Dipole Antenna - Short Dipole

Short Dipole

A short dipole is a physically feasible dipole formed by two conductors with a total length L very small compared with the wavelength λ. The two conducting wires are fed at the centre of the dipole. We assume the hypothesis that the current is maximal at the centre (where the dipole is fed) and that it decreases linearly to be zero at the ends of the wires. Note that the direction of the current is the same in both the dipole branches: to the right in both or to the left in both. The far field E_θ of the electromagnetic wave radiated by this dipole is

Emission is maximal in the plane perpendicular to the dipole and zero in the direction of wires which is the direction of the current. The emission diagram is circular section torus shaped (right image) with zero inner diameter. In the left image the doublet is vertical in the torus centre.

Knowing this electric field, we can compute the total emitted power and then compute the resistive part of the series impedance of this dipole due to the radiated field, known as the radiation resistance:

where is the impedance of free space. Using a common approximation of ohms, we get