What is your antenna type and frequency?
Copper piece length: from quarterwave down to ~1mm (by cutting)
width: may be the same as 50ohm line
You may move it along the line,solder,cut,move again. Each half wavelength along main line you probably obtain similar results, as impedance repeats. I've done some tuning this way, but it is not convenient. Simulation + manufacturing prototypes with stepped sizes based on simulated layout was much more productive.
Also you may try to change resistor in power divider, resistance +-20 Ohm of existing one.