Yagis and Hex Beams

A dipole or a vertical sends your signal more or less everywhere at once. A beam lets you choose. Point it at Europe and Europe hears you louder, while the stations and noise behind you get quieter in your headphones.

A beam helps on receive as much as on transmit. This article covers the classic Yagi, its lighter multiband cousin the hex beam, and how to put either one up safely.

How a Yagi Works

Every Yagi starts with a driven element: a half-wave dipole, fed with coax, mounted across a boom. The rest are parasitic elements, plain rods or wires with no feedline at all.

Each parasitic element picks up energy from the driven element and radiates it again, slightly delayed. Making an element a little longer or shorter than resonance sets that delay. Get the timing right and the signals add up in one direction and partly cancel in the other.

• Reflector: slightly longer than the driven element and mounted behind it. It pushes energy forward.
• Driven Element: the fed dipole, about a half wavelength long.
• Directors: slightly shorter than the driven element and mounted in front of it. Each one draws the beam a little tighter.

A two-element beam usually pairs the driven element with a reflector rather than a director, because that arrangement gives more gain. Adding directors adds gain and narrows the beam, but each one adds less than the one before, and the boom gets longer, heavier and harder to turn.

Where the Name Comes From

The design came out of Tohoku Imperial University in Sendai, Japan, where work began in 1924. Shintaro Uda, a lecturer, first described the antenna in a Japanese journal in 1925, working under the guidance of Professor Hidetsugu Yagi. Yagi’s English-language paper in the Proceedings of the IRE in 1928 is what made it known around the world.

Hams usually just say “Yagi,” but the fuller name is Yagi-Uda. The IEEE, which honors the work with one of its Milestones, credits both men, and so should we.

Gain and Front-to-Back

Gain is always measured against something. The isotropic radiator is an imaginary, lossless antenna that radiates equally in every direction, and gain against it is written dBi. Gain against a half-wave dipole in free space is written dBd. Because a dipole already has about 2.15 dB of gain over the isotropic radiator, the same antenna always reads about 2.15 dB higher in dBi than in dBd. You will also see 2.14 dB; the difference is rounding.

Watch for figures measured over ground, too. Reflections from the earth add what is called ground gain, so an over-ground number can look far better than the same antenna in free space. In Analyzing Antenna Gain Specifications, Bill Rinker, W6OAV, walks through how to read these claims.

Front-to-back ratio compares the signal off the front of the beam with the signal off the back, in dB. On a crowded band it can matter as much as gain, because a good front-to-back lets you hear a weak station ahead while a loud one behind you fades away.

The Hex Beam

A hex beam looks like an umbrella turned inside out. Six fiberglass spreaders hold wire elements in a hexagon, with a driven element and a reflector for each band. The ends of each element fold back toward each other, which is what keeps the antenna so compact.

That folding is the whole appeal. A full-size 20 meter Yagi has elements around 33 feet long, so it needs a stout rotator and plenty of clear space to turn. A hex beam covering 20 meters and several higher bands is much smaller across and light enough that many hams run one on a modest mast. Modern broadband designs also hold a usable match across each whole band.

The trade-off is performance. A hex beam is a two-element beam on each band, so a long-boom monoband Yagi will beat it on gain and front-to-back. Its wires and flexible spreaders also suffer more from ice and high wind than aluminum tubing does.

Rotators, Towers and Safety

A beam is only useful if you can turn it. Choose a rotator rated for your antenna’s weight and wind load with room to spare, use a heavy-wall mast, and leave a loop of coax at the rotator so the cable does not bind as the antenna turns.

Tower work can kill. Treat these as rules, not suggestions:

• Never climb alone, and never install a beam by yourself. Have a ground crew who knows what to do if you get into trouble.
• Wear a full-body climbing harness and stay tied off every moment you are off the ground.
• Everyone on the ground wears a hard hat and stays out from under the work.
• Never ride a hoist or rope, and never climb a crank-up tower; work on it lowered.
• Keep the tower, the antenna and every rope well away from power lines. Plan as if the whole thing could fall, because some day it might.

A tower is also a lightning target, so ground it properly; see Grounding and Lightning Protection. A beam concentrates RF, so do the RF exposure evaluation that Part 97.13 requires. Under Part 97.15, structures over 200 feet or near an airport have extra FAA and FCC steps, and local rules must reasonably accommodate amateur antennas.

Small Yagis for Satellites

Yagis shrink as frequency rises. On 2 meters and 70 centimeters, a dual-band Yagi is light enough to hold in one hand, and that makes it a favorite for the FM satellites. These work much like repeaters in orbit, with an uplink on one band and a downlink on the other.

A handheld radio with its stock antenna can work a satellite on a good pass, but a small Yagi pointed by hand is a real improvement. If the signal fades, try rotating the boom a little; the change in polarization often brings it back. Our article Your First Satellite Contact walks through a whole pass.

Choosing Between Them

• Pick a Yagi if you care most about one or two bands, want the best gain and front-to-back, and have a tower and rotator that can carry it.
• Pick a Hex Beam if you want several bands from one antenna, have limited space or a light mast, and can accept a little less performance.
• Start Small on VHF and UHF, where a handheld or roof-mounted Yagi costs little and teaches you how a beam behaves.

Whichever you choose, read gain claims carefully and plan the installation around safety first.