In order to efficiently deliver power to a different part of a circuit without reflection, the impedances of all circuit elements need to be matched. Free space can be regarded as a further element, since a transmitting antenna eventually should radiate all power from the transmission line into it.
Now, if the impedances in the transmission line and in the antenna are matched at 50 Ω, but the impedance of free space is 377 Ω, won't there be a impedance mismatch and consequently a less-than-optimal radiation from the antenna?
As far as I gathered from the answers, literature and discussions online, the antenna acts as a impedance transformer between the feed line and free space. The argument goes: no power from the feed line is reflected and must go to the antenna. The antenna can be assumed to be resonant and therefore radiates all its power into free space (disregarding heat losses etc). This means that there is no reflected power between antenna and free space, and the transition between antenna and free space is therefore matched.
The same should be true in the reverse direction for a receiving antenna (Reciprocity Principle): a wave in free space (\$Z_0\$) impinges onto an antenna, and the received power is fed into the transmission line (again through impedance transformation). At least in one paper (Devi et al., Design of a wideband 377 Ω E-shaped patch antenna for RF energy harvesting, Microwave and Optical Letters (2012) Vol. 54, No. 3, 10.1002/mop.26607) it was mentioned that a 377 Ω antenna with a separate circuit to match it to 50 Ω was used to "achieve a wide impedance bandwidth" with a high power level. If the antenna normally is already the impedance transformer, what is the matching circuit needed for then? Or alternatively, under what circumstances is the antenna not also the impedance transformer?
Some helpful sources and discussions I found:
- Klaus Kark, Antenne und Strahlungsfelder (in German)
- Impedance Matching (http://www.phys.ufl.edu/~majewski/nqr/reference2015/nqr_detection_educational/Impedance_matching_networks.pdf)
- Forum discussion that mentions impedance transformation for an inverted-F antenna (http://www.antenna-theory.com/phpbb2/viewtopic.php?t=776&sid=dede0d4127170d16cc3a583ab0929f3e)
- Some general notes about antennas 8http://fab.cba.mit.edu/classes/862.16/notes/antennas.pdf)