Calculator Inputs
Example Data Table
| Model | Frequency | Geometry / Input | Loss Resistance | Approx. Radiation Resistance | Approx. Efficiency |
|---|---|---|---|---|---|
| Short Dipole | 100 MHz | Length 0.15 m | 1.20 Ω | 1.974 Ω | 62.19% |
| Half-Wave Dipole | 14.20 MHz | Reference resonant dipole | 2.00 Ω | 73.130 Ω | 97.34% |
| Quarter-Wave Monopole | 145 MHz | Reference monopole over ideal ground | 1.50 Ω | 36.560 Ω | 96.06% |
| Small Loop | 7.00 MHz | Radius 0.50 m, 5 turns | 0.80 Ω | 0.143 Ω | 15.16% |
Formula Used
1) Free-space wavelength:λ = c / f
2) Effective electrical wavelength used here:λeff = λ × velocity factor
3) Short dipole radiation resistance:Rr = 80π²(l / λeff)²
4) Half-wave dipole reference:Rr ≈ 73.13 Ω
5) Quarter-wave monopole reference:Rr ≈ 36.56 Ω
6) Small loop radiation resistance:Rr = 31200(NA / λeff²)²
7) Derived measurement method:Rr = Prad / I²
8) Total feed resistance and efficiency:Rtotal = Rr + Rlossη = Rr / (Rr + Rloss) × 100
These formulas are useful for early engineering estimates. Final antenna design should still be checked with measurements or full-wave electromagnetic simulation.
How to Use This Calculator
- Choose the antenna model that matches your design or measurement method.
- Enter the operating frequency, velocity factor, feed current, and loss resistance.
- Provide the geometry inputs for the selected model, such as element length or loop radius.
- Click the calculate button to show results above the form.
- Review radiation resistance, total feed resistance, radiated power, and efficiency.
- Inspect the Plotly graph to see the resistance trend across the sweep or the resistance components for the derived model.
- Download the result table as CSV for spreadsheets or as PDF for reports.
- Use the engineering notes to spot approximation limits before moving to detailed design tools.
FAQs
1) What is radiation resistance?
Radiation resistance is the part of feed resistance that represents useful power converted into electromagnetic radiation. It behaves like a resistance in power calculations, even though the energy leaves as radio waves.
2) When should I use the short dipole model?
Use it when the element is electrically short, usually below about 0.1 wavelength. Beyond that range, current distribution changes and the simple approximation becomes less reliable.
3) Why is a small loop radiation resistance often tiny?
Small loops radiate inefficiently because radiation resistance scales with the square of loop area and the fourth power of electrical size. Loss resistance can easily dominate unless the loop is carefully built.
4) Why do I need a loss resistance input?
Loss resistance lets the calculator estimate efficiency and heat loss. Without it, you only know how strongly the antenna can radiate, not how much of your input power is wasted.
5) Why does velocity factor appear in this tool?
It helps estimate effective electrical length and practical resonant dimensions. It does not replace full electromagnetic modeling, especially for antennas affected by loading, insulation, or nearby conductive structures.
6) Why are the half-wave and quarter-wave values nearly constant?
Those modes use classic resonant reference values for thin antennas in ideal conditions. Real installations can differ because of element thickness, ground quality, height, nearby objects, and matching networks.
7) Can this calculator replace NEC, CST, or HFSS?
No. It is best for fast engineering estimates, early comparisons, and teaching. Final performance should still be validated with measurement or a full-wave electromagnetic solver.
8) What current should I enter?
Enter the RMS feed current at the antenna input or at the measurement reference plane. Keep the value consistent with your power measurements and loss assumptions for meaningful efficiency estimates.