Quick guide to building a dual-band J-pole Daniel Marks, KW4TI

I made a dual-band J-pole (e.g. DBJ-2 style) using 300 ohm TV twin lead, here are the measurements from my antenna:

53 mm Shorted end of twin lead to feed point 470 mm Total length of shorted section 235 mm Length of 70 cm antenna wire 130 mm Length of RG-316 shorting stub, you can use RG-58 or RG-174 also 333 mm Length of 2 m antenna wire portion

The easiest way I found to make it is to make a 2 m J-pole first, cut the element, and then add the 70 cm and 2 m elements.

First make 2 m J-pole

1. Cut a shorted section of twin lead slightly long (490 to 500 mm) which is the matching section. 2. Use a soldering iron to melt away the insulation at the end of the twin lead to expose the two wires. 3. Solder the two wires together to short them. 4. Up 50 to 53 mm from the shorted end use the soldering iron to melt the insulation at two points on the twin lead. 5. Take a coaxial cable (e.g. RG-58) and expose the center conductor and shield. Twist the shield together and solder into a single conductor so that the center conductor and braid form a "V" shape. You may need to twist a solid wire with the braid to give it some more strength when soldering it together. 6. Solder the center conductor and braid to the two sides of the twin lead exposed in #4 7. On the same side of the twin lead that the center conductor is soldered to, expose the end of the twin lead by melting the insulation. 8. Solder a 1 m length insulated copper wire onto the J-pole. 9. Twist 3-4 turns of the coaxial cable together near the feedpoint to form a choke balun and hold them together with zip ties, or even better, put them through a few turns through a ferrite toroid to form a choke balun. This greatly helps reduce feedline radiation and stabilize the SWR. 10. To measure the SWR, hang the J-pole so that it at least 3 feet from any nearby object. A convenient way to do this is to tie two plastic spring clamps together, clamp one to hold the end of the J-pole antenna wire, and clamp the other to something on the ceiling. Then using minimum power from the radio, measure the SWR at 146 MHz. 11. Tightly coil the end of the antenna wire to effectively shorten it until you get the minimum SWR. At this point you may need to shorten the matching section by a few millimeters several times and reattach the antenna wire and try again. You should be able to get the SWR down to 1.5:1 or less. 12. If all you want is a 2-m J-pole, you’re done. Take some hot glue and fix the coiled end of the antenna wire in place to maintain the correct resonance frequency. If you put it into a tube, you can tweak the resonance frequency by coiling the antenna wire a little more to compensate for the velocity factor of the PVC tube. Make it into a dual band J-pole

13. Cut the antenna wire so it is about 260 mm long. 14. Testing at 445 MHz, trim the antenna wire until you get a minimum SWR. You should be able to get it below 2:1. You could try trimming the matching section as well, but if you trim too much you may have to start over again. 15. Once the SWR is minimized at 70 cm, attach the center conductor of a 150 mm length of RG- 316, RG-174, or RG-58 coax stub to the end of the antenna wire. The center conductor should not be shorted to the shield at the end that attaches to the antenna wire, but it should be shorted together on the other end. 16. Test the SWR at 445 MHz. If the SWR is too high, trim the coax stub a few millimeters, short the center conductor and shield, and test again. You should be able to get the SWR very close or identical to when the stub was not attached. 17. Finally, attach a 400 mm length of wire for the 2 m portion of the antenna, and coil the end to minimize the SWR at 146 MHz. Use hot glue to hold the coil at the end of the antenna in place. 18. You can use hot glue on the joints to insulate the connections, but I have found the resonance frequency tends to drop by a tiny amount, so expect this.

End of the antenna with choke, shorted end of twin lead, and coax cable attached. Attachment point between matching twin lead stub and antenna wire

Coax decoupling stub between 70 cm and 2 m portions of the antenna. The entire antenna shown against a ruler for reference.