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 in free space is approximately 164 mm (6.4 inches). Note that a real resonant element is about 5% shorter than the free-space half-wave due to end effect, so a driven dipole element typically cuts to around 155 mm rather than the full 164 mm. Each element is cut to a specific length and spaced precisely on the boom, then tuned with a NanoVNA.
5-Element Yagi Plans for 915 MHz
A typical 5-element yagi provides approximately 9-10 dBi (roughly 7-8 dBd) gain with a tight forward beam. Reaching 12 dBi generally requires 7-8 elements. With clear line of sight, a full Fresnel zone, adequate height, and a matching antenna on the far end, such a yagi can support links of 10 km or more; the realistic range depends on transmit power, terrain, and spreading factor as much as on antenna gain, so treat long-range figures as best-case line-of-sight, not routine.
FCC compliance note: A yagi above 6 dBi exceeds the 6 dBi reference gain in FCC 15.247(b)(4)(i). When used at 902-928 MHz, conducted transmit power must be reduced dB-for-dB for every dB of antenna gain above 6 dBi (for example, a 10 dBi antenna requires conducted power no greater than 26 dBm). The EIRP ceiling of about 36 dBm (4 W) is a derived limit (30 dBm conducted + 6 dBi), not a bonus you add gain on top of, and there is no relaxed point-to-point antenna allowance at 915 MHz.
Element dimensions (915 MHz, 5-element, nominal - tune with a NanoVNA):
Reflector: 178 mm (7.01")
Driven element: 163 mm (6.42") - center-fed dipole (trim toward ~155 mm for resonance)
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: Built as a split (center-fed) dipole fed through a gamma match or 50-ohm hairpin match - see the construction steps below.
- Feedline: Use RG-174 or LMR-195 only for a short (under ~1 m) pigtail, since RG-174 loses roughly 1 dB per metre (~30 dB per 100 ft) at 915 MHz. For any real cable run, use low-loss LMR-240 or LMR-400-class coax. SMA connector at the antenna end.
- Hardware: 1/4-20 stainless bolts and nylon locknuts to mount elements to boom.
Construction Steps
- Cut all elements to specified lengths using a hacksaw or pipe cutter. Deburr ends.
- Mark boom at element spacing positions.
- Drill 3/16" holes through boom at each position.
- Thread the reflector and directors through their boom holes and secure with nylon locknuts (finger-tight then 1/4 turn more). These are single, continuous rods.
- Build the driven element as a split dipole: instead of one continuous rod, use two collinear half-elements (each about a quarter-wavelength, ~38 mm) mounted on an insulating block at the boom center, with a small gap (~3-5 mm) between their inner ends.
- Feed it with one of these matches:
- Direct/gap feed (simplest): Solder the coax center conductor to one half-element and the coax shield to the other half-element across the center gap. A bare split dipole presents roughly 50-70 ohms, close enough to test; add a current balun (a few turns of the coax through a ferrite at the feed point) to keep RF off the coax shield.
- Gamma match (for a continuous-rod driven element): If you prefer one solid driven rod grounded to the boom at its center, run a parallel gamma rod (about 1/10 the driven-element length) spaced ~10-15 mm alongside one half, connect the coax center to the gamma rod through a small series capacitor (a few pF, often a short coax-sleeve trombone), and bond the coax shield to the driven element at center. Adjust the gamma rod length, spacing, and capacitance for lowest SWR.
- Weatherproof the feed point and connector after tuning.
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. Adjust the match (and trim the driven element toward resonance) as needed.
- Compare RSSI at a fixed test point vs. a reference omni - the yagi should show roughly 6-10 dB improvement in its forward direction. Measured improvement varies with the environment, multipath, and reference antenna, so this is a typical figure, not a guarantee.
- Note the half-power beamwidth: a 5-element yagi has roughly 55 degree horizontal beamwidth. For best performance, aim within about +/-15 degrees (roughly half the half-power beamwidth) of the far station.
Building a Collinear Vertical Antenna
This page covers two simple, easy-to-build omnidirectional verticals for 915 MHz - a J-pole and a 5/8-wave vertical - both a significant improvement over the stock rubber duck antennas included with most LoRa boards. (A true multi-element collinear, 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 Vertical for 915 MHz
The J-pole is one of the simplest omnidirectional verticals to build. It is an end-fed half-wave radiator fed through a quarter-wave parallel matching stub (the "J") - it is not a collinear. Its gain is essentially that of a half-wave dipole, about ~0 dBd (~2.15 dBi); 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 ground plane, not 3.5 dBd.
915 MHz J-Pole dimensions:
Radiator: 163 mm (6.42") - connects to matching section
Matching section: ~82 mm (3.23") - quarter-wave parallel stub (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 offers roughly 3 dB of gain over 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-wave, 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 ~82 mm (quarter-wave), angled 45 degrees downward
Feedpoint: SMA or N connector at base
Impedance: a 5/8-wave element is NOT naturally 50 ohms - it presents
capacitive reactance and generally needs a base matching/loading coil.
(Drooping radials alone match a quarter-wave, not a 5/8-wave.)Weatherproofing a DIY Antenna
Any antenna installed outdoors needs weatherproofing to survive years of 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 to a 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 a reliable weatherproof seal.
- Mounting: Stainless steel hardware is the practical choice; it is only moderately compatible with aluminum, so in coastal or salty environments use anti-seize or dielectric isolation to limit galvanic 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) | ~0 dBd / ~2.15 dBi | Omnidirectional | Easy | Home repeater |
| 5/8 wave vertical | ~1 dBd / ~3 dBi (≈3 dB over a quarter-wave whip) | Omni, low angle | Medium | Long-range omni |
| 5-element yagi | ~7-8 dBd / ~9-10 dBi | 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.