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Saskia "crunch-daddy" Dwarshuis. A rare photo where daddy-pops, for the moment, seems to have the upper hand.

A High-Performance Half-Wave

By Lawrence Morris and Jared Dwarshuis
March, 2010

Click here to watch a video of the coil in operation

Theory of Operation and Construction


We will start with the ballast coil. This is made with two 500 foot lengths of insulated #6AWG stranded wire laid side by side. The ballast is wired in an unconventional manner. If we call the ends of the wires on the left side of the ballast A initial and B initial and the wires on right side of the ballast A final and B final, we would say that A Initial and B final go to the wall outlet and A final and B initial go to the low voltage side of the utility transformer. All of the current travels in the same direction around this ballast.

Reactance for 60 Hz is rather low, so for 60 Hz the ballast acts something like a long extension cord, merely getting power to the utility transformer. But when the spark gap fires across the high voltage side of the utility transformer a high current spike is seen on the low voltage side of the utility transformer. This high frequency di/dt has a large reactance in the ballast and induces voltage across both wires A and B in the bifilar winds. The induced voltage matches the voltage at the wall outlet, so for the brief instant the gap fires no current is drawn.

Our ballast is very efficient. During extended runs on a hot summer day it climbed only fifteen degrees Fahrenheit before stabilizing. We gave the ballast sufficient surface area so that no cooling fan was required.

A peek inside the ballast shows nothing but air.

High-voltage transformer

Our utility transformer is a 10 KVA unit operating at 14,400 volts.

We put this unit on industrial casters so we could move it around without breaking our backs.

Tank Capacitor

For our tank capacitor we used a total of (90) CDE-940 series 3000 volt 0.1uf capacitors wired in series/parallel for a total capacitance of 0.09uf at 30,000 volts

We originally used Maxwell capacitors but found no noticeable difference in performance when we changed to the 940 series capacitors. We are fully aware that many coilers use the 942 series.

The tank capacitor sets the power level of the system. The equation I = V ω C is meant for sine waves and certainly the spark box causes a slight deviation from sine wave behavior. Nonetheless, the equation gives a fair approximation of the reactive power available to the system.

Our voltage is 14,400 and our tank capacitor is 0.09uf. The line frequency is 60Hz so the current is limited to no more than 488ma. Thus the reactive power is restricted by the tank capacitor to very close to 7.2KVA (7200 watts).

Spark Gap

We use a vacuum (forced air) gap with five spark gaps in series.

The blower is from an industrial dust collector. It is a two horse power model rated at 1550 cubic feet per minute.

A rather large flow of air is required to remove ionized gas, heat and conductive deposits.

The spark box is made of 1/4 inch grade CE Garolite and is held together with epoxy.

Each electrode has a total of (25) 1/8th inch pure tungsten welding rods, normally used for tungsten-inert-gas (TIG) welding.
The welding rods are wrapped around a 3/4 inch copper tube. The copper tube stops about 1/2 inch shy of the hot end. The rods are held around the copper tube with three heavy duty hose clamps per electrode.

Two sets of electrodes on one side of the box are retracted to a hard stop by a vacuum diaphragm (made with 1/8th inch neoprene, 50A durometer).

The electrode retraction gives a smooth transition from start-up to full power.

A vacuum leak is introduced with a valve to set the rate at which retraction occurs, and the vacuum is, of course, provided by the blower.

Tungsten wear is negligible with our system.

The tungsten rods never get past being warm to the touch, except very near the end with the sparks.

Filters and Tuners

The large tube structures are a pair of magnetically isolated one-to-one current baluns. The two one-to-one baluns are wired in series on the tank capacitor side, and in parallel on the primary side to produce a four-to-one current balun.

The baluns provide an impedance match that keeps counter-EMF from returning to the spark box (allowing the spark box to perform it's task efficiently). Each bifilar balun is made with a total of 240 feet of 1/4 inch type L tubing with 1.125 inch between tube centers. The balun diameter is 31.5 inch (diameter chosen to fit through a 32 inch wide door).

All of our tube structures are joined with compression fittings so we can easily assemble/disassemble.

There are a total of 5 half wave inductors in our system. Each half wave uses 10 lb of 23 AWG wire around a 12.625 inch by 4 foot cardboard concrete form (approximately 1.9 Km wire each).

Each one-to-one balun has a half wave inside and a half wave outside. These half waves are wired across both the front end and the back end of the primary feed line.

Since the half waves operate at their natural resonant frequency they provide extreme signal selectivity.

Neither the baluns nor the half waves climb more than a few degrees above ambient temperature. We use an infrared thermometer. If a component is liberating heat, it likely needs re-engineering. Clearly the energy that goes into heating components will not be available for making sparks.


The main structure supporting the primary is 6 inch PVC pipe and is made conductive using a mix of HVAC AF 100 metal tape and aluminum foil held to the pipe with 3M #77 spray adhesive. We keep the metal foil to a minimum as magnetic fields induce eddy currents into the aluminum (less so when the aluminum is thin).

Top-end Capacitors (toroids)

Our top end half wave has a consistent load; this allows us to choose a top end capacitance for maximum power through. The frequency is set by the array of half wave coils.

The top end capacitance is chosen entirely for minimizing the impedance for a given spark jump. We picked an opening of 52 inches and kept raising the top end capacitance until the unit no longer fired. Then we closed the jump to 48 inches (the length of the 1/2 wave inductor). This method ensures that voltage is the minimum required for reliable firing and the current is as large as possible (notice that our sparks are white hot).


We use Styrofoam for supporting the primary and for blocking the top end half wave, Styrofoam has excellent RF qualities. The top end disks are also aluminum foil covered Styrofoam.

Primary and coupling

The primary needs a lot of turns and would normally be considered very over-coupled. However k between the magnetically isolated baluns lowers the overall system k to the point where tight coupling on the primary becomes a necessity.

Control circuitry

A limit switch actuated by the vacuum diaphragm operates an industrial control relay to provide proper sequencing.

The high powered circuit for the transformer is switched by an additional heavy-duty relay.

Circuit topologies

We tried many different circuit topologies over the last few years and so far this is the best we have found.

Construction Methods

Why we use English (imperial) units

Our apologies for the non-metric units; everything in the U.S.A is in inches and feet.

Winding Inductors

Let's start with the half wave inductors. You will need five medium sized furniture casters and a board that's about five feet long. Mount four of the casters upside down with one caster on the end oriented at 90 degrees. You will be using the fifth caster to keep the form from rolling off as you wind.

Your spool of wire should be mounted in a box with bearings so that it rolls easily. We always wind from left to right. It takes two people, one to spin the form and the other to guide the wire with their fingernail (or stick). If your form is slightly out of round (typical) you can make four or five Styrofoam circles to stick inside the form to straighten it. We typically use the same Styrofoam disks as hubs for the P.V.C. pipe that supports the secondary winding.

It takes us about two hours to wind an inductor, but we have a lot of practice (25 miles of wire in the last 6 years). It is rather tedious.

Varnish the inductor with a coat immediately after winding to avoid problems. You could put on a zillion coats of high gloss and make it look like a piece of candy, but we don't do this anymore since the polyurethane varnish is a dielectric and gets polarized. Five coats varnish is plenty, make them thin coats or they will run under the wires, pool on the underside, and look nasty.

Winding tubing

Forming tubes is a bit tricky. Get spools of tubing that come in a large diameter box as they are less work hardened. Inspect the box before purchasing; some boxes have banged up tube. Unless you are Mr. Muscle, I would suggest handling no more than 60 or 70 ft. of tubing before sweating in a splice. A pair of normal mortals will get pretty wiped out after winding 240 ft of type L tube.

It is best to have a very smooth form. If anything with a minuscule amount of taper as this makes life easier when removing the tube from the form.

Secure one end of the tube to the form with screws or packing tape. Wind the coils around the drum carefully. When you are finished winding, put a Bungee cord on the loose end of the tube and pull the Bungee cord tight. Now get a rubber mallet and start (lightly) tapping the tube to get out any slack. Place the tubes shoulder to shoulder and tap on the side of the tube (gently) with a soft contoured wood block to get rid of any gaps. It takes an hour or two of (gentle) tapping per balun. If you ding up some of the tube, stick in a splice.

Now move your bungee cord so that you can put pre-cut spacers between the tubes while still keeping the tube under tension. Once you have the spacers in place you tape the tube with big X's between the legs to keep the tube from shifting when you remove the Bungee cord. Now you can glue the top half of the legs on the spacers and work the tube off the form. Do not glue the spacers to the tube or glue the tube to the form. Once removed from the form you can glue the inside leg parts and remove the tape. You will need solvent to remove any tape residue. Best to wear gloves as the acids from your hands leave nasty prints on the copper. Varnish the tube or it will lose its shine rather quickly.

Building the spark box

When you build your box, make sure to leave a few inches space between electrodes. In fact make your box just a bit taller and deeper (but not wider), than the box shown as this makes adjusting the stops easier.

A real nice trick (borrowed from manufacture of electric motors) is to drill the electrode holes over-sized in the box partitions. Then use flanges of the correct size on both sides of the partitions. You use a long copper pipe that goes all the way across the box surrounded by electrodes and clamps. Then glue the flanges. When the glue hardens you get perfect electrode alignment.


The baluns are very dangerous, keep away from them when the coil is running. Get hit with this level of power from either end of the coil, and you are dead! We have no control over your safety; it would break our heart if you got hurt, so please be careful!

Closing thoughts

Good luck from Lawrence Morris and Jared Dwarshuis March, 2010.

Other things we have done

Here are some other Tesla coil related things we have done:

Here are some other things we have done:

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