For an antenna with a wider bandwidth than a single wire dipole, build a folded dipole antenna. You will be able to use it as low as 1.8MHz in 160 meters to as high as 30MHz in 10 meters without having to worry about a fluctuating standing wave ratio (SWR). A typical folded dipole antenna will consist of two parallel wires connected together and spaced 6 inches apart, although other wire separation configurations are common.
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Things you need
- #10, #12 or #14 AWG wire
- Center insulator
- 6-inch spacers
- Antenna feedline
Determine the preferred frequency for your antenna. In the 40-meter band, for instance, that preferred frequency can be 7.15MHz, although any frequency on any band can be chosen.
Calculate the length of the antenna by using the formula L=468/MHz, where L will be length in feet. Using the chosen frequency of 7.15MHz, length is calculated to be 468/7.15 or 65.45 feet, which is 65 feet, 5.4 inches. Multiply this length by 2 since the folded dipole is made with two parallel wires. The new total wire length will be 65.45 feet times 2, which equals 140.90 feet or 140 feet, 10.8 inches.
Add 1 foot to the total wire length to accommodate the 6 inches of separation between the wires. The final wire length will be 141.90 feet or 141 feet, 10.8 inches.
Fold the wire on each end so that the two ends meet in the middle. Insert the 6-inch spacers between the two wires equidistant from one end to the other. The final configuration will be one long wire folded into two parallel wires that are both 65 feet, 5.4 inches long and 6 inches apart. There will be a top wire of an unbroken length and a bottom wire that will be open in the middle where the two ends come together and where the feedline will be attached.
Connect the centre insulator to the open ends of the antenna and solder them to the feedline. The insulator will provide strength at the antenna-feedline junction.
Attach the free end of the feedline to your radio. Your radio is now connected to your folded dipole antenna.
Tips and warnings
- You will probably want to hoist the antenna onto supports at least 30 feet into the air for the best reception.
- The feedpoint impedance of a typical single-wire dipole antenna is 72 ohms. Since a folded dipole exhibits a four-times "step-up" of impedance over the single-wire antenna, the folded dipole antenna's feedpoint impedance is 288 ohms and is thus best fed with a 300 ohm twin-lead feedline. If you want to use another feedline, however, such as coax, open wire or window line, use an antenna tuner to match the 50 ohm radio to the 288 ohm antenna.
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