A key concept for antennas is the concept of resonance. Here is one simple way of thinking about resonance in antennas (or other types of circuits) using an analogy with which you are sure to have experience: A swing.
Think of pushing someone on a swing, where you can time your pushes so that the swing continues to go back and forth with only a slight reinforcing shove each back-and-forth cycle. The oscillating period of the swing and your timed pushes are in resonance with one another, and you can keep the swing going very efficiently with very little effort – just a tiny push each back-and-forth cycle, timed just right. Your little pushes are an applied frequency that is perfectly timed to the swing’s period of oscillation.
Think of what happens when you try to push the swing out of resonance with its natural back-and-forth cycle. Rather than reinforce the swing’s motion you may disrupt it altogether, or you may need to exert a lot more force to make it swing in agreement with your less-than-coordinated timing of pushes. Let’s extend this concept to feeding antennas.
The oscillating period of the swing is determined by the length of the rope or chain by which it hangs – a long rope produces a long period back-and-forth, while a short rope results in quick back-and-forth. With an antenna, the period of oscillation for electric charges running back-and-forth is determined largely by the antenna’s physical length. With a longer antenna the charges need a longer time to travel the full extent of the antenna’s length, just as a longer swing requires a longer time to heave to and fro. With a shorter antenna the end-to-end travel time for electrical charges is reduced, just as a shortened swing requires a more brief time for each back-and-forth oscillation.
The timing of the pushes that the antenna gets is determined by the radio frequency fed into it by the transmitter. The transmitted frequency is the applied frequency, just as your equally timed shoves are applied to the swing. If the applied frequency from the transmitter is well matched to the length of the antenna, giving a properly timed back-and-force voltage “shove” each cycle, the electric charges in the antenna will have just the right amount of time to travel the length of the antenna and back as the applied voltage pushes in one direction and then reverses. The antenna is resonant with the applied frequency, and the result is an optimization of charge surging back and forth with very efficient reinforcement from the transmitter.
If the applied frequency from the transmitter is much different from the natural oscillating frequency required to reinforce charges moving end-to-end in the antenna, efficiency of energy transfer suffers. This is analogous to trying to force the swing to oscillate at a frequency other than its natural resonance determined by its length.
So an antenna should be trimmed to the proper resonant length for the frequency it is intended to radiate in order for it to radiate efficiently and not waste your transmitter’s energy!
Lower frequency bands where wavelengths are long will need a longer antenna. High frequencies with short wavelengths resonate with shorter antennas. You can change the resonant frequency of an antenna by lengthening or shortening its conductive element.
Note that for any given antenna there is a single resonant frequency, but an antenna will typically operate quite well over a range of frequencies (bandwidth) near the resonant frequency. The common scenario is to have a single antenna that operates well over a significant portion of, or the entirety of, an Amateur Radio Service allocated band.
-- Stu WØSTU