# DIY Antenna Construction # Building a 915 MHz Yagi Antenna A yagi antenna provides significant directional gain for point-to-point links - ideal for connecting two backbone nodes across a valley, mountain, or city. Building your own 915 MHz yagi is a rewarding project that costs $10-20 in materials vs. $50-150 for a commercial equivalent. ## Yagi Design Fundamentals A yagi consists of three element types mounted on a boom: - **Reflector** - Behind the dipole; slightly longer than a half-wavelength; increases gain in forward direction - **Driven element (dipole)** - The active element connected to the feedline - **Directors** - In front of the dipole; slightly shorter than a half-wavelength; focus the beam forward At 915 MHz, a half-wavelength is approximately 163mm (6.4 inches). Each element is cut to specific length and spaced precisely on the boom. ## 5-Element Yagi Plans for 915 MHz A 5-element yagi provides approximately 8-10 dBd (10-12 dBi) gain with a tight forward beam - good for links of 10-50+ km. ``` Element dimensions (915 MHz, 5-element): Reflector: 178 mm (7.01") Driven element: 163 mm (6.42") - center-fed dipole Director 1: 151 mm (5.94") Director 2: 147 mm (5.79") Director 3: 144 mm (5.67") Spacing from reflector: Driven element: 49 mm (1.93") Director 1: 115 mm (4.53") Director 2: 210 mm (8.27") Director 3: 330 mm (13.0") ``` ## Materials - **Elements:** 3/16" (4.8mm) aluminum rod or welding rod. Available at hardware stores. - **Boom:** 1/2" (12mm) square aluminum extrusion, 400mm long. Also available as wooden dowel (slightly less rigid but fine for hobby use). - **Driven element:** Requires a folded dipole or gamma match; or use a commercial 50-ohm hairpin match. The easiest approach: use 1/8" aluminum rod with a coax connector at the center. - **Feedline:** RG-174 or LMR-195; SMA connector at the antenna end. - **Hardware:** 1/4-20 stainless bolts and nylon locknuts to mount elements to boom. ## Construction Steps 1. Cut all elements to specified lengths using a hacksaw or pipe cutter. Deburr ends. 2. Mark boom at element spacing positions. 3. Drill 3/16" holes through boom at each position. 4. Thread elements through boom holes and secure with nylon locknuts (finger-tight then 1/4 turn more). 5. Center-solder SMA connector to driven element at the exact center point. 6. Attach feedline to driven element. ## Testing Your Yagi After construction, verify performance: - Use a NanoVNA to check SWR at 915 MHz. Target: SWR less than 2:1, ideally below 1.5:1. - Compare RSSI at a fixed test point vs. a reference omni - the yagi should show 6-10 dB improvement in its forward direction. - Note the half-power beamwidth: a 5-element yagi has approximately 50-60 degree horizontal beamwidth. Alignment must be accurate to within 25 degrees for full performance. # Building a Collinear Vertical Antenna A collinear vertical antenna provides omnidirectional coverage with moderate gain (3-6 dBd) - a significant improvement over the stock rubber duck antennas included with most LoRa boards. A 3-element collinear is straightforward to build with basic tools. ## How a Collinear Works A collinear antenna consists of multiple half-wave dipole elements stacked vertically and fed in phase. Each additional element increases the gain and makes the radiation pattern more disk-shaped (more horizontal, less toward sky/ground) - which is exactly what you want for a terrestrial mesh network. ## Simple J-Pole Collinear for 915 MHz The J-pole is the simplest high-gain collinear to build. It uses a matching stub (the "J") to feed a half-wave radiator. Performance: ~3.5 dBd gain over a basic quarter-wave. ``` 915 MHz J-Pole dimensions: Radiator: 163 mm (6.42") - connects to matching section Matching section: 163 mm (6.42") - parallel to radiator Shorting bar: 40 mm (1.57") - connects bottom of radiator to top of short arm Feed point: 37-42mm from bottom of matching section (tune for min SWR) Material: 3/32" or 1/8" brass rod, or stiff copper wire (14 AWG solid) ``` ## 5/8 Wave Vertical A 5/8 wavelength vertical with a ground plane provides approximately 3 dBd gain with a lower takeoff angle than a quarter-wave - excellent for long-range terrestrial links: ``` 5/8 wave vertical at 915 MHz: Vertical element: 203 mm (7.99") Ground plane radials: 4x at 163 mm (6.42"), angled 45 degrees downward Feedpoint: SMA or N connector at base Impedance: ~50 ohms with 45-degree radials (some designs use a coil matching section) ``` ## Weatherproofing a DIY Antenna Any antenna installed outdoors needs weatherproofing to survive years of exposure: - **UV protection:** Coat metal elements with cold galvanizing compound or clear lacquer spray. Aluminum naturally oxidizes, which is protective; copper and brass oxidize to patina that increases resistance - coat with lacquer. - **Connector protection:** Wrap SMA/N connector base with self-amalgamating tape (silicone rubber tape that bonds to itself). Apply starting from the cable, overlapping onto the connector, then back. Provides IP67+ waterproofing. - **Mounting:** Use stainless steel hardware only (no galvanic corrosion with aluminum). Coat any carbon steel hardware with cold galvanizing compound. - **Housing:** For clean installations, insert the antenna inside a length of PVC pipe (Schedule 40, 3/4" inside diameter for most quarter-wave to collinear antennas). PVC is RF-transparent at 915 MHz with minimal loss. ## Gain Comparison: Antennas for 915 MHz
Antenna TypeGainPatternBuild DifficultyBest Use
Stock rubber duck-3 to 0 dBdOmnidirectionalNone (included)Portable/indoor only
Quarter-wave with radials0 dBdOmnidirectionalEasyBasic outdoor fixed
J-Pole3.5 dBdOmnidirectionalEasyHome repeater
5/8 wave vertical3 dBdOmni, low angleMediumLong-range omni
5-element yagi9 dBdDirectional 55°MediumPoint-to-point link
Commercial 5 dBi fiberglass5 dBi (~3 dBd)OmnidirectionalNone (buy)Outdoor repeater