Currents, Charges, and Near Fields of Cylindrical Antennas and the Electric Field Very Near a Driven Cylindrical Antenna

摘要

The commonly made assumption that the distribution of current along a dipole antenna is sinusoidal is examined critically together with the associated distribution of charge and the electromagnetic field. Measured distributions of current and charge are compared with those assumed in the sinusoidal theory. It is shown that the distributions of charge differ so greatly that it must be concluded that the electric field near and along the surface of the actual antenna is not well approximated by the near field obtained with the sinusoidal assumption. A new approximate solution of the integral equation for the current in a center-driven cylindrical antenna is obtained in terms of simple trigonometric functions. It is shown that this and the associated distribution of charge are in much better agreement with measurements than results obtained from the sinusoidal assumption. Approximate expressions are then derived for the electromagnetic field near the antenna in a form that satisfies the boundary conditions. A first-order formula is derived for the component E sub z(rho,z) of the electric field very near to the surface and parallel to the axis of a perfectly conducting center-driven cylindrical antenna with radius a and length 2h. It has the form E sub z(rho,z) = E sub z(z) ln (rho/a) where rho is the radial distance from the axis of the antenna and Ez(z) is a complicated expression that involves generalized sine and cosine integral functions.