A DIPOLE ANTENNA SUSPENDED LOW OVER A METAL SCREEN

The strong dependence of the input impedance of a dipole on its height of suspension limits the development of directional antennas in the f orm of balanced dipoles suspended over a metal screen [1]. The design of an antenna with a height of suspension equal to several hundredths of the wavelength encounters serious technical difficulties and in man y cases is not possible in practice [usually, the dipole is suspended at a height in the range (0.2-0.3)lambda, where lambda is the waveleng th]. This phenomenon can be attributed to the ''mirror'' reflection of the dipole current in the metal screen which shows up as a tangential component of the electric field arising near the dipole in antiphase with the tangential electric field due to current flow through the dip ole. In this paper we show how one can reduce the effect of ''mirror'' reflection on the input impedance of an antenna by reducing the heigh t of suspension of the dipole over the metal screen.