J.S. Snpw wrote:
"---I unplugged the antenna cable, the signal didn`t change."

You need to measure carrier level to determine FM signal strength. Once
the "FM improvement threshold" signal is reached, very little increase
in audio level results from increases in RF. It`s the magic which makes
RF repeaters work well. All signals, weak and strong, are nearly as
strong in audio levels.

Next. If you build kits, you probably have a way to test continuity.
Check your antenna circuit for opens and shorts in all the right and
wrong places.

Best regards, Richard Harrison, KB5WZI