On the Wireless Beam of Short Electric Waves. (IV)

Abstract

The action of a straight metallic rod of finite length vertically erected in the radiation field of a sending antenna is studied theoretically and tested by experiments. The results are as follows:-
When the natural frequency of this metal rod is equal to the wave frequency, the current induced in it will be in phase with the E.M.F. caused by the electric wave, and, as is well known, the wave is reflected by this rod. Even if the natural frequency is made somewhat lower than the resonance frequency by increasing the length of rod, the current induced in it will be lagging behind the E.M.F., and the reflecting action will still be manifest.
On the other hand, if the natural frequency is made higher, to a certain extent, than the resonance frequency by shortening the length of rod, the current induced in it will be leading before the E.M.F., and when the phase angle the magnitude of the current obtain suitable values, the wave is further transfered forwards beyond the rod. The rod is now acting as a means of converging the wave energy and transmitting it further with more concentration and is therefore now called a wave director.