I'm working with RFID for the first time. All the app notes I've read show a separate region of PCB allocated for the trace antenna, with no planes above/below. I want to understand why this is, and if it's strictly necessary. I understand that the plane will shield the antenna from one direction. For my application, this is ok.
I can fathom the following scenario: 1) An electromagnetic wave is incident upon a trace antenna with a copper plane below it 2) The distance between the plane and antenna is half the wavelength (not realistic for RFID, but this is a thought experiment) 3) The field seen at the antenna is 0, because the incident wave and reflected wave are 180 deg out of phase.
So, in this scenario, there's no changing field and no current induced in the trace antenna, so nothing is received, but this is only because the separation between the copper plane and antenna is lamba/2. When this is not the case, I would imagine there should be current induced in the antenna.
I'm particularly interested in this because I want to save board space. Specifically, I want to put components and signals on top, and the antenna on the bottom, with a copper plane in between.
If I cannot do this, can anyone explain, with precise language and in EM fundamentals (no handwaving armchair engineer stuff please!) why this will not work?