In this thesis the following model for an antenna is proposed:
It is seen as a 2 port network, in which only port 1 is accessible (port 2 is an imaginary connection between the antenna and the free space). Precisely:
- $Z_0$ is the feed line impedance.
- $jX_a$ is the antenna reactance.
- $R_l$ is the antenna parasitic resistance (which accounts for losses).
- $Z_{fs}$ is the free space impedance (377 Ohm)
- The transformer represents what an antenna really does: it transforms the free space impedance $Z_{fs}$ to a radiation resistance (which is the impedance seen at the primary of the transformer.
This description is quite clear. But then, the author says:
Since free space represents a matched termination, there would not be any reflection occurring ($\Gamma_L = 0$).
It does not seem too obvious for me.
As seen above, an antenna may be seen as a two - port network in which port 2 is closed on 377 Ohm. I'd say that usually there exists a mismatch on port 2, and so there is reflection at port 2. This doesn't mean there will be reflection at port 1 too, it depends if the antenna is able to match those 377 Ohm to the feed line impedance.
But I don't understand why there shouldn't be reflection at port 2. Maybe the author is considering a specific assumption he hasn't written, even because I've read somewhere that reflection along (not at the feeding port) some antennas exists and is a problem.
For instance, an infinite biconical antenna is perfectly matched to free space because it's like an infinite transmission line
A real biconical antenna is not matched to free space because it's truncated.