Thursday, March 3, 2011

W5LAN's 2-meter mobile antenna


Brass tubing is available in some hardware and hobby stores. It comes in sizes from 1116 to 21/32 inch outside diameter(OD), in l/32 inch steps. Each size slip fits within the next larger size. It is usually sold in 12- or 36-inch lengths.The antenna is made from two 12-inch lengths of 5/32-inch tubing and two 12-inch lengths of 118-inch tubing. A V-shaped horizontal dipole is formed when the tubes are mounted through a short piece (6 inch or so) of 7/8-inch OD plastic pipe (see Fig). It is V shaped to reduce the overall size and provide a better match to 50 R coax.

Begin by drilling two 5/32-inch holes through the plastic pipe at right angles to each other. (Position one hole slightly below the other so that the dipole elements cross inside the plastic pipe without touching.) Enlarge the holes of two solder lug and force each over one end of the S/32-inch tubes and solder them in place.

Push the other end of those tubes through the holes in the plastic pipe until the solder lugs are flush against the pipe. Strip the end of a length of coax, then solder the braid to one solder lug and the center conductor to the other. Use sealant to weatherproof the coax end and feed point.

The antenna is adjusted to resonance by sliding the 118-inch tubing in and out of the larger tubing to achieve minimum SWR. If the fit is too loose, nick the end of the smaller tube slightly with diagonal cutters, and force it into the larger tubing. After performing the adjustment, cut the smaller tube to a length that leaves about an inch inside the larger tube and solder it to the larger tube. The element lengths on my antenna are about 20.5 inches each, and the SWR was near unity over most of the 2-meter band, with a slight rise at the high frequency end.

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