Principles of Antenna Gain

A characteristic by which antennas are frequently rated is gain. Since antennas are passive devices, do they have gain?

In fact, antennas do not have gain in a general sense. They do not amplify; there is never more power being radiated out than is being fed in.

However, what antennas can do is to concentrate the power being fed into them. An antenna need not radiate evenly in all directions (isotropic); instead, we can focus the power in a desired direction. To do this, however, there is a trade-off. By focusing power in one direction, there will be less power available for the unfocused directions.

Directivity

The antenna gain for a given antenna type is measured at the point of highest power concentration relative to the isotropic case mentioned above, where all of the power is being radiated evenly in all directions. This means, therefore, that if we measure the signal of a highly focused or directional antenna at the point of maximum signal and compare this number to the signal radiated by our isotropic antenna, we will find that the directional antenna is radiating more power; hence the antenna has higher gain — or, more correctly, appears to.

Obviously, this power came from somewhere; the directional antenna has less power in other areas, which can be proven by measuring the signal in all directions and comparing the results with the isotropic case. Thus, for a given increase in power in one direction, there is proportionately less signal being radiated in other directions. As stated before, we are merely taking the same amount of power and refocusing it. Antenna gain is directly related to directivity.

Loss

Antenna gain, however, has another factor: loss. Our theoretical isotropic antenna that radiates evenly in all directions has zero loss. There is always some loss in even the best antennas that prevents them from performing at their theoretical optimum. For a lossless antenna gain and directivity would be equal. However, in reality the antenna gain falls short of the theoretical optimum.

Efficiency

Loss is frequently measured by efficiency, or how much of the power that enters the antenna is actually being radiated. Efficiency is never more than 1, where 1 represents the perfect, lossless antenna. The formula for calculating gain is then:

Gain = Directivity x Efficiency

In summary, antenna gain is not “created” power, but rather a measure of directivity — how concentrated the power is at the point of highest radiated signal — minus the amount of loss or the amount of power wasted.