PIFA Calculator | Planar Inverted-F Antenna Sizing

What Is a PIFA and Why Calculate Its Dimensions?

A planar inverted-F antenna (PIFA) is a quarter-wave resonant patch shorted to the ground plane along one edge or via a shorting pin. It delivers a low-profile, moderate-gain radiator that fits inside phones, IoT modules, Wi-Fi access points, and GPS receivers. The critical dimensions—patch length L, patch width W, shorting-pin position, and feed offset—set the resonant frequency and input impedance. Getting them right before layout saves simulation iterations and prototyping spins.

The PIFA calculator below takes your target frequency, substrate dielectric constant (Dk), and antenna height above the ground plane, then returns starting dimensions you can drop straight into a full-wave solver for fine-tuning.

PIFA Operating Principle

A PIFA is the PCB-friendly evolution of the wire inverted-F antenna (IFA). Where an inverted-F antenna uses a thin trace as the radiating element, a PIFA replaces that trace with a wider rectangular patch. The wider conductor increases bandwidth and gain while keeping the antenna height well below λ/10.

The shorting wall (or pin) connects one edge of the patch to the ground plane, folding the electric-field distribution so that the total resonant path is roughly λ_eff/4 rather than the λ/2 of a conventional patch. This halving of the resonant length is the PIFA’s main size advantage.

Key Geometry Parameters

• L (patch length) — the dimension along the resonant path, from the open edge to the shorting edge. Dominates the resonant frequency.
• W (patch width) — perpendicular to L. Increasing W widens bandwidth and raises radiation resistance.
• H (height above ground) — the air or substrate gap. Larger H improves bandwidth but increases profile.
• Feed offset (d_f) — distance from the shorting edge to the coaxial or via feed point. Adjusting d_f tunes the input impedance toward 50 Ω.
• Shorting-pin width (W_s) — a full-width shorting wall gives the lowest resonant frequency; a narrow pin raises it and adds an inductive component useful for dual-band tuning.

PIFA Resonant-Length Formula

The classical first-order estimate for the resonant condition is:

L + W − W_s ≈ λ_eff / 4

where λ_eff = c / (f × √ε_eff). For a PIFA suspended in air (ε_eff ≈ 1), at 2.45 GHz this gives L + W − W_s ≈ 30.6 mm. On a substrate with Dk = 4.4 (standard FR-4), the effective wavelength shortens and the sum drops to roughly 14.6 mm—almost half the air-spaced value.

This formula is a starting point. Fringing fields, finite ground-plane size, and nearby components shift the resonance by 3–8 %, which is why the calculator output should always be verified in an EM simulator such as Altium Designer’s integrated 3D solver or a dedicated tool like HFSS or CST.

PIFA Antenna Calculator

Enter your target center frequency, substrate Dk, antenna height, and desired patch width. The tool returns the resonant patch length and a suggested feed-point offset for a 50 Ω match.

For a quick sanity check: if you entered 2.45 GHz, Dk = 1.0 (air), H = 8 mm, and W = 15 mm, you should see a patch length near 15.6 mm and a feed offset around 4–6 mm from the shorting edge. On FR-4 (Dk ≈ 4.4) the length compresses noticeably. If the calculator returns a length that seems too short, double-check that you entered the correct Dk—using the substrate’s published value at your operating frequency, not the 1 MHz datasheet number, matters at 2.4 GHz and above.

Design Trade-Offs and Practical Guidelines

Bandwidth vs. Height

PIFA bandwidth scales almost linearly with the height-to-wavelength ratio H/λ. A typical air-spaced PIFA at H ≈ 0.04 λ delivers 4–6 % impedance bandwidth (VSWR < 2). Doubling H to 0.08 λ can push bandwidth past 10 %, but the antenna is no longer "low profile" for many handset designs. For wideband applications (e.g., covering the full 2.4–2.5 GHz ISM band), aim for H ≥ 5 mm at 2.45 GHz.

Shorting Pin vs. Shorting Wall

A full-width shorting wall yields the most predictable quarter-wave behavior and the lowest resonant frequency for a given L. Replacing the wall with a single narrow pin (1–2 mm wide) raises the resonant frequency and introduces an inductive loading that can be exploited for dual-band operation—one resonance from the patch mode, a second from the pin inductance combined with the patch capacitance. If you need dual-band PIFA coverage (e.g., 2.4 GHz + 5.8 GHz Wi-Fi), start with a narrow shorting pin and optimize its width and position.

Ground-Plane Size

The ground plane is part of the antenna. A ground plane smaller than roughly 0.25 λ × 0.25 λ will detune the PIFA and distort its radiation pattern. In mobile devices the PCB ground (typically 100 mm × 50 mm) acts as the counterpoise and significantly influences the low-band resonance. Always model the actual ground-plane outline in simulation.

Substrate Selection

Most PIFAs for consumer wireless operate on standard FR-4 (Dk 4.2–4.5, Df 0.018–0.025 at 2.4 GHz). The loss tangent matters less here than for transmission lines because the patch is a radiator, not a guided-wave structure—some dielectric loss actually helps smooth the impedance curve. For higher frequencies (5.8 GHz, UWB at 6–8.5 GHz), a lower-loss laminate such as Rogers RO4350B (Dk 3.48, Df 0.0037) reduces efficiency droop. Use the PCB thickness calculator to verify that your chosen core and prepreg combination delivers the exact H you need.

Step-by-Step PIFA Design Flow

1. Define requirements — center frequency, bandwidth, maximum antenna volume, polarization, gain floor.
2. Choose substrate and height — balance bandwidth needs against profile constraints.
3. Run the PIFA calculator — obtain initial L, W, and feed offset.
4. Model in a 3D EM solver — include the real ground plane, nearby copper features, and any plastic radome or housing.
5. Tune — adjust L for frequency, d_f for impedance, and W or H for bandwidth.
6. Prototype and measure — compare S₁₁, radiation pattern, and total efficiency against simulation.

Common PIFA Frequency Bands and Typical Dimensions

These are air-spaced estimates. On a dielectric substrate the dimensions shrink by roughly 1/√ε_eff.

Integrating PIFAs With Other RF Structures

A PIFA rarely exists in isolation. The feed trace is a 50 Ω microstrip or coplanar waveguide routed from the RF front-end to the antenna feed via. If your system splits the signal to multiple antennas (MIMO), a Wilkinson power divider can distribute power with good isolation. For phased-array variants that tile several PIFA elements, run the array factor calculator to check grating-lobe-free scan range before committing to element spacing.

Frequently Asked Questions

How accurate is a first-order PIFA calculator?

The L + W ≈ λ_eff/4 estimate is typically within 5–10 % of the simulated resonant frequency. Fringing, finite ground planes, and plastic housings shift the result, so always follow up with full-wave simulation.

Can a PIFA work on FR-4?

Yes. FR-4 is the most common substrate for PIFAs at 900 MHz, 2.4 GHz, and even 5.8 GHz. Its higher loss tangent reduces total efficiency by 1–2 dB compared to PTFE laminates, which is acceptable for most short-range wireless products.

How do I increase PIFA bandwidth?

Increase the antenna height H, widen the patch (larger W), use a lower-Dk substrate, or add a parasitic strip to create a stacked resonance. Each approach trades volume or complexity for bandwidth.

What is the typical gain of a PIFA?

A single-element PIFA on a handset-sized ground plane produces 1–4 dBi peak gain depending on frequency and ground-plane dimensions. Gain rises with ground-plane size.

PIFA vs. IFA — when should I choose which?

Choose a PIFA when you need wider bandwidth and slightly higher gain; choose an IFA when board area is extremely tight and a narrow-band solution suffices. Both are quarter-wave shorted antennas, but the PIFA’s wider radiating element gives it a bandwidth edge of roughly 2× for the same height.