Hello home builders, this is a vertically polarized omnidirectional free space antenna which offers approximately 1.8dB of gain. It has a radiation efficiency 50% better than a ground-plane antenna due to its low radiation angle. The slim jim antenna is a very old design from the 30's and plenty of information can be found about it on the net. This site however, will actually show you how to build your slim jim antenna from Home Depot items!
Antenna Description: Basically it is an end-fed folded dipole operated vertically. The matching stub provides a low impedance feed point (50 ohms) at the base and couples to the antenna section at high impedance at one end. As with all folded dipoles, the currents in each leg are in phase, whereas in the matching stub they in phase opposition, so little or no radiation occurs from this. Correctly matched, the VSWR (Voltage Standing Wave Ratio -Steve) will be much less than 1.5:1, and remains so across the band.
Slim Jim Antenna Feed
Here is a great diagram from Free Radio Berkeley of the antenna feed
The how to build information is derived from
an old FM-10 archive page titled "3 Meter Slim-Jim Antenna"
Construction: The Slim Jim should be constructed from 1/2" copper pipe. The bends are made with soldered 90 degree copper elbows. A slip sleave made from copper can be added to the element above the gap for tuning purposes, although the average length of the gap and spacing between the elements is 3" at 90MHz. No part of the antenna should be grounded to the tower or mast. The recommended mount is the use of PVC pipe and PVC pipe "T's." Make sure the space between the tower or mast and the antenna is one "freespace" 1/4 wavelength.
3 Meter Slim-Jim Antenna (JIM = J Intergrated Match)
1/2 wave section = 5610/MHz Example: 89.5MHz = 5610/89.5 = 62.68"
1/4 wave section = 2805/MHz Example: 89.5MHz = 2805/89.5 = 31.34"
The Slim Jim Antenna setup for portable use.
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