Electronic – Ferrite core inductor for antenna

An inductor fabricated by etching a spiral in a PCB works fine. This has occasionally been used in tight budget high current applications, where EMI radiating from the "coil" is not a major concern, and nor is the precise value of the inductance.

The calculator mentioned by OP in comments is optimistic compared to results obtained in practice.

Yes, 50A in pulses with a sufficient cool-off period between them, on 2 oz quarter inch traces will work fine. If 4 oz copper is an option, that works better, both due to lower resistance and higher thermal capacity.

• One risk of such a design is delamination / gassing of layers in cheaper boards and thinner copper
• Expect perhaps one third to two-third of the inductance that calculator shows, going by actual results I've seen for a 20 Amp 100 microHenry planar PCB inductor
• Coupling with nearby metal is minimal - note that the "coil" is a planar inductor, thus the magnetic lines of force are highly distorted and flattened compared to a circular wire coil profile
• The addition of a squat, rivet-shaped core of suitable material can be very beneficial, especially if the maximum diameter of the spiral can be kept relatively small. As outer diameter increases, efficacy of the core drops drastically.

Disclosure: I've never seen that particular calculator before

The basic reason for using a parallel tuned circuit as the exciter is one of efficiency. If your inductor and tuned capacitor are 33uH and 75nF, resonant frequency will be about 101kHz. If you do the math you'll see that a lossless tuned circuit like this exhibits infinite impedance but still circulates high current between cap and inductor.

Lossless circuit are of course impossible but making the losses as low as possible means that if your inductance is 33uH and your applied voltage is (say) 40VRMS at 100kHz, the current in the inductor is: -

\$\dfrac{40V}{2\cdot\pi\cdot F\cdot L}\$ = 1.93 Amps

Your H bridge won't even be breaking into a sweat because it won't be supplying anything like this current. This current is due to the voltage across the inductor but the capacitor has the effect of performing "power-factor" correction and because the losses are low maybe the H bridge will be supplying in the order of 50mA to a couple of hundred mA.

However, your H bridge is exciting the coil/cap with a square wave and there will be losses due to the harmonics within the square wave. Because of this it makes sense to feed the coil/cap via an inductor too - somewhat smaller than the coil (maybe 1 quarter). You will also need to retune the capacitance to compensate for this. Some experimentation in this is required to get best results but, you should aim to reduce the H bridge's current to avoid it overheating.

I'd also say make one larger coil suitable for all three inductive loads. The larger coil can be any regular shape that suits your requirements for placement of the receiving coils.

Optimum performance is when the receiving coils are also tuned with a capacitor but, because the induced voltage is in series with a receive coil, the tuned circuit behaves like a series tuned circuit and, if the coupling is too great it will heavily detune the transmit coil when it is close by. You should aim for a minimum gap or incorporate circuits in the H bridge that current limit.

I strongly advise you to use something like LTSpice for simulating this - you'll learn a lot about the various interactions. I'd also recommend you read a bit about tesla coils because that is what are are intending to build (when tuned as per my thoughts).