[RFI] How much RF does your power company deliver along with the AC?

Christopher E. Brown cbrown at woods.net
Fri Aug 21 11:15:20 PDT 2009

Had an interesting opportunity last night.

Station is currently an Icom 756PROIII with an inverted L like (30ft 
vertical, 50ft to the side fed at the base through a SGC-237 autocoupler), 
and a 20M dipole in the attic.

On 20M the L has about 20k total common mode choking (10k at base, 10k 
distributed along the feedline, the dipole about 10k split between the 
attic and shack ends.

We had an extended power outage last night, a HV -> MV feeder transformer 
buried about 800 feet from the house feeds all of the final 2 - 4 home 
step-downs in 2 subdivisions.  It failed with enough umphh to rattle the 
windows here.

Result, everything within line of site without power, closest anything 
with power over 1000ft away and on the other side of a hill.

After shutting down all of the UPS equipped gear, and unplugging 
everything in the house I checked some of the AC runs w/ the HT and a 
small faraday loop pickup.  Found a few UPS sounding spikes, but these 
went away within 30 min or so.

Fired up the rig (deep-cycle bank) and checked 20M, < S1 across the whole 
band with _both_ pre-amps on on *both* antennas.  Listened to a surprising 
number of stations given the time (several hours after sunset).  Europe, 
South America and HI stronger on the L, central and eastern US stronger on 
the dipole.  Pretty much as expected.

Spent a good hour running from 1 - 10 Mhz in AM wide, checking on which 
problem noise sources were really local.  A few that I though were very 
close were not.  (PWM like hash on 80 and 40)

Connected the loop pickup to the 756 on a run of LMR400-UF and re-checked 
the AC lines, dead silent.  (Checked later, the failure cut all 
connections including neutral on both sides of the xformer)

Much later, power returns.  Everything still unplugged in house, all CFLs 
off, radio still running on Batt.

Not that I trust the scope to be exact, but dipole now shows ~ 25db higher 
across 20M than the L.  Distance to nearest homes is almost the same for 
both antennas, only real diff is the dipole is in the attic and sits above 
the AC wiring, the L is 45 feet away from the dipole.

With power back, even with everything but a couple lights (filament, not 
fl), strong RF on the AC lines, and s5 - s7 on the dipole.

Anyone know of a source for good "whole house" filter?  The incoming is 
underground, and the feeder from the disconnect to the sub-panel is too 
large for the "largest clamp-on", otherwise I would already have ordered a 
number of them.

Starting to think the best option would be a good filter between the 
disconnect and sub-panel, but have not had any luck finding a unit capable 
of filtering a standard 2 phase 200amp service (standard home 2 hot, one 
neutral + gnd 120/240).  I would love to find something I can have 
installed before the breaker panel that would provide 40db or better 1 - 
30 mhz common and diff supression.

My options for antennas far enough not to closely couple to the AC line 
noise are pretty much exausted with the L, anything else I can do will be 
close enough to make it an issue.

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