Slim Jim Antennas

2 Meter Slimjim (2)2 Meter Slimjim (1)I am a big fan of the J-Pole antenna style and have built a few of them in the past (see here and here). Electrically the J-Pole is a half wavelength antenna that is end-fed by a quarter wavelength stub to achieve a feed point impedance of 50 Ohms. The Slim Jim is very similar except that there are two half wavelength segments with the second folded next to the first. This arrangement makes them ideal to be constructed from 450 Ohm ladder line. This also allows them to be incredibly compact and portable since they can be rolled up very easily.

2 Meter Slimjim (4)2 Meter Slimjim (3)I found a very useful calculator that gives you all of the dimensions needed to build a Slim Jim or a J-Pole from ladder line (you will have to convert from metric to imperial units yourself). Using this calculator set for 146 MHz (the middle of the 2 meter band), I assumed my 450 Ohm ladder line had a velocity factor of 0.91 and cut it to a length of 56 inches (not including the 0.5 inch on each end to be stripped and soldered together). I then cut the half wave radiator 36.75 inches from the top and the quarter wave matching section 18.375 inches from the bottom. This left a gap of 0.8 inches between the two. The 50 Ohm feed point was calculated to be about 1.8 inches from the bottom, however, using my antenna analyzer I found the the feed point should be 2.1875 inches from the bottom. With a piece of RG-8X feedline soldered at the feed point this antenna has an SWR under 2:1 across the entire 2 meter band. To complete the antenna I drilled a 0.25 inch hole at the top so that the antenna can more easily be hung from a mast or a tree or clipped to a wall if used as an indoor antenna.

This type of antenna is a very economical way to build a VHF or UHF antenna. It also has a considerable amount of gain compared to a standard quarter wavelength vertical and is just as easy to build and transport.

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