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Things You Should Know About Antenna Gains
"The term Antenna Gain describes how much power is transmitted in the direction of peak radiation to that of an isotropic source. ----- FMUSER"
#Antenna Gain
The term 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 than directivity in an antenna's specification sheet because it takes into account the actual losses that occur.
A transmitting 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. Note that a lossless antenna would be an antenna with an antenna efficiency of 0 dB (or 100%). Similarly, a receive antenna with a gain of 3 dB in a particular direction would receive 3 dB more power than a lossless isotropic antenna.
Antenna Gain is sometimes discussed as a function of angle. In this case, we are essentially plotting the radiation pattern, where the units (or magnitude of the pattern) are measured in antenna gain.
See Also: >> What's the difference between "dB", "dBm", and "dBi"?
Often manufacturers of antennas (be they wifi antennas, gps antennas, or tv antennas) specify the antenna gain. For instance, manufacturers of wifi antennas may market the wifi antenna as a "high gain antenna", which is more expensive than a similar low gain antenna. The question is: do we want high gain?
See Also: >> Is More Antenna Gain Better?
The answer is: it depends. If you know exactly where your desired signal is coming from, you would like to have maximum gain (towards the desired) direction. However, if you don't know where the desired signal will be coming from, it is better to have a low gain antenna. A couple
examples will make this clear.
{ Example #1 } - TV Antennas. If you mount a TV antenna on your roof, and know the tv broadcast antennas are to the south (for example, on some hill south of the city), then it is preferred to have a high gain antenna. Antennas with gain of at least 12-15 dB are preferred.
{ Example #2 } - GPS (Global Positioning System). GPS antennas for mobile devices are receive only. The job of the gps antenna is to triangulate your position by measuring the received signal from multiple gps satellites, which are all in different directions relative to the receive antenna. For this case, a very highly directional antenna would not be preferred.
{ Example #3 } - Mobile Cellular Antennas. The cellular antenna on your smartphone communicates with a single
cellular network tower. However, the cellular antenna can be held in any orientation, and can be in any position relative to the network tower. Hence, for your mobile device, it is preferred to have a low gain antenna.
#A note on Units for Antenna Gain
If you look at the specification sheet for an antenna, you will typically see units for antenna gain listed in dB, dBi or dBd. I define these terms below:
dB - decibels, as we have been discussing. 10 dB means 10 times the energy relative to an isotropic antenna in the peak direction of radiation.
dBi - "decibels relative to an isotropic antenna". This is the same as dB as we have been using it. 3 dBi means twice (2x) the power relative to an isotropic antenna in the peak direction.
dBd - "decibels relative to a dipole antenna". Note that a half-wavelength dipole antenna has a gain of 2.15 dBi. Hence, 7.85 dBd means the peak gain is 7.85 dB higher than a dipole antenna; this is 10 dB higher than an isotropic antenna.
You may also like: >>Basic Antenna Theory: dBi, dB, dBm dB(mW)
>> Tips on Antenna Gain Measurement