The effects of the physical parameters on the bandwidth of a folded dipole

An expression is derived which yields the feedpoint admittance of a folded unipole or dipole at any desired frequency. This expression shows that the input admittance is composed of two parts-a radiation part and a transmission line part. The radiation part is analyzed as a vertical radiator while the transmission line part is analyzed by transmission line theory. The terms of this expression allow the input admittance of the antenna to be studied in terms of the physical parameters. By plotting the feedpoint admittance on a Smith chart it is possible to observe at what frequency the currents in the feeding transmission line exceed some desired standing wave ratio and thus obtain a measure of the bandwidth of the antenna. The findings show that the broad-band characteristics of the folded diple are obtained primarily through the increase in equivalent cross section caused by the folding of the two elements. The bandwidth is further aided by the reactive component of the transmission line part being opposite to the reactive component of the radiation part, even though the reactive component of the transmission line part is considerably smaller than desired. Experimental and calculated results show that for a given size conductor the bandwidth is improved as the spacing between conductors is increased. The spacing should be as wide as possible but still provide the desired field pattern and input admittance. The bandwidth of a folded dipole is improved if the conductor size is increased regardless of the spacing between elements. Maximum bandwidth is achieved by using large conductors with wide spacing.