RF link and range
RF Link and Range at https://rotorlab.app/rf calculates how far your radio links actually reach. Transmit power alone does not set range; the link budget does: transmit power and antenna gains on one side, receiver sensitivity on the other, less the path loss between them and a fade margin held in reserve for real-world conditions. An aircraft must close every radio link it carries (a control uplink, a video/telemetry downlink, and sometimes a separate data link), and the usable range is set by the shortest of them.
Every figure recomputes from the standard radio equations as you type: free-space path loss (Friis), the line-of-sight horizon from the two antenna heights, first Fresnel-zone clearance over flat ground, and the two-ray ground-reflection model. Terrain, interference, and multipath are not modeled, so measured range remains definitive.
Choosing where the radios come from#
The Radios from selector offers:
- Standalone (edit manually): two default links, a control uplink and a video/telemetry downlink, seeded from presets for you to edit.
- Current build (from the app): the working build. The tool recognizes radio systems by component name (an ELRS or Crossfire control link, a digital HD or analog video system, and separate telemetry radios such as SiK, RFD900, or Microhard units) and builds one card per link, each seeded from the closest preset. Where an obvious transmit power appears in a part name, such as "600mW", it is parsed and applied. A note above the cards says how many links were read from the build.
- Library: (name): any saved build, recognized the same way.
If a build names no radios at all, you get the two standalone defaults.
Each link's inputs#
Every link card starts from a Radio preset (a starting point to edit). The presets are class-nominal figures for common systems, not claims about any one product:
- ELRS 915 MHz (long range control)
- ELRS 2.4 GHz (control)
- TBS Crossfire 915 MHz (control)
- DJI O3/O4 5.8 GHz (video + control)
- Analog 5.8 GHz (video)
- 900 MHz telemetry (SiK)
After picking a preset, edit any field:
| Field | Unit | What it is |
|---|---|---|
| Frequency | MHz | The link's operating frequency; path loss grows with it |
| TX power | mW | Transmitter output power |
| TX antenna gain | dBi | Gain of the transmitting antenna |
| RX antenna gain | dBi | Gain of the receiving antenna |
| RX sensitivity | dBm | The weakest signal the receiver can still use (more negative is better) |
| Cable loss | dB | Loss between the transmitter and its antenna |
| Other loss | dB | Polarization, pointing, and other real-world extras |
| Fade margin | dB | Signal held back above sensitivity as a safety reserve |
| Ground antenna height | m | Height of the ground-station antenna |
| Drone altitude | m | The aircraft's altitude |
The two height fields are labeled by direction: on the control uplink the transmitter is on the ground, on a downlink it is the aircraft.
Reading the outputs#
The big number on each card is the link's usable range in km, with a chip saying what limits it: limited by link budget (the signal runs out of margin first) or limited by line-of-sight (the horizon gets in the way first). The detailed outputs are:
| Output | Meaning |
|---|---|
| Link-budget range | The distance at which free-space path loss uses up the whole budget |
| Line-of-sight horizon | The geometric horizon from the two antenna heights, used as the hard limiter because it is the conservative one |
| Radio horizon (4/3 earth) | Slightly farther than geometric, because the atmosphere bends radio downward |
| EIRP | Effective radiated power: TX power plus TX gain minus cable loss, in dBm |
| TX power | The transmit power expressed in dBm |
| Fresnel clearance | Clearance of the first Fresnel zone at the midpoint, as a percent; 100% or more is good, below 60% is flagged |
| Two-ray breakpoint | The distance beyond which a low antenna's ground reflection changes the roll-off |
| Two-ray range | Range under the two-ray model, where signal falls as the fourth power of distance past the breakpoint |
Each card also draws received power versus distance: a free-space curve, a two-ray curve, a dashed line at sensitivity plus fade margin, and a marker at the resulting maximum range.
Above the cards, the headline verdict states the usable range of the whole system and which link limits it. When more than one link is shown, each link's range and limiter is listed, because the drone can go only as far as the shortest.
Note
A link needs more than a clear straight line; it needs a clear ellipsoid around it. The Fresnel clearance figure tells you whether flat ground already intrudes on that ellipsoid at your antenna heights. Raising the ground antenna is often the cheapest range upgrade.
Sanity checks#
Two quick checks tell you whether a result is believable:
- If the chip says limited by line-of-sight, more transmit power changes nothing. Only antenna height (either end) moves the horizon.
- Doubling transmit power adds 3 dB, which under free-space loss multiplies link-budget range by about 1.4. If a small power change produced a huge range change, look at whether the limiter switched between line-of-sight and link budget.