# Antenna and RF FAQ # Do I need an external antenna? The stock antenna that comes with most LoRa boards is a rubber duck (flexible whip) antenna, typically 1-3 dBi gain (often a quarter-wave stubby around 2 dBi). For many use cases, this is adequate - but upgrading to an external antenna is one of the most cost-effective improvements you can make. ## When the Stock Antenna is Fine - Indoor portable use (office, home) within 200-500m of your nearest mesh node - Temporary deployments where you're moving frequently - Testing and development before a permanent installation - Dense urban areas with many nearby nodes (short hop distances) ## When You Should Upgrade - **Fixed outdoor installation** - Any permanent outdoor node should use an external antenna rated for outdoor use. Stock rubber ducks are not weatherproof. - **Coverage issues** - If you can't reach nodes you'd expect to reach, a better antenna is the first thing to try. - **Backbone repeater** - Repeaters covering a neighborhood or city need the best possible antenna. A 5-8 dBi fiberglass omni provides several dB more gain than a stock whip, which can substantially extend range (roughly doubling it under line-of-sight conditions; the gain in cluttered terrain is smaller). - **Point-to-point link** - If you're trying to bridge two specific locations, a directional yagi (commonly 6-10 dBi for compact 915 MHz models) extends range significantly. **FCC compliance:** on US 915 MHz, FCC 15.247(b)(4)(i) requires you to reduce conducted transmit power 1 dB for every dB of antenna gain above 6 dBi - and, unlike the 2.4 GHz band, the 902-928 MHz band has *no* point-to-point exception that relaxes this. For example, with a 12 dBi yagi you must drop conducted power roughly 6 dB below 1 W to stay within the EIRP limit. Most firmware lets you set TX power accordingly; do not run full power behind a high-gain antenna. ## What External Antenna to Buy For most fixed outdoor deployments, a 915 MHz fiberglass omnidirectional antenna is the right choice: - **Taoglas TI.92.2113 (3 dBi)** - $15-20, compact, good for moderate ranges - **Proxicast 5 dBi (ANT-DB5-5)** - $25-35, good all-around outdoor omni - **Taoglas FXP73 (5 dBi, mag base)** - $25-40, great for vehicle or temporary mounts - **L-com HG908U-PRO (8 dBi)** - $45-60, excellent for high-gain omni backbone nodes ## Connector Adapters Most LoRa boards use a standard SMA connector (male pin on the antenna/pigtail, female body on the board) or a u.FL connector. External antennas typically use an N-connector or SMA. **Watch out:** SMA and RP-SMA (reverse-polarity SMA) look almost identical but do not mate - RP-SMA swaps the center pin and socket, so an SMA antenna will not connect to an RP-SMA board (and vice versa). Check which gender and polarity your specific board revision uses before ordering a pigtail or antenna. Match your connectors: - **Heltec V3, T-Beam:** SMA female on board - use SMA male on pigtail or antenna (verify your revision, as some units ship RP-SMA) - **RAK4631:** u.FL (IPEX) connector - needs u.FL to SMA pigtail (~$5) to connect to any standard antenna - **T-Deck, T-Echo:** SMA female - use SMA male pigtail or direct-connect SMA antenna # What is the difference between dBi and dBd antenna gain? Antenna gain specifications use two different reference points - dBi and dBd - and confusing them leads to incorrect [link budget calculations](https://wiki.meshamerica.com/books/network-planning/page/link-budget-calculations). Here's what each means and how to convert between them. ## The Reference Antennas - **dBi (decibels relative to isotropic)** - Compares gain to a theoretically perfect isotropic radiator (a point that radiates equally in all directions - a perfect sphere). This is a theoretical reference that doesn't exist in practice. - **dBd (decibels relative to dipole)** - Compares gain to a half-wave dipole antenna, which is the most common practical antenna type and a natural reference for antenna engineers. ## The Conversion ``` dBi = dBd + 2.15 Examples: 0 dBd (dipole reference) = 2.15 dBi 3 dBd = 5.15 dBi (approximately 5 dBi) 5.85 dBd = 8 dBi 9 dBd = 11.15 dBi (approximately 11 dBi) ``` ## Which is Used in Practice? Most commercial antenna manufacturers use dBi because the numbers look higher (marketing benefit). For the 902-928 MHz ISM band that matters here, FCC Part 15 expresses its EIRP and antenna-gain limits using the isotropic (dBi) reference - so convert any dBd spec to dBi (add 2.15) before checking it against the 4 W (36 dBm) EIRP ceiling or the 6 dBi antenna-gain threshold. Most link budget calculators accept either unit, as long as you're consistent. **Rule of thumb:** When comparing antennas, make sure you're comparing the same units. A "5 dBd" antenna and a "5 dBi" antenna are NOT equivalent - the dBd antenna is 2.15 dB better. This difference can mean the difference between a reliable link and a marginal one. ## Practical Antenna Gain Reference
Antenna TypeTypical Gain (dBi)Typical Gain (dBd)
Stock rubber duck~0 to 2 dBi~-2 to 0 dBd
Quarter-wave with ground plane~5 dBi (ideal ground plane; less in practice)~2.85 dBd
Half-wave dipole2.15 dBi0 dBd
5/8 wave vertical4-5 dBi2-3 dBd
3-element yagi7-8 dBi5-6 dBd
5-element yagi10-11 dBi8-9 dBd
Commercial 5 dBi fiberglass5 dBi2.85 dBd
Commercial 8 dBi fiberglass8 dBi5.85 dBd
Note: a quarter-wave monopole over an *ideal* (infinite, perfectly conducting) ground plane radiates into a half-space and so has roughly 3 dB more gain than a dipole - about 5 dBi. Real, finite ground planes deliver less than this, but it is not equal to a plain dipole. Use this table as the single canonical reference for stock-antenna gain figures across the wiki. ## What Gain Actually Buys You Every 3 dB of additional gain (all else equal) doubles the effective radiated power. Because free-space range scales with the square root of the power ratio (range ∝ √EIRP), gain translates to range as: - 3 dB gain improvement ≈ 41% range increase in free space (√2 = 1.41x) - 6 dB gain improvement ≈ 100% range increase / double in free space (√4 = 2x) - 10 dB gain improvement ≈ 216% range increase in free space (√10 = 3.16x) These are free-space figures. In practice real-world gains are lower due to terrain and building losses, and higher-gain antennas are also constrained by the 4 W (36 dBm) EIRP limit - you often cannot legally or usefully realize the full theoretical range gain. Still, the relative improvement from a better antenna (within the legal limit and with good siting) is significant.