Antennas & RF Basics
General Guides
The antenna is the best upgrade
Transmit power on these boards is fixed and small. The antenna and where you put it are the variables you actually control, and they matter far more than any firmware setting.
Gain, in one paragraph
Antenna gain is measured in dBi. An antenna cannot add power; it reshapes where the power goes. Higher gain squashes the radiation pattern from a ball toward a flat donut aimed at the horizon. On flat terrain that is a win. In hilly terrain a high-gain antenna can overshoot nodes above or below you, so moderate gain is often the better choice. Every 3 dB is a doubling of effective power in the favored direction, and in free space roughly 6 dB of extra link budget doubles range.
Stock antennas
The bundled antennas vary from acceptable to decorative. Two checks before blaming the firmware for bad range: confirm the antenna is actually built for your band (868 or 915 MHz antennas are not interchangeable with 433 MHz ones, and some bundled units are tuned poorly for either), and confirm it is screwed on snugly. A known-good aftermarket whip in the right band is a cheap experiment.
Ignore listings promising huge gain from a hand-sized whip. A small vertical claiming 12 dBi or more is lying about physics; realistic figures for compact whips are 2 to 5 dBi.
Polarization
These networks use vertical polarization. Keep antennas pointing straight up, on both ends. A node lying flat on a desk with its antenna horizontal is cross-polarized with every upright node around it, which can cost 20 dB. Standing the node up is a free, large improvement.
Height, then gain, then power
At these frequencies, range is mostly a line-of-sight problem (see the Range Expectations guide). Getting the antenna above roofline, on a mast, or up a hill routinely does more than any antenna swap, and any antenna swap does more than a transmit power increase. Spend effort in that order.
Cable is not free
Coax attenuates signal, and at 900 MHz the thin stuff is punishing: RG174 pigtail cable loses roughly 1 dB per meter, so a 3 meter run eats half your power before the antenna sees it. Thick low-loss cable (LMR-400 class) loses closer to 0.13 dB per meter. Practical options, best first:
- Put the whole node at the antenna (PoE or a battery up the mast) and skip long coax entirely.
- Short run of low-loss cable.
- Long run of thin cable: only if you enjoy wondering why range got worse after mounting the antenna higher.
Ground planes
Quarter-wave verticals (including mag-mount whips) need a ground plane under them: a car roof, a metal plate, or radial wires. A quarter wave at 915 MHz is about 8.2 cm (8.6 cm at 868 MHz), so even a modest metal sheet under the mount helps. Half-wave and end-fed designs are more forgiving without one.