Design Inputs
Use consistent fabrication values. Dimensions are returned in millimeters.
Example Design Data
Use this 2.4 GHz FR-4 example only as a starting point.
| Parameter | Example value | Reason |
|---|---|---|
| Center frequency | 2400 MHz | Common ISM-band target. |
| Relative permittivity | 4.4 | Typical nominal FR-4 reference. |
| Board thickness | 1.6 mm | Common low-cost board thickness. |
| Radiator width | 4.0 mm | Balances compactness and bandwidth. |
| Feedline impedance | 50 ohm | Matches common RF connectors and radios. |
Formula Used
The calculator starts with free-space wavelength, then applies substrate loading and practical shortening. It also estimates feedline width with standard microstrip equations.
λ₀ = c ÷ f
εeff ≈ (εr + 1)/2 + ((εr − 1)/2) × [1 ÷ √(1 + 12h/w)]
Ltotal ≈ [λ₀ ÷ (2√εeff)] × correction ÷ mask factor
Larm = (Ltotal − gap) ÷ 2
Here, c is light speed, f is frequency, h is substrate thickness, and w is radiator width. Final tuning should use measured S11 data.
How to Use This Calculator
- Enter the required center frequency in megahertz.
- Copy dielectric, thickness, copper, and loss values from the laminate datasheet.
- Choose a realistic radiator width and center gap.
- Enter the desired feedline impedance and expected efficiency.
- Select a conservative correction factor for the first layout.
- Calculate the geometry, then place the radiator away from ground copper.
- Simulate the layout and trim arm lengths after measuring S11.
Printed Dipole Design Considerations
Start With the Correct Electrical Length
A PCB dipole is usually near one half wavelength long. The required physical length is shorter than a free-space dipole. The substrate changes the electric field distribution. Its relative permittivity slows the wave near the copper. This calculator estimates that effect through effective permittivity. A high-permittivity board creates a smaller antenna. It can also concentrate more energy inside the dielectric. That can reduce bandwidth and radiation efficiency. Use the laminate value at your operating frequency. Nominal values can differ from real production material.
Choose Geometry Before Routing
Both arms should have similar length and width. Keep the feed gap centered. Maintain a balanced shape around the feed point. A narrow radiator is compact but often more sensitive. A wider radiator can broaden the impedance response. However, a very wide arm may affect nearby traces and enclosure spacing. Place the feedline carefully. A direct unbalanced microstrip feed can disturb the dipole currents. Consider a balun, a sleeve, or a differential feed. These choices reduce common-mode radiation on the cable or ground return.
Protect the Radiating Region
Ground copper, batteries, displays, shields, and screws can detune the antenna. Leave a generous copper-free area around each arm. Keep fast digital routes away from the feed gap. Avoid vias through the radiating sections. Solder mask also changes loading. The effect is smaller than a metal enclosure, but it still matters at higher frequencies. Use the mask loading field when copper remains covered. Set it to zero when the radiator is exposed. Preserve the same stack-up during prototypes and production.
Treat Calculated Bandwidth as a Starting Estimate
The bandwidth result is not a replacement for electromagnetic simulation. It is a preliminary comparison tool. Wider traces and thicker substrates may broaden the response. Material loss and nearby components can narrow or shift it. The displayed frequency range assumes a simple printed dipole. Real matching networks can improve the measured result. Measure return loss with a calibrated network analyzer. Trim equal lengths from both arms to raise resonance. Add length to lower resonance. Make small changes between measurements.
Validate the Finished Assembly
Test the antenna inside its final product enclosure. Hold cables and test fixtures away from the radiator. Compare bare-board and assembled measurements. Record the actual board material, copper finish, and enclosure revision. Check efficiency in an appropriate test environment when possible. A good match alone does not guarantee useful radiation. A lossy or blocked antenna can show low VSWR. Confirm range, link stability, and received signal strength. Production samples should include normal component tolerances and mechanical variation. Keep tuning pads or controlled trim zones in the first board revision. Document each adjustment in a simple tuning log. Record arm length, feed implementation, enclosure state, frequency minimum, and return loss. That history prevents repeated experiments and helps future board revisions converge faster and with less risk.
Frequently Asked Questions
1. What is a PCB dipole antenna?
It is a two-arm antenna etched or printed on a circuit board. Each arm carries opposite RF currents. The structure commonly operates near a half wavelength after substrate effects are included.
2. Why is the printed antenna shorter than free-space length?
Electric fields partly travel in the dielectric. That lowers wave velocity. The guided wavelength becomes shorter, so the physical radiator can be shorter than a wire dipole in air.
3. Is FR-4 suitable for a 2.4 GHz dipole?
Yes, it is widely used for prototypes and low-cost products. Its dielectric properties vary more than specialized RF laminates. Expect to tune the final arm length with measurement.
4. Why does the calculator show a feedline width?
The feedline must have a controlled impedance. The calculator estimates a microstrip width for your substrate and target impedance. Verify it against your board stack-up and fabricator data.
5. Do I need a balun?
Often, yes. A dipole is balanced while a microstrip line is unbalanced. A balun or equivalent balanced feed can reduce unwanted current on the ground return and improve repeatability.
6. How much clearance should surround the radiator?
More clearance is generally safer. Keep ground copper and metal parts well away. The suggested board clearance is a planning guide, not an absolute rule. Simulate and measure your complete assembly.
7. What does the length correction factor do?
It accounts for end effects, trace geometry, and practical shortening. Start near 0.95 for many printed dipoles. Adjust it only after comparing simulation or measured resonance.
8. Can I use the result without testing?
No. The result is useful for a first layout. Board tolerances, enclosure metal, cables, batteries, and nearby components can shift resonance. Always verify the finished design.
9. How can I lower the resonant frequency?
Lengthen both dipole arms equally. You can also increase dielectric loading, but geometry changes are usually easier to control. Measure after each small adjustment.
10. How can I increase bandwidth?
Try wider radiator arms, appropriate substrate thickness, and a cleaner surrounding region. Bandwidth also depends on matching and enclosure coupling. Avoid assuming one geometry change solves every issue.
11. What should I measure after fabrication?
Measure S11, resonant frequency, and impedance first. Then check efficiency or range in the final enclosure. Compare multiple boards to understand fabrication variation and tuning margin.