Every long-range video link datasheet carries a headline number — 25 km, 30 km, 40 km — and every integration team eventually discovers that the number was measured on a clear day over water, with a matched antenna pair and a favourable regulatory power setting. When the same link flies at 300 m over hilly terrain in a crowded band, the usable range halves. This guide breaks the 25 km claim into the three budgets that actually decide link closure, using published specifications from production hardware.
The three budgets behind a range claim
Range is not a property of a transmitter; it is the result of a power budget, a bandwidth budget and an antenna budget that must all close at the same time. Get one wrong and no amount of extra milliwatts rescues the flight.
Power budget
Start with radiated power and work backwards. A COFDM air unit rated at 33 dBm EIRP is not 2 W of RF at every regulatory region: the same hardware ships limited to under 30 dBm for FCC, under 20 dBm for SRRC, under 25 dBm for MIC and under 14 dBm for CE. If your integration is certified in Europe, plan the range model at 14 dBm, not 33 dBm. This single line item explains most of the discrepancy between demo flights and delivered systems.
Bandwidth and sensitivity budget
COFDM trades bandwidth for robustness. A 5 MHz channel carrying 1080p60 spends much of its capacity on forward error correction and pilot carriers, which is precisely why it survives multipath and low signal-to-noise conditions that would break a wider analogue link. The receiver sensitivity you need is therefore a function of the video bit rate and the modulation you accept, not a fixed number in the brochure.
Antenna budget
Two 2 dBi LHCP antennas on the aircraft and a 10 dBi ground panel give a healthy link margin only if the ground antenna is pointed. Circular polarisation also buys several dB of multipath rejection, which matters more than raw gain in urban and canyon flight. A 14 dBi ground antenna tightens the beam further: better range on axis, far less tolerance for tracking error.
Reference specification set: SJ-TC403A
A concrete example of how the numbers sit together in a 25 km class system.
- Frequency 2.7–7 GHz, 5 MHz channel bandwidth, 36 frequency points.
- Transmission distance ≥25 km; TX power max 33 dBm EIRP (FCC <30, CE <14, SRRC <20, MIC <25 dBm).
- Video: visible 1080p60 plus infrared 640×512 at 50 fps, simultaneous.
- Air unit 34×34×18 mm, ≤30 g, 11.1–40 V input, CNC aluminium housing with micro turbo fan.
- Ground receiver: HDMI, USB-C PD display, USB 3.2, UART 3.3 V TTL, TF card to 1 TB, 9–30 V at 2 A.
- Antennas: LHCP 3 dBi air unit, LHCP 14 dBi ground unit; operating range −20 to +60 °C.
- OSD in canvas mode refreshed three times per second, Betaflight and INAV compatible.
Putting thermal and visible on the same link
Dual-sensor transmission is where bandwidth budgets fail first. The SJ-TC403 kit handles it by pairing a 4 MP 1/1.88-inch visible camera (1080p60, 149° FOV, 0.0001 lux) with a 720×576 thermal camera (62.4° FOV, F1.0) on a 2.7–7 GHz link with 36 channels, air unit at 22.4 g and 8–40 V input, ground receiver recording to 1 TB. If the mission needs higher thermal resolution, the Eclipse 009-HD module delivers 960×768 at 50 Hz through a 9.1 mm F1.0 lens with 48.3° × 38.6° FOV, feeding Avatar/Ascent HD VTX systems and a UVC output at the same time, in a 28.43×22×6 mm package drawing under 1.5 W.
Commissioning checklist
Before you sign off a 25 km claim, fly these four checks: (1) record actual EIRP after regional limiting, not the datasheet maximum; (2) measure received signal strength at 5, 10 and 15 km on the real antenna mount; (3) confirm the thermal and visible streams both hold frame rate at maximum range; (4) log ground-station temperature, since an actively cooled receiver in a sealed case will throttle long before the RF link does.
FAQ
Is a 25 km link legal in my country?
Range is not regulated, radiated power is. Check the EIRP limit for 2.7–7 GHz in your jurisdiction; the same unit ships at 14 dBm for CE and 33 dBm maximum EIRP elsewhere.
Why does my link drop at 8 km when the datasheet says 25 km?
Usually antenna pointing, Fresnel zone obstruction, or a regional power limit. Re-run the power budget with the certified EIRP before blaming the hardware.
Can I run telemetry and video on one radio?
Yes. The SJ-TC403 family carries video and data simultaneously with canvas-mode OSD, so flight-controller telemetry rides the same link.
Does circular polarisation really matter?
On multipath-heavy routes, yes — LHCP antennas reject reflected signals that would otherwise arrive at opposite polarisation and degrade the constellation.
Next step
Tell us your flight profile, the video resolution you must deliver and the regulatory region you will certify in, and we will return a link budget with the antenna pair and power setting that fits. Our team supports both kit-level and OEM air-unit integrations.
Related hardware: SJ-TC403A 25 km imaging transmission system, SJ-TC403 imaging transmission kit, and Eclipse 009-HD thermal FPV camera.
Related reading
Antenna selection decides more of the link budget than transmitter power. Our sister site lists the CoFDM imaging transmission system rated for 25 km with dual visible and thermal cameras, including the LHCP antenna pair used in the reference budget above.