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). ************************************************ Here are the Instructions for the Better J-Pole. Remember, the spacing is important, If you use any element material other than what is shown here, you may have to change the spacing to get a low SWR. Please e-mail me, and let me know how this works out. and let me know if you have any questions. You may want to print the page showing the J-Poles at.... Arrow Antenna or http://Members.aol.com/Arrow146/index.html Construction Plans for the Model J146 2 Meter Solid Aluminum J-Pole Parts Needed: Radio Shack # 21-961 3/8"-24/SO239 Mount (Mobile CB Antenna) Radio Shack # 15-826 TV Mast U-Bolt Clamp Assembly About 80 inches of 3/8" diameter Solid Aluminum Rod. 5 1/2 inch piece of strut angle Aluminum 1 1/2" X 1 1/2" X 3/16" Thick. 2 Stainless Jam Nuts 3/8-24 thread Tools needed: Drill Press with drill Bits up to 1/2". Tap & Die for 3/8-24 thread. Sander or file to de-burr the cut edges. 2 9/16 wrenches & a 3/4 wrench. Vice-grips Make sure the piece of angle Aluminum is exactly 5 1/2 inches long. Drill 2 1/4" holes on one side, centered to fit the U-Bolt for mounting. On the other surface of the angle Measure in 1/2" in from each end and drill a 1/4" pilot hole. This should give you 4 1/2" from center to center of the two holes. Redrill one of the pilot holes to 11/32" Then tap the hole with a 3/8-24 Tap. Redrill the other pilot hole to 1/2" Finish as required. Cut a piece of the 3/8" Aluminum rod to about 59". Attach a vice-grip to one end. Using a 3/8-24 Die cut in about 8 threads in the other end. Screw on the Coupling Nut that came with the SO239 Mount Tighten as much as you can. Measure the rod & coupling nut & cut to exactly 57 1/2" Cut a piece of the 3/8" Aluminum rod to about 21" Attach a vice-grip to one end. Using a 3/8-24 Die cut in about 14 threads in the other end. (If you don't have jam nuts, cut enough threads for 2 nuts & the thickness of the angle bracket.) Screw on one of the Jam nuts & tighten as much as you can. Measure from the bottom of the nut and cut the rod to exactly 19 1/4" (Remember the nut must be included in the Measurement.) (Put a small amount of grease on the threads) Screw the element into the threaded hole of the angle bracket and tighten. Screw on the other nut and tighten it. Attach the 57 1/2" element in the 1/2" hole using the rest of the SO239 Mount. Mount & test your new antenna. SWR should be Less than 1.2:1 at 146 MHz and less than 1.3:1 at 144 &148 MHz. This Model J146 is available from Arrow Antenna for only $ 35. The J146/440 Dual Band with single SO239 feed is only $ 43. Please add $ 6. Shipping & Handling per order (for US Funds). 73 Allen Lowe NOIMW Arrow Antenna 1803 S. Greeley Hwy. # B Cheyenne, WY 82007 (307) 638-2369 Fax (307) 638-3521 Arrow146@aol.com