If I understood it correctly, the 50ohm only appear at high frequencies as impedance. If I measure the resistance normally with a Mulitmeter the Mulitmeter shows 0 ohm. Since the resistance of 50ohm only occurs at high frequencies, I can't measure it with a Mulitmeter, can I? How do I measure the impedance and at which frequency?

regards, Jürgen

The 50ohm (50+j0) appears at all frequencies, it is frequency independent.

Your multimeter shall show 50ohm if the impedance is resistive (thus your termination will show 50ohm when measuring the mid pin against the gnd).

Generally an "impedance" is a "complex value" R+jX or R-jX, based on the sign it could be capacitive or inductive (X is the reactance).

When there is the reactance X the impedance is frequency dependent.

For example your antenna impedance could be 180-j200 at 90MHz, or 340+j12 at 105MHz, etc.

Your 50ohm BNC termination is theoretically 50+j0 (pure resistive, not dependent on frequency), in reality there is a capacitive or inductive portion of reactance based on parasitic capacitancies and/or inductancies. You may see the influence of the parasitic reactancies usually at higher frequencies, based on the manufacturing quality of the termination.

Measuring impedance is not an easy exercise for a beginner, unless you have got the equipment and you know well what you are doing.

PS:

https://en.wikipedia.org/wiki/Electrical_impedance