So they are good for making a antenna into a multi band antenna eh?
Not always. It depends on the antenna and how it is constructed. What I pointed out are generalities; there is more to it than that. But that is a starting point in simplest of terms.
An Imax 2000 or an Antron A99 have a built-in capacitor and coil that allows them to work fairly close to five Ham bands (6, 10, 12, 15, and 17 meters) and the CB band as well. But using a tuner to make up for the slight impedance difference allows those antennas to be used by Hams (I use an Imax 2000 with a tuner). Not all antennas can do that. A 1/2 wave CB aluminum ground plane might be able to do 10 and 11 meters WITH a tuner ('CB'); but not the others bands mentioned as the the A99/Imax 2000 can do. Construction differences dictate limitations as well as strengths too.
Hams use the term '10 meters' or '6 meters' or '40 meters' - because the length of the wave to those frequencies are 10 meters = ~33.2 ft and 6 meters = ~20 ft and 40 meters = ~132 ft.
As the frequency used increases; then the length of the antenna decreases.
As the frequency used decreases; then the length of the antenna increases.
Some Hams operate on 160 meters; how long is a dipole for that frequency if a dipole is 1/2 the length of that frequency?
Ans: 80 meters - or ~265 ft.
Each half if the dipole is ~132.5 ft; because a dipole uses two 1/4 waves to make a 1/2 wave.
The resonant frequency is ~1.8mhz.
It all gets easier when you get used to it.