The input impedance of the antenna is the ratio of the signal voltage induced at both ends of the feeding point to the signal current at the connection end between the antenna and the feeding line. The input impedance has both resistance and reactance components. The reactance component of the input impedance will reduce the effective signal power entering the feed line from the antenna. Therefore, it is necessary to make the reactance component as zero as possible and make the input impedance of the antenna pure resistance.

　　The input impedance is related to the structure and working wavelength of the antenna. The basic half wave oscillator, which is a half wavelength wire fed symmetrically in the middle, has an input impedance of (73.1+j42.5) ohms. When the length of the oscillator is shortened by 3% to 5%, the reactance component can be eliminated, making the input impedance of the antenna pure resistance, even if the input impedance of the half wave oscillator is 73.1 ohms (nominal 75 ohms).

　　A half wave oscillator with a total length of about one wavelength and bent into a U-shaped tube shape and symmetrically fed in the middle can be seen as a parallel connection of two basic half wave oscillators, with an input impedance four times that of the basic half wave oscillator, which is 292 ohms (nominal 300 ohms).

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