Calculator Inputs
Formula Used
This calculator uses a common transmission-line approximation for a rectangular microstrip patch antenna.
Patch width:
W = c / (2f) × √(2 / (εr + 1))
Effective permittivity:
εeff = ((εr + 1) / 2) + ((εr - 1) / 2) × (1 / √(1 + 12h / W))
Fringing extension:
ΔL = 0.412h × ((εeff + 0.3)(W / h + 0.264)) / ((εeff - 0.258)(W / h + 0.8))
Effective length and physical length:
Leff = c / (2f√εeff)
L = Leff − 2ΔL
Inset feed relation:
Rin(y) = Redge × cos²(πy / L)
Inset distance from the radiating edge:
y = (L / π) × arccos(√(Z0 / Redge))
Where: c is the speed of light, f is resonant frequency, εr is substrate dielectric constant, h is substrate height, Z0 is target feed impedance, and Redge is edge input resistance.
Important: The automatic edge resistance is a simple first-pass estimate. Use measured or simulated edge resistance for more reliable feed placement.
How to Use This Calculator
- Enter the resonant frequency in GHz.
- Enter substrate dielectric constant and substrate height.
- Enter the target feed impedance, such as 50 ohms.
- Optionally enter edge resistance if you already know it.
- Optionally enter manual width and length if your patch geometry already exists.
- Click Calculate Feed Point.
- Read the inset distance from the radiating edge.
- Place the feed on the patch width centerline.
- Export the result as CSV or PDF for documentation.
- Validate the final geometry with simulation or measurement.
Example Data Table
The sample rows below use the automatic edge resistance estimate.
| Frequency (GHz) | εr | Height (mm) | Target Z0 (ohms) | Edge R (ohms) | Width (mm) | Length (mm) | Inset (mm) |
|---|---|---|---|---|---|---|---|
| 2.40 | 4.40 | 1.60 | 50 | 197.18 | 38.01 | 29.42 | 9.77 |
| 5.80 | 2.20 | 1.60 | 50 | 151.79 | 20.43 | 16.47 | 5.03 |
| 1.575 | 2.94 | 1.524 | 50 | 168.43 | 67.81 | 55.06 | 17.43 |
Patch Antenna Feed Point Design Guide
Why This Calculator Matters
A patch antenna feed point calculator helps engineers place the feed where impedance matches the line. That position matters because poor placement increases reflection and wastes RF power. This page estimates inset feed depth for a rectangular microstrip patch. It also calculates width, effective dielectric constant, fringing extension, guided wavelength, and physical patch length for practical engineering work.
Key Inputs for Better Results
The main design inputs are resonant frequency, substrate dielectric constant, substrate height, and target feed impedance. Higher dielectric values usually shrink the patch. Thicker substrates change fringing fields and effective permittivity. The calculator also accepts optional manual width, manual length, and edge resistance. That flexibility helps when you already have geometry from a prototype, simulation, or previous layout.
How Inset Feeding Changes Impedance
For many rectangular patches, the highest input resistance appears near the radiating edge. An inset feed moves the connection inward and lowers that resistance. The change follows a cosine-squared relationship in the transmission-line model. When the target line is 50 ohms, the correct inset point can produce a much better match. Better matching can improve power transfer, return loss, and measurement consistency.
Where This Engineering Tool Fits
The calculated dimensions are useful during early engineering studies, classroom work, and antenna layout planning. They are also helpful when comparing substrates for WLAN, ISM, GNSS, and other narrowband applications. Still, this is a first-pass design tool. Real antennas are affected by copper thickness, losses, feed geometry, ground size, fabrication tolerances, and nearby enclosures.
Use the Results Correctly
Use the results as a starting point, not as the final answer. After finding a feed point, validate the structure with an EM solver or measured prototype. Check S11, bandwidth, efficiency, and radiation pattern. If you know the edge resistance from simulation, enter it directly for a better inset estimate. That step can make this patch antenna feed point calculator much more useful in real design flows.
Practical Layout Transfer
Because the tool reports normalized feed position and distance from the center region, it is easy to transfer the result into CAD. You can center the feed across the patch width and place the inset along the patch length. Then export the values, document your assumptions, and compare several substrate options quickly. That speeds repeatable engineering decisions and supports cleaner antenna development workflows.
FAQs
1. What does the feed point control in a patch antenna?
The feed point controls input impedance at the connection location. A better location improves matching with the transmission line. That reduces reflected power and helps the antenna perform more consistently near the design frequency.
2. Why is inset feeding popular for rectangular patches?
Inset feeding is popular because it lets designers reach a lower impedance without external matching parts. By moving inward from the radiating edge, the input resistance drops to a value closer to common feed lines.
3. Why can the calculator reject my target impedance?
The inset-feed equation needs the target impedance to be less than or equal to the edge resistance used in the model. If it is larger, there is no real inset distance that satisfies the equation.
4. Should I enter manual edge resistance?
Yes, when you have it from simulation, measurement, or a trusted design source. Manual edge resistance usually gives a better inset estimate than a simple automatic approximation, especially for refined layouts.
5. Do I still need electromagnetic simulation?
Yes. This calculator is a first-pass engineering tool. Final antenna performance depends on feed geometry, losses, ground size, conductor thickness, fabrication tolerances, and surrounding structures.
6. Can I use manual patch width and length?
Yes. Manual width and length are helpful when the patch shape is already chosen. The calculator will then use your geometry to estimate the inset location and related transmission-line values.
7. What units does the calculator use?
Inputs use GHz for frequency and millimeters for physical dimensions. The results table also reports patch dimensions, wavelength values, and inset distance in millimeters for easy layout transfer.
8. Is this calculator valid for every patch antenna shape?
No. It is intended for a rectangular microstrip patch using a dominant-mode transmission-line approximation. Circular, stacked, slotted, or strongly modified patches need different equations or full-wave analysis.