For the reciprocity theorem, if an antenna has a low efficiency in transmission, it has the same efficiency when receiving.
In literature, it is usually said that one shouldn't use a low-efficiency antenna for transmitting because a lot of power would be wasted.
However, I often read that if it's only for receiving signals, a low-efficiency antenna might be used. Why? In such a case, you're wasting the same amount of power you'd have wasted by using the same antenna in transmission!
Also, with a low-efficiency antenna as a receiver, you'll have a smaller S/N ratio compared to what you'd have with a higher-efficiency antenna.
Just because you're receiving the power that someone else transmitted (i.e. someone else spent to transmit it) shouldn't be a good enough reason to use a low-efficiency antenna to receive it.
Best Answer
The reciprocity theorem is about the gain of the antenna. Let's say we have an antenna with 50% power efficiency, so 3dB more loss than a perfect antenna.
If used for reception, it will lose half the power it receives, so decrease the signal to noise ratio due to the receiver input noise by 3dB. Not ideal, but no biggy, it just means a reduction in range compared to a perfect antenna in a quiet environment.
If we're using it in a busy mobile radio environment, and there are lots of other users creating interference on the same and nearby channels, then it's attenuating those signals as well, so the system performs just as well as when using a perfect antenna.
Now consider it for transmission. It will lose half the power we put into it. If we want to radiate 100mW (for a mobile phone) or 100kW (for a TV transmitter), we will need our RF power amplifier to generate 200mW (which will give us half the talk time on the same battery), or 200kW (would you buy a 200kW amplifier when you only really needed 100kW, never mind the extra 200kW of mains power to run it).
While the antenna gain is reciprocal, how you use it certainly isn't.