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• General Knowledge
• Antenna knowledge
• Antenna Gain vs Length - Quick Reference
• Antennas reviews
• ✅ Gizont 167mm 915mHz VNA sweep
• ✅Gizont 40cm 915mHz VNA sweep
• ❌ ABBREE 40cm 915 mHz VNA sweep
• ✅Heliumtech 50cm VNA sweep
• ❌ Gorilley (Fake ALFA 5 dbi) VNA sweep
• ✅ RF Explorer 8dBi 1300mm VNA sweep
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General Knowledge

Antenna knowledge

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.

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.

Antennas reviews

✅ Gizont 167mm 915mHz VNA sweep

The Gizont 167mm is a very good narrow band antenna. It is likely a 1/2 wave dipole, They say it is a 10 dBi but is is more likely a 2-3dBi by lenght...

Purchase link: https://www.aliexpress.com/item/1005004607615001.html

Result: -34.2 dB return loss at 910mHz, a perfect result (1.04 VSWR at 32.925230828 cm)

✅Gizont 40cm 915mHz VNA sweep

The Gizont 167mm is a very good (slightly wider band) antenna. This is likely an impedance matched 5/8-wave or a colinear (multi-element). They say it is a 10 dBi but is is more likely a 5dBi by lenght...

The bigger Gizont 40cm
-24dB of return loss is pretty good
It is much wider band the the 167cm, it is a good one.

Purchase link: https://www.aliexpress.com/item/1005007121522456.html

Result: -24dB of return loss at 910mHz is pretty good.
(Calibrated for a span of 100MHz, but displays a span of 200MHz, so good enough)

❌ ABBREE 40cm 915 mHz VNA sweep

❌ DO NOT BUY

This is a sweep of the 40cm ABBREE antenna, the result of -7 dB of return loss is garbage, the antenna does not resonate anywhere from 1hz to 8GHz... Unusable. disappointing. Disgustang!

This is a bad antenna, do not purchase.

Purchase link just for reference: https://www.aliexpress.com/item/1005012228888082.html

✅Heliumtech 50cm VNA sweep

Heliumtech 5.8 dBi (50cm) 915 MHz Waterproof fiberglass Antenna with Mount Bracket Kit (N Female) from Canadian Wireless Supply on Amazon

Purchase link: https://www.amazon.ca/dp/B0BSYKSS22

Result: -20.9 dB at 915mHz is good enough

❌ Gorilley (Fake ALFA 5 dbi) VNA sweep

❌ DO NOT BUY

This is a sweep of the "5dBi" Gorilley antenna, which looks like an ALFA 5 dbi knockoff, which ended up being a fake.

Result: -6.4 dB at 915mHz is BAD, and it only slightly resonates at 1.2GHz...

This is a bad antenna, do not purchase.

Purchase link just for reference: https://www.amazon.ca/dp/B0G4CWQMDX

✅ RF Explorer 8dBi 1300mm VNA sweep

     

Fiberglass Antenna Kit For 902-928MHz 8dBi 1300mm

 

Comes with an antenna pole bracket and a 1m N female to RP-SMA male (RP is a weird inversion)

This antenna is the backbone of our core Meshcore network in Ottawa.
It has been proven to allow for long distance links while maintaining omnidirectionality.

Purchase Link: https://www.seeedstudio.com/RF-Explorer-LoRa-Fiberglass-Antenna-Kit-902-928MHz-8dBi-1300mm-p-5278.html

 

Result: -26.5 dB of return loss at 910.525MHz is excellent

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