K1DU J-Pole: 450 Ohm Ladder Line Antenna for 144 & 440 MHz

This article by Fred Delaney, K1DU, details the construction of a J-Pole antenna using 450 Ohm ladder line, designed for operation on the 2-meter (144 MHz) and 70-centimeter (440 MHz) bands. The design is based on an earlier concept using 300 Ohm TV twinlead by KD6GLF. The J-Pole is presented as a suitable indoor, freestanding antenna, particularly useful in areas prone to lightning or for hams with antenna restrictions. It can also be configured for portable use, such as a backpack version for camping. The antenna is vertically polarized and consists of a ¾ wavelength radiator and a ¼ wavelength matching stub, operating as an end-fed half-wave antenna. It does not require ground radials and offers a gain of 2.4dB over isotropic. The article provides the formulas and calculations for the ¾ wave radiator and ¼ wave stub, using a velocity factor of 91% for 450 Ohm ladder line. Specific dimensions for 146 MHz are given: a ¾ wave radiator of 55 3/16 inches and a ¼ wave stub of 18 3/8 inches. Construction involves cutting a 57¾-inch piece of ladder line and stripping 4 inches from one end.