Voltage and current in non-resonant dipole

If the antenna is a resonant half wavelength the currents and voltages are easy to figure. The voltage is normally zero every half wave. The current is also zero at every half-wave point. So the current is quite happy ☺ if its natural zeros are a half wave apart as at the two open ends of a wire. Likewise the voltage has peaks every half wavelength and is also quite happy with those peaks occurring naturally at the ends of the dipole. Under these ideal conditions all is well in the world. To top it all off the ratio of voltage to current at the center (that’s where you normally connect coax) is low. Remember that the current is as large as it will ever be at the center and the voltage is at its low point. We said zero but anytime we say zero it may actually be really small. So what is the ratio of voltage to current? Hint: V/I Another hint E/I. Remember Ohms law? R=E/I Resistance is voltage divided by current. Resistance is impedance to current flow. Resistance impedes current. Resistance is also Impedance. So if the ratio of voltage to current is low, the impedance is low. Coax cable likes to be connected to low impedances. Remember anytime anyone mentions impedance you need to simply think voltage to current ratio. High voltage and low current means high impedance. Low voltage and high current means low impedance. Impedance is not really as complicated as you might think. It is just the relationship of voltage to current.
Now what happens if the wire dipole is increased in length so that it is longer than a half wavelength. Well the current will enter the wire at the center and move toward the end where the first time it gets there it will be reflected, equal and out of phase (adds to zero at the end point). It then races toward the other end at almost the speed of light where it hits the other end. This time when it reaches the center of the antenna it is lagging behind the current from the next cycle that entered the center just before this reflected wave reached the center. In the resonant case we talked about these two currents would have been in phase at the center and simply added together. The two maximum values would have occurred at the same time. So where each current may have been 2 amps there would have been 4 at the center on the resonant antenna. Now in the non resonant case, when the reflected 2 amps passes the center the incoming current from the coax would have been 2 an instant before, but because of the increased travel time of the reflected wave, it arrives at the center a bit late and if the incoming current has decreased to 1 amp. There will only be 3 amps at the center due to the addition of these two waves. Because the first reflected wave is late it disrupts the timing of everything. It gets still worse because the next incoming wave must travel longer than in the resonant case before it is reflected and each set of reflections is late arriving back at the center and is again out of phase with the incoming current. After a few cycles of this everything settles into a grove so to speak. The end result is that the resulting standing waves of voltage and current do not have the same relationship in the non-resonant case as in the resonant case. In the resonant case the voltage to current ratio was low, maybe even 50 ohms. It would even act like it was a pure 50 ohm resistance connected to the coax. In the non-resonant (long) case, the phase relationship between the voltage and current would be shifted somewhat like it is in an inductor. The current would seem to be behind where it should be or at least where it was in the resonant case. That is why we say the long antenna is inductive or its impedance is inductive. Its voltage to current ratio is its impedance as always but the points where the voltage maximum and current maximum occur are shifted such that the current peaks later than it would have in a resonant antenna. In addition to this inductive effect, the current is lower at the center (remember the current peaks of the incoming and reflected wave are not both occurring at the center at the same time) so the voltage divided by the smaller current at the feed point gives a larger number for the impedance (voltage divided by current).
Its like (not exactly) 200 volts divided 4 is 50 ohms in the resonant case, while 200 volts divided by 3 is 66 ohms. Do not take this example to literal. The resulting voltages and currents in the non-resonant case are not easy to calculate and both voltage and current will change. They also change in different directions depending on the antenna being long or short.