Calculator Input
Use the fields below to estimate inset-fed patch input impedance and feed matching behavior.
Example Data Table
This sample represents a common inset-fed rectangular patch design case.
| Frequency (GHz) | εr | h (mm) | W (mm) | L (mm) | Edge R (Ω) | Inset y0 (mm) | Target Z0 (Ω) | Estimated Zin (Ω) |
|---|---|---|---|---|---|---|---|---|
| 2.45 | 4.40 | 1.60 | 37.26 | 28.83 | 300.00 | 10.60 | 50.00 | 50.93 |
| 3.50 | 3.48 | 0.76 | 28.80 | 21.40 | 260.00 | 8.05 | 50.00 | 51.82 |
| 5.80 | 2.20 | 0.80 | 20.10 | 17.30 | 220.00 | 6.18 | 50.00 | 49.06 |
Formula Used
This page uses the transmission line view of a rectangular inset-fed patch. The feed point moves away from the radiating edge. That motion changes the input resistance with a cosine-squared relationship.
Input impedance at inset depth
Zin(y0) = Redge × cos²(πy0 / L)
Inset depth for a target line impedance
y0,target = (L / π) × arccos √(Z0 / Redge)
Effective permittivity
εeff = (εr + 1)/2 + (εr - 1)/2 × (1 + 12h/W)-1/2
Fringing extension
ΔL = 0.412h × [((εeff + 0.3)(W/h + 0.264)) / ((εeff - 0.258)(W/h + 0.8))]
Effective length
Leff = L + 2ΔL
When estimated edge resistance is selected, the page uses a simple slot conductance approximation. That estimate is useful for screening designs. Final layouts should still be checked in a full-wave simulator.
How to Use This Calculator
- Enter the operating frequency, dielectric constant, and substrate height.
- Keep manual patch size values or enable automatic patch sizing.
- Choose manual or estimated edge resistance mode.
- Provide a manual edge resistance if that mode is selected.
- Enter the inset depth and target line impedance.
- Click Calculate Impedance to generate results.
- Review the input impedance, suggested inset depth, return loss, and VSWR.
- Use the graph to see how impedance changes across the feed path.
- Export the result table as CSV or PDF for reports.
Frequently Asked Questions
1. What does this calculator estimate?
It estimates the input resistance seen at an inset feed point on a rectangular patch. It also suggests inset depth for a chosen line impedance and reports matching quality.
2. Why does inset depth change impedance?
The standing wave along the patch length changes current distribution. Moving the feed inward shifts the local voltage and current ratio, which changes the input resistance.
3. Why is the center of the patch not a good edge-fed point?
Near the center line, the cosine term approaches zero. That makes the modeled input resistance very small, which is usually unsuitable for common 50 ohm feeds.
4. When should I use manual edge resistance?
Use manual edge resistance when you already have simulated, measured, or trusted reference data. That choice usually gives better feed predictions than a rough analytical estimate.
5. What is the estimated edge resistance mode for?
It gives a quick starting value from simple slot conductance equations. It is helpful during early sizing, but it should not replace full-wave validation.
6. Why does the page show effective permittivity and fringing?
Those values explain why electrical length differs from physical length. Fringing fields make the resonant patch appear slightly longer than the copper dimension alone.
7. Can this calculator replace an EM simulator?
No. It is a fast engineering estimate. Real antennas also depend on feed width, finite ground size, losses, conductor thickness, mutual coupling, and fabrication tolerance.
8. What result values usually indicate decent matching?
A lower mismatch, a return loss better than about 10 dB, and a VSWR under about 2 are common starting targets for practical feed matching.