SSTC Tesla coil top load: what it does, what it costs, what QCW changes

A topload adds capacitance to the top of the secondary. That lowers the resonator's impedance, and a lower impedance means less voltage for the same power. Voltage is what makes the arc long, so a topload costs you length.

In return the resonant frequency moves less when the arc grows. Whether that is a good deal depends on how much your coil detunes.

What it is and why

Z = sqrt(L / C),   and volts at a given power go as sqrt(P · Z)

More capacitance gives a lower impedance and fewer volts. What you get for those volts is a resonant frequency that does not run away as the arc grows.

How far a coil detunes is a property of the machine under the topload, not of the topload itself.

What you decide

Whether to fit one at all

The criterion is how far the arc drags the resonance.

• A slayer exciter at a few watts has nothing to detune. A point, and nothing else.
• A 4 MHz HFSSTC at 500 W is stable enough that Richie Burnett runs his resonator with no toroid at all, having "not found large toroids to have a beneficial effect on CW coil designs". His experience is at megahertz, so be careful applying it to a 300 kHz CW coil.
• A DRSSTC detunes noticeably, and wants one.
• A QCW throwing a metre detunes badly, and goes the other way entirely: capacitance is added, so that the arc is a smaller fraction of the total.

In practice, if you are wondering whether to take the toroid off: if you have branching and a drifting frequency, keep it. If you are short of length and have power to spare, try without. Never run without a breakout point.

One toroid or a stack

A stack is usually a small toroid below and a large one above, and the two do different jobs. The small one screens the top turns of the secondary, where the potential is highest and the gradient sharpest. The large one carries the capacitance and holds the breakout, which decides where the discharge goes. The lower one, being nearer the primary, also takes the induction heating.

That division of labour is our own reading of the geometry, worked out from what each toroid sits near. None of the builds we have read describes it directly, and it has not been checked against a coil built for the purpose.

The spacing between them is the adjustment, and it does not behave the way a calculator says (see the next section).

How much clearance it gets

There are three clearances, each for a different reason.

A gap to the primary of about the secondary's radius. This one is about heat. The toroid sits in the primary's field and heats inductively, and the gap keeps that down. Slot the toroid as well, so it does not act as a shorted turn. None of the six Steve Ward build pages we have read mentions slotting a toroid, so this is not his advice; the reason holds without him.

Height above the winding of about the same. This one is about where the discharge starts. If the toroid sits too low, its field falls back onto the top turns, where the potential is highest, and the breakout starts there instead of off the point.

No other metal closer than half the secondary's diameter, anywhere. This one is about foreign objects in the field. A conductor in the near field loads the resonator and pulls its Q down, wherever it sits.

The fitting that joins toroid and winding counts separately. The clamp or lug at the top of the winding is at the highest potential on the coil and is the sharpest thing there, so it breaks out before anything you designed to. The one builder in this corpus who chased the problem reports the fix as a distance, not a shape: "No flashovers due to increased distance between topload and connection clamp". davekni's suggestion in the same thread is to remove it as an exposed object instead: "That connection could be insulated, or even an entire horizontal plastic layer (big washer) could fit (and seal to) the primary coil form" (HVF 1988). Either way, measure clearance from the metal that is actually there, not from the toroid's underside.

If you search the forum for this, you will find the question but not an answer. "What should be the ideal distance between the upper end of the secondary winding and the top load?" was posted in that thread and got no reply. The rules above are ours, worked out from the mechanism.

Bigger is harder to break out of, and throws longer

A larger toroid resists breakout, and the arc it throws when it finally goes is longer:

Steve White, posting as MRMILSTAR on HVF 1937: "As the radius of the toroid gets larger it becomes harder for the streamer to break out but when it does it will be longer."

Why is not known, and he offers it as speculation. The thread gives two explanations. His is field shape: a larger radius spreads the surface field, so nothing breaks down until the voltage is higher. Marek Novotny, earlier in the same thread, suggests stored energy: a larger toroid holds more charge, which helps the air break down and pushes more current into the spark as it loads. He doubts it himself, since coil streamers are far from the DC discharges the argument applies to. This is a separate issue from stacks not adding up. That was about the number a calculator gives; this is about field shape and stored energy, which no capacitance figure captures.

What will get you

Stacked toroids do not add up. Two toroids of the 10.879 pF used here do not make 21.758 between them. Capacitances add when you parallel capacitors in a circuit, but not when their electric fields overlap, as two toploads at the same potential do. Some field lines end on the other body instead of on ground, so at the same voltage there is less charge. For two equal spheres touching, electrostatics gives 1.386 times one of them instead of twice, so nearly a third is lost.

That is a reason to distrust the calculator, not to avoid stacks. Stacks work and are common. Measure the assembled secondary's resonance, and expect the calculator to read high, more so the closer the bodies are.

The skin effect argument about seams gets the right answer for the wrong reason. People often say that the seams of a home-made toroid need not be properly joined because of skin effect. What actually carries current across a seam is the capacitance between the overlapping sections. While that capacitance is large compared with the topload's total, the voltage across the seam is small. When it is not, you get arcs between the sections and less stability than before. In practice, fold one or two millimetres of tape over the edges so foil meets foil directly instead of through the adhesive.

That edge is an observation, not an explanation. Branching has not been shown to be the failure past it: why the sword bends found no report that lists frequency among the causes of branching, and our own table calls 1 to 4 MHz a thin straight flame, well above the band. Take 300 kHz as the edge of where the shape has been observed, and do not attach a mechanism to it.

The third job: how long the arc can get

It is not an accessory. It is most of the resonator

On one coil, 150 mm of winding on a 90 mm former:

Ces total             14.031 pF
of which the toroid   10.879 pF   =  78 %
the winding itself     3.152 pF   =  22 %

The 22 per cent is not a constant, and davekni, who worked hard to pin it down, explains why it cannot be. He needed a topload-only figure to turn his charge captures into a top voltage. His coil's frequency before breakout matches 32 pF in total. He first guessed 28 pF for the topload alone, then revised it to 27 on a measured top to bottom current ratio of 84 per cent. That leaves 4 to 5 pF in the winding, 12 to 16 per cent [derived], against the 22 here. His caveat is the useful part. Measuring the frequency without the topload does not give you the winding's capacitance with it, because that capacitance is partly to ground, partly to the topload and partly between turns, and once the arc forms some of the topload's own share moves from ground to the arc. He says plainly that he does not know how to estimate it. On every coil that reports a split, including the 22 above, the split is a modelling choice.

With that caveat, the frequency, the impedance and the coil's tolerance of arc loading are mostly set by the part on top, with the winding contributing 22 per cent. In practice "tuning the secondary" means choosing the toroid, and a rewind is almost never the right fix for a frequency you do not like.

An earlier version of this section showed two versions of one published build, an 8 by 2 inch toroid at 406 kHz against an 11.25 by 3.4 inch at 343 kHz, with impedances of about 48.1 and 40.7 kΩ fitted in JavaTC, labelled "same winding, same wire, same turns". That example has been removed. The build is Gao Guangyan's QCW 1.5, and his own log contradicts most of it. Experiment one ran a new secondary of 3.5 by 6 inches, about 840 turns of AWG 34, under the small toroid. For experiment two he went back, in his words, to the original 32 AWG 5.55 inch secondary from his QCW 1.0, and built a new primary as well. The winding, the wire, the turns and the primary all changed along with the toroid. His measured secondary resonances also run the other way from the old table: 302.7 kHz standing alone with the 8 by 2 inch toroid and 339.3 kHz with the larger one, because the shorter winding underneath more than made up for the bigger part on top. Neither 406 nor 343 kHz is a secondary resonance anywhere in that log. The figures near them are pole and operating frequencies, and the ones near 408 to 413 kHz in that log belong to the fat-toroid setup, not to the small toroid the table paired with 406.

The two rows are not two unrelated machines either. The small-toroid row came from QCW 1.0, whose 32 AWG 5.55 inch winding is the one he returns to in QCW 1.5. He measured it standing alone at 408 kHz in 2014, against the 339.3 kHz the fat toroid gives it in 2015. That pair does isolate the toroid on one resonator, across two of his pages: a drop of 16.8 per cent [derived], with a year and a change of room in between. The secondary is a resonator covers that pair and its caveats. The old rows stay out, because a fitted 406 and 343 with impedances beside them are not those readings. This section rests on the 78/22 split alone, and that split is JavaTC's.

Capacitance and breakout are the two jobs everyone knows about. The third one decides whether the coil is still working at the end of the ramp.

An arc is a load, and a growing arc is a growing load. Past some conductance it damps the resonator enough that the two poles merge, and the drive loses what it was tracking. The topload sets where that happens. The Z here is the secondary's own characteristic impedance, which the secondary's page works out at 26.6 kΩ for this coil; the figure comes from that article:

the poles hold while   R_arc  >  Z / k
here                   Z / k  =  26.6 kΩ / 0.421  =  63.2 kΩ

Put in Anders Mikkelsen's arc model, about 75 kΩ·m in parallel, so that a longer arc is a lower resistance, and the threshold becomes a length. Read the paragraph under the table before trusting the metres, because that constant is the weakest input here.

With twice the toroid, the arc reaches 1.58 m instead of 1.19 before the pole structure gives way. Besides capacitance and field shape, the topload sets how much arc the resonator will tolerate.

The metres are derived, not measured, and the constant behind them comes from one builder. L_collapse = 75000·k/Z is our algebra over Anders Mikkelsen's arc model, 8 pF per meter in parallel with 75 kohm*meter, fitted to his own measured phase angles, 50 degrees near 340 kHz and 60 near 460 (HVF 3140). The constant does have a source, but the source is the outlier: 75 kΩ·m puts a 1.5 m arc at 50 kΩ, below every other published arc resistance once they are all converted to the same parallel form. There is no secondary frequency does that conversion and gets 77, 79, 103, 225 and 258 kΩ at 1.5 m on this coil. All five are above the 63.2 kΩ threshold, so all five leave the poles intact at a metre and a half. On the harshest of the five the collapse comes at 1.83 m instead of 1.19 [derived, 77 kΩ at 1.5 m is 115.5 kΩ·m, and 115500 · 0.421 / 26600]. The 2.81 m figure is not one of the published models either; it is our own result from the pole-structure calculation in that same article. Use the ratios from the table, not the metres. The whole column scales by one factor, and where it sits is open until the constant is either confirmed or replaced by the converted one.

The same lever, without the aluminium

The formula says C. It does not say toroid.

Z = sqrt(L / C)

Anything that adds capacitance between the top of the winding and ground moves Z, and the collapse length with it. A string of ceramic capacitors from topload to ground does exactly that. It is used on the largest coils now being designed, where a secondary capacitor bank of C0G ceramics does the job a toroid would otherwise do. Electrically the two are the same, with the same Z, the same frequency and the same collapse length:

The 300 kHz edge is the same edge. Capacitors do not get around it, because the frequency falls for the same reason either way. What changes is the cost:

On a QCW there is not much room for the topload. It needs clearance from the primary and height above the winding, and a coil built for coupling has a short, fat secondary with the primary right against it. A toroid big enough to matter pushes the geometry apart, and the geometry sets k. A capacitor string lowers Z without changing any dimension.

The string does not replace the toroid completely. It has no breakout, shapes no field and does not screen the top turns. The toroid can shrink to a small field-control ring, but it stays.

The next point is our own reasoning, not from a source. A toroid's capacitance is its field in the room: the current that leaves it returns through the floor, the walls and whatever else is nearby. A capacitor string carries the same current down a wire instead. A smaller toroid plus a string therefore gives the same total C with less of it coupled to the room, and the return path is one you chose instead of one the room decided. This is the same idea as the capacitance that does not care about the room. It does not make the coil any quieter. The topload is still at the same voltage, so its near field is unchanged; what changes is where the return current goes, and how repeatable the capacitance is. The string also does nothing useful until you shrink the toroid to match. Put it beside the same toroid and you have just added capacitance.

The current in the string is real. Six picofarads at a hundred kilovolts and 357 kHz is about 1.35 A through those ceramics [derived, I = 2πfCV]. The practice described here (C0G only, 3 kV parts, derated to about half for AC) has no citation and should be treated as unsourced. On those figures it takes roughly 67 parts in series to stand off the topload [derived, 100 kV over 1.5 kV a part], so a 330 pF string comes to about 5 pF [derived, 330 / 67], and useful capacitance means several strings in parallel. Mount them on rigid board, pot each in its own tube so a failed one can be removed on its own, and degas the epoxy.

Coupling also has an unresolved contradiction across the ramp. Worked at one bridge voltage, the gain each pole draws changes sign with the load:

k =         0.2    0.3    0.4    0.5    0.6
light 1 M   104.6  105.0  105.5  106.1  106.8   higher k, higher gain
heavy 50 k   92.8   90.0   88.3   87.5   87.3   higher k, lower gain

Strong coupling helps at the start of the ramp, where the load is light, and hurts at the end, where it is heavy, because it reflects the heavy load into the primary and cuts the Q. Elsewhere in this corpus coupling is treated as pole-structure margin, which pulls the other way. One possible reconciliation, ours and not checked against any source, is that a QCW wins on how long it keeps pouring energy in rather than on peak gain, and that coupling is what keeps that going. That is a guess until someone measures it or finds it stated, so treat the contradiction as open. The table is our own computation.

On a QCW, where the same cost buys something else

On a QCW the topload does the same three jobs as on any coil, plus one more that only shows up once an arc is a metre long. It sets how much arc the resonator can carry before its two poles damp together into one and the driver loses the structure it was tracking. Every coil that grows a long channel runs into that limit. On a QCW, crossing it costs more, and that shapes the design.

The three jobs

It stores charge. A discharge is charge leaving, and a topload is the reservoir it leaves from. Without one, the resonator has only its own winding capacitance, which is a small fraction of what a topload adds.

It sets a good part of the frequency. That capacitance and the secondary's inductance are the resonator. Change the topload and you change the coil's frequency, so you also change where the primary has to be tuned.

It holds the breakout and keeps the discharge off the winding. The field on a smooth toroid is spread out, so nothing breaks down until you put a point on it. Without a topload the discharge leaves from the top turns of the secondary and burns the wire down.

And why this department buys it anyway

A metre and a half of arc adds about 6.2 pF to the secondary derived, 4.14 pF per metre times 1.5 m, and that rate is [one coil's calibration rather than a property of arcs], against the toroid's own 10.879 and a cold Ces of 14.031. The arc adds another 44 per cent of the cold resonator's capacitance [derived, 6.2 / 14.031], and the resonance moves a long way while the arc grows.

How far depends on which frequency you mean, and there are two:

• The secondary's own resonance falls 16.7 per cent across the ramp.
• The upper pole, which is what the bridge is actually commutating, falls from 519.1 kHz to 463.3, which is 10.7 per cent. It moves less because the primary stays where it was tapped, at 349.9 kHz, so a pole made from both resonators is anchored at one end.

The tracking has to follow the 10.7, and it is not a rounded version of the 16.7. Both figures, and the reason for keeping them apart, are in the frequency slides down the ramp.

The bigger the resonator's own capacitance, the smaller the fraction the arc adds, and the less the frequency moves. Double the toroid, taking Ces to 24.9 pF, and the same 6.2 pF of arc is 25 per cent instead of 44. The secondary's fall drops from 16.7 per cent to 10.5 [derived, sqrt(24.9 / 31.1)]. A QCW accepts the voltage penalty on purpose for this reason, and can go further with capacitance added as discrete parts, which avoids the interlayer voltage stress that winding for capacitance would bring. That is the capacitor string described above.

Where each kind of coil sits

• Slayer exciter, a few watts: nothing to detune. A point, and nothing else.
• Burnett's HFSSTC, a 500 W demonstration unit at 4 MHz: the frequency is stable enough. A breakout bolt, no toroid at all. He writes that he has "not found large toroids to have a beneficial effect on CW coil designs", and that a coil this small does not seem to detune by very much even at full power. The detuning is what decides it, not the toroid (richieburnett.co.uk).
• QCW throwing a metre: severe detuning. A large toroid, and on some designs a capacitor string beside it.

In every case you pay for the topload in volts, and volts make the arc long. The difference between coils is how much frequency stability you get for them, which is what the first half of this article works out.

The 4.14 pF per metre is one coil's calibration, not a measurement. Two arcs have been measured and both come out above it, but neither builder published a rate. What each measured is a total, 16 pF on Anders Mikkelsen's arc and about 7 pF on Weston's, and the per-metre figures of 5.03 and 7.66 are our own division of those totals by the arc lengths [derived, 16 pF over 3.18 m and about 7 pF over 3 ft]. They should not be called published measurements, and the 3.18 m in that division is the one input in the row we have not yet confirmed in the thread. Both sit above the 4.14 used here, so every figure above shifts the same way across that range. The spread does not change the basic picture: the arc adds capacitance, that capacitance pulls the secondary down, and the pole you are switching follows it. Only the decimals are uncertain.

The job that decides how long the arc can get

This job also belongs to every coil. The first half of the article counts it as the third job, treating charge and frequency as one; here they are split because on a QCW they are separate decisions. The numbering differs, the job is the same.

An arc is a resistance across the resonator, and a growing arc is a falling resistance. Past a point it damps the two poles together into one, and the driver loses the pole structure it was tracking. The toroid sets where that happens, because the toroid sets Z:

the poles hold while   R_arc  >  Z / k
here                   Z / k  =  26.6 kΩ / 0.421  =  63.2 kΩ

The 63.2 kΩ is derived, not measured. It is the secondary's characteristic impedance divided by the coupling, both given above.

A longer arc is a lower resistance, so the threshold can be read as a length. The full table and the widget for it are above: 1.19 m as built, 1.58 m with twice the toroid. A bigger toroid lowers Z and pushes the length out.

It is not that only a QCW gets there

A DRSSTC reaches a metre and a half too. It stacks about fifteen 300 microsecond bangs on a channel that stays alive between them, each burst adding another ten centimetres to the end of the last (see what a QCW arc is and an arc grows as fast as charge arrives). Every arc model in this corpus sets the arc's resistance by its length alone, so a metre and a half of channel presents the same resistance to the resonator whichever machine grew it. A DRSSTC can reach the threshold too, so an argument that starts by assuming it cannot is wrong from the start.

Where the threshold sits at that length is also uncertain, and the spread is wide. The 1.19 m above comes from the harshest arc model in circulation, and our own pole-structure calculation puts it as far out as 2.81 m for this same coil. Converted to one form, the five published arc resistances all leave the poles intact at 1.5 m, with margins of 1.22 to 4.08 times and this coil's own at 1.85; the working is in there is no secondary frequency.

What differs is the price of crossing it

A DRSSTC's event is about 300 microseconds into a flat bus, and its current feedback loop starts from nothing at the beginning of every bang anyway, from a startup oscillator, from noise, or from a comparator modified to free-run (see every bang starts open loop). The next interrupter pulse starts the next bang regardless. So losing the pole structure costs one flash, and the next bang starts from the same conditions as the last.

On a ramp the conditions do not repeat. The buck voltage has moved on, the arc has grown, and the pole structure is not the one the driver lost. What a QCW arc is makes the same argument about pulse skip, and it applies here for the same reason: a QCW's event is 6 to 25 milliseconds of one continuous lock, tracking a frequency that slides the whole way. If it gives way two thirds of the way along, you lose the last third, which has the highest bus voltage and is where the arc was going to get longest.

Any coil can reach the collapse threshold, but only a QCW has to design for the margin, because its event is long, not because its arc is.

That step, from those two articles to "it costs the rest of the ramp", is our own reasoning and not a measurement. None of the sources here has published a capture of a ramp collapsing partway and what the rest of it then did.

Past the threshold you can still operate. Anders Mikkelsen ran 3.18 m beyond the collapse, and the diagnostic he gives is better than any threshold calculation because it needs no capacitance model: when the frequency stops responding to arc length, the coil has left the two-pole regime. That is also covered in there is no secondary frequency.

And it pulls against the frequency

A QCW has to stay in the band where the arc grows as a straight sword instead of branching. What a QCW arc is puts that band at 200 to 500 kHz, and we put a practical lower edge on it at 300 kHz, past about 28 pF of total secondary capacitance on this coil. A bigger toroid adds collapse margin but drops the frequency towards that edge, and the two limits are closer together than the cold numbers suggest.

Take the doubled toroid again: Ces of 24.9 pF, Z down from 26.6 kΩ to 20.0, collapse out to 1.58 m, and 320 kHz cold, all from the table above. Then add the arc. Another 6.2 pF gives about 286 kHz by the end of the ramp [derived, 320 kHz times sqrt(24.9 / 31.1)], below the edge.

That last step is our arithmetic on the table's figures. The 300 kHz edge is drawn on the coil's cold resonance, and the arc pulls the resonance down further while it burns, so a coil comfortably above the edge on the bench can be under it by the end of a ramp.

A QCW design has to sit between the two limits: enough capacitance that the arc's damping does not merge the poles, but not so much that the end-of-ramp frequency drops out of the sword band. That is why QCWs look the way they do. The short, fat secondary gives high coupling, which widens the pole split and makes each pole stiffer against the arc, and it carries a big toroid held just off the floor. The poles are covered in there is no secondary frequency, and how far they move down a ramp in the frequency slides down the ramp.

The move that is not a move

Doubling the toroid to halve the detuning does not simply work, for the reason in the previous section: the absolute frequency at the end of the ramp falls with it, and the absolute frequency is what keeps arcs straight.

You could compensate with turns. The doubled toroid raises Ces by a factor of 1.775 [derived, 24.9 / 14.031], so holding the cold 426.49 kHz needs the inductance down to 0.563 of what it was [derived, 1 / 1.775], which is about 25 per cent fewer turns [derived, turns go as sqrt(L) on a fixed former, and sqrt(0.563) is 0.750]. That changes the impedance:

Z = sqrt(L_sec / Ces)
  as built           26.6 kΩ
  doubled, retuned   15.0 kΩ   [derived, 26.6 · sqrt(0.563 / 1.775)]

A low secondary impedance means a higher peak current for the same voltage and a shorter ontime, the wrong direction for a machine built around a long ramp. Three things move, and you only wanted one of them to.

A big topload lowers the sensitivity to the arc and lowers the frequency; compensating with turns lowers the impedance. You find the best compromise by measuring, not by choosing one of the three in advance.

And still not by winding

A QCW cannot get its capacitance from the secondary winding. Capacitance gained in the winding comes with lower impedance, and low impedance needs more current for the same voltage. Two layers wound for capacitance end up with more interlayer capacitance than the topload has, and hundreds of volts per turn across thin varnish at the layer transitions. That failure is covered in the secondary is a resonator.

Capacitance added as discrete parts is different from winding for capacitance, because it does not add interlayer voltage stress. The general version, a string of ceramics doing the job a toroid would otherwise do, is described above.

The breakout has a harder life here

On a coil that bangs, the breakout point fires in bursts of a few hundred microseconds, hundreds of times a second. On a QCW it carries a full ramp of 6 to 25 milliseconds out to a metre or more, five to seven times a second (see the interrupter is the note). Each event is twenty to eighty times longer than a bang [derived, 6 to 25 ms against 300 µs] and finishes at the top of the bus. The tip wears out, in the sense what a DRSSTC breakout point is for describes: performance drifts over an evening with nothing changed, because a spike that has burned back changes where the arc starts and whether each ramp lights at all.

How far it should stand out is on its own page: clear of the field that hugs this surface, not tucked inside it.

The breakout point's own capacitance is the same trap here as on any coil; the measured case is in the first warning above.

The numbers

• The trade: Z = sqrt(L/C), volts as sqrt(P·Z).
• Two equal touching spheres: 1.386 times one, not twice.
• Breakout capacitance in the failed case: 12.6 to 20 pF, against a topload of the same order. It wants to be far larger.
• Primary to toroid gap: about the secondary's radius.
• Metal clearance: further than half the secondary's diameter.
• The impedance lever: Z = sqrt(L/C) = 26.6 kΩ, and Q under arc ≈ R_arc / Z.
• Pole collapse: the poles hold while R_arc > Z/k = 63.2 kΩ. As a length that is 1.19 m as built on Anders Mikkelsen's 75 kΩ·m, and 1.83 m on the harshest of the five published models once converted. The threshold in ohms is solid; the length is not.
• The wall: about 28 pF total, where this coil drops below 300 kHz.

The measured figures here are other people's; the impedance and collapse-length numbers are ours, derived as noted above.

What goes wrong

• Arcing inside the structure rather than off the point. The breakout has too little capacitance to the topload. Measure it.
• The top turns of the secondary are burnt. It was run with no breakout point.
• The measured resonance is well below the calculated one on a stack. The calculator added the capacitances. They do not add.
• The toroid gets hot. Induction heating from the primary. Increase the gap and slot it.
• Fitting a bigger toroid made the arc shorter. You paid for stability you did not need. On a coil that was not detuning, the lost volts are all you got.
• Branching and a drifting frequency after removing the toroid. Put it back; that coil needed it.

Where next

• Winding and measuring a secondary, where the other half of sqrt(L/C) is set, and why capacitance gained in the winding is worse than capacitance gained here.
• What a DRSSTC breakout point is for.
• Is it CW or not, on what the CW label does and does not tell you about whether to fit a topload.

The topload on the department diagrams explains itself if you point at it.