Is there any effect if I change dielectric constant value in Radar level transmitter? I have the doubt that dielectric constant is only effected when we are using it for interface level measurements.
It is the change in dielectric constant that produces the signal change in radar devices. The simple app, shooting water, is the change in the dielectric of air (low) versus the dielectric of water (high).
What your particular device does with a setting for dielectric constant is an unknown, but the whole basis or radar measurement is the change in dielectric constant.
Some radar transmitters have a general media dielectric parameter that affects the transmitter's sensitivity (e.g., lower dielectric results in higher sensitivity so that the transmitter can detect smaller signals).
There may also be a separate parameter for the dielectric of the upper media when configured for interface measurement. Such a parameter could mainly affect the calculated position of the interface boundary (different dielectric materials displace the signal from the interface by different amounts).