Xi’an, China — HOAN Microwave has expanded its custom RF coil manufacturing capabilities to support RF and microwave designs where standard catalog inductors cannot meet electrical, mechanical or PCB integration requirements. The capability covers miniature air-core coils, high-Q custom inductors and broadband conical winding structures, with support from winding development through RF characterization.

At GHz frequencies, changing a coil by a few turns can solve one problem and create another.

More turns increase inductance, but they can also increase inter-turn capacitance and move the first self-resonance closer to the operating band. Shortening a coil can save PCB space while changing pitch and RF impedance. Longer leads may simplify assembly but introduce additional inductance after installation.

For custom RF coil development, the winding geometry therefore has to be considered together with the target frequency, Q, current and available installation space.

Custom Air Core Inductors for GHz RF Designs

HOAN manufactures miniature air-core inductors with configurable winding dimensions and lead structures.

Published HOAN designs illustrate how relatively small physical changes can produce different RF components:

For the HALA2000303R, the published specification states an SRF above 22 GHz, while the specified operating range is 1–18 GHz.

That distinction matters. The first self-resonant frequency should not automatically be treated as the usable frequency limit of an RF coil.

The required impedance, Q and installed response may become unacceptable before the first resonance is reached.

Custom Winding Geometry for SRF, Q and Size Constraints

A simplified RF coil contains more than its nominal inductance.

Its electrical behavior also includes conductor resistance and distributed parasitic capacitance:

L + R + Cp

The approximate first self-resonant frequency can be expressed as:

fSRF ≈ 1 / (2π√LCp)

Increasing inductance by adding turns may also increase Cp, depending on turn spacing and winding geometry.

In a compact bias network, for example, the additional turns may provide higher impedance at the lower end of the required band while bringing resonance closer to the upper end.

Custom development may therefore involve adjusting:

· turn count;

· wire diameter;

· winding diameter;

· winding pitch;

· lead length and orientation;

· mechanical envelope;

· current capacity;

· required operating frequency.

For applications where loss is critical, a high-Q custom inductor may require a different balance between conductor size, turn count and winding dimensions than a coil optimized primarily for maximum inductance in limited space.

Conical winding structures provide another option for broadband RF choke applications. Their changing winding geometry can be used where a conventional cylindrical coil does not provide the required combination of broadband response, impedance and available installation space.

From Winding Geometry to Measured RF Performance

A coil can meet its inductance target on an LCR meter and still behave differently after installation on a microwave PCB.

At higher frequencies, the effective structure is closer to:

RF coil + leads + solder joints + PCB pads + transmission line

Pad capacitance and additional lead inductance can shift the installed resonant response away from the component-only measurement.

This becomes particularly important for:

· RF amplifier bias networks;

· microwave matching circuits;

· broadband RF chokes;

· high-speed receiver front ends;

· communication modules;

· test and measurement equipment.

HOAN uses high-frequency measurement equipment, including Vector Network Analyzers, for RF component characterization.

For selected RF inductors, S2P Touchstone data can also be provided so that engineers can evaluate frequency-dependent behavior in RF simulation software instead of relying only on an ideal lumped-inductor model.

In ADS, HFSS or another RF simulation workflow, an S2P model can provide a more realistic representation of the characterized component as operating frequency approaches the upper end of its specified band.

Why PCB Information Matters in a Custom RF Coil Project

A request for “20 nH” does not define enough information for many microwave designs.

Two coils with the same nominal inductance can behave differently because of:

turn count, pitch, conductor diameter, lead geometry and mounting parasitics.

PCB information becomes especially valuable when:

· the operating frequency is close to the component SRF;

· lead length must be minimized;

· the available footprint is tightly constrained;

· the coil is used in a matching network;

· the circuit requires controlled impedance across a wide band.

A PCB layout or module drawing can reveal restrictions that are not visible from an inductance specification alone.

For example, rotating or repositioning a lead to fit a compact module may change the installed parasitic structure. At several GHz, that mechanical adjustment may also become an RF adjustment.

RF Coil Parameters to Provide for Custom Development

Providing the main design conditions at the beginning reduces unnecessary prototype iterations.

A prototype can then be evaluated for inductance, geometry and RF response before the winding parameters are fixed for production.

If measured resonance or impedance does not fall where the circuit requires it, changes to turn count, pitch, diameter or lead geometry can be evaluated during the next iteration.

Supporting RF Bias, Matching and Microwave Module Designs

Custom RF coils are commonly considered when a standard component fits one requirement but misses another.

A bias network may require more broadband impedance.

A matching circuit may need a small inductance with low loss.

A compact microwave module may have almost no freedom for lead length or component orientation.

In these cases, selecting by nominal inductance alone leaves too much of the RF behavior undefined.

HOAN’s custom RF coil manufacturing capability is intended to connect the winding design more closely with the final electrical and mechanical requirements of the circuit.