How to caculate the antenna Efficiency and antenna Gain

Here, we'll introduce two fundamental antenna parameters: antenna efficiency and antenna gain.
 

Antenna Efficiency

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

[Side Note: Antenna Impedance is discussedin a later section. Impedance Mismatch is simply power reflected from anantenna because it's impedance is not the correct value; hence, "impedancemismatch". ]
 

The losses associated within an antenna aretypically the conduction losses (due to finite conductivity of the antenna) anddielectric losses (due to conduction within a dielectric which may be presentwithin an antenna).

 

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

Efficiency is ultimately a ratio, giving anumber between 0 and 1. Efficiency is very often quoted in terms of apercentage; for example, an efficiency of 0.5 is the same as 50%. Antennaefficiency is also frequently quoted in decibels (dB); an efficiency of 0.1 is10% or (-10 dB), and an efficiency of 0.5 or 50% is -3 dB.

 

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

 

 

Since  is always anumber between 0 and 1, the total antenna efficiency is always less than theantenna's radiation efficiency. Said another way, the radiation efficiency isthe same as the total antenna efficiency if there was no loss due to impedancemismatch.

 

Efficiency is one of the most importantantenna parameters. It can be very close to 100% (or 0 dB) for dish, hornantennas, 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 oftendue to the electronics and materials that surround the antennas; these tend toabsorb some of the radiated power (converting the energy to heat), which lowersthe efficiency of the antenna. Car radio antennas can have a total antennaefficiency of -20 dB (1% efficiency) at the AM radio frequencies; this isbecause the antennas are much smaller than a half-wavelength at the operationalfrequency, which greatly lowers antenna efficiency. The radio link ismaintained because the AM Broadcast tower uses a very high transmit power.

 

Improving impedance mismatch loss isdiscussed in the Smith Charts and impedance matching section. Impedancematching can greatly improve the efficiency of an antenna.

 

Antenna Gain

The term Antenna Gain describes how muchpower is transmitted in the direction of peak radiation to that of an isotropicsource. Antenna gain is more commonly quoted in a real antenna's specificationsheet because it takes into account the actual losses that occur.

 

An antenna with a gain of 3 dB means thatthe 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 sameinput power.

 

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

 

 

The gain of a real antenna can be as highas 40-50 dB for very large dish antennas (although this is rare). Directivitycan 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 anantenna can be arbitrarily low because of losses or low efficiency.Electrically small antennas (small relative to the wavelength of the frequencythat the antenna operates at) can be very inefficient, with antenna gains lowerthan -10 dB (even without accounting for impedance mismatch loss).

Leave a message 

Message List