Amplifier and antenna gain may sound the same, and both may be expressed in decibels, but they actually are measures of two different things.
In an amplifier, gain is simply the ratio of the output divided by the input. Gain has no units as it is a ratio. However, amplifier gain is often expressed in decibel units, abbreviated dB. This is the base-10 logarithm of the output/input ratio multiplied by a factor of 10.
Gain = 10log10[Output/Input] dB
Expression in dB allows gain to be plotted on a semilog or log-log chart versus amplifier frequency, time, or other parameter of interest. This method is used for convenience because the gain of amplifiers can vary over a wide range of values. Use of a logarithmic scale makes for compact graphs when plotting gain relationships.
The gain relationship is straightforward when the input and output are both voltages or when the input and output are both electrical current. Where things get potentially confusing is in the case of the transresistance amplifier, where input is current and output is voltage, and the transconductance amplifier, where input is voltage and output is current. You’ll also find references to transimpedance amplifiers. These are basically transresistance amplifiers that typically convert current from a photodiode (and sometimes other current-generating sensors) into a voltage. In all cases, the gain is still a ratio of output to input, and gain in dB is still calculated the same way, even though the units of the denominator and numerator may be different.
Antenna gain is a bit of a misnomer because it implies an increase in voltage at the output port with respect to the input port. In actuality, antenna gain is calculated by comparing the measured power transmitted or received by the antenna in a specific direction to the power transmitted or received by a hypothetical ideal antenna in the same setting. Thus in contrast to the case of amplifiers, antennas have no power supply providing the energy for amplification.
If the comparison is to an ideal (lossless) antenna radiating or receiving energy equally in all directions, the gain is measured in dBi (decibels-isotropic). Sometimes the comparison is to an ideal half-wave dipole antenna, defined as having 2.15 dB gain. In this case, the gain is measured in dBd (decibels-dipole).
A point to note is that the land mobile industry generally expresses antenna gain as dBd. When antenna manufacturers list gain in units of dB, they often mean dBd. But dB and dBi are different units of measure. The relationship between the two is dBi = dBd + 2.15.
Thus a 3-dB antenna has the same gain as a 5-dBi antenna. The direction of the power propagation is a key antenna quality. Gain is often represented plotted as a radiation pattern where the radius of the plot is on a decibel scale often normalized to a maximum value for the particular antenna under test or to an isotropic radiator. The direction that has the most power is considered the main lobe; exactly opposite the main lobe is the back lobe, and any other unwanted or unintended radiation features are called sidelobes. If the antenna spec doesn’t specify a direction, gain refers to peak value in the direction of the antenna’s main lobe.
For example, a collinear antenna oriented east-west with 6.41 dBd gain would be able to transmit or receive more than 4 times the signal power of an ideal dipole antenna in the east and west directions. Little signal power would radiate in the north and south directions.
Due to reciprocity, the gain of a receiving antenna is the same as when the antenna transmits. There are a few other antenna parameters that are defined in terms of antenna gain. For a given frequency, the antenna’s effective area A (also called effective aperture) is equal to the ratio of the received power available at the terminals of an antenna to the power per unit area in the incident wave. For all antennas, A is related to gain G at a given wavelength λ by A/G=λ2/4π.
Antenna gain can also be expressed as the ratio of power transmitted in a certain direction with a specific reference point. The relation between antenna gain and antenna efficiency is G = ε x D where D is the directivity and ε is the efficiency. Antenna efficiency is how much RF power delivered to the antenna (from the transmitter) actually passes into the air. The efficiency of a transmitting antenna is the total radiated power divided by the input power, ε =Po/Pin. Antenna directivity D is the maximum radiation intensity of the antenna, Imax, divided by the antenna’s average radiation intensity, Iave. Thus D= 4πImax/Iave.
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