Table of Contents
CHAPTER 1 — GENERAL INFORMATION
1-1. SCOPE
This manual covers design, construction, and calibration of the Moxon Rectangle Antenna directional antenna for operation on 20M, 15M, 10M (14–29 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: moxon.nec (in antenna directory)
- FCC Part 97 — Antenna installation requirements
1-3. SAFETY PRECAUTIONS
CHAPTER 2 — THEORY OF OPERATION
2-1. PARASITIC COUPLING PHYSICS
Modified cubical quad with folded tips to form a compact rectangular two-element beam; one driven, one reflector. 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: 7–10 dBi (F/B > 35 dB). 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 direct feed (driven element naturally 50 Ω resonant). 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) | 7–10 dBi (F/B > 35 dB) | ____ |
| Front-to-back ratio | ≥15 dB (3-element minimum) | ____ |
| Impedance at resonance | 45–55 Ω, X <±10 Ω | ____ |
| Efficiency | 90–97% | ____ |
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 |