Abstract :
This investigation was carried out for the purpose of finding whether the assumption usually made regarding the distribution of current in an antenna was a sufficiently close approximation to the conditions that obtain in practice. The current distribution of a straight, vertical antenna was found experimentally by placing ammeters at various points along the antenna and reading them by means of a telescope. In order that the effect of the ammeters might be calculated, the antenna was constructed in the form of a cage with the ammeters inside. Thus the capacity effects of the ammeters were eliminated. The self-inductance of the ammeters was measured and a correction for it was applied to the theoretical curves. The comparison of the experimental and theoretical curves, calculated on the usual simple theory that the current distribution is sinusoidal, showed remarkably good agreement over the whole range of wave-lengths used (15 m to 800 m). The highest frequency was nearly three times the natural frequency of the antenna. According to the simple theory, the natural wave-length of a simple antenna is 4 times its length. More rigorous mathematics led Macdonald. to put the figure in some cases as high as 5 times its length, and this figure is often quoted in textbooks. This was reduced to 4.2 times by Abraham and Föppl. The experimental results of this paper agree with the latter figure. The effect of a change in diameter of the wire at a point in an antenna was investigated and found to agree with theory. Various shapes of antenn¿ were also tested, including the common inverted ¿L¿ and ¿T¿ antenna¿. The effect on the current distribution of a conductor placed near the antenna was also examined with the conductor insulated, earthed and tuned, and was found to be negligible. A theoretical analysis, which is unfortunately very laborious to apply, agreed with the experimental results obtained.
