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 dBi** 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.

 

Thoughts?