10 years back I was doing a lot of spherics
recording using both E-field probes and loops. The VLF spectrum
is, of course, absolutely saturated with hum from the power grid.
There were some interesting things I learned about the nature
of electrical grid hum during that time. More or less as follows
in no particular order....
1) The primary frequency (60Hz in the US)
and the third harmonic have roughly the same "signal"
strength since we use 3-phase power distribution. 2nd harmonic
levels are considerable lower in level but sound much dirtier.
next highest level that I encountered in my recordings was at
720Hz using either an E-field probe or a loop sensor. The fundamental
sounds rough, the third harmonic sounds fairly clean and 720Hz
The E-field probe was more sensitive to the higher harmonics than
the loop sensor... probably mostly due to circuitry differences
in the amplifiers. The E-field probe became less sensitive to
the fundamental and lower harmonics they higher it was held.
Using the E-field probe I found the noise envelope surrounding
tall buildings tended to have a radius approximating the height
of the building. With a loop sensor I could be much closer to
a building before the hum level equalled that picked up the E-field
this to the fact that E-field strength decreases as the square
of the distance from the source whereas the magnetic field decreases
as the cube of the distance.
4) With the loop sensor I could rotate the
loop to partially null hum pickup but only if the loop was mounted
or held upright. If, OTOH, it was held or mounted with the turns
parallel to the earth it was easily saturated by ground-conducted
hum currents even when located several hundred feet from tall
areas I could trace out the apparent paths being traversed by
the earth currents.
Note from VA3ZNW:
my 2 pens added. In the 80- 90-s I actively explored simple DC
and regenerative receivers. Main luck of the receivers was the
hum. The receivers pick up the hum (50-Hz in Russia) ever without
an antenna. Ever the receiver was fed from a battery. Any one
receiver- transistor or tube one.
impossible to eliminate the hum. I used rejected filters at input
and between stages at Audio Amplifier. I used Audio Amplifiers
that did not work below 300- Hz. No success! Receiver roared at
the higher harmonics. It was in my city shack.
when I try the receivers at a field conditions, far away from
the city and any electrical distribution wires I was amusing.
first my impression was- the receiver did not work. No any hum.
But then when turn around tuning capacitor I heard stations. Lots
stations that I cannot receive at the city. I understand, that
the receiver works fine, just no any hum. No lots of intermodulation
very amusing! Receiver that at the city conditions roared and
noised the same receiver in the field gave very good reception
without any noise. However, as I noticed, at cloudy and rainy
weather I could pick up the electrical hum. However the hum was
not strong as it was in the city. As well it was appeared intermodulation
DC and regenerative receiver do not like city with the hum from
main from electrical power equipment and modern electronic devices.
My nostalgia is the times when I received interferences only from
power tube horizontal generator of the TV.