Fan Dipole Design Basics
A fan dipole uses several half wave dipoles from one feed point. Each pair of wires is cut for a different band. The pairs spread apart like a fan, so nearby wires interact less. This layout lets one feed line serve many bands. It is popular with field stations, small gardens, and simple home stations.
Why Length Matters
A dipole becomes resonant when its electrical length suits the target frequency. The common starting point is 468 divided by frequency in megahertz. Real wire is not perfect free space. Insulation, nearby roofs, trees, soil, and bends change the final length. That is why this calculator includes velocity factor, sag allowance, and trimming margin. These values help you cut long before tuning.
Handling Multiple Bands
Fan dipoles are not separate antennas in practice. They share the feed point. Current chooses the wire pair with the lowest impedance near each frequency. Close spacing can shift resonance. Wider spread usually gives cleaner tuning. The highest frequency wires often need the most careful adjustment. Small cuts change them quickly.
Tuning Tips
Start with the lowest band first. Raise the antenna to normal height. Measure resonance with low power. Fold wire ends back instead of cutting at once. Then tune the next higher band. Repeat until all bands are close. Changes on one pair can move another pair, so check every band again.
Practical Use
This calculator gives a strong starting design. It estimates total wire, each leg, build length, height ratio, and fan spacing. It also gives a simple current estimate for the selected power and impedance. The numbers are planning values, not a final guarantee. Always test the finished antenna in its installed position.
Safety Notes
Keep wires away from utility lines. Use good strain relief at the feed point. Add drip loops, weather sealing, and safe supports. A well planned fan dipole can be simple, efficient, and easy to repair.
Installation Checks
Record each measured resonance after installation. Note the weather, height, and final folded length. These notes make later repairs easier. Use a balun or choke when feed line radiation appears. Retest after storms, because stretched rope and wet supports can change tuning. Keep all hardware corrosion resistant.