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.