Antenna Efficiency and Antenna Gain

On this page, we'll introduce two fundamental antenna parameters: antenna efficiency and antenna gain.
The
[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:
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 is the antenna's
total efficiency, is the antenna's
loss due to impedance mismatch, and is
the antenna's radiation efficiency.
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.
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.
The term
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 (
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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## Antenna Efficiency

**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.
[Equation 1]
[Equation 2]
## Antenna Gain

**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. *G*) can be related to
directivity (*D*) by:
[Equation 3]

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