How to Calculate an RF Link Budget for 40km+ Long-Range FPV Flights
Key Takeaways
- A link budget adds every gain and subtracts every loss; the result is the signal level at the receiver.
- Free-space path loss at 5.8GHz over 40km is about 139.8dB — the dominant term in any long-range calculation.
- Free-space path loss is 20×log10(distance in metres) + 20×log10(frequency in MHz) − 27.55.
- You want at least 10–15dB of margin above the receiver’s sensitivity for a link that holds through aircraft manoeuvres.
- Receiver sensitivity matters as much as transmit power: Aomway specifies −115dBm at 2MHz on the Apexlink digital links and −98dBm on the 7–8.8GHz VRX.
Most long-range FPV range problems are arithmetic, not magic. A link budget tells you before you fly whether a given transmitter, antenna and receiver combination can close the link at your target distance — and by how much margin. The single largest term is always free-space path loss, and it grows with the square of distance, which is why doubling your range costs 6dB no matter what else you change.
| Term | Value at 40km on 5.8GHz | Effect |
|---|---|---|
| Transmit power (10W) | +40.0 dBm | Gain |
| TX antenna gain (12dBi Yagi) | +12.0 dB | Gain |
| RX antenna gain (12dBi Yagi) | +12.0 dB | Gain |
| Free-space path loss | −139.8 dB | Loss |
| Feeder and connector loss | −1.5 dB | Loss |
| Received signal level | −77.3 dBm | Result |
| Receiver sensitivity | −98 dBm | Threshold |
| Link margin | +20.7 dB | Headroom |
The formula
Free-space path loss in dB is:
FSPL = 20×log10(d) + 20×log10(f) − 27.55
where d is distance in metres and f is frequency in MHz. For 40km at 5800MHz:
20×log10(40000) = 92.04
20×log10(5800) = 75.27
92.04 + 75.27 − 27.55 = 139.76 dB
That is the term that dominates everything else. Note the two consequences: doubling distance adds 6dB of loss, and doubling frequency also adds 6dB. That is the whole reason lower frequencies reach further.
Working the budget
Add the gains, subtract the losses, and compare the result with the receiver sensitivity:
Received power = TX power + TX antenna gain + RX antenna gain − FSPL − feeder losses
For a 10W transmitter (+40dBm) with a 12dBi Yagi at each end, 1.5dB of total feed loss, at 40km on 5.8GHz:
40 + 12 + 12 − 139.76 − 1.5 = −77.26 dBm
Against a receiver specified at −98dBm, that leaves about 20.7dB of margin. That is a comfortable link — on paper.
Why you still need 10–15dB of margin
The calculation above assumes free space, perfect alignment and no interference. Reality subtracts from it:
- Antenna misalignment. A high-gain directional antenna has a narrow beam. Bank the aircraft and you lose several dB instantly.
- Polarisation mismatch. An RHCP/LHCP mismatch costs around 20dB — enough to consume your entire margin in one mistake.
- Fresnel zone intrusion. The radio path is not a line; it is an ellipsoid. Trees and terrain near the path block it even when there is visual line of sight.
- Atmospheric and rain attenuation. Small at 5.8GHz, but not zero.
- Interference. Other transmitters raise the noise floor, which directly reduces your effective sensitivity.
This is why a 20dB calculated margin is not excessive. It is the difference between a link that holds when the aircraft turns and one that breaks the moment it does.
Sensitivity is half the equation
It is tempting to chase transmit power, but receiver sensitivity moves the threshold just as effectively and costs less power and less heat.
| Receiver | Sensitivity | Where it matters |
|---|---|---|
| Apexlink Pro | −115 dBm at 2 MHz | Digital link, 40km+ rated range |
| Apexlink Lite | −115 dBm at 2 MHz | Digital link, 18km rated range |
| 7–8.8GHz VRX | −98 dBm | High-band analog video reception |
Both Apexlink models share the same −115dBm figure, and both are specified at 2MHz bandwidth — sensitivity degrades as you widen the channel, so quote the number at the bandwidth you actually use. See the Apexlink Pro documentation for the full specification set.
Getting the most from the budget you have
In order of return on effort:
- Fix polarisation. Match RHCP to RHCP at both ends. This is free and worth up to 20dB.
- Raise antenna gain. Moving from a 4dBi omni to a 12dBi Yagi is about 8dB on the ground end. Every unit is VNA tested to SWR below 2.0 across the full band.
- Improve receiver sensitivity. A better receiver raises the threshold without adding heat or current draw.
- Reduce feed loss. Shorter, better coax. 1.5dB saved is 1.5dB earned.
- Only then consider more transmit power. Doubling power from 5W to 10W buys 3dB — less than a better antenna, and it costs you heat, weight and current.
Frequently Asked Questions
What is the free-space path loss formula?
FSPL in dB equals 20 times log10 of the distance in metres, plus 20 times log10 of the frequency in MHz, minus 27.55. For 40km at 5800MHz that works out to about 139.8dB.
How much link margin do I need for FPV?
Aim for at least 10 to 15dB above the receiver’s sensitivity, and treat 20dB as comfortable. Antenna misalignment, polarisation mismatch and Fresnel zone obstruction all consume margin in flight, so a link that only just closes on paper will drop out in practice.
Is more transmit power the best way to extend range?
Usually not. Going from 5W to 10W adds only 3dB, while fixing a polarisation mismatch recovers around 20dB and upgrading from a 4dBi omni to a 12dBi Yagi adds about 8dB. Power also costs heat, weight and current draw.
Why does doubling frequency add 6dB of loss?
Because free-space path loss depends on the square of frequency, so a doubling is a factor of four in power terms, which is 6dB. The same logic applies to distance: doubling range also costs 6dB.
What receiver sensitivity should I look for on a long-range build?
For digital links, Aomway specifies minus 115dBm at 2MHz bandwidth on both Apexlink models. For high-band analog video, the 7 to 8.8GHz VRX is specified at minus 98dBm. Always compare sensitivity figures at the same bandwidth.
Hardware Testing Application
We ship free transmitters, antennas and datalinks to pilots and builders who will test them honestly and publish what they find. No scripted PR, no paid positive bias — and you keep the hardware. Apply to the Take It. Test It. Keep It. programme.
Product Specsheet Download
Full specification tables, frequency coverage and engineering reference notes for the hardware discussed here are published in the Aomway documentation centre. If you need a document that is not listed — a specific revision, a custom frequency table, or a manual for a discontinued model — contact our engineering team and we will send the correct document rather than have you work from a third-party copy.
About Aomway
Aomway has been developing and manufacturing video transmission and UAV radio hardware for over 15 years, producing equipment for any frequency band from 1 GHz to 9 GHz at power levels from 3W to 15W. We build high-power FPV video transmitters, directional and circularly polarised antennas, digital links, fiber-optic tether systems and detection hardware for pilots, integrators and commercial operators worldwide.
