The efficiency of the antenna

Antenna efficiency with the antenna to provide power and antenna radiation power. An efficient antenna radiates most of the energy delivered to the antenna. An inefficient antenna absorbs most of the power lost within the antenna. An inefficient antenna may also have a lot of energy reflected off due to impedance mismatches. Less efficient antennas radiate power than more efficient antennas.

[Side note: Antenna impedance is discussed later in the chapter. Impedance mismatch is the power reflected from the antenna because the impedance is not the correct value. Therefore, this is called impedance mismatch.]

Conduction loss of the type of loss in the antenna. The conduction loss is due to the limited conductivity of the antenna. Another mechanism of loss is dielectric loss. Dielectric loss in an antenna is due to conduction of dielectric material. Insulation may be present in or around the antenna.

The ratio of antenna efficiency radiated power can be written as antenna input power. This is the equation [1]. Also known as radiation efficiency antenna efficiency.
Equation [1]
　　Efficiency is a ratio. The ratio is always a number between 0 and 1. Efficiency is often given at one percent. For example, a 0.5 efficiency is up to 50% the same. Antenna efficiency is also often cited in decibels (dB). An efficiency of 0.1 is equal to 10%. This is also equal to minus 10 decibels. An efficiency of 0.5 equals 50%. This is also equal to minus 3 decibels.
 
　　The first equation is sometimes called the radiation efficiency of the antenna. This distinguishes it from another commonly used term called the antenna's total efficiency. Total efficiency antenna radiation efficiency Impedance mismatch loss of an antenna times. Impedance mismatch losses occur when the antenna is physically connected to the transmission line or receiver. This can be summarized in the formula [2]. The total efficiency can be written as. The loss due to impedance mismatch is written as. The radiation efficiency of the antenna reads:
Equation [2]
　　The impedance mismatch loss is always a number between 0 and 1. Therefore, the overall antenna efficiency is always less than the radiation efficiency. To reiterate this point, if there is no loss, impedance mismatch due to radiation efficiency equal to the total antenna efficiency.
　　Improving efficiency is one of the most important antenna parameters. It can be very close to 100% satellite dish antennas, horn antennas, or half-wavelength dipoles with no lossy material around them. Cell phone antennas or consumer electronics antennas are typically 20 to 70 percent efficient. This is equivalent to -7 decibels -1.5 decibels (-7, -1.5 decibels). This is often due to the loss of electronics and materials surrounding the antenna. These tend to absorb some radiated power. Energy is converted into heat without radiation. This reduces the efficiency of the antenna. Car radio antennas can have an antenna efficiency of 0.01 at AM radio frequencies. [That's 1% or -20 decibels.] This inefficiency is due to the antenna being smaller than the half wavelength at the operating frequency. This greatly reduces the efficiency of the antenna. Wireless links are maintained because AM broadcast towers employ very high transmission power.
　　Impedance mismatch loss enhancement is discussed in the Smith diagram and impedance matching section. Impedance matching can greatly improve antenna efficiency.