Current on a wire obeys the law

I hope the last post gave a good idea of why an antenna can act like a capacitor when short and like an inductor (coil) when long. It is all about the voltage waveform and the current waveforms either being in phase (increasing to a maximum, decreasing to zero and to a minimum all at the same time) or being somewhat out of phase (having one or the other reach a maximum, zero or minimum before the other).
There are various reasons for this to happen. I think I explained why it happens in a capacitor or inductor. You may wonder why it happens in an antenna or wire when that wire may not look very much like a capacitor or coil. (Note I will use the terms inductor and coil interchangeably. A coil is an inductor and an inductor is usually a coil.)
There are certain things that just happen in this world. You just need to recognize it and believe it. We call these things natural laws. You throw a ball up in the air and it will come down due to the Law of Gravity. There are also electrical laws just like that. They all have names but I will not try and name them just talk about them as the occasion arises.
The total current at the end of a wire is zero. The current at the end of any antenna wire (as long as that end is not connected to a feed line or some other conductor) is zero. That is a law. Anytime I see the end of a wire, I know one thing for sure. The current at the end is zero. The reason is simple. Suppose current is flowing down the wire toward the end. (It does that in an antenna you know) Lets suppose that you are looking at this wire and it is running from left to right in front of you. Lets send 5 amps of current down the wire from left to right. The end on the right is not connected to anything. (Like the end of a dipole antenna) When the 5 amps of current gets to the end of the wire what happens? Does it just stop and remain at 5 amps? No. That cannot happen or it would violate one of our electrical laws. (Maybe you know the name of the law, if not maybe you can look it up) Remember at this time we have 5 amps flowing from left to right. At the open end of the wire, those 5 amps is reflected or bounced back to the left. Now at this instant we have current of 5 amps flowing left to right and 5 amps flowing from right to left. We have equal and opposite currents existing right at the end of the antenna. Any time you have equal and opposite anything they tend to cancel. The total current at the end of the wire is +5 amps -5 amps = zero amps. This will be the case for all open-ended conductors. Now what happens to that current that is reflected back from the end of the wire? It will travel all the way to the other end of the wire or until it reaches some major change in the wire like a feed line another conductor attached or some other physical thing that will disrupt it for some reason. In the case of a center fed dipole antenna, it will go all the way to the other end and be reflected again. Every time it is reflected the total current will add to zero. So at both ends of any dipole of any length, the current will be zero. Visualize a center fed dipole (you have to visualize because I can not draw on a blackboard like I would like to here).
Current enters the dipole at the center and travels out to the ends where it is reflected and the value of current is always zero at the ends. If the dipole just happens to be a half wavelength long, several neat things happen. We will have zero current at the ends because the currents are always equal and opposite. When the currents are opposite we call that “out of phase” specifically we call it 180 degrees out of phase. You cannot get any more out of phase than 180 degrees.
At the center of the antenna the currents will always be totally in phase. We have what is termed a current maximum right at the center. So if you were to plot values of current all along the antenna you would start with zero current at say the left end, gradually build up to a maximum in the center and down again to zero at the right end. Can you visualize that? Sure. You could actually measure this effect by taking the temperature of the wire when it is being used for a transmitting antenna. The center of the antenna would be warmer than the ends. Current flowing through a wire of conductor generates heat. For example if the resistance was 1 ohm and 5 amps flowed, the heat generated would be 25 watts. The heat generated at the end of a wire will always be zero. So the wire would be much cooler at the end than it would be at the center. I have exaggerated this a bit. The resistance of a wire antenna is extremely small and the amount of current flowing from a high-powered amateur transmitter may be only 5 or 10 amps. So you probably could never detect it. You can however run your hand on a piece of coax used for transmitting and feel the warm and cool spots. A warm spot will be felt where there is a current maximum. These warm spots will occur every half wavelength along the line.
I found this out the hard way many years ago. I was running high power (about 300 watts) on AM and actually melted a piece of coax at one of these “warm”spots!

Next I will talk about how the voltage acts on a dipole. It is a little different than the current, however it always obeys the laws also.