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Use this tag for questions relating to dipole antennas.

Basic Dipole Theory

The dipole is one of the simplest and most widely used types of antenna. A dipole is a very basic antenna structure consisting of two straight, symmetrical, collinear wires. The driving current from the transmitter is applied between the two halves of the antenna where each side of the feedline is connected to one of the two wires.

How can we explain the fact that you can drive current into a dipole when the ends are open circuited? The simplest way is to consider the parasitic capacitance between the two arms of the antenna as the return current path. At high frequency this capacitance will represent a low impedance. Current through this uncontrolled parasitic capacitance represents radiation.

Therefore, a dipole antenna requires two parts in order to radiate and the amount of radiation will be proportional to the dipole current. Note also that, a dipole does not require a “ground” to work.

Dipoles are resonant antennas, meaning that the elements serve as resonators, with standing waves of radio current flowing back and forth between their ends. The length of the dipole elements is determined by the wavelength of the radio waves used. The most common form is the half-wave dipole, in which each of the two elements is approximately λ/4 long, so the whole antenna is λ/2 long.

Without diverting into serious math, the length of a λ/2 dipole in meters = 143/f(MHz). In practice, the actual length will be somewhat shorter due to ohmic losses in the wire itself and the connection to the feedline.

A common practice is to construct the dipole using the above formula, hang it temporarily and measure the resonant frequency. Then determine the difference between the resonant frequency and the desired frequency, calculate the difference and trim half that amount off both ends. The result should be resonant at the desired frequency.

The maximum theoretical gain of a λ/2-dipole is 2.15 dBi. In practice, it's going to be somewhat less due to ohmic losses in the wire itself and the connection to the feedline.

Ideally, a λ/2 dipole should be fed with a balanced line matching the theoretical 73Ω impedance of the antenna. In practice, that's unlikely. Good results can be obtained by directly feeding the antenna using 50Ω coax. This will give a 1.5:1. It's common to place a choke or current balun at the feedpoint to transform the unbalanced coax feed into a balanced line feed suitable for feeding a dipole antenna.

This won't change the 1.5:1 impedance match, but it will be more efficient and reduce the chance that your feedline becomes part of the radiator.