A half-wave dipole is two quarter-wave wires fed in the centre with coax. It costs about $30, is easy to build, and is the reference every other HF antenna is measured against.
Length
Total length in feet ≈ 468 / frequency in MHz. On 20 m (14.1 MHz) that is about 33 feet, or 16.5 feet each side. Cut long, then trim a few inches at a time and measure SWR.
Height and pattern
- At a half wavelength or more above ground, a dipole radiates usefully at low angles — good for DX.
- Below about a quarter wavelength it becomes NVIS: nearly all the energy goes straight up and
- A dipole is broadside-directional; it is quietest off the ends.
comes back down within a few hundred miles. Excellent for regional emergency nets, poor for DX.
Variations
- Inverted V — one support in the middle, ends sloped down. Needs one mast instead of two, and
- End-fed half-wave (EFHW) — fed at one end through a 49:1 transformer; a single support point
- Doublet with ladder line — any length, fed with open-wire line and a tuner; works on every
- Fan dipole — several dipoles on one feed point for multiband use.
- Random wire with a 9:1 unun — cheap, effective, needs a counterpoise and a tuner.
works nearly as well.
and multiband operation make it the favourite portable antenna.
band.
Practical notes
- Use insulated stranded wire; it stretches less and survives weather.
- Support ends with rope, not wire, and leave slack for tree movement.
- Add a choke balun (coax coiled or a ferrite choke) at the feed point, or the coax shield
- Keep the ends away from people and from anything conductive; the voltage there is high.
radiates and RF gets into the shack.