CURRENT AND VOLTAGE DISTRIBUTION
If the wire in the first illustration had been infinitely long the charge (voltage) and the current (an electric current is simply a charge in motion ) would both slowly decrease would result from dissipation of energy in the form of radio waves and in heating the wire because of its resistance. However , when the wire is short the charge is reflected when it reaches the far end, just as the ball bounced back from the barrier. With radio –frequency exitantion of a half –wave antenna, there is of course not just a single charge but a continuous supply of energy, varying in voltage according to a sine—wive cycle. We might consider this a series of charge, each of slightly different amplitude than the preseding one. When a charge is now haveling in the opposite direction. However, the next charge is just reaching the end of the antenna, so we have two current of practically the same amplitude flowing in opposite direction. The resultant current at the end of the antenna there fore is zero. As we move father back from the end of the antenna the magnetudes of the out going and returning current are no longer the same because the charge causing them have been supplied to the antenna at different part of the Rf cycle. There is cancellation, therefore,and a measurable current exist. The greatest different – that is, the largest resultant current--- Will be found to exist a quarter wavelength away from the end of antenna. as we move back still father from this point the current will decrease until, a half wavelength away from the antenna, it will reach zero again. Thus, in a half-- wave antenna the current is zero at the end and maximum at the center. This current distribution along a half-- wave wire is shown in Fig.1.
Fig.1.Current and voltage distribution on a half--wave wire . In this conventional representation the distance at any point ( x , for instance ) from the wire , represented be heavy line, to the curve gives the relative intensity of current or voltage at the point. The relative direction of current flow ( or polarity of voltage ) is indicated by drawing the curve either above or bellow the line that represent the antenna. The curve above, for example, show that the instantaneous polarity of the voltage in one half of the antenna is opposite to that in the other half .
this distance measured vertically from the antenna wire to the curve marked “ current “, at any point a long the wire represent the relative amplitude of the current as measured by an ammeter at the point . This is called a standing wave of current. The instantaneous value of current of at any point varies sinusoidally at the applied frequency, but is amplitude is different at every point along the wire as shown by the curve. The standing wave curve it self has the shape of a half sine wave , at least to a good approximation.
The voltage along the wire will behave differently ; it is obviously greatest at the end since at this point we have two practically equal charge adding. As we move back along the wire , however,the out going and returning charges are not equal are their sum is smaller. At the quarter-wave point the returning charge is of equal magnitude but of opposite sign to the out going charge, since at this time the polarity of the voltage wave from the source has reversed ( one-half cycle ). The two voltage therefore cancel each other end the resultant voltage is zero . Beyond the quarter- wave point, away from the end of the wire,the voltage again increases,but this time with the opposite polarity.
It will be observed, therefore, that the voltage is maximum at every point wire the current is minimum,and vise versa. The polarity of the current or voltage reverses every half wavelength along the wire , but the reverses do not occur, at the same point for both current and voltage ; the respective reversals do not occur, in fact, at point a quarter wave a part .A maximum point on standing wave is called a loop ( or antinode ) ;a minimum point is called a node.
