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My 12kv/90ma spark gap Tesla coil (SGTC) is nearing completion and I was thinking that I could do some sort of experiment with wireless power transmission.  I recall someone talking about rigging up another coil to attach to a light bulb which could be lit up from a fairly significant distance.  I've researched this a bit but I can't find anything about Tesla coil transmitters and receiving coils, and using them to light up light bulbs.  Any information would be greatly appreciated.

It is important to keep in mind that your 12kv/90ma SGTC is a partially damped or damped-wave oscillator, depending upon the break rate.  This means that the electrical vibrations in the resonance transformer's secondary coil ring down partially or completely before the next primary pulse occurs.  This is in contrast to an undamped or continuous wave (CW) oscillator.  The faster the break rate the faster the primary pulses, and the closer one comes to achieving continuous wave operation.  A significant advantage to the solid state Tesla coil (SSTC) (and the vacuum tube Tesla coil (VTTC) as well) over the SGTC is that the break rate is easily brought up to the secondary's resonant frequency, allowing for continuous wave (CW) operation.  This is preferable if one is interested in investigating Tesla's non-radiating form of wireless energy transmission.  Another big advantage of the SSTC is the ease of primary circuit tuning.  One doesn't have to fiddle around making adjustments to the primary capacitor and a primary tap.  The exact same result is achieved by a simple adjustment of the function generator's pulse repetition frequency.  Furthermore, the possibility of spurious emission in the form of electromagnetic radiation is greatly reduced.

The procedure for modifying a Tesla coil for wireless transmission involves increasing the size of the topload terminal and raising it up slightly above the top turn of the secondary.  The next step is to increase the primary capacitance to slow down its rate of vibration and bring the primary circuit back into tune with the modified secondary-topload combination.  The idea is to create a high potential on the elevated terminal at the greatest break rate possible but with no sparks or streamers issuing forth.

When I was on the Board of Directors of the International Tesla Society in 1990 I did improvise a damped-wave Tesla coil RF transmitter.  It was based upon a fellow board member's tabletop TC display unit.  The approximately 1" knob topload was replaced with a copper flush-valve float, and moved off of the secondary on to an improvised insulated stand a few inches higher than its original position and a little bit off to the side.  I modified its glass-plate capacitor by adding another sheet of aluminum foil.  The receiving transformer, prepared beforehand, had an AWG 30 gauge primary wound on a 1.75" cardboard tube.  The secondary to which the load was connected was wound on top of the helical-resonator primary towards the lower end of the single-layer coil in exactly the same fashion as Tesla did when he was in Colorado Springs in 1899.  The electrical load consisted of a 6-volt pilot light.  The elevated terminal was a sheet of aluminum foil hung off of another improvised stand made of wooden sticks.  The lamp was brightly lit at a range of about 15 feet.  It's not at all hard to do.

On the left is a solid state Tesla coil transmitter with an elevated terminal. The resonator has a 7:1 aspect ratio, which is the same as one of the experimental coil forms that Tesla used in Colorado Springs.  The primary coil form or spool is the upper portion of a 5-gallon plastic pail.  The secondary or helical resonator coil form was fabricated from a 1/8" thick cardboard cylinder (a Grace Ice & Water Shield inner cylinder) with a few light coats of polyurethane finish and 3/32" acrylic end disks glued in with epoxy.

On the right is a Tesla receiving transformer with widely-adjustable variable elevated terminal. The terminal is not very robust but it does give a very wide range of tuning and this is particularly useful when first configuring a Tesla coil transmitter-Tesla receiving coil pair.  

For the load, it is best to use a very small incandescent lamp connected to the receiving transformer's secondary winding.  I like to use a single low voltage clear Christmas tree light because they are cheap, plentiful, and perform well.  A low intensity LED will also perform quite well and will light at more than twice the distance because of the lower current drain that is placed upon the receiver.

Visit the Tesla's Wireless Work website for the specifications of this particular Tesla transmitting-receiving outfit.

All of the Tesla coil design programs I've seen seem to be for maximum spark length, but what about maximum RF output?  Are Tesla coils for wireless transmission built differently?  I know usually one would put a large toroid on top, but is there anything else?

One big difference is that spark gap Tesla coils are partially damped or damped oscillators.  Greater spark length is achieved by reducing the break rate resulting in a longer interval between discharges of the primary capacitor.  This is the case for the typical spark gap Tesla coil (SGTC) and also the DRSSTC.

For wireless transmission it is desirable to increase the break rate up to the vibration rate of the resonator.  It is also important to charge the primary capacitor with direct current.  The objective is simply to achieve the greatest potential on the elevated terminal with the greatest possible break rate with no sparks issuing.

Once you have the transmitter built and in place with a robust ground connection and an elevated terminal, adjust the primary capacitor and the primary tap so that the primary vibration is the same as the secondary vibration.  The objective is to develop the maximum e-field in the vicinity of the transmitter.  Use a cheap analog voltmeter set on a lower AC volts scale with the COM terminal lead to ground and a long V terminal lead supported up in the air as an e-field probe.  A fluorescent tube on a grounded stand also works fairly well.  Also, the transmitter does not have to be powered up to the point where the thing is almost sizzling and sparks are just about to break out.  When everything is properly tuned up the transmission and reception takes place even at very low power levels.

The basic Tesla coil design is the essentially same whether it be used for the creation of artificial lightning or wireless transmission, with the general exception of the topload being elevated above the helical resonator's top turn.  For a constant diameter of resonator, the length of the vertical cylindrical conductor between its top turn up to the elevated terminal can be shortened and at the same time the resonator itself lengthened until a point is reached at which the topload is once again directly adjacent to the top turn.

Do I have to build a giant Tesla coil transmitter, like Tesla did, to demonstrate wireless transmission?

The individual experimenter doesn't need a large high-power Tesla-coil transmitter to learn a lot about the wireless transmission of electrical energy Tesla style, and to produce meaningful results.  Anyone with a small solid-state Tesla coil can do this.  Actually smaller may be better because this allows the close in near-field demonstrations to be performed inside of a Faraday cage.  Here are some guidelines.

For the transmitter:

1) Use a 7:1 to 9:1 aspect-ratio coil form wound with about 1,100 to 1,800 turns of wire.

2) Position the transformer about 25 feet or more from a good Earth connection, and connect the two using a piece of insulated #12 or larger stranded wire laying on the ground.

3) Use a well filtered DC power supply or battery bank in conjunction with a high precision pulse generator to drive a unidirectional switching circuit, i.e., don't use the four-device bridge circuit. Connect the pulse generator to the SSTC driver circuit with about 25 feet of RG-58 coaxial cable.  Use optical isolation; the coax could be replaced with fiber optic cable. Start out with the pulse generator duty cycle set to less than the maximum 49% duty cycle.  The duty cycle can be used as a means to control transmitter power output.

5) Elevate the topload so it's about 1.5 times the bottom-to-top secondary height above the secondary's top turn.

4) For high power oscillators increase the size of the topload so that streamers do not escape from it when the transmitter  is operating at maximum power.

6) Tune the oscillator to its fundamental resonant frequency by observing the reaction of an oscilloscope, or analog voltmeter set to the lowest scale, with one lead connected to ground and the other connected to a nearby elevated terminal. A grounded fluorescent or neon lamp supported in the air a few feet away from the transmitter is good for course adjustment.  A frequency counter can also help.

7) If your solid state Tesla coil's coefficient of coupling is tight, try loosening it.

For a passive receiver or wavemeter:

1) Construct a coil stand out of 2 1/2" white PVC pipe and 6 pieces of 22" x 1' x 1/2", fastened with 1/4" x 20 brass bolts to form a tripod.  This type of stand is great for the Tesla coil transmitter as well (click here for an example).

2) Construct an adjustable topload using piece of heavy-duty aluminum foil, 4' long by 18" wide, wrapped around a 20" wooden dowel, mounted on a small board with end brackets. Before starting the wrap, fold one end of the foil around a piece of bare copper wire, with one end flush and the other extending out 2", and solder it to a copper slip-ring installed on one end of the dowel.  (The topload will be connected to the coil with a piece of wire run to a strip of springy metal pressing on this slip ring.)  Now roll a few inches of the foil's free end on to a 20" wooden batten and then sandwich it with a second batten, and attach a pull cord of light nylon line.  Mount the entire assembly on a vertical section of 1 1/2" black PVC pipe.  Use a 1 1/2" to 2 1/2" slip bushing and a 2 1/2 " to 2 1/2" coupling to mate to the 2 1/2" pipe.  Tuning is accomplished by pulling the foil out from the roller like a window shade.  This arrangement works well, especially if there is little or no wind.  One difficulty is the need to tip the whole apparatus to its' side in order to roll back the foil when its adjusted past peak resonance.  An alternative arrangement is two telescoping PVC pipes with a cord and pulley arrangement (click here for an example).  This more robust elevated cap assembly allows a topload of fixed dimensions to be raised and lowered at will.  The coil-to-topload connection is with a festoon possibly made with a conductor stripped from a piece of hard elevated telephone drop wire.

3) Obtain a 1.5:1 to 3:1 aspect-ratio coil form on which to wind the receiver's resonator coil.  Fill it with a piece of wire the size and length of which will result in a coil that, with the adjustable topload attached and set at or near its smallest capacity, is resonant at a slightly higher frequency than the transmitter frequency.  This is the receiving transformer's primary coil.  A higher aspect-ratio primary can also be used.  A miniature receiving coil can be created using an empty aluminum foil tube wound with AWG #40.  A removable secondary can be wound on a short section of paper towel roll.  Tuning can be accomplished with a moveable ferrite rod, such as used for AM radio loop stick antennas.

4) Ground the resonator using a piece of insulated #12 cord about 25' to 50' in length as is done with the transmitter. Good results can be achieved using a standard ground rod 8' or longer driven into soaking-wet earth. Fire hydrants and steel well casings also work well.

5) Wind a secondary coil around the primary, close to its base. Instead of a solid conductor, it may be better to use a piece of insulated wire from a split-in-two zipcord, or a long piece of test-lead wire.  A small low-voltage incandescent Christmas tree lamp is connected to the secondary as a load.  A small permanent magnet DC motor can be run through a 4-diode bridge rectifier. Work with this arrangement for a while to hone your tuning skills.

6) For long distance reception it is not necessary to use the secondary described in step #5.  A conventional long-wave communications receiver can be capacitively coupled to the primary circuit instead.  This is done by running a lead from the receiver's antenna terminal across and up to a point on or near the PVC pipe, about 2 ft. above the top turn of the resonator.  A second lead is run from the receiver's ground terminal to a common grounding point fastened to one of the tripod legs. Tune both the helical resonator and the receiver to the transmission frequency; try adjusting the antenna-to-resonator coupling for effect.  Nearby objects such as hillsides, trees and buildings have a noticeable effect on tuning, so get as much out in the open as possible.  You'll find that you have to step back from the coil to avoid detuning of the resonator, and to get maximum resonant rise.  Watch the "S" meter as you move forward and back, as this helps with fine tuning.  Have an assistant sweep the pulse repetition rate through the resonator's center frequency and observe the effects on the signal strength and background noise level.  An oscilloscope can be used in place of the receiver.  A sensitive e-field probe will also produce good results.

Alternatively, the antenna terminals of the long-wave communications receiver can be connected directly to the secondary described in step #5 via an RF transmission line.  

A few additional suggestions:

1) Join the Yahoo! Wireless Energy Transmission Tech Group located at http://tech.groups.yahoo.com/group/wireless_energy_transmission/ ;

2) Pursue an amateur radio operator's license, General Class or higher;

3) Familiarize yourself with the following:

Federal Communications Commission Spectrum Policy Task Force Report of the Unlicensed Devices and Experimental Licenses Working Group, November 15, 2002 [ http://transition.fcc.gov/sptf/files/E&UWGFinalReport.pdf ]

FCC 47 CFR Part-5 Rules: EXPERIMENTAL RADIO SERVICE (OTHER THAN BROADCAST) [ http://www.access.gpo.gov/nara/cfr/waisidx_02/47cfr5_02.html ]

OET EXPERIMENTAL LICENSING SYSTEM (Under Part-5 Rules) [ https://apps.fcc.gov/els ]

APPLICATION FOR NEW OR MODIFIED EXPERIMENTAL RADIO STATION AUTHORIZATION (Form 442) [ https://fjallfoss.fcc.gov/oetcf/els/forms/442Entry.cfm ]

FCC 47 CFR Part-15 Rules: RADIO FREQUENCY DEVICES [ http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr&sid=96b9ed1af62c5d232f886fcd5ba5755c&rgn=div5&view=text&node=47:1.0.1.1.14&idno=47 ]

FCC 47 CFR Part-18 Rules: INDUSTRIAL, SCIENTIFIC, AND MEDICAL EQUIPMENT [ http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr&sid=92ce5be3f621b2578565a96aeb26a623&rgn=div5&view=text&node=47:1.0.1.1.16&idno=47 ]

"Methods of Measurements of Radio Noise Emissions from Industrial, Scientific and Medical Equipment" http://www.fcc.gov/Bureaus/Engineering_Technology/Documents/measurement/mp5/mp5-1986.pdf ]

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