not sure precisely what you are measuring, but in general the phase will increase as the frequency increases, if for no other reason than the transmission line has more phase shift at higher frequencies.
But if the antenna is matched over only a small frequency band, or if there are other mismatches in the system, you can have standing waves set up on the tramission lines, which correspond to significant phase ripple.
also, of course, you could have wireless multipath that would make the transmitted signal phase look non-linear with frequency.