How to build a dipole antenna for a CB Radio
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15 November 2009
Things You'll Need: 1]Wire. About 133 feet for the 80 meter band, 67 feet for the 40 meter band, 34 feet for 20 meters, 17 feet for 10 meters, etc. 2]Wire cutters. 3]Calculator. 4]Coaxial feedline (RG-58 is fine for most HF use) with the proper connector on one end to fit the antenna port on your radio. 5]Soldering iron and solder. 6]Sharp knife. 7]Electrical tape or Coax Seal. 8]Parachute cord or other nonconductive, preferably UV-resistant rope. 9]One or three insulators, or you can just use rope. 1] Decide your operating frequency. This should probably be the lowest frequency that your license class allows, in the sub band corresponding to your desired operating mode. For example, if you are a Technician and wish to operate SSB phone on 10 Meters, you should use 28.300 MHz as your starting frequency. If on the other hand you would like to experiment with your CW privileges on 40 Meters, use 7.025 MHz. 2] Calculate the correct wire length in feet by dividing 468 by the operating frequency, in megahertz. In the above examples, 28.300 MHz would be 16.537 feet and 7.025 MHz would be 66.62 feet. Rounding to the nearest half foot is fine, but add about 6 inches to facilitate making attachment points . With the cutters, cut the wire to the calculated length, then fold the wire in half and cut it again in the exact center. 3] Using the knife, strip one end of each wire if insulated, or scrape it to prepare for soldering, if bare. Attach that end of one wire through one of the holes in the insulator, and twist it into a loop. Repeat for the other wire, in the other hole. 4] Strip a few inches of the end of the coax that does not have the connector, and separate the center conductor from the braid. Solder the center conductor to where you twisted the loop on one of the wires (near the insulator), and solder the braid to the other wire in the same location. Tape up this connection to waterproof it, or use Coax-Seal as directed. 5] Make a loop in the free end of each wire, attaching it to the other two insulators if used, or just tie the rope to the loop if you are only using one insulator. 6] Temporarily hang the dipole for tuning. You can either hoist it by the ends in the flat-top configuration, using the ropes you attached in the previous step; or hoist it by a third rope attached to the center insulator, as an inverted-V. If you hoist it as an inverted-V, pull out the ends and stake their ropes to the ground, keeping the wire ends well above ground. 7] Connect the other end of the coax to your transmitter or antenna analyzer, and check the SWR at your start (lowest) frequency. Then check it at the high end of your frequency range. SWR should be higher at the lowest frequency. Clip wire a few inches at a time from both ends, checking SWR as you go, and making sure you don't go too far by making the SWR too high at the upper frequency! The SWR will probably be acceptable from the beginning, so you may decide to skip the tuning step, after checking it. An SWR not exceeding 1.5:1 across the range is fine, and even 2:1 is probably OK. 8] Once you are satisfied with the SWR, tighten everything so it doesn't move too much, and you're finished! Tips & Warnings You may be able to scrounge wire. Copper-plated steel electric fence wire is a good wire to use. I have also bought 16 gauge lamp cord, available in bulk at the hardware store, and separated it into its two conductors. Use your imagination. As alluded to before, the end insulators may be deleted, simply using the non-conductive rope as end insulators. A short piece of rope may also be used as a center insulator. However, if you are going to run high power, use all three insulators. The ends of the antenna are high voltage points, and could have a corona discharge at high power levels. Follow the standard safety procedures for working with all antennas, such as never crossing a power line, observing ladder safety procedures, etc. As mentioned in the tips, if you are going to use an amplifier, don't use any of the frugal tips. Use minimum 14 gauge wire, the proper insulators at all three points, and coax rated for the power you will be using. Ground your station properly, and disconnect this antenna if you expect lightning in the vicinity.
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