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- 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 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.
-
- To get the best possible match, in step three above simply MARK the
- "solder points" and measure from the mark for step 4 and 5. Now solder
- straight pins to your conductor and your shield. Insert the pins at
- the marked point and test for VSWR at the design frequency (146MHz).
- If necessary, probe up or down till you reach 1:1 (close as possible).
- Solder at the best points. To try this, you may want to start with
- the twin lead a little long and trim down to resonant length - note:
- you'll need to trim in a 3:1 ratio to maintain the 3/4 to 1/4 wave.
-
- 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. This antenna is relatively broad-banded
- and will be more than adequate if simply built as noted in steps above.
-
- 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).
-
- --
- Ed Humphries Texas Instruments, Inc. 512-250-6894
- N5RCK Internet ed.humphries@hub.dsg.ti.com
-