Video Transmitter (VTX)

🟢 Start — zero knowledge, plain words. 🟡 Hands-on — building or buying, specifics and tradeoffs. 🔴 Specialist — the physics and math behind it.

🟢 Start. The VTX takes the camera's picture and broadcasts it on 5.8 GHz. It has selectable power (25–1000+ mW) and channels so several pilots can fly side by side. In digital systems, camera and transmitter usually come as one module (an air unit).

🟡 Hands-on. In the EU the legal limit on 5.8 GHz is 25 mW EIRP — and counterintuitively, that's usually enough, because range depends far more on the antenna and line of sight than on watts. Analog channels are organized into bands (A/B/E/F/R); when flying together, stick to Raceband with spacing. You change power and channel from the OSD menu thanks to the VTX's UART connection (SmartAudio/Tramp). Practical note: a VTX on the bench, with no airflow, overheats within a minute or two — use pit mode/25 mW for service work.

🔴 Specialist. Range is decibel arithmetic, not watt arithmetic. The scale: $P_{\text{dBm}} = 10\log_{10}(P/1\,\text{mW})$, so 25 mW = 14 dBm and every +3 dB doubles power. Free-space path loss:

$$\text{FSPL(dB)} = 20\log_{10}(d_{\text{km}}) + 20\log_{10}(f_{\text{MHz}}) + 32.44$$

At 5.8 GHz over 1 km: FSPL ≈ 108 dB. The link budget $P_{rx} = P_{tx} + G_{tx} + G_{rx} - \text{FSPL}$ shows that 10× the power buys only +10 dB, while a directional antenna on the goggles adds 8–14 dB legally and cheaply. That's why specialists invest in antennas, not watts.