Dipole antenna
| Dipole antenna | |
|---|---|
| Type | Half-wave wire antenna, fed in the center |
| Length | 468 ÷ frequency in MHz, in feet (143 ÷ MHz in meters), then trim |
| Feed point impedance | About 70 ohms in free space; near 50 ohms at typical heights |
| Feed line | Coaxial cable, ideally with a balun or choke |
| Typical lengths | |
| 80 m (3.7 MHz) | About 126 ft (38 m) |
| 40 m (7.1 MHz) | About 66 ft (20 m) |
| 20 m (14.2 MHz) | About 33 ft (10 m) |
| 2 m (146 MHz) | About 38 in (97 cm) |
The dipole is the simplest effective antenna and the one most hams build first. It is a wire one half-wavelength long, cut in the middle, with the feed line connected to the two halves. Strung between two trees, a dipole for the 40-meter or 20-meter band will work the world for the cost of some wire, a center insulator and a length of coax.
How it works
A half-wave dipole is resonant: at its design frequency, current flows strongly at the center and falls to nothing at the ends, and the antenna presents a feed-point impedance close to the 50 ohms that radios and coax expect. Off that frequency, the impedance rises and the SWR climbs, which is why a dipole is essentially a single-band antenna, though a 40-meter dipole also works on 15 meters, where it is three half-waves long.
The dipole radiates best broadside to the wire and least off the ends. At low heights, under about a quarter wavelength, most of the signal goes upward, which is ideal for regional contacts within a few hundred miles. Raise it to a half wavelength and the pattern flattens out, favoring long-distance skip.
Building one
- Calculate the length: 468 divided by the frequency in megahertz gives the total length in feet. For 7.1 MHz that is 65.9 feet, or about 33 feet per side.
- Cut each half a little long. Attach the halves to a center insulator and the far ends to end insulators, with a few extra feet for tying off.
- Connect the coax at the center: center conductor to one half, shield to the other. A 1:1 balun or a choke made from a few turns of coax at the feed point keeps the feed line from radiating.
- Hoist it up as high as you can, check the SWR, and trim both ends equally until the lowest SWR lands where you want it. Shorter raises the frequency; longer lowers it.
The ends carry high voltage when transmitting, so keep them out of reach.
Variations
- Inverted V. The center is hoisted on a single mast and the ends slope down at 90 to 120 degrees. It needs only one support, takes less room, and is a little less directional. Cut it about 5 percent shorter.
- Multi-band dipoles. Parallel dipoles for several bands share one feed point; trap dipoles use tuned circuits to electrically shorten the wire on higher bands; off-center-fed dipoles use a 4:1 balun to work several bands with one wire.
- Folded dipole. Two parallel wires joined at the ends, with a 300-ohm feed point, common at VHF.
- Doublet. A dipole of any convenient length fed with ladder line and matched by a tuner, which turns one wire into an all-band antenna.