[RFI] Mystery Noise (Aren't they all)

Jim Brown jim at audiosystemsgroup.com
Wed Mar 6 16:32:04 EST 2024


Wes,

This smells like a case that the late Neil Muncy tracked down in several 
different recording studios, including one where I sat next to him as he 
did it.

The mechanism was noise in form of triplen harmonics of 60 Hz on the 
neutral of a single-phase feed from a high-leg delta power system. This 
distribution is widely used in neighborhoods where there are both 
single-phase and relatively low power 3-phase customers. One of the legs 
has a center-tapped transformer, the others don't, and single phase 
customers are fed by it.

Because single phase customers get the neutral and 3-phase customers do 
not, the path to ground for the triplen harmonic current generated by 
3-phase loads is via that neutral (3-phase delta customers don't get a 
neutral).

The mechanism for triplen harmonics is that loads in modern power 
systems are nearly all non-linear, where load current is dominated by 
capacitor-input PSUs, distortion the sine wave. Triplen harmonics (order 
divided by 3) add in the neutral of 3-phase systems, and it's not 
unusual for neutral current to exceed phase current. Years ago, I 
attended an SBE meeting at the studio of the Chicago Fox station, where 
the Chief Engineer described how they had almost gone off the air, and 
what they had to do to fix it.

In the recording studios, noise coupling was magnetic to single-coil 
guitar pickups, and dynamic mics without corresponding cancellation 
coils. Top line dynamics from EV and Shure, the major US companies, have 
those coils, lower cost ones do not.

In my Chicago shack, I ran into that same mechanism with an end-fed 
top-loaded wire ending in the shack that used on 80 and 160. It coupled 
into the keying circuit from computer to my rig, which was a shielded 
audio cable, and locked the rig into keydown. I solved the problem by 
replacing that keying line with a high quality braid-shielded twisted 
pair, with the shield connected only at the sending end, and pair 
carrying the keying signal. As a test, I verified that this fixed the 
issue up to 17M.

In addition to magnetic coupling as a mechanism, it could be IR drop in 
the grounding and bonding system.

73, Jim K9YC

On 3/6/2024 12:05 PM, Wes Stewart wrote:
> To Chris, I have a Sony ICF-SW7600GR that I have tried to use.  With the 
> built-in loopstick it is pretty deaf.  Local BC is received fine but the 
> only way I can hear the noise in to hold the radio close to the 
> vertical.  An interesting, but unexplained, observation is that when I 
> disconnect the coax feeder the noise is reduced.  A resistive 
> termination on the antenna made no difference, still lower noise.
> 
> Really reaching for something, I considered that since the buried 
> powerline to my house runs somewhere under the coax, which is on the 
> ground, and perhaps under some of the radials, there may be some 
> coupling.  A stretch I know.  I did listen using my 80-meter inverted-V 
> and the noise is still there at greatly reduced strength.




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