Building a Collinear Vertical Antenna
AThis collinearpage covers two simple, easy-to-build omnidirectional verticals for 915 MHz - a J-pole and a 5/8-wave vertical antenna- provides omnidirectional coverage with moderate gain (3-6 dBd) -both a significant improvement over the stock rubber duck antennas included with most LoRa boards. (A 3-true multi-element collinearcollinear, which stacks several half-wave sections in phase to reach ~3-6 dBd, is described conceptually below but is a more advanced build not detailed here.) The J-pole and 5/8-wave verticals below are 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. Note that the two single-element verticals built on this page (the J-pole and the 5/8-wave) are not multi-element collinears; they are simpler designs with dipole-class gain, included here as practical starting points.
Simple J-Pole CollinearVertical for 915 MHz
The J-pole is one of the simplest high-gainomnidirectional collinearverticals to build. It usesis an end-fed half-wave radiator fed through a quarter-wave parallel matching stub (the "J") to- feedit is not a collinear. Its gain is essentially that of a half-wave radiator.dipole, Performance:about ~3.50 dBd gain(~2.15 overdBi); the widely-repeated "~3 dBd J-pole" claim is a myth. Its real advantages are a clean omnidirectional pattern and an easy feed, not extra gain. A J-pole gives roughly 2-3 dB more than a basic quarter-wave.wave ground plane, not 3.5 dBd.
915 MHz J-Pole dimensions:
Radiator: 163 mm (6.42") - connects to matching section
Matching section: 163~82 mm (6.42"3.23") - quarter-wave parallel tostub (half the radiator length)
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)Tuning the feedpoint: attach the coax at about 40 mm up from the bottom of the matching stub, sweep SWR with a NanoVNA (see the Testing & Tuning pages), and slide the tap a few millimetres up or down toward the lowest SWR at 915 MHz. The exact feed position depends on conductor diameter, so expect to fine-tune. These dimensions are a starting point - verify against a 915 MHz J-pole/Slim Jim calculator and tune with a NanoVNA.
5/8 Wave Vertical
A 5/8 wavelength vertical with a ground plane providesoffers approximatelyroughly 3 dBddB of gain withover a quarter-wave whip - that is about 1-1.5 dBd (~3 dBi) over a dipole, not 3 dBd. Its main benefit is a lower takeoff angle than a quarter-wavewave, -which is excellent for long-range terrestrial links:
5/8 wave vertical at 915 MHz:
Vertical element: 203 mm (7.99") nominal; trim ~5% shorter for end effect, tune with a NanoVNA
Ground plane radials: 4x at 163~82 mm (6.42")quarter-wave), angled 45 degrees downward
Feedpoint: SMA or N connector at base
Impedance: ~a 5/8-wave element is NOT naturally 50 ohms with- 45-degreeit presents
capacitive reactance and generally needs a base matching/loading coil.
(Drooping radials (somealone designs usematch a coilquarter-wave, matchingnot section)a 5/8-wave.)Weatherproofing a DIY Antenna
Any antenna installed outdoors needs weatherproofing to survive years of exposure:exposure. Any DIY antenna installed outdoors must also be grounded and surge-protected (see Grounding and Lightning Protection) and kept well clear of overhead power lines during and after installation; solder in a well-ventilated area.
- UV protection: Coat metal elements with cold galvanizing compound or clear lacquer spray. Aluminum naturally
oxidizes,oxidizeswhichtoisaprotective;protective oxide layer against bulk corrosion, but that oxide raises contact resistance at joints and connectors, so bare aluminum joints still need protection; 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+awaterproofing.reliable weatherproof seal. - Mounting:
Use stainlessStainless steel hardware is the practical choice; it is only(nomoderately compatible with aluminum, so in coastal or salty environments use anti-seize or dielectric isolation to limit galvaniccorrosion with aluminum).corrosion. 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
Gains below are given in both dBd (relative to a half-wave dipole) and dBi (relative to an isotropic source). Convert with dBi = dBd + 2.15.
| Antenna Type | Gain (dBd / dBi) | Pattern | Build Difficulty | Best Use |
|---|---|---|---|---|
| Stock rubber duck | -3 to 0 dBd / -0.85 to 2.15 dBi | Omnidirectional | None (included) | Portable/indoor only |
| Quarter-wave with radials | ~0 dBd / ~2 dBi | Omnidirectional | Easy | Basic outdoor fixed |
| J-Pole (end-fed half-wave) | Omnidirectional | Easy | Home repeater | |
| 5/8 wave vertical | ~1 dBd / ~3 | Omni, low angle | Medium | Long-range omni |
| 5-element yagi | Directional ~55° | Medium | Point-to-point link | |
| Commercial 5 dBi fiberglass | ~3 dBd / 5 dBi | Omnidirectional | None (buy) | Outdoor repeater |
Note: antennas above 6 dBi require a dB-for-dB reduction of conducted power under FCC 15.247(b)(4)(i) in the 902-928 MHz band - see the FCC Regulations and EIRP Reference page.