A Pancake Coil Is Strongest at the Center. An Inch Off-Target Cuts the Strength in Half.
When you place a disc on a knee for a 40-minute session and the patient feels less relief than expected, the instinct is to blame the device. Improper placement of the disc is the more likely culprit.
A brochure that shows peak Gauss without sharing where the peak field is doesn't provide the info needed for proper coil placement. Two discs of the same outer size can deliver very different fields. A pancake coil has a center focus point, while a donut coil spreads its field into a ring around the center.
It's a spiral wound into a disc.
A pancake coil is a disc-shaped coil wound in a spiral. Most are built up in several stacked layers, some in a single layer. Either way, the layered turns reinforce each other, and the disc-shaped form decides everything that happens above the coil face. The field has a strong peak at the center and a smooth, predictable falloff toward the edges. There is nothing else to the pattern.
Why the field is strongest at the center.
Picture each turn of wire as a small contribution to the total field. At the center of the spiral, every turn pushes the same direction, straight out of the coil face. The contributions stack. Off-center, the turns on the far side of the coil start pushing against the turns on the near side. The contributions partially cancel. The peak sits at the geometric center because the center is the only place where all of those contributions line up.
See the same pattern on the bench.
You can read the pancake pattern straight off this heatmap. The peak hits 3,370 Gauss at the geometric center. By 40mm (1.6 in) out, the field has dropped to half that. By 68mm (2.7 in) out, the field is down to one tenth, and what is left past that point is too weak to count. For this disc at its top setting, 77.8 percent of all the measured strength on the coil face sits inside the strong zone, the band where the field is at or above half its peak. That figure is high. It is the signature of a tightly focused field.
Place the center over the knee.
The strong zone for this disc is roughly 80mm (3.1 in) across at top setting. That is small enough that placement matters and large enough to give you some margin. When the center of the disc sits over the target, the strongest part of the field lands on the target. That is the whole reason the geometry exists. A pancake is the geometry that targets.
A placement miss looks like a weak device.
Slide the disc 50mm (2 in) to one side, and the joint that used to sit at the peak is now in the 50 percent zone. It is the same disc, the same power setting, the same 3,370 Gauss peak. The peak is just no longer on the joint. The session still runs the full 40 minutes. The field still goes where the geometry sends it, which is now an inch off the knee. You might look at the brochure again and wonder whether the device is the problem. It usually isn't. The right answer is a calmer hand on the placement.
On a spec sheet, the coil is the question, not the housing.
"Disc" describes the housing. A disc applicator can hold a pancake coil, a donut coil, or something else. The spec sheet should say which one. If it doesn't, ask. Two pieces of evidence are worth asking for. The first is a heatmap or a falloff chart, which shows the field shape in one glance. The second is the therapeutic core diameter, which tells you how wide the strong zone is. If the peak doesn't sit at the geometric center of the heatmap, the construction is a donut, not a pancake, no matter what the housing looks like.
A short checklist for buying a pancake coil
- Confirm the coil construction. Ask whether the disc is a pancake or a donut. "It's a disc" doesn't answer the question.
- Ask for a heatmap or a falloff chart. A pancake's signature is a center peak and a smooth outward decline. The heatmap shows you in one glance.
- Ask for the therapeutic core diameter. That number is the placement margin you'll have over a target.
- Match the shape and size to the target. A small joint pairs with a small strong zone and careful placement. A larger soft-tissue area may call for a different geometry.
A pancake isn't a better coil than a donut or a loop. It's a different tool. A donut delivers broader ring coverage. A loop drapes over a large area. Each one answers a different question. The question a pancake answers is "I have a specific target, and I want the strongest field to land on it."
What the full report adds.
Our published example reports include the heatmap, a chart of how the field falls off from the center, and tables that give the field's strength, its coverage width, and how tightly it is focused, at every power setting. The pancake example on the public reports page is the cleanest illustration of the geometry. It shows a peak at the center, a smooth falloff from there, a field tightly focused in the middle, and a narrow strong zone. Once you've read one of these reports, the empty space on a brochure that prints only peak Gauss becomes loud.
Ask for the heatmap, not just the number.
Our example reports show what a fully tested pancake looks like, with the heatmap, the falloff chart, and every measurement published end to end. If you'd like help reading a specific spec sheet, we'll take the call.
Schedule a Call See Example ReportsA pancake concentrates its field at the center of the disc face. Place its center over the target, and the geometry does the job it was designed for. Place it elsewhere, and the device gets blamed for a placement problem.
A future post in this thread looks at what happens when the coil construction is a donut instead, and the peak shifts off the geometric center into a ring around it.