Key takeaways
- The coil is the sensing element, so inductance, Q and stability are the measurement, not support values.
- Q has to be specified at the operating frequency, because AC resistance rises with skin and proximity effect.
- Self capacitance sets the usable band, and the winding style sets the self capacitance.
- Separate the drift sources and state the mounting condition, or the bench reading will not match the application.
The coil is the sensing element
In an inductive sensor, a variable reluctance sensor or an RFID antenna, the coil is not a support component. Its inductance, its quality factor and its stability are the measurement. A proximity sensor reads the change in inductance when a metal target enters the field. An antenna matches the reader to the tag at a frequency. A coil that is correct on the bench but drifts with temperature will move the reading or the tuning.
The design starts from the physical quantity: the change in inductance per unit of target movement, the field strength at a distance, or the resonant frequency. Those requirements set the geometry, the core and the winding. The electrical performance then has to hold across the temperature range and the production spread, not only in the prototype.
Inductance, Q and AC resistance
Inductance sets the frequency of an LC circuit and the reactance at the operating frequency. Q is the ratio of reactance to resistance, so it measures how much energy the coil stores against how much it loses. A high Q gives a sharper resonance, a stronger field for a given drive and a more stable oscillator.
At DC the resistance is the copper resistance. At the operating frequency the current crowds to the surface through the skin effect, and the field of adjacent turns pushes it further through the proximity effect, so the effective resistance is higher. Q therefore falls with frequency once the AC resistance grows faster than the reactance. The number to specify is Q at the operating frequency, not at 1 kHz.
Self capacitance and the usable band
Every winding has capacitance between turns, between layers and between the winding and the core or shield. That capacitance resonates with the inductance at the self resonant frequency. Above it the coil behaves as a capacitor, so the usable band ends well below it.
Winding style decides the capacitance. A single layer has less turn to turn capacitance than a multilayer bank winding. A shield adds capacitance to the winding. Keeping the winding away from the core, or using a segmented bobbin, lowers it. For an antenna, the self resonance is part of the design, not a defect to be removed.
Core material and stability
A ferrite core concentrates the flux and raises the inductance for a given number of turns. The grade decides the loss at the operating frequency, the permeability and how much the permeability changes with temperature. A high permeability grade gives more inductance but usually more loss and a lower frequency limit.
Mechanical stress changes permeability. A core that is clamped too hard, potted under stress or mounted on a flexing bracket will drift. A gap, if there is one, stabilises the inductance against permeability change but adds fringing and loss. Temperature, humidity and nearby metal all move the reading, so the design has to fix the ones that matter or compensate them.
Constructions and where they fit
The table compares the constructions that appear most often in sensor and antenna work. The bands are directional, not limits, because the winding and the core grade move them.
| Construction | Useful band | Q behaviour | Stability | Note |
|---|---|---|---|---|
| Air wound solenoid | Low to mid MHz | High with few turns | Good, no core loss | Needs volume for the inductance |
| Ferrite core solenoid | kHz to low MHz | Falls near core loss | Depends on grade and gap | Concentrates the field |
| PCB spiral | MHz and above | Limited by copper and dielectric loss | Good on a stable substrate | Repeatable and low profile |
| Bobbin wound, shielded | kHz to low MHz | Reduced by shield loss | Good against external fields | Shield adds capacitance |
Directions are illustrative. Measure the finished construction at the operating frequency.
Tolerance and drift
Inductance tolerance is usually wider than the tolerance on the dimensions that produce it, because permeability varies between cores and the winding build varies between bobbins. The tolerance is also widened by temperature, which is why a coil that is trimmed at 20 °C can sit outside the window at 70 °C.
Separate the drift sources: core permeability with temperature, copper resistance with temperature, dimensional change, moisture and the proximity of metal. A coil in a plastic housing near a steel bracket will read differently from the same coil on a bench. State the mounting condition and the trim range, then check the extremes.
- Inductance at the operating frequency and bias
- Q at the operating frequency
- DC resistance and its temperature correction
- Self resonant frequency
- Temperature coefficient of inductance
- Shield, core gap and mounting condition
Measurement
Measure with an LCR meter or an impedance analyser at the operating frequency, with the same bias and the same fixture the application uses. A four terminal connection removes the lead resistance. The fixture has to hold the coil in the same relation to any metal or shield, because both change the reading.
Calibrate with a traceable standard and record the temperature. For a sensor, a functional check against a target at a known distance is more useful than a bare inductance reading, because it includes the field geometry and the effect of the housing.
What the drawing should state
A useful coil drawing states the electrical values, the test conditions and the mechanical envelope. Without the test conditions, two suppliers can meet the same inductance and still deliver coils that behave differently in the circuit.
- Inductance and tolerance at a stated frequency and bias
- Minimum Q at that frequency
- Maximum DC resistance
- Minimum self resonant frequency
- Core grade, gap and winding style
- Operating temperature range and mounting condition
- Termination, encapsulation and any shield
Frequently asked questions
Why does Q fall at higher frequency?
The reactance rises with frequency, but the AC resistance rises faster once skin and proximity effect become significant. Core loss also increases with frequency. The result is a peak in Q, and the operating point should sit below it.
What is the self resonant frequency of a coil?
It is the frequency where the winding capacitance resonates with the inductance. Above it the coil looks capacitive, so the useful range ends below it. Winding style, layer count and any shield all change it.
Should I specify inductance or turns?
Specify inductance with a tolerance at the operating frequency and bias, and let the supplier choose the turns for the core. Turns alone do not define the field, because permeability and geometry also decide the result.
How does a metal target change a sensor coil?
The target carries eddy currents that oppose the field, which lowers the inductance and changes the loss. The amount depends on the target material, the distance and the frequency, so the functional check should use the real target.
Sources and further reading
- IEC 62044-3:2000, Cores made of soft magnetic materials, measuring methods
- IEC 60205:2016, Calculation of the effective parameters of magnetic piece parts
- Qumek coil design notes for sensors and antennas
Qumek Engineering Desk
Engineering team
The engineering team behind Qumek drawings, DFM notes, tolerances and inspection standards. Every guide is reviewed against the drawings and processes used on live programmes.
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