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 Type | Gain | Pattern | Build Difficulty | Best Use |
|---|---|---|---|---|
| Stock rubber duck | -3 to 0 dBd | Omnidirectional | None (included) | Portable/indoor only |
| Quarter-wave with radials | 0 dBd | Omnidirectional | Easy | Basic outdoor fixed |
| J-Pole | 3.5 dBd | Omnidirectional | Easy | Home repeater |
| 5/8 wave vertical | 3 dBd | Omni, low angle | Medium | Long-range omni |
| 5-element yagi | 9 dBd | Directional 55° | Medium | Point-to-point link |
| Commercial 5 dBi fiberglass | 5 dBi (~3 dBd) | Omnidirectional | None (buy) | Outdoor repeater |