You can make a make a simple dipole antenna into a multi-band antenna by adding an additional set of elements for the band you want to operate. A couple of years ago, I added 30m elements to my 40m dipole and now it works on both 40m and 30m. The reason this works is that when operating 40m, the 30m elements present a relatively high impedance, while the 40m elements a relatively low impedance. RF current, like any kind of electrical current will take the path of least resistance.
As shown below, the 30m elements hang down below the 40m elements. If you space the elements close to one another, you may have to tweak the lengths of the elements for the best SWR. In my case, that wasn’t a problem.
If two bands is good, why not four or five? Yes, you can do that. You can add as many bands as you have space and wire for.
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