Slim-Jim is easy to built, no radial 145Mhz antenna. It has almost equal gain of a 5/8 lambda (Did not measure with any professional equipment, but my FT23R S-meter shows equal readings on both antennas) and much higher gain than 1/4 lambda whip. But it is much easier to buld than a 5/8 or 1/4 antenna. This looks like a modified J pole. I am sorry I don�t know who invented this, but some one who has done a good job.|
To built this antenna you need 3/4 inch (even 1 inch is okay) diameter, 5 and half foot or longer PVC pipe. No 14 to 18 copper wire slightly longer than 10 feet. As shown in the picture make a hole (just enough to put the no 20 copper wire) across the PVC pipe from the one end of it. Make another one exactly 60 inches (5 feet) below the first hole (That is the bottom end of the antenna). At the bottom end you should have at least about half foot PVC left to fix it somewhere. Between two holes is Length A (see Pic).
When you connect the coax, do not connect it as shown in the picture. I draw it only to show the connection. But connect the coax up side down so the water will not go in to the coax. Center connector of the coax should connect to the opposite side of the Gap. If you want to change the matching, try to change the distance of D.
Also at the end you will have to put some tape, around the PVC from few places, so the wire will not move here and there. - Good luck!