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How is gain defined for a receiver antenna? If we look at a radiating antenna, there is a Pin, and the gain in a particular direction would be the radiation intensity Ur, divided by the radiation intensity of a lossless isotropic antenna Ui, in that particular direction. G = Ur/Ui = (4pi * Ur)/Pin.

But, what is the baseline for the receiving scenario. I know it is the isotropic antenna, but in which way? What does Pin (as the very basis for the comparison) translates to in the receiving scenario?

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For an antenna that can transmit an EM wave it's receive gain is exactly the same as its transmit gain. This is down the antenna reciprocity: -

Reciprocity is one of the most useful (and fortunate) property of antennas. Reciprocity states that the receive and transmit properties of an antenna are identical. Hence, antennas do not have distinct transmit and receive radiation patterns - if you know the radiation pattern in the transmit mode then you also know the pattern in the receive mode.

Quote taken from here.

Be aware that this is for the transmission and reception of an EM wave and not, for instance for a medium wave ferrite rod antenna - it can only intercept the "M" part of the EM wave when used as a receiver and it does it very well but, it makes a poor transmitter because it cannot generate a suitable "E" value due to it's small size.

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  • \$\begingroup\$ I understand that the gain value is the same for both scenario. What I don't know is that what does gain really mean for a receiving antenna, if one wants to be more precise then just saying that a larger gain in one direction means the antenna is more efficiently converts EM waves coming from that direction to electric power than waves coming from a direction with a smaller gain. How is gain defined for a receiver antenna? \$\endgroup\$
    – Cerike
    Commented Nov 4, 2018 at 17:56
  • \$\begingroup\$ When I see that one of the antennas have a gain of 3 dBiC, and the other has a gain of 5 dBiC does that only tell me, that one of them is more directed? Or does it also tell me, and if yes in what way, that how efficiently the antenna produces electric power form EM power? My intuition is, that a losses antenna should have an isotropic gain of 1 in the receiving scenario, because it will transform all the EM energy into electric energy (with 100% efficiency) no matter what the radiation patter might be. I am confused. \$\endgroup\$
    – Cerike
    Commented Nov 4, 2018 at 17:57
  • \$\begingroup\$ A bigger receiving gain means that it has a wider aperture. The aperture is akin to a fishing net therefore the more area the net has the more fish it catches or, in terms of an antenna, the more power it collectes from the ether. More gain means a higher directivity. Remember that it is watts per sq metre that is hitting the antenna and therefore the antenna aperture (in sq metres) converts the incident EM wave to electrical power at the amplifier signal terminals. The antenna also converts the impedance of free space (377 ohms) to an electrical impedance of normally lower value...... \$\endgroup\$
    – Andy aka
    Commented Nov 4, 2018 at 18:35
  • \$\begingroup\$ ..... I think my comment is likely to spark more questions from you so I urge you to do some research into understanding antennas a bit more and remember that this site is a Q and A site and not a training ground that might meander thru a series of ongoing questions and answers. This will likely mean that I will suggest you raise a new question for some likely spin-off thoughts you may develop. \$\endgroup\$
    – Andy aka
    Commented Nov 4, 2018 at 18:37

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