# Antenna Gain vs Length - Quick Reference

### Antenna Gain vs Length - Quick Reference for 915 MHz Omnis

If you've ever wondered how to ballpark an antenna's gain just by looking at it, here's a cheat sheet:

**2 dBi** → ~120-170 mm  
**3 dBi** → ~200-250 mm  
**4 dBi** → ~280-350 mm  
**5 dBi** → ~400-450 mm  
**6 dBi** → ~500-600 mm  
**7 dBi** → ~800-900 mm  
**8 dBi** → ~1200-1350 mm  
**9 dBi** → ~1500-1650 mm

#### Why does this work?

At 915 MHz, one wavelength (λ) is about 328 mm. Omnidirectional collinear antennas get their gain by stacking half-wave elements vertically with phasing stubs between them. More elements = longer antenna = more gain.

\- **2 dB**i is basically a half-wave dipole (λ/2 = 164 mm). This is where most stock LoRa whip antennas land.  
\- **3 dBi** gets into 5/8-wave territory (~205 mm plus matching network)  
\- **4-5 dBi** covers short collinears and 2-element designs  
\- **6 dBi** around 550 mm is a 3-element collinear (~1.68λ)  
\- **7 dBi** around 850 mm is a 4-5 element collinear (~2.6λ)  
\- **8 dBi** around 1275 mm is pushing 7+ elements (~3.9λ)  
\- **9 dBi** around 1600 mm is an 8+ element array (~4.88λ)

The general rule is that doubling your elements gets you about +3 dB of gain. You'll notice the gaps get smaller as you go up because at higher element counts you hit diminishing returns from losses in the phasing network.

#### One caveat

Below 5 dBi, the length-to-gain correlation gets less reliable. Manufacturers use loaded and helical designs to shrink things down, so you'll see "2 dBi" antennas that are only 50-80 mm because they're coiled internally. The eyeball method really shines from 5 dBi and up where you're looking at full-size collinear builds.

#### TL;DR

Under 200 mm? Probably 2-3 dBi. Around 300-450 mm? 4-5 dBi range. Half a metre, ~6 dBi. Approaching a metre, ~7 dBi. Over a metre, ~8 dBi. Pushing 1.5m+, you're in 9 dBi territory. No spec sheet needed, just eyeball it.

#### Choosing the Right dBi and Wave Shape for Your Environment

• **2–3 dBi** (Omni / Rubber Duck)

 • Wave Shape: Tall, rounded sphere.

 • Best For: Dense urban areas, indoor gateways, mobile nodes (backpacks, cars, or kayaks), or hilly terrain where nodes are located above or below you. A wide vertical beam forgives tilt and varying node elevations.

• **5–6 dBi** (Short Fiberglass)

 • Wave Shape: Balanced, medium donut.

 • Best For: Suburban environments or mixed terrain. It clears local ground obstructions better than a 3 dBi antenna while maintaining enough vertical beamwidth to reach nodes that aren't on the exact same flat plane.

• **8–9 dBi** (Tall Fiberglass)

 • Wave Shape: Thin, flat horizontal disk.

 • Best For: Open, flat, rural terrain with clear line-of-sight. Warning: If mounted very high on a tall mast in non-flat terrain, the flat beam will literally shoot straight over nearby nodes, resulting in zero coverage for devices close to the ground.