Near resonance, the resistance and reactance terms of an antenna vary much in the manner shown in Figure 11. If the antenna Q is low enough, the radiator is termed a broadband antenna. The curves in Figure 12 indicate the theoretical feed- point resistanee of a dipole antenna for various heights above a perfect ground plane. In free space, the feed- point resistance of a thin dipole is approximately 73 ohms. Feedpoint Resistance of Dipole Suspended Above a Perfect Cround Figure 10.
Reactance varies rapidly for center-fed antenna one wavelength long.
Reac'ance is zero when the antenna is slightly less than wavelength long. In free space the feedpoint resistance of a half-wave dipole is about 73 ohms. The modifying effeds of the ground change this, as shown above, with the value approaching 73 ohms as the dipole is far removed from the ground. The ground has less effect on the feedpoint impedance of a vertical antenna. Because of the manner in which current ffows in an antenna, radiation from practical antennas is not uni- form, but is directive to a certain degree.