Thank you both Dave and Bojan! I found a good article explaining this idea (https://www.rfcafe.com/references/electrical/vswr-reduction.htm). Is my understanding correct that an attenuator helps lowering VSWR because the reflected power goes through the attenuator again so it's improved? Since the 50W PA already has a 3dB attenuator, adding another 3dB basically means my 705 has to output at about 10W to get 50W. This is not exactly I'm looking for to improve the input impedance match. I think the Class-C circuit in 50W PA has about 13dB gain, so ideally I want my 705 to output only at 2.5W to get 50W yet with a good SWR. Seems what I'm trying to achieve is to replace the 3dB attenuator in 50W PA with an impedance match network. I guess I can try removing the attenuator and add an ATU there. But if there's a way to analyze/measure the input impedance directly, that would be more helpful.