The efficiency of an antenna is related to the power supplied to the antenna and the power radiated by the antenna. A highly efficient antenna will radiate 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 out due to impedance mismatch. Reduce the radiated power of an inefficient antenna compared to a more efficient antenna.
[Side note: Antenna impedance is discussed in a later chapter. Impedance mismatch is reflected power from the antenna because the impedance is an incorrect value. Therefore, this is called impedance mismatch. ]
Type of loss within the antenna is conduction loss. Conduction losses are due to the finite conductivity of the antenna. Another mechanism of loss is dielectric loss. Dielectric losses in the antenna are due to conduction in the dielectric material. Insulating material may be present within or around the antenna.
The ratio of the efficiency of the antenna to the radiated power can be written as the input power of the antenna. This is equation [1]. Also known as radiation efficiency antenna efficiency.
[Equation 1]
Efficiency is a ratio. This ratio is always a quantity between 0 and 1. Efficiency is often given at a percentage point. For example, an efficiency of 0.5 is up to 50% the same. Antenna efficiency is also often quoted in decibels (dB). An efficiency of 0.1 equals 10%. This is also equal to -10 decibels (-10 decibels). An efficiency of 0.5 equals 50%. This is also equal to -3 decibels (dB).
The first equation is sometimes called the radiation efficiency of the antenna. This distinguishes it from another commonly used term called the total effectiveness of the antenna. Total Effective Efficiency Antenna radiation efficiency multiplied by the impedance mismatch loss of the antenna. Impedance mismatch losses occur when the antenna is physically connected to the transmission line or receiver. This can be summarized in formula [2].
[Equation 2]
formula [2]
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, if there are no losses, the radiation efficiency is equal to the total antenna efficiency due to impedance mismatch.
Improving efficiency is one of the most important antenna parameters. It can be very close to 100% with a satellite dish, horn antenna, or half wavelength dipole without any lossy material around it. Cell phone antennas or consumer electronics antennas typically have an efficiency of 20%-70%. This is equivalent to -7 dB -1.5 dB (-7, -1.5 dB). Often due to loss of electronics and materials surrounding the antenna. These tend to absorb some radiated power. The energy is converted into heat energy and there is no radiation. This reduces the efficiency of the antenna. Car radio antennas can operate at AM radio frequencies with an antenna efficiency of 0.01. [This is 1% or -20 dB. ] This inefficiency is because the antenna is smaller than half a wavelength at the operating frequency. This greatly reduces the efficiency of the antenna. Wireless links are maintained because AM broadcast towers employ very high transmit power.
Impedance mismatch losses are discussed in the Smith Chart and Impedance Matching sections. Impedance matching can greatly improve the efficiency of the antenna.
Antenna gain
Long-term antenna gain describes how much power is transmitted in the peak radiation direction, relative to an isotropic source. Antenna gain is more commonly quoted in the specification sheet of an antenna. Antenna gain is important because it takes into account the actual losses that occur.
An antenna with 3 dB gain means the power received from the antenna is 3 dB much higher than it would be received from a lossless isotropic antenna with the same input power. 3 dB is equivalent to twice the power supply.
Antenna gain is sometimes discussed as a function of direction or angle. However, when a single number specifies the gain, then that number is the peak gain for all directions. The "G" of antenna gain can be compared with the directivity of "D" of futuristic type.
[Equation 3]
The gain of a real antenna, which can be as high as a very large satellite dish, is 50 dB. Directivity can be as low as 1.76 dB like a real antenna (such as a short dipole antenna). Directionality can never be less than 0 dB. However, the peak antenna gain can be arbitrarily small. This is due to losses or inefficiencies. Electrically small antennas are relatively small antennas that operate at the wavelength of the frequency at which the antenna operates. Small antennas can be very inefficient. Antenna gain is often below -10 dB, even when impedance mismatch is not taken into account.
Post time: Nov-16-2023
