Patch Antenna Impedance Calculator

Calculator Form

Design mode

Frequency

GHz

Relative permittivity

Substrate height

Patch width

Patch length

Feed inset from radiating edge

Source or line impedance

ohms

Conductor efficiency

Loss tangent

Edge resistance calibration factor

Reset

Formula Used

Effective dielectric constant: εeff = (εr + 1) / 2 + [(εr - 1) / 2] × [1 + 12h / W]^-0.5

Fringing length: ΔL = 0.412h × [(εeff + 0.3)(W / h + 0.264)] / [(εeff - 0.258)(W / h + 0.8)]

Effective length: Leff = L + 2ΔL

Resonant frequency: fr = c / [2Leff√εeff]

Approximate edge resistance: Redge = 90 × [εr² / (εr - 1)] × (L / W)² × calibration factor

Inset feed resistance: Rin(y) = Redge × cos²(πy / L)

Reflection coefficient: Γ = |(Rin - Z0) / (Rin + Z0)|

VSWR: VSWR = (1 + Γ) / (1 - Γ)

Return loss: RL = -20log10(Γ)

Mismatch loss: ML = -10log10(1 - Γ²)

How to Use This Calculator

Select auto mode for a starting patch size, or custom mode for your real board dimensions.
Enter the operating frequency, dielectric constant, and substrate height.
Enter patch width and length when using custom mode.
Enter the feed inset distance from the radiating edge.
Set the expected line impedance, usually 50 ohms.
Add loss tangent, efficiency, and calibration factor if known.
Press calculate and review resistance, return loss, VSWR, and bandwidth.
Download the CSV or PDF report for your design notes.

Example Data Table

Case	Frequency	εr	Height	Patch W	Patch L	Feed inset	Expected note
FR4 WiFi trial	2.45 GHz	4.4	1.6 mm	37.2 mm	28.8 mm	8.5 mm	Common low cost board estimate
Rogers style board	5.8 GHz	3.38	0.8 mm	17.8 mm	13.9 mm	4.0 mm	Smaller patch with lower loss
Air spaced model	1.575 GHz	2.2	3.2 mm	75.0 mm	61.0 mm	17.0 mm	Wider bandwidth starting point
Compact sensor patch	0.915 GHz	6.15	1.9 mm	78.0 mm	61.5 mm	18.0 mm	Compact but bandwidth limited

Patch Antenna Impedance Guide

A patch antenna looks simple, yet its feed point is sensitive. Small changes in substrate height, dielectric constant, and feed inset can move the input resistance a lot. This calculator gives a practical first estimate before simulation or measurement.

Why impedance matters

Transmitters and receivers expect a known load, often 50 ohms. When the patch input differs from that load, energy reflects toward the source. Return loss falls, VSWR rises, and useful radiated power drops. A matched feed keeps the system stable and efficient.

Main design inputs

Frequency sets the electrical size of the patch. The dielectric constant controls wave speed inside the substrate. A higher value usually makes the patch smaller, but it can reduce bandwidth. Substrate height also matters. A thicker substrate can improve bandwidth, though it may increase unwanted radiation and surface waves.

How inset feeding works

The radiating edge of a rectangular patch usually has a high resistance. Moving the feed inward samples a lower standing wave value. The common model uses a cosine squared relation. It is useful for microstrip inset feeds and quick layout planning. The target inset result shows where a chosen source impedance may occur.

Understanding the output

Edge resistance is the estimated resistance at the radiating edge. Inset resistance is the estimated resistance at your selected feed depth. The reflection coefficient compares this value with the feed line impedance. Return loss, VSWR, mismatch loss, and accepted power explain match quality in familiar radio terms.

Use with care

Closed form patch equations are approximations. Real boards include solder pads, connector launch effects, copper thickness, fringing fields, finite ground planes, and fabrication tolerance. For high power, high frequency, or narrow bandwidth designs, use electromagnetic simulation and a network analyzer. Still, this calculator is valuable for sizing, checking trends, and preparing design notes.

Practical workflow

Start with the auto estimate option. Review the suggested width and length. Then enter your real layout dimensions. Adjust feed inset until the input resistance approaches the desired line impedance. Save the CSV or PDF report for comparison. Repeat the process after measurement, using the calibration factor to align the model with test data. Document assumptions, units, and limits beside every exported result carefully today.

FAQs

1. What does patch antenna impedance mean?

It is the input resistance and reactance seen by the feed line. This calculator focuses on the resistive part near resonance, where the reactance is usually tuned close to zero.

2. Why does feed inset change resistance?

The standing wave current changes along the patch length. Moving the feed inward changes the sampled current and voltage ratio. That changes the input resistance.

3. Is the edge resistance exact?

No. It is a practical closed form estimate. Ground size, connector launch, copper thickness, etching tolerance, and nearby objects can change real measured impedance.

4. What is a good VSWR value?

A VSWR below 2 is often usable. A value near 1.5 or lower is better. Critical radio systems may require stricter return loss targets.

5. Can I use this for any patch shape?

This page is intended for rectangular microstrip patches using an inset feed model. Circular, stacked, shorted, and slotted patches need different models.

6. Why does substrate height affect bandwidth?

A thicker substrate can increase stored field volume and radiation bandwidth. However, it may also increase surface waves, spurious radiation, and fabrication sensitivity.

7. What does calibration factor do?

It lets you scale the edge resistance estimate. Use it after comparing calculator results with simulation or network analyzer measurements for a similar layout.

8. Should I still simulate the antenna?

Yes. Use this calculator for planning and early tuning. Final designs should be checked with electromagnetic simulation and measured using proper RF test equipment.