I’m designing an RF device that should be able to use multiples sets of antennae, one a low-gain internal PCB antenna and the other an external high gain antenna attached via an SMA connector.
I’d like to be able to automatically detect when the external antenna is connected and disconnected (so that I can electronically switch between antennas) be able to switch to using it automatically.
I have come across ways of doing this by adding a known resistance to the antenna and then simply detecting this resistance through the use of a potential divider. This however would require the antenna to be precisely calibrated and be a short circuit antenna. Ideally I would like to be able to detect any kind of antenna attached to the SMA connector.
How might I go about doing this? Are there any other electrical properties of the antenna which could be exploited to detect its presence? (For example, would it be possible to somehow detect the capacitance of the antenna when it is connected or would this change in capacitance be insignificant?)