Calculator Inputs
Plotly Graph
This graph shows how the output dB difference changes while the reference gain moves across your selected range.
Example Data Table
| Subject Gain (dBi) | Reference Gain (dBi) | Reference Type | Output (dB) | Power Ratio |
|---|---|---|---|---|
| 9.00 | 0.00 | Isotropic | 9.00 | 7.9433 : 1 |
| 12.00 | 2.15 | Dipole | 9.85 | 9.6605 : 1 |
| 6.50 | 3.00 | Custom 3 dBi | 3.50 | 2.2387 : 1 |
| 15.00 | 5.00 | Custom 5 dBi | 10.00 | 10.0000 : 1 |
Formula Used
Relative dB from dBi: dB = dBiinput − dBireference
Power ratio: Power Ratio = 10(dB / 10)
Field or voltage ratio: Field Ratio = 10(dB / 20)
dBi to dBd: dBd = dBi − 2.15
dBi is an absolute antenna gain reference relative to an isotropic radiator. A plain dB value is only meaningful when it compares one gain to another reference.
How to Use This Calculator
- Enter the antenna gain in dBi.
- Select isotropic, dipole, or a custom reference.
- Enter a custom reference value when needed.
- Choose decimal precision for cleaner output formatting.
- Set the graph range to explore reference sensitivity.
- Click Calculate Now to show results above the form.
- Review the dB difference, ratios, and dBd conversion.
- Use the CSV or PDF buttons to export the result.
FAQs
1. Can dBi be converted directly into dB?
Not by itself. dBi is referenced to an isotropic radiator, while dB is a relative comparison. You must choose a reference before the value becomes a useful dB difference.
2. Why does 8 dBi look the same as 8 dB sometimes?
When the chosen reference is isotropic, the reference value is 0 dBi. Subtracting zero leaves the same numeric value, so 8 dBi becomes a relative difference of 8 dB.
3. What is dBd and why is it included?
dBd measures antenna gain relative to a half-wave dipole. It is common in antenna work, so the calculator also shows dBd using the relation dBd = dBi − 2.15.
4. What does the power ratio mean?
Power ratio tells how many times stronger or weaker one gain is compared with the selected reference. A result of 2 means twice the power ratio, while 0.5 means half.
5. Why include a field or voltage ratio?
Some engineering work uses amplitude relationships instead of power. The field or voltage ratio is based on 20 log calculations, so it helps when comparing signal amplitudes.
6. Can I enter negative dBi values?
Yes. Antenna gains may be negative when they are below the isotropic reference in a specific direction. The calculator accepts negative entries and processes them normally.
7. What does the graph show?
The graph shows how the output dB changes as the reference gain changes across your chosen range. It is useful for sensitivity checks and design comparisons.
8. When should I use a custom reference?
Use a custom reference when comparing against a known antenna, system baseline, or specification target. It helps turn the dBi input into a practical engineering difference.