Radio Where you learn about radio, and the hardware we use. General Knowledge To share trick on how to maximize your signal Antenna knowledge For information about how antennas work and how to choose one 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 Learn about antenna models and sweeps ✅ 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 Cables Filters To share good special purpose filters  Cavity Filters To share sweeps of specific specialized Cavity Filters