11nH Air Coil Inductor Ultra High SRF RF Choke Coil Low IMD

HALA0800503R 11nH Air Core Inductor Zero Magnetic Saturation Total Linearity Ultra-High SRF Lowest IMD 4-20GHz RF Coil

Intermodulation Distortion in Passive Components: Why Ferrite Cores Generate PIM and Air Cores Don't

Passive intermodulation distortion (PIM) is not just a connector and cable problem. At high RF power levels, ferrite-core inductors generate measurable third-order intermodulation products that fall directly into the receive band, desensitizing the receiver and reducing cell-site capacity. The physics is straightforward: ferrite is a nonlinear magnetic material whose permeability μ varies with the instantaneous magnetic field strength H. When two or more RF tones pass through a ferrite inductor simultaneously, the time-varying μ modulates the inductance, which in turn phase-modulates the RF signal — generating PIM products exactly where they do the most harm.

The HALA0800503R 11nH ±20% air core inductor eliminates PIM generation by eliminating the nonlinear material. With an air core operating at the fundamental free-space permeability μ₀ = 4π*10⁻⁷ H/m — a universal constant that does not change with magnetic field strength, temperature, or frequency — there is zero permeability modulation and therefore zero PIM. The 8-turn helical winding of 0.05mm enameled copper wire maintains perfectly linear inductance from 0mA to 400mA DC bias, from -55°C to +125°C ambient, and across the full 4-20GHz operating band.

Zero Magnetic Saturation and Total Linearity: The Measurement Difference

To quantify what "zero saturation" means in practice, consider a GaN Doherty power amplifier bias tee operating at 3.5GHz with 300mA drain bias. A typical 11nH ferrite multilayer chip inductor at 300mA exhibits approximately 35-55% inductance droop due to partial core saturation. At 3.5GHz, the RF impedance drops from j242Ω (ideal 11nH) to approximately j121Ω — less than half the required isolation impedance, resulting in significant RF leakage into the DC supply path.

The HALA0800503R maintains its full 11nH inductance at all DC bias currents. At 3.5GHz, that means consistent j242Ω impedance — no saturation, no droop, no bias-dependent detuning. This is not a marginal 5% improvement; it is a fundamental qualitative difference that changes the design margin from negative to strongly positive. The total linearity extends to temperature: μ₀ does not change with temperature, meaning the 8-turn air-core helix exhibits zero temperature coefficient of inductance — a property no ferrite inductor can claim.

Ultra-High Self-Resonant Frequency: Why 8 Turns on Air Beats 8 Layers on Ferrite

Self-resonant frequency is determined primarily by parasitic inter-winding capacitance. In the HALA0800503R, three design factors push the SRF well above 20GHz:

• Air dielectric between turns: The inter-turn capacitance is proportional to the dielectric constant of the material between adjacent conductors. Air has εᵣ = 1.0, the lowest possible value. By comparison, ferrite chip inductors embed their windings in materials with εᵣ = 10-15, producing 10-15* higher parasitic capacitance for the same geometry.
• 0.05mm ultra-fine wire with 0.30mm coil diameter: The turn-to-turn spacing is large relative to the conductor cross-section, minimizing the parallel-plate capacitance between adjacent turns. Each turn is adjacent to exactly two neighbor turns in the single-layer helix — no overlapping buried layers where capacitance concentrates.
• Non-overlapping single-layer helical geometry: Multilayer chip inductors stack conductors vertically with only thin ceramic layers separating them. A single layer of 8 turns on air has fundamentally lower parasitic capacitance than 4 layers of 2 turns each separated by 10-20μm of high-εᵣ ceramic.

The practical consequence: the HALA0800503R operates as a pure inductor throughout the specified 4-20GHz range with no parasitic resonances in-band. For 5G FR2 applications (24.25-29.5GHz), the SRF margin ensures the inductor never approaches self-resonance where Q degrades rapidly and phase response becomes unpredictable. Engineers designing bias tees and matching networks can model the inductor as a simple series R-L with confidence that no hidden parasitic resonance will invalidate the simulation.

Lowest IMD: Zero Nonlinearity by Design

In full-duplex FDD systems, transmitter power leaks into the receiver through finite duplexer isolation. Any PIM product generated by passive components in the transmitter path that falls into the receiver band adds directly to the noise floor. For a 5G n78 TDD base station with 200W EIRP, even -110dBc PIM from a single bias tee inductor translates to -60dBm at the receiver input after 50dB duplexer isolation — 30dB above the typical -90dBm receiver sensitivity threshold.

The HALA0800503R contributes negligible PIM because its inductance is not a function of current. The governing equation for the air-core solenoid — L = μ₀ * N² * A / l — contains only constants (μ₀) and geometry (N=8 turns, area A, length l). None of these terms vary with instantaneous current, voltage, or temperature. This is fundamentally different from ferrite inductors where L = μ(H) * N² * A / l and μ(H) depends on the instantaneous magnetic field, which depends on the instantaneous sum current of all RF tones passing through the inductor.

For wideband systems with 200MHz instantaneous bandwidth and 256QAM modulation, the -38dB EVM requirement leaves no margin for passive PIM above -120dBc. The HALA0800503R eliminates this PIM budget line item entirely — the inductor contributes zero intermodulation beyond the thermal noise floor of the system.

Key Specifications

HALA0800503R vs. HALA0600503R: Choosing the Right Platform

The HALA080 and HALA060 share identical wire diameter (0.05mm), inner diameter (0.30mm), current rating (400mA), and operating temperature range (-55°C to +125°C). The key differences — turn count and inductance — determine which platform best suits your frequency band and impedance matching requirements:

The 8-turn HALA080 generates 57% more inductance than the 6-turn HALA060 on the same 0.30mm mandrel, providing correspondingly higher RF impedance at any given frequency. For bias tee designs where DC supply isolation impedance directly determines RF leakage, the HALA080's j276Ω at 4GHz provides an additional 4.5dB of isolation compared to the HALA060's j176Ω. Choose the HALA080 when your design demands maximum choke impedance; choose the HALA060 for absolute maximum SRF margin when operating above 15GHz.

Understanding Skin Effect at 20GHz: Why 0.05mm Wire Matters

At 20GHz, the skin depth in copper is approximately 0.46μm. The 0.05mm (50μm) diameter wire has a radius of 25μm — more than 50* the skin depth. This means the RF current flows exclusively in the outermost 0.46μm shell of the conductor, and the effective RF resistance is significantly higher than the DC resistance. This is a fundamental physics limit that affects all inductors regardless of core material. The HALA0800503R addresses this through its short total conductor length — approximately 7.5mm for the 8-turn helix plus leads — which keeps the total skin-effect resistance manageable. At 10GHz where skin depth is ~0.66μm, the effective AC resistance is approximately 1.5-2* the DC value. The S2P Touchstone data captures the actual frequency-dependent resistance, enabling accurate simulation without skin-effect approximations.

PCB Assembly Guidelines

• Lead Trimming: The standard leads are ≥8.0mm. After soldering, trim flush with the PCB pad edge to minimize parasitic lead inductance. Each millimeter of excess lead adds approximately 0.8-1.0nH of series inductance — a significant fraction of the 11nH nominal value. For precision RF designs above 10GHz, trim tolerance should be tighter than ±0.2mm.
• Prevent Solder Bridging to Active Turns: Because the leads are tinned continuously to the coil root, uncontrolled solder wetting can allow solder to wick up onto the active 8-turn winding — effectively shorting turns and reducing inductance. Control solder fillet height to remain on the lead below the first turn. Use a dwell time under 3 seconds for manual soldering and verify under 10* magnification that solder has not reached the helical winding. For reflow assembly, ensure paste stencil aperture design prevents excess solder volume that could promote run-up.
• Tinned-to-Root Leads: The tinning extends to the coil root, ensuring a continuous solderable surface from the PCB pad to the first turn. This eliminates the bare-copper gap that can corrode under condensing humidity. Avoid solder run-up onto the active turns — precisely control solder dwell time and fillet volume.
• Perpendicular Mounting: Orient the 8-turn coil body perpendicular to the RF signal trace. In multi-channel phased-array layouts, maintain ≥1.5mm spacing between adjacent inductor bodies to achieve >30dB channel-to-channel isolation at 28GHz.
• Mechanical Securing: After RF tuning verification, immobilize the coil body with a micro-dot of low-dielectric RF adhesive (εᵣ < 3.0, loss tangent < 0.005). This prevents microphonic detuning in fan-cooled equipment and vibration-induced S-parameter drift.
• Storage: Store in ESD-safe carriers at 20-25°C and 40-60% RH. Tinned leads maintain solderability for 1 year minimum under recommended conditions.

Applications

• 5G FR1/FR2 Bias Tee Networks: 400mA DC handling with zero saturation and zero PIM makes the HALA0800503R the ideal choke for GaN PA drain bias injection at 3.5GHz (n78) and 28GHz (n257). Superior IMD linearity preserves ACLR margins in 256QAM transmitters.
• Doherty Power Amplifier Output Matching: The 11nH inductance with perfectly linear current response supports the dynamic load modulation of Doherty combiners where the peaking amplifier bias current varies from 0mA to 400mA across the envelope cycle.
• Satellite Communication Up/Downconverters (C/Ku/Ka-band): Wide -55°C to +125°C range with temperature-independent inductance (μ₀ constant) ensures consistent frequency conversion and filter response from cold-start to full-power thermal steady-state.
• Optical Transceiver Bias Networks (100G/400G): Ultra-high SRF and low PIM contribute to clean laser bias injection without introducing bit-error-rate-degrading intermodulation in EML-based 56Gbaud PAM4 transmitters.
• Linear Wideband Test Equipment: Per-lot S2P Touchstone data enables accurate de-embedding of the inductor response from VNA measurements. The zero-PIM air core eliminates inductor non-linearity as a measurement uncertainty source in 2-tone and multi-tone IMD test setups.

Contact us for evaluation samples, per-lot S2P data, or to discuss custom turn counts and inductance values on the HALA080 platform.