Why The G5RV, designed by Louis Varney (callsign G5RV), is a 102-foot center-fed wire with a specific length of open-wire or ladder line (about 31 feet of 450-ohm window line, or roughly 34 feet of true open-wire) acting as a matching/transformer section. That stub transforms the feedpoint impedance so the antenna presents a workable load on several bands, and it is then joined to coax, usually through a balun, for the run to the shack. On 20 meters the flat top is three half-waves in phase, which is where the original design length came from.
Watch out The trap antenna answer describes a different multiband scheme, where LC traps electrically isolate sections of the wire; the shorted-coax wideband dipole fed with a 4:1 balun is the double bazooka, not the G5RV.
G5RV = 102 ft wire + ~31 ft ladder line + balun + coax. No traps, no coax radiators.