The air core inductor symbol in schematic diagrams consists of a series of adjacent semi-circles (humps) with no straight line (solid or dashed) running beneath or through them. This absence of a core line indicates a non-magnetic core—typically air, ceramic, or plastic—used for high-frequency, high-Q applications where core saturation and hysteresis losses must be avoided. Because the relative permeability ($\mu_r$) is approximately 1, the inductance remains linear regardless of DC bias current.
The Complete Inductor Symbol & Core Reference Table
Before troubleshooting or ordering replacements, you must correctly identify the core type from the schematic. The table below maps the visual symbol to its physical core material and practical circuit behavior. Read the "Symbol Description" column carefully; the presence or absence of a single line completely changes the component's saturation characteristics.
| Symbol Description | Core Type | Standard | Practical Meaning & Application |
|---|---|---|---|
| Adjacent humps, no line underneath | Air / Non-Magnetic (Ceramic/Plastic) | IEEE 315 / IEC 60617 | No magnetic saturation. High Q-factor, low inductance. Used in RF tuning, VHF/UHF filters, and high-frequency snubbers. |
| Humps with solid straight line underneath | Laminated Iron / Steel | IEEE 315 / IEC 60617 | High inductance, low frequency (50/60Hz to audio). Prone to eddy currents at high frequencies. Used in power supply chokes. |
| Humps with dashed straight line underneath | Ferrite / Powdered Iron | IEEE 315 / IEC 60617 | Moderate to high frequency (kHz to low MHz). Higher saturation current than solid iron, but will still saturate if DC bias is too high. |
| Humps (no line) with tap wire extending from a hump | Tapped Air Core | IEEE 315 | Used in autotransformer configurations or impedance matching networks in RF amplifiers. |
| Humps (no line) with diagonal arrow crossing them | Variable Air Core | IEC 60617 | Slug-tuned or mechanically adjustable coil. Used in legacy radio tuners and custom RF tank circuits. |
Regional and Standard Variants: IEEE vs. IEC
While the basic air core inductor symbol (humps with no line) is universal, the notation for adjustable or variable air core inductors diverges slightly depending on whether your schematic follows US or international standards.
- IEEE 315 (US): A variable air core inductor is often depicted with an arrow pointing parallel to the humps, or with a specific arrowhead indicating a moving core slug. The basic fixed air core remains just the humps.
- IEC 60617 (Global): A variable inductor is strictly denoted by a diagonal arrow crossing through the humps at a 45-degree angle. The arrow must not touch the core line (if one exists). For an air core variable, the diagonal arrow crosses the humps with no underlying line.
In modern 2026 CAD tools like Altium and KiCad, the default libraries often mix these standards. Always verify the BOM against the schematic symbol, as automated library generators frequently apply the IEC diagonal arrow to IEEE-style schematics.
Rows People Get Wrong: Faded Markings and Misread Schematics
Misidentifying an inductor's core type from a schematic or a faded PCB silkscreen leads to catastrophic circuit failure, usually via magnetic saturation or unexpected self-resonance. Here are the three most common bench-level mistakes:
1. The "Hidden" Ceramic Core Assumption
When you see the air core symbol (no line) on an RF schematic, you might assume you need to wind a literal air-spaced copper coil. In modern SMD designs, the air core symbol is routinely used for ceramic-core chip inductors (like the Coilcraft 1812CS series). Ceramic (alumina) has a relative permeability of $\mu_r \approx 1$, making it magnetically identical to air, but it provides the mechanical stability required for automated pick-and-place assembly. If the BOM specifies an SMD part for an "air core" symbol, buy a ceramic-core chip inductor.
2. Confusing Faded Dashed Lines for Air Cores
On older, sun-baked, or poorly printed schematics, the dashed line denoting a ferrite or powdered iron core can fade or blur. If the dashes blur together, it looks like a solid line (iron core). If they fade entirely, it looks like an air core. Never guess. If the circuit operates in a switching power supply (10kHz - 1MHz), it almost certainly requires a ferrite/powdered core to handle the energy storage. An actual air core in that position would result in massive ripple current and immediate MOSFET failure due to lack of inductance.
3. PCB Trace Inductors
In microwave and 5G RF front-ends (2.4GHz and above), discrete inductors are often replaced by meandering PCB traces. The schematic will still use the standard air core inductor symbol, but the physical PCB will just show a zig-zag copper trace with no component mounted. Do not attempt to solder a wire coil over a PCB trace inductor; you will destroy the designed characteristic impedance and parasitic capacitance of the trace.
Decision Path: Specifying a Replacement Air Core Inductor
When an RF circuit fails and you must replace an inductor marked with the air core symbol, use this decision tree to select the exact replacement part. Do not substitute ferrite-core inductors in these positions; the core losses will destroy the circuit's Q-factor and thermal stability.
| Condition / Circuit Context | Diagnostic Check | Concrete Part Selection |
|---|---|---|
| VHF/UHF Transmitter (Through-hole, < 150 MHz, high power) | Measure physical diameter. If > 5mm and handles > 500mA RF current. | Bourns 78F Series (e.g., 78F101K-RC for 100µH). Heavy-gauge wire, non-magnetic bobbin. |
| WiFi / Bluetooth RF Tank (SMD, 2.4 GHz, 0603 footprint) | Requires ultra-high Q and tight tolerance (±2% or better). | Coilcraft 0603CS Series (e.g., 0603CS-10NX for 10nH). Ceramic core, wirewound. |
| IF Filter / AM Radio (Through-hole, < 2 MHz, low cost) | Circuit requires high inductance (mH range) but low DC current (< 50mA). | Fastron 77A Series (e.g., 77A-103M for 10mH). Note: Often uses a non-magnetic phenolic core to mimic air. |
| Broadband RF Choke (SMD, > 1 GHz, signal line isolation) | Needs high self-resonant frequency (SRF) and low DC resistance (DCR). | Murata LQW15AN Series (e.g., LQW15AN10NG00). Unshielded, non-magnetic core structure. |
Safe Interpretation When Physical Markings are Missing
When you are handed a legacy PCB with rubbed-off silkscreen and no schematic, you must empirically verify whether an unmarked inductor is truly an air/ceramic core or a magnetic core. You can do this safely at the bench using an LCR meter with a DC bias function.
- Baseline Measurement: Connect the inductor to your LCR meter (e.g., Keysight E4980A or a benchtop DER EE DE-5000). Measure the inductance ($L$) at 100 kHz with 0A DC bias. Record the value.
- Apply DC Bias: Slowly ramp the DC bias current up to the circuit's expected operating current (e.g., 100mA to 500mA).
- Observe the Delta:
- If the inductance remains perfectly flat (changes less than 1-2%), the component is an air or ceramic core. The relative permeability is 1, and it cannot magnetically saturate.
- If the inductance drops significantly (e.g., falls by 10% to 50% as current increases), the component contains a magnetic core (ferrite or powdered iron) that is entering saturation. It is not an air core, regardless of what the faded schematic might imply.
By combining the visual schematic reference (the presence or absence of the core line) with empirical DC bias testing, you eliminate the guesswork from RF and power circuit repairs. Always default to the physical saturation test when the schematic documentation is ambiguous or degraded.
