Since the secondary of a Tesla coil is a resonant device, and being that the usual goal is to get as high a voltage as possible with the least input of energy and stress on the parts, it makes sense for the coiler to try to make this resonator as efficient (high Q factor) as possible. Generally, this is not that difficult. Most any secondary coil wound with even a little common sense will be satisfactory for a Tesla resonator, but careful resonator design, with focus on minimizing losses, can result in excellent performance. Use of form materials with low RF dissipation factors, and the configuration of the winding itself, can make a noticeable difference. Tweaking for performance is one of the fun things about the Tesla coil hobby, just as it is in many other pursuits.
Some coilers have proposed multifilar ('multiple filament') winding as one method for construction of a Tesla secondary resonator. A multifilar winding consists of two or more insulated conductors wound alongside each other. This is generally done to increase the the amount of copper and surface area in the winding, lowering resistance and (hopefully) increasing the Q factor. However, all other things being equal, this lengthens the coil, lowering the inductance (and the Q!), and offsetting the gains from the lowering of resistance. Some believe multifilar winding may also reduce RF 'skin effect' losses. Litz wire could be considered an extreme example of multifilar winding, but some sources indicate that the RF benefits of using Litz become trivialized if the wire length is a significant fraction of the wavelength, which it is in a typical Tesla resonator. Ah, the rich milieu of tradeoffs!
So, whether or not a multifilar winding is appropriate must be determined for the application. If a multifilar winding is being considered, the following must be kept in mind.
A multifilar winding can be more work to make, and may (or may not) yield a higher Q resonator depending on how it is implemented.
The wire positions must be chosen so that each wire sees the same conditions electrically. Otherwise, each wire will resonate at a slightly different frequency, and instead of a single combined resonant response at one frequency, there will be a closely spaced group of peaks, which add together to produce a wider and somewhat less strong response peak. This lowers the effective Q of the winding.
The situation described above was manifest in an experimental secondary winding consisting of 421 turns of five #31 enameled wires laid side by side as a flat group. The group turns were spaced one wire diameter apart. This resulted in a ribbon-like conductor configuration. At the ends of the winding, the group was cut flush and the wires soldered together. When the resonant frequency of the coil was measured, it was found to have multiple, closely spaced peaks, even though the wires were all the same length (the winding still performed well in an actual setup).
This result is easily explained. The outer wires of one turn, being closer to the outer wires of the adjacent turns, share a certain amount of capacitance due to the voltage difference between turns. The wires closer to the center of the group are farther away from the wires of the next turn, are somewhat shielded by the outer wires, and share less capacitance. The central wire shares the least capacitance with the wires of the other turns since it is farthest away from them. The different capacitances cause the wires to have slightly different resonant frequencies, even though they are the same length and tied together at the ends. The voltage stress on the individual wires varies in a similar manner.
Obviously, this simple method of multifilar winding might be avoided (it could be 'fixed' by soldering the wires together at intervals).
There are two other construction alternatives available
which can ensure that each conductor operates in an identical electrical
1. The wires of the group may be twisted together into a cable (as in Litz wire). Care must be taken that each wire visits the interior/exterior parts of the cable to the same extent. This grouping method results in a more compact winding than a ribbon-like arrangement, for the same number and size of wires. The turns may be spaced if desired or needed.
2. For more than two wires wound in a flat, ribbon-like
arrangement, each wire must start and end at a point on the coil form equidistant
from the others. For example, if four wires are used, each wire must start
at a point 90 degrees from the others on the form, and end 90 degrees apart
from the others at the other end of the form, each wire making the same
number of turns. Since four side-by-side wires of the same size take up
more space, the winding must be made longer, wider, or both. The group
may be spaced if desired or needed, but the same spacing must
then be used between each wire in the group, for reasons discussed
earlier. Ground point connections to each wire must be the same length,
and top load connections to each wire must be the same length.
These two strategies tend to equalize the total capacitances and voltage stresses on the individual conductors; the second method further equalizes and reduces these factors along each conductor. The second method reduces energy stored in the winding's distributed capacitance (Cdist) by dividing the voltage evenly across the wires. Adjacent wires can act as capacitors if there is a voltage difference between them. Consider a single-wire winding at 1000 volts per turn; the capacitance between each turn is charged to this voltage. For a four-wire, ribbon-like winding constructed as in method 2 above, the voltage difference between each wire is 250 volts at 1000 volts per group turn. The resulting stress on the wire insulation of this winding is reduced by a factor of 4.
The energy stored in a capacitor, in joules,
is represented by ½CV². For purposes of simple illustration
let us set the between-wire capacitance value for one turn at 1 pF. For
the single-wire winding at 1 pF per turn and 1000 volts, the stored energy
in the capacitance of one turn is 0.5 x .000000000001 x 1000000, or .0000005
joule. Each of the four between-wire capacitances in one turn of the four-wire
winding, at 1 pF and 250 volts, stores 0.5 x .000000000001 x 62500, or
.00000003125 joule. The total stored energy for one turn of the four-wire
winding is then .00000003125 x 4, or .000000125 joule. This is 1/4 of the
energy stored between the turns of the single wire winding.
David McNamee March 17, 2001