End Fed Half Wave (J-Pole, Slim Jim) Antenna

The end fed half wave antenna is an antenna with a slightly higher gain compared to a half wave dipole. At the end of a half wave radiator the impedance is very high. By using a quarter wave stub the feedline impedance can be matched to the radiator. A 50 ohm impedance can be found approximately at λ/40 from the shortened end of the stub. Usually tuning is required. The impedance of the stub feedline is not critical but it should be of a low loss type, preferably an open air isolated line. Velocity factors of various stub insulation materials Air 1.0 Polyethylene (PE) 0.659 RG6,RG8,RG58,RG59,RG174,RG213,RG214 Foam polyethylene (FE) 0.79-0.88 Air spaced polythene (ASP) 0.84-0.88 Polystyrene (PS) 0.626 Foam polystyrene (FS) 0.910 Solid PTFE 0.695 RG142,RG400,RG393 Air spaced PTFE (AST) 0.85-0.90 PMMA (Acrylic, Plexiglass) 0.60 Nylon 0.47-0.53 Vinyl (PVC) 0.35-0.58 Glass 0.35-0.51 Snow 0.91 Ice 0.56 Water 0.11