit coupling
the RFI
>  > into or out-of the power strip.  I hooked up a multimeter and the
>  > extension cord measured out as about 150 pF if the receptacle-end was
>  > not shorted.  The maximum RFI hash seemed to be around 22500 kHz, and
>  > with extension cord plugged into the power strip the signal in the
>  > vicinity of the hash measured -115 dB dBm compared to a noise floor of
>  > -131 dBm, for an overall SNR of about 15 to 16 dBm.  Without the
>  > extension cord, the signal strength was about -130 dBm compared to a
>  > noise floor of -137 dBm, or SNR of about 7 dBm.  Looking at it a
>  > different way, plugging in the extension cord raised the RFI hash
signal
>  > strength from about -130 dBm to -115 dBm, while increasing the noise
>  > floor from -137 dBm to -130 dBm.
>  >
>  > I can’t say for sure whether the extension cord is coupling some RFI
>  > **OUT OF** the plug strip and ‘broadcasting’ out into the ether to be
>  > picked up by the SDR, or if the extension cord is picking up the
RFI and
>  > coupling it back **INTO** the AC power to be passed through the
>  > PC-to-SDR interface.  Regardless of the mechanism, it is worth noting
>  > that plugging extension cords into an AC outlet can cause some strange
>  > things to happed, and are worth considering as a potential source
of RFI
>  > when using an SDR or other receiver.
>  >
>  > Thanks again Dave.  Have a great weekend and best of 2022.  73
>  >
>  > Ken  -- WBØOCV
>  >
>  > *From: *Dave (NK7Z) <mailto:dave@... <mailto:dave@...
<mailto:dave@...>>>
>  > *Sent: *Friday, December 31, 2021 11:39 AM
>  > *To: *main@SDR-Radio.groups.io <mailto:main@SDR-Radio.groups.io
> <mailto:main@SDR-Radio.groups.io <mailto:main@SDR-Radio.groups.io>>>
>  > *Subject: *Re: [SDR-Radio] Interesting "source" of RF interference
>  >
>  > Hi,
>  >
>  > I would put a high Z, ferrite suppression choke on the extension
cord at
>  >
>  > the plug end as a test.  If the RFI goes away or is reduced, then
as you
>  >
>  > suggested, the cord is probably a transmitting antenna for the RFI...
>  >
>  > It is possible your not adding enough choking impedance to the main
>  >
>  > antenna feedline, and it is still part of the main antenna system, and
>  >
>  > picking up the radiation from the extension cord.  I might move the
>  >
>  > feedline around and see if the RFI level is changing as a test of
this.
>  >
>  > If the extension cord test reduces the RFI level, I might add a
few more
>  >
>  > suppression devices to it, and test again.  If the RFI is reduced,
then
>  >
>  > I would conclude the extension cord is radiating the EMI from the
>  >
>  > charger, and my feedline is picking it up.
>  >
>  > 73, and thanks,
>  >
>  > Dave (NK7Z)
>  >
>  >
>
>
>
>
>


Ken Sejkora
 

Good points Kriss and Allan.  However, the power strip is screw-mounted into the underside of the computer hutch, and I’m not yet ready to unplug everything and dissect the power strip. 

 

I did perform some additional testing this morning with different configurations of extension cords plugged into the power strip, and obtained some interesting results.  The 9-foot two-wire cord still yielded the strongest introduction of RFI into the system.  I also tried a 6-foot/2-wire cord, 20-foot/2-wire cord, and a 15-ft/3-wire cord.  I used the ‘Signal History’ feature in SDR Console to measure the noise floor and signal level over a 30-second period, using the default sampling interval of 50-milliseconds.  This yielded around 500 data points to calculate some meaningful averages for each configuration.  I set the mode to ±8 kHz AM to accommodate the fact that the RFI hash tends to ‘wander’ about a bit in frequency, so the wider AM setting was able to capture the entirety of the RFI signal during the sampling period.  In all tests, the extension cord was not terminated or providing power to any appliance.  Anyway, here are the results, all results in dBm:

 

Configuration    Noise Floor         Signal Level        SNR

No cord                    -132                         -119                  13

6-ft/2-wire             -129                         -114                  15

9-ft/2wire              -126                         -108                  18

20-ft/2wire            -131                         -119                  12

15-ft/3-wire          -132                         -119                  12         

 

The 9-ft cord added about 6dBm to the noise floor, 11 dBm to the signal level, and raised the RFI SNR by about 4 to 5 dBm over the configuration where no cord was plugged into the power strip.

 

I tried another test with the 9-ft cord plugged in, in which I positioned the cord to run over and adjacent to the SDR and antenna coax feeding it.  I did see an increase of about 5 to 7 dBm in both the noise floor and RFI signal levels compared to the cord running diagonally across the center of the room away from the coax and SDR.  Thus, it appears that I’m probably coupling some noise from the power strip into the antenna coax/common-mode-choke/SDR.  Looks like I’ve got a bit of work to do, perhaps clean the coax connections, swap out some coax jumpers, and maybe try a different CMC.

 

Nonetheless, I find it interesting that different lengths of extension cord behave differently in their ability to pick up and/or radiate RFI into my system.  Again, it points out the need to check everything when trying to chase down RFI.

 

73  --  Ken

 

From: Allan Isaacs
Sent: Friday, December 31, 2021 07:21 PM
To: main@SDR-Radio.groups.io
Subject: Re: [SDR-Radio] Interesting "source" of RF interference

 

I thought much the same Kriss. Some extension cords have a suppressor for a phone lead or similar and some now even have a built-in USB power charging socket.

Or even maybe a pair of capacitors to ground which may be leaking in some way.

Allan G3PIY

From: main@SDR-Radio.groups.io [mailto:main@SDR-Radio.groups.io] On Behalf Of Kriss Kliegle KA1GJU
Sent: 31 December 2021 22:51
To: main@SDR-Radio.groups.io
Subject: Re: [SDR-Radio] Interesting "source" of RF interference

 

Power Strip have any hardware inside for filtering, surge, etc?
What's the extension cord do when plugged into the same outlet the power strip is plugged into?

73 Kriss KA1GJU

 


HERB KEATON N2GEW
 

Happy New Year to All!
Perhaps not as "interesting" (but certainly an RFI source to be reckoned with) is my shack computer speaker system. It happens to be an Asus Desktop with a
Dell 5.1 powered speaker system into the Realtek HD Audio Manager. Upon turning the speaker system on, I have always detected a low and slight "hum" associated
with these speakers, but easy to ignore as it is only detectable when the speaker volume is extremely low.

Whether that "hum" is associated with the RFI or not is indeterminate. But take a look at 14290 Khz and 7150 Khz approximately. 

Note, the easy workaround is to turn off the system. See 10.02.28 when I turn off to 10.02.34 turn on: the 40-meter screen shot. I don't have to 
unplug anything, just shut off the speaker system. 

I will look at eliminating this in more depth moving forward.
73'


jdow
 

Somebody mentioned the possibility of a failing MOV in the plugstrip. For that you'd have to take it apart. The easier solution is to toss the bugger. (And if it is new and not been through a thunderstorm, do not get the same brand again.) What you may be seeing is a (1/4 wave?) resonance effect in the antenna formed by the cord you had plugged in. Stray capacitance can reduce the resonant frequency of such cords.

{^_^}

On 20211231 21:33:15, Ken Sejkora wrote:

Thanks jdow.  Although I am indeed interested in identifying and take the steps necessary to eliminate/minimize the source of the interference, the primary objective of my original and follow-up posts was to illustrate that RFI can crop up from what might initially appear to be a very unlikely source.  Who would have thought that simply plugging in an unterminated extension cord would generate 15 dBm RFI at 22 MHz?  That concept would not have occurred to me before yesterday, and in this case it was easy to at least reduce the RFI through a very simple approach – just unplug the stupid cord.  I didn’t need any special shielding, ferrites, chokes, etc. to cure this particular problem.

 

However, now I’d like to go the next step and understand why the cord is introducing RFI, and even more importantly, to let others know of my experience so that they can also learn and hopefully benefit from my experience.

 

Thanks again for your insights, Joanne.  Happy 2022 to all.  73

 

Ken --  WBØOCV

 

From: jdow
Sent: Friday, December 31, 2021 10:11 PM
To: main@SDR-Radio.groups.io
Subject: Re: [SDR-Radio] Interesting "source" of RF interference

 

You'll hate me for this Microsoft answer, 100% accurate and 100% useless. "Where it works."

I guess what that says is work with some trial and accidental success, admitting to no errors.

In general I would start with the choke close to the front end device if it is on a wire to the front end device. Otherwise, try it to find out what works best for your personal nest of wires.

{O.O}   Wish I could give a magic quick solution; but, such does not exist.

 



jdow
 

The traditional sovereign cure for audio hum is 1 or more 1:1 audio transformers. It means you have a ground loop with non-negligible mains current on the shield connection of the #)*$)**&@##^ single ended pointy thing plugs and their sockets.

Treating the ends of the cables feeding inputs to devices treating the single ended wires as if they were balanced can also work. But that means RF problems will appear as you have to unground the socket and feed the socket's ground to the input of a differential amplifier. That's a lot more work so I've never really done it.

{^_^}

On 20220101 07:54:01, HERB KEATON N2GEW wrote:
Happy New Year to All!
Perhaps not as "interesting" (but certainly an RFI source to be reckoned with) is my shack computer speaker system. It happens to be an Asus Desktop with a
Dell 5.1 powered speaker system into the Realtek HD Audio Manager. Upon turning the speaker system on, I have always detected a low and slight "hum" associated
with these speakers, but easy to ignore as it is only detectable when the speaker volume is extremely low.

Whether that "hum" is associated with the RFI or not is indeterminate. But take a look at 14290 Khz and 7150 Khz approximately. 

Note, the easy workaround is to turn off the system. See 10.02.28 when I turn off to 10.02.34 turn on: the 40-meter screen shot. I don't have to 
unplug anything, just shut off the speaker system. 

I will look at eliminating this in more depth moving forward.
73'

Attachments:



Allan Isaacs
 

Hi Ken

Thinking a little more on your observations.

Any electrical system will be subject to ground currents and because of this you’ll find in a decent hi-fi system that ground connections are important. That usually means only a single feed for the system chassis safety earths. To meet this various safety earth connections will be opened at the mains plugs to allow only the single feed. The main problem I recall was in a turntable where you can get hum superimposed on the audio.

With an SDR you will get noise at the aerial connector from ground current.

In an old hi-fi it used to be just hum but with loads of switching power supplies the noise will include harmonics of the basic oscillators.

I did a study of a ship to shore radio problem a few years back when a harmonic of a switching power supply was blocking the radio channel used by an Isle of Wight Red Funnel ferry.

What was very odd was the amazing stability of the VHF harmonics in the circuit used in the power supply but that is another story.

The noise you are looking at is being radiated or induced from the cable. It looks like the cable length is resonant at a specific group of harmonics.

 

I can recommend a Tiny SA if you want to locate an RFI source.

Because it has an internal battery and a short aerial you should be able to see the effect of ferrite suppressors on particular mains leads.

All the best for 2022

Allan G3PIY