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Antenna Efficiency
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# Antenna Efficiency

The efficiency of an antenna is a ratio of the power delivered to the antenna relative to the power radiated from 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.

One nice property of antennas is that the efficiency is the same whether we are using the antenna as a transmit or receive antenna. Hence, we could define antenna efficiency as the ratio of "potential power received from all possible angles", but that's more complicated. Just remember transmit and receive antenna efficiency is the same, and since it is easier to understand efficiency in terms of power radiated vs. power supplied, we'll simply use that definition. This property of antennas is known as antenna reciprocity.

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

[Equation 1]

Being a ratio, antenna efficiency is a number between 0 and 1. However, antenna efficiency is commonly 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 is the antenna's total efficiency,  is the antenna's loss due to impedance mismatch, and is the antenna's radiation efficiency.

[Equation 2]

From equation [2], since 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.

In practice, unless otherwise specified, antenna efficiency typically refers to the total efficiency (including mismatch loss).

What causes an antenna to not have an efficiency of 100% (or 0 dB)? Antenna efficiency losses are typically due to:

*conduction losses (due to finite conductivity of the metal that forms the antenna)

*dielectric losses (due to conductivity of a dielectric material near an antenna)

*impedance mismatch loss

Examples of dielectrics include glass, plastics, Teflon, and rubber. The strong Electric Fields near an antenna lose energy to heat due to the conductivity of the dielectric. If the electrical conductivity is zero, the dielectric loss within a material is zero. However, many materials (such as silicone and glass) have a conductivity that is low but still enough to significantly decrease the antenna efficiency.

Efficiency is one of the most important antenna parameters. It can be very close to 100% (or 0 dB) for dish antennas, 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). Car radio antennas can have an 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.

Finally, a note on dB vs. percentage. It is very common in the industry to quote antenna efficiency in percent. However, there are two strong reasons why antenna efficiency should be measured in decibels (dB):

{1} everything associated with the RF (radio frequency) world is measured in dB: transmit power is dB, isolation is in dB, defense is in dB, radiosensitivity is in dB. Hence, it follows antenna efficiency should be in dB.

{2} If a change to an antenna is made, and someone says "how much did the efficiency change" and the response is "5%", that is ambiguous. An increase from 1% to 6% is a huge change (7.8 dB), whereas an increase from 85% to 90% is small (0.24 dB).