17nH RF Choke Coil Low Parasitic Capacitance Cylindrical Core Coil Radiation Hardened

HALA1200503R 17nH Air Core Inductor Low Parasitic Capacitance High Dielectric Strength Radiation Hardened 12T RF Coil

Why 0.05mm Wire Changes Everything: DCR, Current, and the 400mA Threshold

Wire diameter is the single most consequential design parameter in a micro air core inductor — and the difference between 0.03mm and 0.05mm is far greater than the numbers suggest. Cross-sectional area scales with the square of diameter: a 0.05mm wire has (50/30)² = 2.78* the copper cross-section of a 0.03mm wire. DC resistance is inversely proportional to cross-sectional area, so the 0.05mm wire has approximately 64% lower DCR. At 400mA, the 0.03mm HALA1200303R is limited to 200mA not by any magnetic constraint (the air core cannot saturate), but by the I²R self-heating of the thinner copper filament. The 0.05mm HALA1200503R doubles the current ceiling — 400mA continuous, with sufficient thermal margin for GaN PA bias networks operating at 28-50V drain with 200-400mA bias current.

The HALA1200503R 17nH ±20% 12-turn air core inductor intentionally trades maximum inductance density (26nH for the 0.03mm version) for current-handling headroom (400mA vs 200mA). With 2.78* the copper cross-section, the thicker wire also produces a slightly wider turn-to-turn pitch that reduces proximity-effect AC resistance at high frequencies. In the 10-20GHz band where skin depth in copper is only 0.46-0.66µm, the current flows exclusively in the conductor surface — and the 0.05mm wire provides more surface area per unit length than the 0.03mm wire, further reducing high-frequency ESR. This is why the HALA1200503R's frequency range extends to 20GHz (vs 18GHz for the 0.03mm version) — the lower AC resistance from the thicker wire preserves Q factor deeper into the K-band.

Minimal Parasitic Capacitance: Single-Layer Air-Spaced Geometry

Parasitic capacitance in an inductor is the enemy of wideband operation. Every adjacent turn pair forms a parasitic capacitor; the total parasitic capacitance degrades SRF, distorts the frequency response, and creates a non-flat passband. The HALA1200503R minimizes this capacitance through three design choices:

• Air dielectric (εᵣ=1.0): The lowest possible dielectric constant. Compare to ferrite chip inductors where windings are embedded in εᵣ=10-15 ceramic — parasitic capacitance scales directly with εᵣ, so the air-core design achieves 10-15* lower inter-turn capacitance for the same geometry.
• Single-layer, non-overlapping helix: Each of the 12 turns is adjacent to exactly two neighboring turns. There are no buried layers, no vias, no stacked conductors separated by thin ceramic layers. The parasitic capacitance network is a simple series chain of 11 small inter-turn capacitors — no parallel coupling paths that multiply total capacitance.
• 0.05mm wire on 0.30mm mandrel: The turn-to-turn spacing is approximately 30-40µm. With 0.05mm conductor diameter, the ratio of gap to conductor size is approximately 0.6-0.8:1 — large enough to minimize fringing-field capacitance while compact enough to achieve 17nH in 12 turns.

High Dielectric Strength and Insulation Integrity

The inter-turn insulation in the HALA1200503R is provided by the polyurethane enamel coating on the 0.05mm copper wire. This coating serves three critical functions simultaneously:

• Dielectric barrier: The enamel coating has a dielectric strength exceeding 100V/µm. With a coating thickness of approximately 3-5µm, the breakdown voltage between adjacent turns exceeds 300V — far beyond the turn-to-turn voltage in any practical RF circuit. At 17nH and 400mA, the voltage drop across the entire 12-turn inductor at 20GHz is approximately j2.14kΩ * 0.4A = 856V peak (reactive), distributed approximately equally across 12 turns — approximately 71V per turn. The >300V inter-turn breakdown provides a >4* safety margin.
• Environmental protection: The polyurethane enamel is hydrophobic, resisting moisture absorption that could create conductive surface leakage paths across the enamel surface under high humidity. Combined with post-assembly conformal coating, the insulation system withstands 85°C/85%RH damp heat testing per IEC 60068-2-78.
• Thermal stability: The enamel maintains its dielectric properties across the full -55°C to +125°C operating range. Unlike some insulation systems that become brittle at low temperature or soften at high temperature, the polyurethane formulation remains mechanically and electrically stable throughout the rated temperature range.

Radiation-Hardened by Design: No Semiconductors, No Damage

Radiation hardness is not achieved through shielding or special processing — it is inherent to the physics of the air-core inductor. The governing equation L = μ₀ * N² * A / l contains only free-space constants (μ₀) and physical geometry (turns N, area A, length l). Total ionizing dose (TID) up to hundreds of kilorads creates no trapped charge because there are no semiconductor junctions. Displacement damage creates no lattice defects that affect inductance because there is no crystal lattice whose properties determine μ. Single-event effects create no current transients because there is no active device to be triggered. The inductor is as radiation-immune as the laws of electromagnetism themselves.

This inherent immunity extends to all harsh-environment conditions: thermal shock (-55°C to +125°C, 1000+ cycles), condensing humidity, salt fog, and low-pressure (high-altitude) operation. The air core has no CTE mismatch to manage, no porous material to absorb moisture, and no volatile compounds to outgas under vacuum. For equipment deployed in environments where repair is difficult or impossible, the HALA1200503R eliminates passive-component degradation as a failure mechanism.

HALA120 Comparison: 0.03mm vs 0.05mm Wire

Key Specifications

Why 17nH with the Same 12 Turns? Understanding the Physics

This is the most frequent question from engineers comparing the two HALA120 variants. Both have 12 turns, yet the HALA1200503R produces 17nH while the HALA1200303R produces 26nH — a 35% difference. The answer lies in the turn-to-turn pitch. The 0.05mm wire is 67% thicker than the 0.03mm wire, which increases the center-to-center spacing between adjacent turns. This wider pitch reduces the mutual inductive coupling coefficient (k) between turns. In a multi-turn air-core solenoid, the total inductance L = L_self * N² where L_self includes both self-inductance per turn and the mutual coupling between all turn pairs. With wider pitch from the thicker wire, the mutual coupling term decreases, and the total inductance drops from 26nH to 17nH. This is not a manufacturing error — it is an intentional trade-off that buys you 2* the current handling (400mA vs 200mA) and 2GHz of additional high-frequency bandwidth (20GHz vs 18GHz) through reduced AC resistance.

Assembly Guidelines: Micro-Soldering for Pre-Stripped Leads

• Geometric Crosstalk Shielding: During PCB layout, position the central axis of the 12-turn coil body strictly perpendicular (orthogonal, 90°) to the RF microstrip signal line. When the coil is parallel to the trace, magnetic flux from the coil couples directly into the transmission line, creating parasitic mutual inductance that appears as unexpected notches in S21 and degraded channel isolation. A 15° deviation from perpendicular increases near-field coupling by 2-3dB at 10GHz. Verify perpendicularity during optical inspection before soldering.
• Pre-Stripped Lead Preparation: The leads are pre-stripped and pre-tinned at the factory — no manual stripping or flux application is required before soldering. The tinning extends continuously from the tip to the coil root, ensuring a uniform solderable surface with no bare copper gaps that could initiate corrosion under humidity exposure.
• Minimum Lead Length: Trim leads to absolute minimum after soldering. Excess lead adds ~0.8-1.0nH/mm parasitic series inductance.
• Micro-Soldering: SAC305 or Sn63/Pb37. Tip ≤260°C, ≤3s dwell. The 0.05mm wire has more thermal mass than 0.03mm and is correspondingly more tolerant of brief temperature excursions.
• Fixation: After RF tuning, micro-droplet of low-dielectric RF adhesive (Epotek H20E). Verify S-parameters post-cure.

Applications

• GaN/GaAs PA Bias Tees (28-50V, 200-400mA): 400mA rating with zero saturation and low DCR enables reliable drain bias injection. 17nH provides j214Ω at 2GHz through j2.14kΩ at 20GHz — isolation improving with frequency where PA gain typically rolls off.
• SATCOM K/Ka-Band LNBs: 2-20GHz frequency coverage supports multi-band LNB designs. Radiation-immune operation for orbital environments.
• 5G FR2 (n257/n258/n260) Bias Networks: 400mA capacity with >20GHz SRF margin ensures clean bias injection at 24-40GHz operating bands.
• High-Reliability Outdoor Base Stations: -55°C to +125°C range with radiation immunity and no ferrite degradation mechanisms supports 10+ year deployment lifetimes in uncontrolled environments.

Contact us for evaluation samples, S2P Touchstone data, or to discuss your specific bias network requirements.