To be pedantic, it draws current which sags the voltage in the reader coil.. but yes basically correct. However, with a booster in the mix, you have a second LC tank circuit with it’s own eddy currents and field generation happening.
Phase 1: Primary Coil Energization (Rising Current)
AC current begins flowing through the primary coil, increasing from zero toward peak.
This moving charge generates a magnetic field (B1) that radiates outward from the primary coil in the classic toroidal pattern – field lines emerge from one face, loop around, and return through the other face.
Because this is basically an air-core transformer, the field is not confined by a ferromagnetic core. It spreads out spatially, following the inverse-cube law for a magnetic dipole at distances greater than the coil diameter.
The rate of change of this field (dB1/dt) is what matters for coupling. During the rising portion of the AC cycle, the field is expanding outward.
Phase 2: Field Reaches the Secondary Coil
A portion of B1’s flux lines thread through the area enclosed by the secondary coil. This fraction is described by the coupling coefficient k (0 < k < 1). In air-core systems, k is typically low (0.01-0.3 depending on distance and alignment).
By Faraday’s Law, the changing magnetic flux (dPhi/dt) through the secondary coil induces an EMF (voltage) across the secondary winding: V_induced = -N * dPhi/dt
This induced EMF drives a current through the secondary L/C tank circuit of the booster board.
Phase 3: Secondary Tank Circuit Charges (First Half-Cycle)
The induced current flows through the secondary inductor (L) and charges the capacitor (C). Energy is being transferred from the magnetic field into the electric field of the capacitor.
As current flows through the secondary coil, it generates its own magnetic field (B2). By Lenz’s Law, B2 is oriented to oppose the change in flux that created it – so B2 partially opposes B1.
The net field in the coupling region is now B1 - B2 (vectorially). This is “reflected impedance” – the primary sees a load because the secondary’s opposing field effectively resists the primary’s field, requiring more energy from the primary source.
Current continues flowing until the capacitor is fully charged. At this instant, all the energy in the secondary is stored in the electric field of the capacitor, and current through the secondary inductor is momentarily zero. B2 = 0 at this instant.
Phase 4: Capacitor Discharges Back Through Secondary Inductor (Field Reversal)
The capacitor now begins to discharge back through the secondary inductor, driving current in the opposite direction.
This reversed current creates a new magnetic field B2 that is now oriented in the opposite direction from the original B2. Instead of opposing B1, it may now be reinforcing it (depending on where in the primary’s AC cycle we are).
The energy oscillates: capacitor (E-field) → inductor (B-field) → capacitor → inductor… at the resonant frequency f = 1 / (2pi * sqrt(LC)).
Each time current flows through the secondary inductor (in either direction), it creates a magnetic field that radiates back toward the primary coil, influencing it.
Phase 5: Resonant Energy Exchange (Steady State)
At resonance – when the primary driving frequency matches the secondary’s natural frequency 1/(2pisqrt(LC)) – something critical happens: the secondary’s oscillations are phase-aligned with the driving signal such that energy transfer is maximized.
The secondary current (and thus B2) is 90 degrees out of phase with the induced EMF, and the voltage across the capacitor can build up to values much larger than the initial induced EMF (voltage amplification by the Q factor of the tank).
The interaction between fields becomes a continuous dance:
Primary cycle: B1 rising → B1 peak → B1 falling → B1 reversed
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Secondary tank: Current in L Energy in C Current in L Energy in C
(B2 opposes) (B2 = 0) (B2 reverses) (B2 = 0)Phase 6: The Field Superposition Picture
At any instant, the total magnetic field at any point in space is the vector sum of B1 + B2. Neither coil “owns” the field – it’s one unified field with contributions from both.
When the secondary is at resonance with no load:
The Q factor determines how much energy accumulates in the tank. High Q means large circulating currents and strong B2.
B2 can become comparable in magnitude to B1 in the coupling region, even though only a fraction of B1 reaches the secondary. The resonant buildup amplifies the response.
The secondary’s field reaches back to the primary, modifying the impedance the primary source sees. This is the mechanism of reflected impedance.
Key Insight
The fields don’t take turns – they coexist and superpose continuously. The useful mental model is:
Primary creates a time-varying field.
Secondary responds with its own time-varying field (phase-shifted).
These two fields add vectorially everywhere in space at every instant.
At resonance, the timing of B2 relative to B1 is such that maximum energy remains circulating in the secondary.
Without a load, the only losses are resistive (wire resistance) and radiative, so the tank rings with high Q and B2 is strong relative to what you’d expect from the weak coupling.
The “collapse and re-expansion” is really the continuous sinusoidal oscillation of energy between the capacitor’s electric field and the inductor’s magnetic field, with the magnetic field component reaching back into the shared coupling space and interacting with the primary’s field on every cycle.
Given that the booster board and proxmark3 are not perfectly matched, and the inclusion of a 3rd coil (the transponder) in the mix.. it’s almost like the 3 body problem.. behavior is going to be difficult to model exactly, which is why it would be very hard to make the hf tune function properly with booster in the mix. There are some gross observations that do seem to hold true though, like a drastically lower measured voltage, and sometimes the voltage appears to go up.. likely because it does (at the time of measure) as the co-mingling of 3 different coils in the same shared field interacting together is a soup of magnetic flux.