Antenna Efficiency and Antenna Gain

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On this page, we'll introduce two fundamental antenna parameters: antenna efficiency and antenna gain.

Antenna Efficiency

The efficiency of an antenna relates the power delivered to the antenna and the power radiated or dissipated within the antenna. A high efficiency antenna has most of the power present at the antenna's input radiated away. A low efficiency antenna has most of the power absorbed as losses within the antenna, or reflected away due to impedance mismatch.

[Side Note: Antenna Impedance is discussed in a later section. Impedance Mismatch is simply power reflected from an antenna because it's impedance is not the correct value; hence, "impedance mismatch". ]

The losses associated within an antenna are typically the conduction losses (due to finite conductivity of the antenna) and dielectric losses (due to conduction within a dielectric which may be present within an antenna).

The antenna efficiency (or radiation efficiency) can be written as the ratio of the radiated power to the input power of the antenna:

antenna efficiency

[Equation 1]

Efficiency is ultimately a ratio, giving a number between 0 and 1. Efficiency is very often quoted in terms of a percentage; for example, an efficiency of 0.5 is the same as 50%. Antenna efficiency is also frequently quoted in decibels (dB); an efficiency of 0.1 is 10% or (-10 dB), and an efficiency of 0.5 or 50% is -3 dB.

Equation [1] is sometimes referred to as the antenna's radiation efficiency. This distinguishes it from another sometimes-used term, called an antenna's "total efficiency". The total efficiency of an antenna is the radiation efficiency multiplied by the impedance mismatch loss of the antenna, when connected to a transmission line or receiver (radio or transmitter). This can be summarized in Equation [2], where antenna efficiency is the antenna's total efficiency, impedance mismatch loss is the antenna's loss due to impedance mismatch, and total efficiency is the antenna's radiation efficiency.

efficiency of an antenna

[Equation 2]

Since impedance mismatch loss is always a number between 0 and 1, the total antenna efficiency is always less than the antenna's radiation efficiency. Said another way, the radiation efficiency is the same as the total antenna efficiency if there was no loss due to impedance mismatch.

Efficiency is one of the most important antenna parameters. It can be very close to 100% (or 0 dB) for dish, horn antennas, or half-wavelength dipoles with no lossy materials around them. Mobile phone antennas, or wifi antennas in consumer electronics products, typically have efficiencies from 20%-70% (-7 to -1.5 dB). The losses are often due to the electronics and materials that surround the antennas; these tend to absorb some of the radiated power (converting the energy to heat), which lowers the efficiency of the antenna. Car radio antennas can have a total antenna efficiency of -20 dB (1% efficiency) at the AM radio frequencies; this is because the antennas are much smaller than a half-wavelength at the operational frequency, which greatly lowers antenna efficiency. The radio link is maintained because the AM Broadcast tower uses a very high transmit power.

Improving impedance mismatch loss is discussed in the Smith Charts and impedance matching section. Impedance matching can greatly improve the efficiency of an antenna.

Antenna Gain

The term Antenna Gain describes how much power is transmitted in the direction of peak radiation to that of an isotropic source. Antenna gain is more commonly quoted in a real antenna's specification sheet because it takes into account the actual losses that occur.

An antenna with a gain of 3 dB means that the power received far from the antenna will be 3 dB higher (twice as much) than what would be received from a lossless isotropic antenna with the same input power.

Antenna Gain is sometimes discussed as a function of angle, but when a single number is quoted the gain is the 'peak gain' over all directions. Antenna Gain (G) can be related to directivity (D) by:

gain of an antenna or antenna gain

[Equation 3]

The gain of a real antenna can be as high as 40-50 dB for very large dish antennas (although this is rare). Directivity can be as low as 1.76 dB for a real antenna (example: short dipole antenna), but can never theoretically be less than 0 dB. However, the peak gain of an antenna can be arbitrarily low because of losses or low efficiency. Electrically small antennas (small relative to the wavelength of the frequency that the antenna operates at) can be very inefficient, with antenna gains lower than -10 dB (even without accounting for impedance mismatch loss).

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