How do you match an LDMOS FET to 50 Ohms?
The manufacturer should provide linear models (S-parameters) in the form of something that can be consumed by an RF/Microwave CAD package which usually covers the designed-for range of operation with a fairly large number of points.  Look for &quot;touchstone&quot; or &quot;s2p&quot; (two-port network).
The S-params file for AFT05MP075N is available from NXP here: https://www.nxp.com/part/AFT05MP075N#/ under software and tools.  The range extends down to a point at 200 MHz (measured).
Given a known system impedance, one can convert S-parameter values into impedances (Z).
The CAD tool then usually interpolates to get values for the precise value you require for calculation.  The tool can usually synthesise the matching network too, although, it would be much more valuable to understand the theory and check the result with the tool.