Yagi Antennas
The Yagi-Uda antenna — universally called simply a "Yagi" — is the most popular directional antenna in amateur radio. If you have ever seen a TV antenna on a rooftop, you have seen a Yagi. By arranging multiple parallel elements along a boom, a Yagi concentrates radiation in one direction, providing gain, directional selectivity, and front-to-back rejection. On VHF and UHF, Yagis are essential for weak-signal work, satellite contacts, and contests. On HF, large Yagis on towers enable competitive DX stations.
How a Yagi Works
A Yagi consists of three types of elements, all parallel to each other and mounted perpendicular to a boom:
Driven Element
The driven element is the one directly connected to the feedline. It is typically a half-wave dipole or a folded dipole. This is the only element that receives power from the transmitter — all other elements are parasitic (they are excited by mutual coupling from the electromagnetic field, not by a direct electrical connection).
Reflector
The reflector is a single element placed behind the driven element (opposite the desired direction of radiation). It is slightly longer than the driven element — typically 5% longer. The reflector's function is to redirect energy that would radiate backward, sending it forward instead. It acts like a mirror for radio waves.
Directors
Directors are placed in front of the driven element (in the direction of desired radiation). They are slightly shorter than the driven element — typically 5% shorter for the first director, with each subsequent director slightly shorter still. Directors guide and focus the electromagnetic wave forward, increasing gain and narrowing the beam.
A Yagi must have at least a driven element and one parasitic element (reflector or director) to function as a directional antenna. Adding more directors increases gain and narrows the beam, but with diminishing returns — each additional director adds less gain than the previous one.
Element Count and Performance
| Configuration | Approximate Gain (dBd) | Beamwidth | Front-to-Back |
|---|---|---|---|
| 2 elements (driven + reflector) | 3–4 dB | ~120 degrees | 10–12 dB |
| 3 elements | 5–6 dB | ~70 degrees | 15–20 dB |
| 5 elements | 7–9 dB | ~50 degrees | 20–25 dB |
| 7 elements | 9–11 dB | ~40 degrees | 20–25 dB |
| 10+ elements | 11–14 dB | ~30 degrees | 20–30 dB |
Yagi Design Parameters
Several dimensions define a Yagi's performance:
- Element lengths: Determine resonant frequency and coupling. The driven element is close to a half wavelength; the reflector is about 5% longer; directors are progressively shorter.
- Element spacing: The distance between elements along the boom significantly affects gain, impedance, bandwidth, and front-to-back ratio. Typical spacing is 0.15–0.25 wavelength between elements. Wider spacing generally gives more gain but requires a longer boom.
- Boom length: The overall boom length (from reflector to last director) is the primary determinant of gain. More boom length = more gain. A 1-wavelength boom yields about 10 dBd of gain regardless of how many elements are on it (though more elements provide better pattern and bandwidth control).
- Element diameter: Thicker elements increase bandwidth. HF Yagis often use tapered aluminum tubing (larger diameter at the center, smaller at the tips) for both structural strength and bandwidth.
Feeding a Yagi
Impedance
A simple dipole driven element in a Yagi has a feedpoint impedance significantly lower than the ~73 ohms of a dipole in free space, often in the range of 15–30 ohms depending on spacing and element configuration. This poor match to 50-ohm coax needs to be addressed.
Common Feed Methods
Gamma match: A single rod runs parallel to one half of the driven element, connected between the boom (ground) and a point on the element. A series capacitor (or adjustable stub) tunes out the reactance. The gamma match allows direct connection to 50-ohm coax and enables the driven element to be electrically connected to (grounded on) the boom, simplifying construction. This is the most common feed method on commercial HF Yagis.
T-match / Omega match: Similar principle to the gamma match but uses two rods, one on each side of the element. Provides a balanced match.
Folded dipole driven element: A folded dipole has approximately four times the impedance of a regular dipole (~280 ohms in free space). In a Yagi, where element interaction lowers impedance, a folded dipole can bring the feedpoint impedance up to a more convenient range. A 4:1 balun then matches to 50-ohm coax. This approach also broadens bandwidth.
Hairpin match: A shorted stub (a U-shaped piece of wire or rod) is placed across the feedpoint terminals. It acts as an inductor that, combined with the naturally capacitive impedance of a short driven element, creates a match to 50 ohms. Used on many commercial VHF/UHF Yagis.
Direct 50-ohm feed: By carefully optimizing element lengths and spacing, some designs achieve a direct 50-ohm match at the driven element feedpoint with no matching network. The DK7ZB design philosophy emphasizes this approach.
VHF/UHF Yagis
Yagis truly shine on VHF and UHF where the wavelength is short enough that multi-element antennas are physically compact. A 5-element 2 m Yagi is about 6 feet long — manageable for a rooftop or even handheld use.
Applications
- Weak-signal SSB/CW on 2 m and 70 cm: Horizontal Yagis pointed toward the other station. EME (Earth-Moon-Earth) communication uses large Yagi arrays.
- Satellite contacts: Yagis for the VHF uplink and UHF downlink, often on a lightweight azimuth-elevation mount. Hand-held "arrow" style antennas work for LEO satellite passes.
- Direction finding (fox hunting): A small handheld Yagi helps locate hidden transmitters.
- Contests and grid square chasing: VHF contesters use horizontally polarized Yagis on towers, often stacking multiple Yagis for additional gain.
HF Yagis
HF Yagis are physically large and require a tower and rotator, making them a significant investment. A 3-element 20 m Yagi has a boom length of about 24 feet and elements spanning 34 feet. Despite the size and cost, the performance gains are dramatic — 6 dB or more of gain over a dipole translates to the equivalent of quadrupling your transmit power and improving your receive by the same margin.
Popular Commercial HF Yagis
- Mosley TA-33: Classic triband (20/15/10 m) trapped Yagi. Affordable, decent performance.
- Cushcraft A3S: Triband trapped Yagi, similar to the TA-33 with slightly different trade-offs.
- SteppIR DB18E: Uses motorized elements that physically adjust length for each band, eliminating traps and their losses. Covers 20–6 m with full-size performance on each band. Premium price.
- OptiBeam OB11-3: High-performance 3-element triband design from Germany.
- Homebrew monobanders: Many serious contesters build single-band Yagis from aluminum tubing. Monobanders outperform triband trapped designs because each antenna is optimized for one band without compromise.
DIY: Building a 3-Element 2-Meter Yagi
Here is a practical build guide for a simple 3-element Yagi for the 2-meter band (144–148 MHz). This design uses a direct 50-ohm feed based on the DK7ZB design approach.
Materials
- Boom: A 36-inch (91 cm) length of 3/4-inch or 1-inch square or round non-conductive material (wood dowel, PVC pipe, or fiberglass tube). If using a conductive boom (aluminum), elements must be insulated from it and element lengths adjusted (conductive booms slightly detune elements).
- Elements: Three pieces of 3/16-inch or 1/4-inch aluminum rod (or #8 to #10 AWG solid copper wire for a lighter build):
- Reflector: 40.5 inches (102.9 cm)
- Driven element: 38.2 inches (97.0 cm) — this is the total tip-to-tip length; the feedpoint gap at the center is about 1 inch
- Director: 36.4 inches (92.5 cm)
- Element mounting hardware: Stainless steel U-bolts, hose clamps, or through-bolts with insulating bushings
- Feedpoint connector: An SO-239 or BNC chassis mount connector at the center of the driven element
- Coaxial cable: RG-58 or RG-8X with appropriate connector
Element Spacing (from reflector)
- Reflector to driven element: 14 inches (35.6 cm)
- Driven element to director: 11 inches (27.9 cm)
- Total boom length: 25 inches (63.5 cm)
Assembly
- Mark element positions on the boom.
- Mount the reflector and director directly to the boom (they are continuous rods, no feedpoint gap).
- Mount the driven element as two halves, each 19.1 inches, with a 1-inch gap at the center. Secure both halves to the boom, insulated from it.
- Connect the SO-239 or BNC connector at the driven element gap. Solder the center conductor to one half and the shield to the other half.
- Connect coax and test.
Tuning
- Connect a NanoVNA to the coax.
- Measure SWR across 144–148 MHz.
- The SWR minimum should be near 146 MHz. If it is too low in frequency, trim the driven element slightly (1/4 inch from each end at a time). If too high, the element is too short.
- Final SWR should be well below 1.5:1 at the target frequency.
Expected Performance
- Gain: approximately 7 dBi (4.85 dBd)
- Front-to-back ratio: approximately 15 dB
- Beamwidth: approximately 70 degrees
- SWR bandwidth (< 2:1): the entire 2 m band (144–148 MHz)
This antenna is perfect for satellite work (hand-held), fox hunting, point-to-point simplex communication, and working distant repeaters. Mount it horizontally for SSB/CW weak-signal work, or vertically for FM (though vertical Yagis are less common).
Stacking Yagis
Two or more identical Yagis can be stacked (mounted one above the other at a specific spacing) and fed in phase to increase gain by approximately 3 dB (for two antennas). Stacking compresses the elevation pattern, directing more energy toward the horizon.
The optimal stacking distance depends on the single antenna's beamwidth — typical stacking spacing is 0.5 to 1 wavelength. Stacking requires a power splitter/combiner (phasing harness) to distribute power equally and in-phase to both antennas.
Stacking is common in VHF/UHF contesting and EME, where every dB of gain matters.
Safety Considerations
- HF Yagis are heavy and mounted high. Tower work is inherently dangerous. If you are not experienced with tower climbing, hire a professional.
- Rotators and control cables add complexity. Ensure the rotator is rated for the antenna's wind load and weight.
- RF exposure: Yagis concentrate energy. Maintain safe distances from the front of the antenna during transmission, per your country's RF exposure regulations.
- Wind loading: Calculate the wind survival rating for your antenna and ensure the tower and rotator can handle it. Ice loading can significantly increase weight and wind cross-section.
Summary
The Yagi antenna offers the best directional gain-per-dollar of any amateur antenna type. On VHF/UHF, even a small homebrew Yagi provides a dramatic performance improvement over an omnidirectional antenna. On HF, a Yagi on a tower is the gold standard for DX and contesting. Start with a simple VHF Yagi project to learn the principles, and scale up from there as your station grows.
Contributors
IUU6