Impedance of an antenna
I need to calculate input impedance of an antenna.


Different antennas have different impedances and the same antenna has widely different impedances depending how long it is relative to the radio frequency it emits or receives.

For instance, a quarter wave monopole has an impedance like this: -



So, when used as a classic quarter wave monopole, the impedance is purely resistive (about 37 ohms). When used at 0.47 wavelengths it is also purely resistive but nearer 2500 ohms.

Monopoles are often used "short" and typically at a length of 0.05 wavelengths it will look like a capacitor having an input impedance of about -j1000 ohms.

So, decide what antenna you are referring to, then decide what effective length the antenna is then try and find a graph like the above.