Why an antenna is inductive or capacitive

Now we know how to calculate wavelength and half wavelength on free space using the formulas:
wavelength in meters = 300/MHz or
wavelength in feet = 982/MHz
It is of course more useful to get a half wavelength in feet from:
Half wavelength in feet = 491/MHz
As an example if we want to know what a half wave is for the middle of the ten meter band we would find it by dividing 491 by 28.5 and the answer is 17.22 feet.

This is the free space half wavelength in feet. It is based on the speed of the wave in space. If we are going to use a wire or some other conductor to carry this signal, then the speed will be less by several percent. Lets assume that we pick a wire with a certain type of insulation, such that the speed is reduced to 95% of the speed in free space. We then need to multiply the 17.22 feet by .95. We get 16.36 feet.
This tells us that a half wavelength for our radio signal is only 16.36 feet. If we were going to make a half wave long antenna it would be about 16.36 feet. (I say about because there are other usually minor factors that could affect the length)
If our wire were only 14 feet instead of 16.36 feet at our chosen frequency of 28.5 MHz it would be “short”. Remember that a “short” antenna acts like a capacitor. If the wire was 20 feet it would be “long” and act like an inductor or coil.
At this point you may be asking the question “what does it mean to be acting like a capacitor or acting like an inductor?”
The short answer is that the relationship between voltage and current is different.
In a pure resistance the current is proportional to the voltage. No voltage, no current. Maximum voltage, maximum current. Minimum voltage, minimum current. The current is said to be in phase with the voltage.
In a capacitor, when we first apply voltage the capacitor acts like a short circuit. Maximum current flows but the voltage is low. Then the capacitor charges up and the current flow stops. The voltage then is at a maximum but the capacitor is fully charged and the current is at a minimum or zero. We say that the current and voltage are out of phase. If we look at graphs of the voltage and current (voltage across the capacitor and current into or out of the capacitor) we will see that the current wave peaks before the voltage wave. The peaks are about ¼ wavelength apart. We like to talk about this type of wavelength lengths by using the term degrees. We take a wavelength and divide it into 360 equal parts. We say a wavelength is 360 degrees. So a quarter wave is 90 degrees. If the current is ¼ wave ahead of the voltage it is 90 degrees ahead. Another way to say the same thing is that the current leads the voltage by 90 degrees in a capacitor.
If we have a short antenna we find that the current peaks before the voltage peaks.
In a resonant antenna we find that the current and voltage are in phase.
In the case of the long antenna, it acts like an inductor. In an a inductor, when you apply the voltage, the current starts slowly then builds up. The voltage peak comes before the current peak. The common way of expressing this situation is to say that the current lags the voltage in an inductor.
If we have a long antenna, the current will peak after the voltage.
What does all this mean? Well If I have an antenna that is long or inductive I can sometimes use just a capacitor to tune it to resonance. If the antenna is short I may be able to only use a coil or inductor to tune it.
A practical example is the mobile whip antenna. Other than on ten meters the HF mobile antenna will be short. A short antenna is capacitive. A coil placed at the base of a mobile whip is frequently all that is needed to tune that antenna. Depending on the frequency, the capacitance of the antenna will vary and a coil will have to be chosen to suit.