A conical inductor may look suitable from its inductance value but behave differently after it is mounted on an RF board. In broadband Bias Tee and microwave bias networks, parasitic capacitance, lead length, PCB geometry, and mounting orientation all affect high-frequency performance.

For practical selection, start with the required frequency range and DC current, then verify the mounted component with measured RF data.

1. Start with Frequency Range and Inductance

At the lower end of the operating band, inductive reactance provides a useful first check:

X_L=2ΠfL

For a 120 nH inductor, the ideal reactance at 200 MHz is approximately 151 Ω.

This calculation is useful at lower frequencies, but it should not be used to predict behavior directly at 20 or 40 GHz.

As frequency increases, inter-winding capacitance, conductor resistance, lead geometry, and PCB coupling become increasingly important. The component no longer behaves as an ideal lumped inductance.

For a Broadband Choke, the measured response across the complete operating band is therefore more useful than the nominal inductance alone.

2. Check S11 and S21 with a VNA

For broadband RF applications, VNA data should be reviewed before finalizing the component.

The HALT20005 specification includes typical test data covering:

· 10 MHz–500 MHz

· 10 MHz–20 GHz

· 10 MHz–40 GHz

S21

Depending on the measurement topology, S21 can help evaluate transmission or RF isolation behavior.

Look across the complete frequency sweep for unexpected peaks, resonant regions, or deterioration near the upper end of the required band.

S11

S11 provides information about the response seen at the RF port. Changes in S11 may come from the inductor, PCB transition, test fixture, or a combination of these effects.

For broadband evaluation, S11 and S21 should therefore be interpreted together with the test setup.

Recommended Figure: HALT20005 measured S11/S21 response from an actual VNA test.

The HALT20005 documentation also contains test data extending to 40 GHz, but the original record notes that fixture effects reduce measurement accuracy at this range. The 40 GHz result should therefore be treated as reference data rather than an ideal component response.

3. Don't Judge a Broadband Conical Inductor by SRF Alone

Self-Resonant Frequency (SRF) is commonly used when selecting conventional RF chip inductors.

For a broadband conical inductor, a single SRF value does not describe the complete RF response.

The conical winding, distributed capacitance, lead geometry, and mounting environment all contribute to frequency-dependent behavior. For a design covering several octaves, measured impedance and S-parameter data across the intended band are more useful than relying on one resonance number.

This is particularly important in broadband Bias Tee and microwave DC-feed circuits.

4. PCB Layout Is Part of the RF Performance

At microwave frequencies, the PCB and component mounting become part of the RF structure.

For HALT20005, the specified installation places the small end of the cone toward and perpendicular to the RF transmission line. The RF-side lead should be kept as short as possible, and the component should be fixed with adhesive.

Recommended Installation

DC Bias → Conical Inductor → Short Lead → RF Microstrip Line

Keep:

· Cone tip close to the RF line

· RF-side lead short

· Inductor perpendicular to the Microstrip Line

· Component mechanically stable

Avoid long leads, excessive loop area, uncontrolled mounting angles, or unnecessary spacing from the PCB.

At 20–40 GHz, even a short connection contributes to the RF structure. A component measured on one fixture may therefore show a different response after being installed on a PCB with different microstrip geometry or lead length.

5. Check the DC Current Rating

A conical inductor used in a Bias Tee normally carries DC current while providing RF isolation from the bias path.

For HALT20005, the specified maximum current is 200 mA.

If the circuit requires more current, the component should not be selected solely because its frequency range is suitable.

A practical selection should therefore consider:

Frequency Range + Inductance + DC Current + Mounted RF Response

6. Conical Inductor or Chip Inductor?

A conical inductor is not automatically the better choice for every RF design.

For compact or narrowband circuits, a chip inductor may be simpler. As the required bandwidth increases, the measured broadband response becomes a more important selection criterion.

FAQ

Can I select a conical inductor only by inductance?

No. For broadband RF designs, also consider DC current, parasitic behavior, PCB layout, mounting geometry, and measured S-parameters across the required band.

Why should the small end of the conical inductor face the RF line?

For HALT20005, the recommended installation places the small end toward and perpendicular to the RF transmission line while keeping the RF-side lead short. This helps control the connection geometry at the RF transition.

When should I consider a conical inductor instead of a chip inductor?

A conical inductor is worth evaluating when a Bias Tee or RF choke needs to operate across a very wide frequency range, particularly when the upper frequency extends well into the microwave region. For narrower-band or highly compact designs, a chip inductor may be more practical.