Antenna Gain Calculator

Gain in dBi/dBd of a parabolic dish or any aperture from size, frequency and efficiency.

What it does

The gain of a dish or aperture antenna grows with its area and with frequency: G = η(πD/λ)². The result is given in dBi and dBd.

Where it's used

Satellite TV dishes, point-to-point microwave links and radio-astronomy dishes.

How to use it

Choose dish (by diameter) or aperture area.
Enter the size and the operating frequency (12G = 12 GHz).
Enter the aperture efficiency (50–70 % for typical dishes).

The formula explained

G: gain relative to an isotropic antenna
η: aperture efficiency
D: dish diameter
λ: wavelength = c / f
dBd: gain relative to a dipole = dBi − 2.15

Worked example

A 1.2 m C-band dish at 4 GHz.

Inputs:

Antenna type: Parabolic dish (diameter)
Dish diameter: 1.2 m
Frequency: 4G Hz
Aperture efficiency: 55 %

The calculator gives:

Gain: 31.44 dBi
Gain: 29.29 dBd
Linear gain: 1,391.6

Tips and common mistakes

Doubling the diameter adds about 6 dB of gain.
More gain means a narrower beam that must be pointed more precisely.
Efficiency above about 75 % is unusual for real dishes.

Questions people ask

What is the difference between dBi and dBd?

dBi compares the antenna with an ideal isotropic radiator; dBd compares it with a half-wave dipole, which itself has 2.15 dBi. So dBd = dBi − 2.15.

How does gain change with frequency?

For the same dish, gain rises with the square of frequency: three times the frequency adds 20 × log10(3) ≈ 9.5 dB. The 1.2 m dish gives 31.4 dBi at 4 GHz and about 41 dBi at 12 GHz.

What is EIRP?

Effective isotropic radiated power: transmitter power plus antenna gain, minus cable losses, all in decibels. 10 W (40 dBm) into a 31.4 dBi dish with negligible cable loss gives about 71.4 dBm EIRP.

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