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Increasing Bandwidth of Vertical RadiatorsAuthor: Edmund A. Laport
The selfimpedance of a radiator of these dimensions at 218 kilocycles is of the order of 11 ohms resistance and 80 ohms reactance. Ground resistance and other loss resistances must be added to this. The antenna current will be equally divided, by symmetry, among the eight vertical wires, and a residual portion of the total current will flow in the steel central supporting mast. The exact proportion of the total antenna current flowing in the mast itself can be computed by means of logarithmicpotential theory, but we shall assume for the present that it is the same as that in one of the vertical wires. The system therefore is equivalent to a ninewire antenna with equal current division. We may choose to use a double tuning system, by including anywhere from one to eight wires in the fed portion, the remainder being tuned directly to ground. There is therefore a range of input impedances available for feed purposes, as shown in Table 1.4.
In the resistances given in the table the ground and other loss components have been omitted for simplicity. It is seen immediately that a wide range of input resistances is available according to the number of wires (with the supporting mast counted as a wire) included in the fed portion of the system, and with the remaining wires multipletuned in such a way as to maintain equal currents in all wires. It is interesting that, with two wires in the fed portion, the resistance is of a value that would permit direct matching of convenient types of openwire feeders.
With four fed wires, the value is suitable for direct matching with coaxial feeders. The antennatuning gear consists of two inductors only, and the full bandwidth capabilities of the radiator are utilized by avoiding the use of more complicated networks having additional energy storage. Figure 1.11 illustrates this arrangement when twowire feed is used to match an openwire feeder.


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