Antenna Design
Antenna Design Software

# Resonant Frequency, Bandwidth and End-Effect of a Wire-Cage Dipole Antenna

Author: R.J.Edwards G4FGQ © 14 October 2002

Each half of a cage dipole consists of a cylindrical, rectangular or triangular cage of parallel wires, the wires being connected together at their ends. The purpose of the cage is to increase the effective diameter of the antenna radiating elements. This causes an increase in capacitance and a decrease in inductance per metre of length. The characteristic impedance Zo = Sqrt(L/C) of the antenna and the equivalent L & C series tuned circuit therefore decreases. The inductive reactance is reduced and so is the Q.

As with other tuned circuits, Q = Inductive-Reactance/Resistance. In this case the resistance is the equivalent radiation resistance which is assumed to be uniformly distributed along the antenna conductor. It is equal to exactly twice the feed-point resistance.

In the analysis program you can download below, the dipole is specified by its length, cage diameter, number of wires in the cage, and wire diameter. If there are only two wires in the cage then they are spaced apart by the diameter of the imaginary cylinder and can be used to investigate the frequency and bandwidth of a folded dipole. When only one wire is present it can be given any diameter to represent rods and tubes.

Note: The computed feed-point resistance applies only to exactly resonant dipoles.

The transmit bandwidth is defined as the band between the two frequencies at which the SWR on the feed-line has risen to stated values, assuming the SWR at the band centre has been previously adjusted by some means to be 1:1.

The receiving bandwidth is defined as the band between the two frequencies at which receiver input power has fallen to 1/2 the level at the band centre. The also is described as the 3dB bandwidth. For an impedance-matched receiver, the 3dB bandwidth is 2*Fc/Q where Fc is the centre frequency and Q is the intrinsic Q of the antenna.

In practice the defined band-edges are not exactly asymmetrically disposed about the resonant frequency and the transmission line impedance is rarely such that SWR equals exactly 1:1 at the resonant frequency. Cones alternatively can terminate cage ends, but the program neglects such minor geometric details. The height above ground affects the antenna input resistance. The feed-line length and antenna tuner will affect the system bandwidth. Therefore high calculating accuracy is not needed. However, the program does show how insensitive bandwidth and resonant frequency are to relatively large changes in antenna diameter.

At HF, the dependence of Q and bandwidth on antenna diameter is sometimes exaggerated, but for TV and other wideband transmissions, shape and diameter are important antenna design features.