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I'm looking for an explanation about what type of relation have antenna gain with it's efficiency.

For example I want to use a ceramic antenna that is a dipole antenna. It has:

  • Average Gain of -0.5 dBi typ. (XZ-V)
  • Return Loss 9.5 dB min.
  • Input Power 3W

and not more details.

Antenna Gain worth something in antenna efficiency? That mean that I have 0.5dB gain in one direction and -0.5dB in another. But if I want an omnidirectional antenna, should I choose 0dB gain?

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    \$\begingroup\$ Gain for antennas is really just a measurement of directionality. \$\endgroup\$
    – PlasmaHH
    Commented Jun 1, 2015 at 11:45
  • \$\begingroup\$ Ok, but from PCB antenna to ceramic antenna, I've some kind of gain? If so, which value tell me that ceramic antenna allow me more distance trasmission? \$\endgroup\$
    – Yaro
    Commented Jun 1, 2015 at 12:01
  • \$\begingroup\$ for tx, if you feed in 100W, you can only ever distribute 100W. There is no way an antenna design could magically make it be 200W distributed. All it can do is distribute most of it into one direction, leaving less into the other. The datasheet of the antenna will tell you details about that. \$\endgroup\$
    – PlasmaHH
    Commented Jun 1, 2015 at 12:04
  • \$\begingroup\$ Gain of -0.5dB suggests it's pretty close to omnidirectional. \$\endgroup\$
    – user16324
    Commented Jun 1, 2015 at 12:08

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Antenna efficiency is generally taken as the amount of total power radiated (in all directions) divided by the electrical power put into the antenna. It is not related to antenna gain.

Antenna gain is something a little more complex. A mathematical device called an "isotropic antenna" is a theoretical antenna that emits power equally in all directions. This is the base line for a lot of comparisons between antennas. No such antenna exists but it is still a good device for making comparisons.

An antenna that bunches the emitted power up so that it produces more watts/sq meter in one direction compared to a different direction is generally specified as having gain. A dipole antenna will concentrate power like this: -

enter image description here

It has a gain that slightly exceeds (by 2.15dB) the 0 dBi mark on the polar diagrams. Note I used the term "dBi" to indicate gain relative to the theoretical isotropic device.

So, an antenna with "gain" must has directionality - it forces more watts per sq meter in some directions and fewer watts per sq metre in other directions.

Antenna gain works equally in transmit as it does receive - an antenna with higher gain will collect more watts and convert them to more volts in its "optimum" direction than it does in other directions.

A receiving antenna has, what is known as, an aperture (think of it as a net which it catches radiated power - the bigger the net the more power it catches).

Both efficiency and gain need to be considered together though - an antenna with significant gain may still not be a good emitter if the efficiency is poor.

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