antenna
I'm looking at a schematic which includes an SMA antenna connector. I'm wondering why this connector needs grounding and whether a simple copper wire without grounding can be used instead
Would I be able to use an internal antenna like this one, with no grounding?
First, I would look up the datasheet of the RF chip to make sure that the output impedance is 50 Ohms (it doesn't have to be).
Then, peel the coax of your antenna in such a way that it looks like a 3 toothed fork with the inner conductor being in the middle. This is really important for best performance (otherwise your antenna may not work well due to poor balancing).
Using a sharp knife, razor blade (etc) remove the mask on the a small section of the trace leading up to the antenna so that bare copper is visible.
Cut that trance after to pad to isolate your old antenna.
Expose copper on both sides of the center trace (your ground).
Solder the center conductor of your coax to the former antenna trace pad and solder the coax outer shield grounds to the pads.
Keep these unwound wires as short as possible.
Keep in mind, that you will still lose a couple of dB in the transition and cabling.
The main consideration here is the frequency of operation - if the unused XBee and the one with the U.FL connection operate at the same frequency, you'll be fine. It won't make an appreciable difference whether you attach a U.FL antenna or if you solder the other wire antenna on.
See here for a similar answer.
Best Answer
Every antenna feed point requires two nets. If it's an unbalanced feed, the second net is the ground return.
By connecting the ground to the SMA connector, the board designer gives you the flexibility to use different antenna implementations. You may have a cable and then a remote antenna, or you may have an antenna right at the connector.
If you have a monopole antenna, you need to have a reference ground, which could be implemented using the board's ground plane. If you're simply going to connect a monopole antenna right at the SMA connector (no cable), then you may be right and the ground connection on the SMA side may not be strictly necessary, since the board ground will be part of the antenna. However, I see no penalty in connecting the SMA ground even in this case. You may actually be saving yourself from some trouble: the antenna you're going to use may have been designed taking the length on the SMA ground into consideration, or there may be matching components connected to the ground inside the antenna.