dBi to dB Calculator

1) Why this conversion matters

RF link budgets mix absolute power and relative gain. Antenna gain is often specified in dBi, while system blocks and losses are listed in dB. Converting helps you keep one consistent scale so additions and subtractions stay correct.

2) What dBi represents

dBi is gain referenced to an ideal isotropic radiator. An isotropic source radiates equally in all directions and is used as a universal reference. A value like 8.15 dBi means the antenna concentrates energy so the peak gain is 8.15 dB above isotropic.

3) What “dB gain” means here

dB is a ratio unit without a built‑in reference. When you say “antenna gain in dB” you are usually describing the same numeric gain, but you are not stating the reference (isotropic, dipole, or something else). This calculator removes the reference label, not the number.

4) Linear ratios you can compare

Many engineers sanity‑check results in linear form. Power ratio uses 10^(dB/10): 3 dB ≈ 2×, 6 dB ≈ 4×, and 10 dB = 10×. Voltage or field ratio uses 10^(dB/20): 6 dB ≈ 2× and 20 dB = 10×.

5) Typical gain values (real numbers)

Small handheld antennas often sit around 0–3 dBi. Common Wi‑Fi “rubber duck” styles are frequently 2–5 dBi. Panel and sector antennas are often 8–18 dBi. Parabolic dishes can range from about 15 dBi for compact units to 24 dBi and higher for larger apertures.

6) Impact on EIRP and coverage

EIRP is typically computed as transmitter power (dBm) + antenna gain (dBi) − feeder losses (dB). For example, 20 dBm radio + 12 dBi antenna − 2 dB cable gives 30 dBm EIRP. That 2 dB cable loss cuts power by about 37% (10^(−2/10) ≈ 0.63).

7) Common mistakes to avoid

Do not confuse dBi with dBd. dBd is referenced to a half‑wave dipole, and 0 dBd corresponds to about 2.15 dBi. Also, remember dB sums only for ratios; you cannot add dB to watts directly without converting power to dBm or a linear scale.

8) When to use each output

Use dB when you are combining gains and losses in a budget table. Use power ratio when explaining “how many times stronger” the antenna peak is. Use voltage ratio for field strength comparisons, impedance‑matched voltage levels, or when a spec uses the 20‑log relationship. Export results to document designs and collaborate with others.

FAQs

1) Is dBi numerically different from dB?

The numeric value is the same. dBi is a dB gain value that explicitly states the reference is an isotropic radiator. If you remove the reference label, you can report the same number as dB gain.

2) What is the difference between dBi and dBd?

dBd is referenced to a half‑wave dipole. A dipole has about 2.15 dB more gain than isotropic, so 0 dBd ≈ 2.15 dBi. Convert between them before mixing specifications.

3) Why does the calculator show power and voltage ratios?

Decibels are logarithmic. Power ratio uses 10^(dB/10) and is best for power comparisons. Voltage or field ratio uses 10^(dB/20) and is useful when specs use the 20‑log relationship.

4) Can I add dB values directly in a link budget?

Yes, when they represent gains or losses (ratios). For example, you can add antenna gain and subtract cable loss in dB. But do not add dB to watts; use dBm or convert to linear power first.

5) What does a negative dBi value mean?

It indicates loss relative to isotropic, often due to antenna pattern, mismatch, or inefficiency. For instance, −3 dBi corresponds to about half the power in the peak direction compared with isotropic.

6) How accurate are common “3 dB = double” rules?

They are close approximations. Exactly, 3 dB is a power ratio of about 1.995 (nearly 2×). Small rounding differences are normal, especially when you display fewer decimals.

7) When should I round to fewer decimals?

Round to match measurement uncertainty. Many antenna data sheets are only reliable to about 0.5–1 dB, so 1–2 decimals is usually enough. Use more decimals when teaching or verifying calculations.