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Pickup Coils
THE PICKUP COIL
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DISCLAIMER: which is to say that if you have not read it already,
let me know now and I will post you a copy. There are sticklers
who would jump all over me if they thought I was documenting
experiments where the instructions appear to direct a person to
act in such a way as would, or could, cause injury to themselves
or others, break laws, etc.. Understand now that I advocate
common sense, and that common sense should be used at all times
around Tesla coils.
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The "pickup coil" is a very useful detection and experimental
device in high-frequency Tesla work. Tesla wound and employed
many of these type coils in his RF work. A careful review of the
COLORADO SPRINGS NOTES will show that he used a "pickup" coil in
a great many of his experiments there.
The "pickup" coil is really nothing more than a very simple
Tesla secondary. These coils usually do not need not be very
electrically strong, high Q, or critically designed and con-
structed. Normally these coils are built quickly, with a minimum
of fuss and expense.
Old Tesla coils that have failed us as secondaries in spark
systems make ideal pickup coils providing that have not
completely broken down, and even spare good coils may be used.
But often a specific experiment requires a resonator be tuned to
a particular frequency, and it is perhaps best to wind a new coil
to meet the experimental requirements. Also, I prefer the pickup
coil to be physically small so that I can move it around easily,
though that is not always an experimental necessity.
I generally wind these coils with Formvar enamel magnet wire (#22
or smaller) on a somewhat fat coil form so that I end up with a
rather squat coil. I usually do not spend time sanding, sealing,
or capping both ends of the coil form. I set the coil form up on
a spindle, tape down the first few turns with plastic electrical
tape, and wind away. The coil is typically wound by hand in less
than 30 minutes, and finished with a few strips of tape at the
top to hold things in place. A top cap with an insulator does
find use; on 6" PVC drain pipe a plastic coffee can lid will fit,
and a small insulator may be easily attached. After I have worked
with a pickup coil for awhile, and have found that it meets
specific needs, I will throw on a few coats of sealer to stick
the wire down on the form so it won't fall off, and generally
spruce up and harden the coil.
So what do you do with these rather shoddy coils? Experiment
with them! Learn the ins and outs of tuning, resonance, and RF
currents. Find where your RFI is getting out and plug the leaks.
The list of possible uses goes on and on, but I will try to cover
a few interesting ones.
TRACING RFI
Most coilers are not using oscilloscopes and frequency generators
to tune their coils, which is OK, but how do you trace RFI
leaking out of your coil system and into the TV, radio, or tele-
phone? I have had RFI so intense that it reset the clock on my
VCR, and the VCR was off. In any case it is a mistake to try to
use any solid state equipment in an area influenced by the pulse
fields of a working Tesla coil, which means any O'scope built in
the last 20 years is out of luck anyway once you turn the coil
on. One way to trace RFI is with a tuned pickup coil and 120 volt
neon indicator bulb.
Wind a pickup coil that is lower in frequency than the Tesla coil
system you are operating. Tune the pickup coil to match your
system frequency by grounding the base wire of the pickup coil to
the same location that you are grounding the base wire from the
Tesla coil secondary, then begin taking off turns of wire from
the pickup coil until the pickup coil is resonating, and you get
a spark from the open end of the coil. If too many turns are
removed, simply splice some wire back on the coil until you get
back to a nice sharp tune.
A word of caution. The tune of a pickup coil can be exceedingly
sharp. The presence of your body close to the coil (or other
large object, including the floor) will be sufficient in many
cases to alter the resonate frequency. It you decide that the
coil is in tune while it is sitting close to the floor, or while
you are standing right next to it, you may lose the sharp tune
when you move it or step away. Some of the experiments performed
with pickup coils may require a very precise tune, so it is best
to have the coil tuned in sharp when it is not affected by nearby
objects. I set the coil up on an insulator (like plastic buckets)
at least 3 feet above the floor, and step back two or more steps
after subtracting or adding wire before I judge the coil output.
Once the pickup coil is in sharp tune I generally harden the coil
a bit to keep it from falling apart when working with it. A few
extra strips of tape are nice, and I sometimes cut small slots
and anchor the top and bottom leads wires with hot glue. A
standard meter probe with an alligator clip at one end, a few
extra alligator clips, and a small 120 volt neon indicator bulb
should be assembled. The first thing I check are the building 60
cycle grounds.
Find the location of the ground connection used by the power
utility inside (or outside) of the structure you are concerned
about. Depending on the local code, the building construction,
and the contractor, this can be just about anywhere. Once
located, set up an insulator near the ground connection and
position your pickup coil. Using an alligator clip, connect the
base wire of the pickup coil to the 60 cycle ground. Step back,
turn out the lights, and have an assistant fire the Tesla Coil
while carefully observing the bare end wire of the pickup coil.
Any sparking of the pickup coil indicates a serious RF leak that
is energizing the neutral wire in the entire building, and could
be a fire hazard. If the pickup coil does not spark, but you can
detect corona at the wire tip, you have an RF leak that deserves
to be resolved as it will cause interference problems with any
grounded circuit board on the 60 cycle system.
If the pickup coil does not resonate to the point where spark or
corona is detectable with the naked eye, a 120 volt neon
indicator bulb may be used to detect resonate rise. Connect one
lead of the bulb to the top of the pickup coil and repeat the
experiment. If the Tesla coil system in use has any connection at
all to the 60 cycle ground, the neon indicator bulb should glow.
An insulated capacitance (large conductive surface, antenna) may
be connected to the second lead of the neon indicator to increase
sensitivity. (see: THE INVENTIONS RESEARCHES AND WRITING OF
NIKOLA TESLA, 1992 edition, ISBN 0-88029-812-X, pp185, fig.124)
An interesting note here is that a pickup coil with a neon
indicator bulb can frequently detect bleedover in the 60 cycle
HOT wire(s) if a serious RF leak is detected on the neutral wire.
If sparking or corona was detected with the naked eye on the
pickup coil when connected to the neutral wire, move the base
wire of the pickup coil to a 60 cycle hot wire and repeat the
experiment. Be advised that the pickup coil will be 60 cycle hot,
appropriate precautions should be taken.
In the event that 60 cycle ground contamination is detected with
the pickup coil, the obvious solution is to re-work the Tesla
coil ground. I have always advised the use of dedicated, heavy
(meaning low impedance, high current) RF grounding for any
serious Tesla work.
Another interesting use of the pickup coil is for experimenting
with transmission of electrical power through ground currents.
For obvious reasons I prefer to conduct these experiments with a
rather small, low power Tesla coil, usually 4 inches in diameter
or so. The reader who has followed some of my Tesla work will
begin to understand why I recommend such system parameters as
large (even huge) primary coils in Tesla coil design, not just
for efficiency, but also because it allows for tuning flexibility
to perform experiments such as this.
Take your small Tesla coil system, with the secondary coil
grounded to a dedicated RF ground, and load discharger onto the
coil until no spark allowed to break out. Then re-tune the coil.
HINTS: If you do not have an oscilloscope and frequency counter,
use a low pressure light, such as a neon or florescent tube
(brighter glow = closer in tune), or draw spark with a grounded
probe, as an indicator of system tune. If the primary on your
existing coil system is not large enough to allow the tank
circuit to tune with the new secondary configuration, lash on a
temporary primary extension, and keep in mind you will want
tighter coupling.
A normal 1/4 wave Tesla coil system with large discharger that is
not allowed to spark is called a "Tesla Transmitter". The system
energy is trapped in the resonator, and is forced to ground by
the high voltage on the discharger. Tesla transmitters pump ex-
ceptionally strong currents through the base wire and ground
path. They will "power up" or energize the RF ground to the point
that spark may be drawn off the ground with a key or other
conductor held in the hand. This condition of a "powered up"
ground is excellent for Tesla transmission experiments.
Once the Tesla coil system is top loaded and tuned for trans-
mission, wind and tune a pickup coil to match the new system
frequency. Using just the bare pickup coil connected to the
dedicated RF ground (or simply "system" ground); sparking from
the pickup coil will be so intense that the coil will be des-
troyed in short order. To get a sharp tune on the pickup coil
without frying it, it will sometimes be necessary to establish a
remote ground so that the coil can be brought into tune without
burning up. A simple remote ground for tuning purposes is easy to
establish: connect the pickup coil to a water pipe and subtract
turns until sparking or corona is detected, drive a short copper
clad ground rod some distance from system ground and connect the
pickup coil to this, or set up a small insulated counterpoise
over the system ground and connect the pickup coil to the
counterpoise.
Once the pickup coil is in sharp tune you may have an assistant
work the controls of the Tesla transmitter and you may take the
pickup coil into the field. I have been able to detect RF ground
currents by connecting the base wire of the pickup coil at the
following locations (objects): basement copper water pipe in the
neighbors houses (300 ft, 600 ft), galvanized storm culvert (500
ft), creek bed (connection to 50 foot AL flashing sunk in bottom,
1500 ft), 3 foot copper clad ground rod (200 ft). The input power
into the Tesla transmitter in this case was below 1.4 kVA into a
4 inch coil with 30 x 4 toroid. By using a high Q six inch coil
(wound with #21 magnet wire on a properly sealed coil form) tuned
to the transmitter frequency with a small toroid, I was able to
light florescent tubes at the creek bed ground connection 1/4
mile away from the transmitter.
Tesla transmitter performance is greatly dependant upon the spark
gaps. In Tesla's transmission experiments with small oscillators
he used a mercury break which produced a continuous wave
(undamped) signal (see: U.S. Patent No. 609,245, "Electrical-
Circuit Controller" ; U.S. Patent No. 568,179 "Method and
Apparatus for Producing Currents of High Frequency"; NIKOLA
TESLA: LECTURE BEFORE THE NEW YORK ACADEMY OF SCIENCES - APRIL 6,
1897, edited by Leland I. Anderson, 1994, Twenty First Century
Books, ISBN 0-9636012-7-X, pp74 fig 17 & 18. The next best thing
is a quench gap with closely spaced electrodes which are sealed
airtight, and lastly is the normal static and rotary gaps used by
most coilers which produce a highly damped signal.
Richard Quick
... If all else fails... Throw another megavolt across it!
___ Blue Wave/QWK v2.12