Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

donate to eham
   Home   Help Search  
Pages: [1] 2 3 Next   Go Down
  Print  
Author Topic: Common mode current on AC wiring?  (Read 3514 times)
KC1BMD
Member

Posts: 608




Ignore
« on: September 03, 2017, 05:50:25 PM »

Situation:
- 2nd floor shack in townhouse condo.
- two separate fan dipoles in attic above (40/20/10m and 17/12m) both center fed with RG-8x and current baluns at feed point for each.
- Power 100W max.
Problem:
- Plug-in CO detectors (with battery backup) occasionally get tripped, only when plugged in, not when unplugged on battery.
- So it appears to me it is not being caused by direct radiation into the detector (e.g. wrapping in foil does not help) but rather caused by RF current coming in via AC wiring.
- Problem mitigated to some degree (at lower power levels anyway) by switching to different brand CO detector.
- Unable to isolate to specific bands or frequencies, or power levels (can happen when tuning up at 10-15W too).
- Neighbor with unit attached to our end unit (3 units in building) also affected but we switched his also to another brand.

I tried to make some measurements and plugged one of the more susceptible CO detectors into an outlet at the end of a few feet long extension cord. I measured some RF current using an MFJ-854 (of questionable accuracy) which varies but appears to be e.g. 10-80mA, or so, when sending CW key down at about 25W. Interesting, the measurement seems to vary as I slide the meter up and down to different spots on the cable but could be my imagination or the MFJ meter quality Smiley.

Questions:
- Is there some level of such RF current that is significant (e.g. some level that can be ignored besides 0Ma)?
- Is this common mode current, and if so is it: a) getting past the current baluns on coax outer shield, or b) being induced/radiated onto the AC cable by the antenna near field radiation?
- Aside from moving to a single family home with more land (which I'm searching for), or moving the antenna away out of the attic and further away from the operating position while still in the condo (not able to do that), are there any other comments or suggestions?
Logged
K6BRN
Member

Posts: 450




Ignore
« Reply #1 on: September 03, 2017, 06:30:53 PM »

Your house wiring is in the (very) near field of the antenna, where capacitive and inductive coupling is dominant and very strong.  You need to 1.  Move the antenna elements away from any wiring, particularly parallel runs, 2. Consider running any nearby wiring through toroid inductors, 3.  Make sure the antenna match is as good as possible,  4.  Reduce power output, and 5.  Perform an RF exposure (safety)  assessment as described on the ARRL site.

In summary... your antenna is probably TOO CLOSE. to your house wiring and to you to run 100 watts safely.

Brian - K6BRN
Logged
AA4HA
Member

Posts: 2381




Ignore
« Reply #2 on: September 03, 2017, 07:36:57 PM »

With the house wiring being so close to the antenna element it is making the house wiring act as part of the antenna system. Even if you achieve a good match with your antenna for minimal SWR the wiring beneath the antenna is some fraction of a wavelength away. It would not surprise me to find that the radiated pattern for your antenna is more like a "cloud burner" and is going more upwards than to the horizon.

As sensitive electronics like a CO or smoke detector is making a decision on an alarm threshold based off of microvolts of differences in its circuity it is not surprising that you are getting false trips. GFCI's are also subject to the same sort of upset.

If your installation "had" to be the way that it is you might be able to reduce the impact by putting the detector inside of an RF-proof enclosure (like a mini-Faraday cage) with PI filters on the enclosure metal for the incoming AC power. It's a tricky design process that is more of an art if you are not totally familiar with the science behind shielding.

You may find it easier to move your antenna. As has been said, your power levels at 100 watts might be above suggested limits for human exposure to the general public. You should run the calculations to be sure.

Also, making a double walled, shielded Faraday cage with a double layered copper mesh window to allow the air flow in and around the detector might also invalidate the performance of the CO or smoke detector. I bet you would never get anyone to sign-off on you modifying a detector that is going in someone else's apartment or condo.
Logged

Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
Free space loss (dB) = 32.4 + 20 × log10d + 20 × log10 f
KC1BMD
Member

Posts: 608




Ignore
« Reply #3 on: September 03, 2017, 08:08:25 PM »

Thanks for the comments.

K6BRN: I have already performed an RF safety assessment and am within safe limits (for both controlled and uncontrolled areas) for 100W and up to an including 12m. At 10m I believe it was 85W max to be within limits, but I haven't been using 10m much due to poor propagation in the sunspot cycle. I don't currently operate any VHF or UHF and if I do, it would likely have to be portable.

AA4HA: Faraday cages for the detectors would not really be feasible for reasons you cited. I doubt it would help since the RF is apparently coming in via the AC wiring (although you mentioned filters -- but implementing that would be problematic, if it would even work.

As an experiment, I added several ferrites to the AC extension cord with as many windings going through them as I could manage an that seemed to reduce the RF somewhat but not to the point that it prevented tripping. I guess I'm stuck with the situation for now until I can find a suitable place to move (a longer term solution).
« Last Edit: September 03, 2017, 08:11:42 PM by KC1BMD » Logged
KC1BMD
Member

Posts: 608




Ignore
« Reply #4 on: September 04, 2017, 07:50:41 AM »

I used this calculator: Amateur Radio RF Safety Calculator
It has an entry for: The distance to the area of interest: From the centre of the antenna, in feet

For my fan dipoles, would that be indicating that the area of concern is the high current portion (center), rather than the high voltage portion (ends)? The reason I ask is because I am closer to the ends of these dipoles than the center. In any case, my numbers showed not much exposure even at 100W PEP (lower avg-pwr on SSB/CW) at relatively close proximity (even at ~10ft).
Logged
W9IQ
Member

Posts: 1707




Ignore
« Reply #5 on: September 04, 2017, 08:06:03 AM »

The radiation of the antenna is proportional to the current in that part of the dipole. In a dipole antenna the current is highest in the center and tapers off to near zero at the ends. So the majority of the radiation occurs in the center of the antenna with the ends contributing very little. As a result, the RF exposure is much less near the ends of the dipole antenna than the center.  However, as frequency increases - the human exposure limits decrease and the distance between the center and the end of the dipole become less so the current distribution in a dipole has a diminutive effect on RF exposure as frequency rises

- Glenn W9IQ
Logged

- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
KC1BMD
Member

Posts: 608




Ignore
« Reply #6 on: September 04, 2017, 08:38:47 AM »

Thanks Glenn. I won't be going over 100W PEP at my current location and usually operate no more than 75-80, sometimes only 25-50 (use only what you need as the guidelines say). At least using the calculator reference above I'm well within safe limits for controlled and uncontrolled areas.

I just wish I could tame the CO detectors. It seems that if I went to battery-operated (not AC plug-in) they would not trip and I believe such devices acceptable in my location (would need to double-check). However, I'm not sure I could convince my neighbor to do that. That would be a last resort if the issue rears it's ugly head again.
Logged
KD0REQ
Member

Posts: 2017




Ignore
« Reply #7 on: September 04, 2017, 09:53:14 AM »

any way you can set up an outdoor vertical? that will get you distance from the wiring, and perhaps reduce the intersection of copper to radio waves. you're pretty tightly coupled right now.
Logged
KC1BMD
Member

Posts: 608




Ignore
« Reply #8 on: September 04, 2017, 10:43:11 AM »

I couldn't put up anything permanent outside on the common land area (i.e. back and side yards). I do occasionally put up a couple of different antennas on a push up mast on tripod on nice days but take them down when done (40-10m EFHW, 20-10m cobweb, 15m PAR rectangle). I don't believe these interfere with anything but haven't tested them enough to see and only at relatively low power so far (well under 100W). If I was able to put up a vertical, it would have to be on my deck (which I have exclusive use of) but that's also adjacent to the living areas (and so wiring). That also brings a new set of challenges like how to bring the feed line in, whether to locate the shack in the basement, instead of 2nd floor, lightning protection, RF grounding, how to manage radials, etc. But thanks for the suggestions.
Logged
KC2MMI
Member

Posts: 812




Ignore
« Reply #9 on: September 04, 2017, 10:58:53 AM »

If these are simple plug-in CO detectors, it could be worth asking some mfrs. and possibly the ARRL folks, if there are another brand or model that are known to suffer less from an RFI problem. And if no one knows a better brand, perhaps to buy a couple from Amazon or Costco or other sources that will let you return them if they don't work any better.

From the next door neighbor's and the association point of view, it shouldn't matter which detectors you wind up, as long as they are a typical approved commercial unit.
Logged
KC1BMD
Member

Posts: 608




Ignore
« Reply #10 on: September 04, 2017, 11:09:50 AM »

I did a lot of research. A few others I tried are no better. I contacted the manufacturer of the original's tech support. They had no solution. I did find one brand/model that is less susceptible but not completely. It was still an improvement even if only marginally.
Logged
WA3SKN
Member

Posts: 6492




Ignore
« Reply #11 on: September 04, 2017, 11:40:59 AM »

Why not just make some filters?
Some .1 to .01 uf disc caps (proper voltage) and a couple of coils like the old "Brute Force" filter in many ARRL handbooks should do the trick.
You don't even need toroids to build one.

-Mike.
Logged
KC1BMD
Member

Posts: 608




Ignore
« Reply #12 on: September 04, 2017, 12:12:44 PM »

Why not just make some filters?
Some .1 to .01 uf disc caps (proper voltage) and a couple of coils like the old "Brute Force" filter in many ARRL handbooks should do the trick.
You don't even need toroids to build one.

-Mike.

If it was only my issue, then I could try that for the 3 required CO detectors I have (one on each of three living area levels). However, I won't modify anything in my neighbor's unit. In ours, it is only one CO detector that trips sometimes (the one on the 2nd floor where my shack is and closest to the attic antennas -- if that matters or perhaps just coincidence -- I say that because at least two of my neighbor's detectors were affected). A complicating factor is that the electrical feeds for the building's main AC panel, in a utility closet attached to my garage) comes through my attic to the neighbor's units (two more units) where it feeds the individual AC panels in each unit's basement. Definitely very sub-optimal situation! I thought about getting one of the massive clamp on ferrites (that I think Fair-Rite sells) but not sure where to place it or if it would even help. Thanks anyway for the suggestion.
Logged
WB6BYU
Member

Posts: 17046




Ignore
« Reply #13 on: September 04, 2017, 12:31:59 PM »

One of my standard RFI test tools is an extension cord wrapped around a ferrite
rod.  Makes it easy to insert it in the power cord and see how much difference
it makes.

That isn't necessarily the most effective arrangement:  a shorter cord run
through a toroidal core would be a better filter, but I couldn't do that
with the existing plug.  (One of the split cores could also work.)

For loaning out to someone, or testing with a neighbor, using an unmodified
commercial cord may have an advantage from the perception of safety.
For my own use, I'd probably make my own cord to suite the required
current draw, frequency range, etc.

Note that, for common mode current, a ferrite choke will be more effective
than bypass capacitors.
Logged
W9IQ
Member

Posts: 1707




Ignore
« Reply #14 on: September 04, 2017, 12:50:14 PM »

I do agree with Dale that a couple of turns of the AC wiring around a 31 toroid core would be worth testing. Fitting the core in the j box and determining if it is permitted is another matter.

- Glenn W9IQ
Logged

- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
Pages: [1] 2 3 Next   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!