When getting S11 measurements, does it matter if the BGM121 is connected to power? Generally I would expect that it doesn't matter because S11 is just a measurement of reflection but because I don't know the internal mechanics of how the BGM121 works I am asking this question.
Yes, it can be recommended to utilize the so-called bazooka (or sleeve) balun for antenna impedance measurements, and it is especially important if the the board size (GND plane) is much smaller than the wavelength (lambda).
Sleeve balun suggestion:
- board size > lambda : not needed,
- lambda > board size > lambda/2 : practically not needed,
- lambda/2 > board size > lambda/4 : recommended,
- lambda/4 > board size : strongly recommended, crucial.
Some details on the bazooka balun are discussed in our antenna design guide (AN853):