Voltage on a wire (especially at the end)

Voltage on an antenna works in a similar manner to the current. When the voltage wave reaches the end of the antenna wire, it also is reflected. Unlike the current which totally reverses its direction resulting in zero total current, the voltages do not cancel but add together. A voltage wave consisting of say 50 volts will be reflected at the open end of a wire. The resultant voltage will be 50 plus 50 or 100 volts. Remember the current had to be zero by law. There is a law that says the sum of all currents, into and out of a point must be zero. This law is called Kirchhoff current law. That law does not apply to voltage. Kirchhof does have a voltage law that we, however must follow. More on that later. At this point think of the voltage wave similar to an ocean wave hitting a breakwater. Water flows in, abruptly hits the breakwater and is stopped. Piles up and bounces back away from the wall. If you have ever watched two water waves passing or crossing each other you have seen that when their peaks cross the wave rises up about twice as high as either wave alone. Voltage acts the same way. In fact so does current with the exception of the fact that when it is reflected it is reflected out of phase and there fore the two currents cancel. Voltage is not reflected out of phase from an open end and there fore the two voltages add together.
This leads us to another interesting thing about antennas. The voltage at the end will always be a maximum. The highest voltage that can exist on an antenna will be at its end. Do not ever touch the end of an antenna while it is being used for transmitting. I had a friend who had on a pair of gloves and when he touched the end of the antenna while it was transmitting on 160 meters he got bit right through the gloves.
At the center of the antenna (remember we are only talking about a half wave resonant dipole at this point. Once we understand that we will increase its length so that it is not resonant) the voltages will be out of phase and add to zero (well almost zero). The voltage at the center of a half wave resonant dipole will always be a minimum.
Lets review what we know for sure about a half wave resonant dipole. The current at both ends will always (by law) be zero. The voltage at the ends will always be a maximum. The current at the center will be a maximum, while the voltage will be a minimum. These currents and voltages are a result of the reflections (of both current and voltage) from the ends. There is a name for this kind of wave. It is a standing wave. You might say the voltage stands at a maximum at the end. The current stands at a maximum at the center. The current stands at a minimum at the end. The voltage stands at a minimum at the center. In reality these current and voltage waveforms do not actually stand still, they sort of “march in place”. At the end where the voltage is maximum, it really varies from a maximum positive voltage to a maximum negative voltage and back again. It does this at the same rate as the frequency of the radio signal. The current does the same, at the center it varies from a maximum positive value to a maximum negative value. At all points on the antenna the voltage or current will “oscillate” between a fixed maximum and minimum value. Think of what it looks like to watch some one jumping rope. The rope in fixed at both ends but the center goes up to a maximum height over the persons head then down to a lowest point almost on the ground and repeats over and over until the person jumping the rope stops transmitting…I mean stops jumping. This may help visualize how the current varies in a dipole. Visualizing the voltage is not quite so easy but bare with me. Think of someone holding two long broom handles. Arms out stretched. The person raises the right arm pointing that broom handle up. With the left arm he (or she) points the broom stick down and touches the ground with its tip end. Then he lowers the right one while simultaneously raising the left one and repeats this exercise several million times a second. What you have is the end points of the broom sticks are “oscillating”up and down between a maximum height of probably 8 volts…..I mean 8 feet and the ground or zero feet. The left side is up when the right side is down. Half way out on the broom sticks that point is “oscillating” up and down between a height if maybe 3 and 5 volts….I mean 3 and 5 feet. This is a good representation of the way the voltage standing wave varies on a half wave resonant dipole.
More tomorrow.