Table of Contents
CHAPTER 1 — GENERAL INFORMATION
1-1. SCOPE
This manual covers design, construction, and calibration of the Yagi-Uda Directional Antenna directional antenna for operation on 6M (50–54 MHz), 2M (144–148 MHz), 70cm (430–440 MHz). The antenna provides significant forward gain and front-to-back rejection for weak-signal, DX, and satellite work.
1-2. APPLICABLE REFERENCES
- ARRL Antenna Book — Yagi and LPDA Chapters
- Leeson, Physical Design of Yagi Antennas (ARRL, 1992)
- NEC2 model: yagi_uda.nec (in antenna directory)
- FCC Part 97 — Antenna installation requirements
1-3. SAFETY PRECAUTIONS
CHAPTER 2 — THEORY OF OPERATION
2-1. PARASITIC COUPLING PHYSICS
Driven element with parasitic reflector and one or more directors providing forward gain; boom-mounted. Gain is achieved through parasitic coupling: the driver excites the reflector and director elements by near-field induction. The reflector (5% longer than the driven element) carries current that lags by approximately 160° and re-radiates energy forward. Directors (5% shorter) carry current leading by approximately 140° and focus energy forward. Each additional director adds approximately 1 dB of gain at optimal spacing (0.2–0.25λ).
2-2. ELEMENT DESIGN RULES
For a 3-element Yagi on VHF: driver = 0.473λ, reflector = 0.505λ, director = 0.440λ. Spacing: reflector to driver = 0.2λ, driver to director = 0.25λ. These values produce forward gain of 7–8 dBd with front-to-back ratio of 20–25 dB. Gain estimate: 8–12 dBi forward. At UHF, dimensional tolerance is critical — elements must be within ±1 mm of design length for proper pattern formation.
2-3. FEED IMPEDANCE AND MATCHING
Feed impedance: 50 Ω (via gamma match or T-match at driven element). The driven element impedance drops below 50 Ω when directors are added (typically 20–40 Ω). A gamma match, T-match, or delta match raises this to 50 Ω. The gamma match uses a parallel conductor tapped on the driven element to form an L-network; the shorting bar position and gamma rod length are adjusted for 50 Ω + j0 Ω at the design frequency.
CHAPTER 3 — MATERIALS AND CONSTRUCTION
3-1. BILL OF MATERIALS
| Qty | Item | Specification |
|---|---|---|
| 1 | Boom | 6061-T6 aluminum tubing, OD 1–1.5 in; length per design; non-conductive for VHF if elements through-bolted |
| Per design | Element stock | 3/16–1/4 in aluminum rod for VHF; tape measure steel for portable UHF |
| 1 | Driven element | Folded or single dipole; see matching section below |
| 1 | Gamma match assembly | 3/16 in rod, 6–10 in from center; 10–50 pF trimmer; beta match alternative |
| 1 | SO-239 or N-type connector | Weatherproof; mount at boom junction |
3-2. ELEMENT DIMENSION FORMULAS
CHAPTER 4 — ASSEMBLY PROCEDURES
- Cut all elements to calculated lengths. Deburr cut ends. Label each element (reflector, driven, director-1, director-2 ...).
- Mark element mounting positions on boom. Verify spacings with tape measure before drilling. Drilling error >5 mm will degrade F/B ratio.
- Mount parasitic elements through boom (metal boom) using nylon or PVC element mounts, or through metallic boom with direct element-to-boom contact (boom correction required for metallic boom).
- Install driven element with insulated center block. Mount gamma match rod parallel to driver at 6–10 in from center; connect shorting bar and trimmer capacitor.
- Attach coax feed: center pin to gamma rod, braid to boom ground. Route cable along boom toward rear to minimize feed-line distortion of pattern.
- Perform initial SWR check per Chapter 5 before installing at height.
CHAPTER 5 — CALIBRATION PROCEDURE
- SOLT calibrate NanoVNA at feed point of antenna (on bench, 3 ft above ground for initial check).
- Set sweep: center frequency ±10%.
- Adjust gamma match: slide shorting bar toward center to increase impedance; away from center to decrease. Adjust trimmer capacitor to null reactance.
- Target: SWR <1.5:1, R = 45–55 Ω, X = −5 to +5 Ω at design frequency.
- Once matched, verify SWR across full target band. Record: SWR at center, SWR at band edges, F/B ratio (compare field-strength meter readings front vs. back).
- At final installed height (away from nearby objects), re-verify SWR — ground proximity affects impedance.
CHAPTER 6 — TUNING AND ADJUSTMENT
Gamma match adjustment: Moving the shorting bar changes the equivalent inductance in the match network; moving the trimmer changes reactance compensation. Typical procedure: set trimmer to midrange; slide shorting bar for minimum SWR; fine-adjust trimmer for minimum SWR; iterate. Director lengths can be adjusted ±2% to trade off gain vs. F/B ratio per NEC2 simulation.
CHAPTER 7 — VERIFICATION
| Parameter | Requirement | Pass/Fail |
|---|---|---|
| SWR at design frequency | < 1.5:1 | ____ |
| Forward gain (NEC2) | 8–12 dBi forward | ____ |
| Front-to-back ratio | ≥15 dB (3-element minimum) | ____ |
| Impedance at resonance | 45–55 Ω, X <±10 Ω | ____ |
| Efficiency | 88–95% | ____ |
APPENDIX A — CALCULATIONS AND FORMULAS
APPENDIX B — EXAMPLE RESULTS
| Freq | SWR | R (Ω) | X (Ω) | F/B (dB) | Notes |
|---|---|---|---|---|---|
| Band center | <1.3:1 | 50 | +2 | 20–25 | After gamma match optimization |
| Band edge low | <1.8:1 | 42 | −15 | 12–18 | Pattern narrows at edges |
| Band edge high | <1.8:1 | 58 | +18 | 12–18 | Director coupling increases |