TV Twin-lead J-pole
        By Ed Humphries - N5RCK
        
        The following is a description of a J-Pole antenna made from 300 ohm
        TV twin-lead. They have quite a few advantages which include improved
        performance for 2-meter HTs, portability, and low cost.
                        |   | do not short this end.
                        |   | (when trimming for vswr, cut both sides)
                        |   |
                        |   |
                        |   |
                    3/4 |   |        Technically-speaking, this is a 1/2
                   wave |   |        wave end-fed antenna with a 1/4 wave
                        |   |        matching section.
                        |   |
                        |     1/4" gap
                        |   |         (trim for vswr _below_ gap)
                        |   | 1/4
                        |   | wave
                        |   |
    coax ctr conductor=>*   *<= coax shield
                 1 1/4"-|   |
                         -*-    solder the twin leads together at bottom
        For a center frequency of 146 MHz:
        1. Start with @54" of TV twin lead (flat, NOT foam core)
        2. Strip 1/2" of insulation at bottom and solder wires together.
        3. Measure 1 1/4" from soldered wires and strip insulation on both
           sides. This is the solder point for a coax feedline.
        4. Measure 16 3/4" from coax shield solder point and cut out 1/4" notch.
        5. Measure 50 1/3" from coax center conductor solder point and trim
           off twin lead at that point.
        6. Feed with a length of RG58U coax.  Tape coax at feedpoint to the
           twin lead for strength and seal coax for weather protection.
        It has been noted that this design can lead to rf coupling onto the
        feedline. To avoid, put ferrite beads on the coax at the feedpoint,
        or use 3-5 turns of coax (1"-2") taped together at the feedpoint.
        You may attach an alligator clip to the plastic on the top of
        the antenna in order to easily hang it.  Alternately, punch a
        hole near the top and use a length of fishing line to hang.
        This design appears on many BBSs, in club newsletters, and in books;
        the earliest reference that I know of is a Jan. 1984 D.A.R.C. antenna
        article by James Burks, KA5QYV.
        FYI, the 1/4 wave sections for other center frequencies are:
        144 MHz =17 inches, 145 =16.88, 146 =16.75, 147 =16.65, 148 =16.54
        I usually just go ahead and solder the coax in place and trim
        down to as close to 1:1 vswr as I can get. I use the MFJ vhf
        antenna analyzer and a frequency counter then afterwards test
        with a radio and in-line swr/power meter. When done, the antenna
        should also present 1:1.2-3 vswr in the center of 444MHz band as
        well (demonstrated on my dual-band meter and Alinco DJ-580). You
        will need to trim in a 3:1 ratio to maintain the 3/4 to 1/4 wave.