Antenna Gain Calculator (dBi)

Understanding antenna gain:

Antenna gain measures how effectively an antenna focuses radiated power in a particular direction compared to a reference antenna. It does not create power. It redirects power out of the directions you do not care about and into the one you do, the way a flashlight reflector turns the same bulb into a beam.

Gain units explained:

A half-wave dipole has a gain of 2.15 dBi (0 dBd). Every 3 dB of gain doubles the effective power in the favored direction.

Common antenna gains:

Effective Isotropic Radiated Power (EIRP):

EIRP (watts) = Transmitter power × Feedline efficiency × Antenna gain (linear, from dBi)

Antenna gain (linear) = 10^(Gain_dBi / 10)

Feedline efficiency = 10^(-Loss_dB / 10)

Example calculation:

A 100-watt transmitter with a 5-element Yagi (10.5 dBi) and 1.5 dB feedline loss:

• Gain linear: 10^(10.5/10) = 11.22
• Feedline efficiency: 10^(-1.5/10) = 0.708
• EIRP: 100 × 0.708 × 11.22 = 794 watts EIRP

This means a 100W station with a good Yagi has the same signal strength as a 794W station with an isotropic antenna. The dipole-referenced cousin, ERP (Effective Radiated Power), uses dBd instead of dBi and comes out 1.64× smaller: 484 W in this example.

The 3 dB rule:

Every 3 dB of gain doubles the effective power:

• +3 dBi = 2× power
• +6 dBi = 4× power
• +10 dBi = 10× power
• +20 dBi = 100× power

Gain vs. beamwidth tradeoff:

Higher gain means narrower beamwidth. A 3-element Yagi is around 64° between the half-power points, and a 10-element narrows to about 38°. Dish antennas get under 5°. Narrow beamwidth needs accurate aiming, and repays it with better signal and better rejection of noise arriving from anywhere else.

Practical considerations:

Adding 3 dB of antenna gain is equivalent to doubling transmitter power, and it is far cheaper. A $50 antenna upgrade often outperforms a $500 amplifier. Antenna height and clear line-of-sight matter as much as gain for real-world performance.

One caution on published gain figures. A number quoted in dBi looks 2.15 dB better than the same antenna quoted in dBd, and some marketing takes advantage of that by leaving the reference letter off entirely. If a mobile whip claims “9 dB” with no i and no d, assume dBi. A genuine 9 dBd mobile antenna would be a large collinear, not something you clip to a car roof.