[RFI] How much RF does your power company deliver along with the AC?
Christopher E. Brown
cbrown at woods.net
Sun Aug 23 01:09:31 PDT 2009
On Sat, 22 Aug 2009, Jim Brown wrote:
> On Sat, 22 Aug 2009 20:48:54 -0500 (CDT), Christopher E. Brown wrote:
>> Backed up right to the
>> house I get the elevated noise floor across the band (mainly looking at
>> 20M here, because it has the least radiated noise outside of a few known
>> sources but wideband noise is there everywhere 20M and down). Pull out 10
>> - 50 feet and (on the short whip) it goes away. Back into a neighbors
>> driveway 4 houses down and get close to the house and same effect (though
>> it was harder to tell as this also brought be close to their home
> It sure sounds like you've done all the power system right. What's your soil
> like? Good ground? Poor ground?
About average, only real good thing is the area is marshy with a high
water table, the soil below a foot or so is never dry.
> Here's the connundrum. If the noise source is outside the house and it's
> return path is earth (likely it's common mode), why is the noise level
> rising on the house side of that ground? Logic suggests either 1) there's a
> noise source on the house side of the ground or 2) it isn't a very good
> ground; or 3) there's another ground on one or more branch circuits, which
> provides a current path for common mode current. I'm guessing you've checked
> that. :)
I have no unplanned paths, and the signal itself is weak, move away from
the house and it goes away. I assume I would have similar coupling to the
power lines, but all of the power lines are 100% underground, except for
the houses. With a small antenna I have to get within about 2 feet of an
AC line to start detecting, it is only the 32 foot dipole that strongly
couples at over 10 feet.
I checked all of the light poles on this street and the next (they feed
from the same transformers as the houses), and I get similar noise at all,
though the level varies. Get the small loop or the ducky on the HT within
a foot or so and the floor rises. I am ignoring any "specific" signals,
looking only at the wideband hash and ignoring anything with audable
modulation or narrow spikes.
> One possibility is that the ground is simply placing a voltage minima on the
> line (long wire antenna) and as you come into the house the voltage is
> rising. That suggests CAPACITIVE (E-field) coupling from the power line
> inside your home to your antennas. You might get a clue by taking a talkie
> with HF RX and a duck antenna and probe along the lines to look for peaks
> and dips.
> Question. Is it practical to put your wiring in EMT? Probably not. :)
> Another observation. I was chasing noise on 160M and took the HF RX in my
> THF6A talkie for a walk around the neighborhood. One thing I did was to
> couple the talkie's mag antenna to every wire I could find that was on a
> wooden power pole (feeders, grounds). Sometimes I would hear noise.
> Sometimes I would hear 160M signals, coupled from that downlead to my
> talkie. One I specifically remember because he's about 1,000 miles from me
> and he was loud, was W7LR. So in that case, the talkie's loop antenna WAS
> inductively coupling to that power pole's downlead.
> Jim K9YC
I keep thinking that whatever small part of the summ of all the conducted
emissions from all of the nearby houses is ending up as low levels of
common mode on the local power distribution, and that this noise is for
the most part ending up in the soil, with a small part making it into each
house and _weakly_ radiating.
Rapid fallof at
HT ducky at 12 in
small (6 inch) loop 2 feet
102 inch whip ~ 5 feet
I need to find where I can borrow a low end spec analyzer and measure the
levels, I expect they are actually pretty low, after all I can hear an
Apple Airport Extreme power supply at 50 feet with the small loop and get
a clean null to DF, and can only detect the line noise at 2 feet or less.
It is only the large antenna that makes it an issue.
Did a quick check with a 15 foot line cord.
Plugged into any circuit in the house with nothing on the end loop gets
strong pickup at 1 to 2 feet from the line cord.
Repeated the same but with 10 turns through a 2.4in toroid in the middle
of the cord.
12 to 18 inches past the choke and I have to place the loop against the
cable jacket to detect the noise and even then it is weaker.
I tried following power runs with the ducky, but is nearly impossable to
follow for a long enough distance with the way things wrap around, but I
do get the same result off of every circuit and anythink near. The garage
door traps (runs parallel to a circuit in the ceiling for 12 feet).
I went into the crawl space and went along the main heating duct for about
30 feet. Seemed to fall off a bit then climb but is hard to tell with
such a rapid fall off.
As i understand it, there should be no real current flow, and I don't see
how the mag field could extend far enough, the idea of capacitive coupling
from the whole of the house wiring to the dipole.
EMT is not an option. I have been considering it for 3 circuits in the
crawl space, and is possible for the attic (though very difficult, it is
less an attic than a truss space, I barely fit to string wire). The best
I could do without tearing the whole house apart would be about 15% of the
wiring, and I would need to hire a 5'2" 110lb electrician to do it. I
would not expect it to help anyway, since the EMT could not be extended
all the way back to the panels.
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