ULF induction coil
construction notes........
In principle the construction of a high inductance coil seems straightforward; lots of turns of thin wire wound
onto a large coil former.............
Reality of course is always a bit different, and initially several questions arose : how big , how many turns
of wire and also the shape factor of such a coil. Searching around in professional publications did not
turn up much else except the statement of "30,000 turns". Well, that was a start anyway. The required
physical size was never found in several web searches, except for one. There a fairly small coil was used,
in fact several coils together in an assembly.
Life is often strange, and I found a very professional amateur in my general metro area who made a real
masterpiece of an induction coil; he is using it for geomagnetic and seismic applications.
The construction of his 100,000 turn coil is described in detail at his site : AJBV-ind.coil
That site should also be of considerable interest to seismic and very low frequency acoustic phenomena
researchers...........
After studying the material at his site and a meeting with him, I decided to make a similar coil.
The added interest was that we could both compare our results over a reasonable distance (~100km).
This is a good distance, not next to our own backyards, and also not on a different continent !
Next the coil former was constructed. This was simply a 800mm length of 50mm dia. electrical conduit.
The high impact PVC type (orange colour). The end flanges are 50mm I.D. an ~ 85mm O.D. , made
from 6mm thick acrylic sheet.
This coil former then had to be mounted in a way so that it could be readily driven by a speed controlled
I used altogether ~ 9.5kg of 0.3mm dia. enamelled copper wire. The winding was controlled by supply-
ing power to the drill from a VARIAC, set to ~ 65Vac, switched on/off by a foot switch. After a bit of
experience it was possible to wind the wire with amazing speed, in the order of several hundred r.p.m.
No attempt was made to layer-wind the coil, considering there is no need to worry about high layer-to-
layer voltages (only up to a few hundred uV ), and conserving coil winding-window space was not an
issue in this case. The whole winding was tapped at 31,500 turns; subsequently no use was found
for this tapping point (so far).. No more wire was left after ~ 69,300 turns, so the coil was finished then.
The total coil resistance was measured as 3.64kOhm, the inductance 10.52H.
The coil was then tried out and found to be quite "deaf" , something I was almost expecting.........
A bit like building a radio with a ferrite rod antenna, but only using the coil without the ferrite.
A huge chunk of ferrite material was never an option for this project, but ordinary steel should be ok
because of the low frequencies. The best material would have been transformer laminations, but
not with the required shape factor for my coil. The next best thing was considered to be 3mm
thick steel "flat bar", which had to be insulated from each other simply with packing tape, to act as
a bit of a barrier for possible circulating ("eddy") currents. Some may think inserting a steel pipe or
even a solid round steel bar would do the trick; but this will act as a shorted turn (secondary winding)
and absorb a lot of signal from the coil !!
As the coil former inside aperture is round instead of square, several different widths of steel bars are
stacked so that an approximation to the circular shape is achieved. The overall length of all the bars
is 2 metres.
Illustrations :
Adding the steel core made an unbelievably large increase in sensitivity. Receiving Schumann resonances was instantly possible. There was also a significant improvement in the ratio of weak
signal to mains power noise observable. The upper frequency limit of the whole assembly is sur-
prisingly high, in the order of several hundred Hz , but in this case it was limited to ~ 150Hz.
Finally, the coil assembly had to be enclosed within a removable PVC pipe cover ,made from 90mm
'stormwater' pipe & fittings. (This would of course not be required for coils operating indoors).