A A cage dipole is basically an attempt to achieve a broader SWR bandwidth by using a thicker radiator. Building a dipole out of, say, a large-diameter aluminum tube isn’t very practical, but you can create almost the same thing, electrically speaking, by using a number of individual wires properly spaced to create a antenna that looks like a round birdcage (Figure 1). That’s the principle behind the cage dipole. A typical HF cage dipole can exhibit a 2:1 SWR frequency range almost 2 times broader than a single-wire dipole (Figure 2). There are other means of creating an electrically “thick” antenna. The bow tie, for example, makes use of the same principle.
Figure 1—A cage dipole resembles a round birdcage. Circular spreaders can be used to separate the individual wires. The spacing between wires should be 0.02-wavelength or less.
Figure 2—The theoretical SWR versus frequency response on 75 meters for a cage dipole that’s 122 feet 6 inches long with a spreader diameter of 6 inches, fed with 50-Ω coax.
From QST October 1999
