The Video Transmission RF PCB is the radio-frequency board that takes compressed or raw video data and broadcasts it from the UAV to one or more ground receivers. Operating predominantly in the 2.4 GHz and 5.8 GHz ISM bands for consumer FPV, and licensed spectrum allocations for military ISR (Intelligence, Surveillance, Reconnaissance), this board must deliver high data rates (10–50 Mbps) with robust modulation schemes (COFDM) while managing significant thermal loads from the final-stage power amplifier. This article covers RF and thermal design strategies specific to video transmitter boards.
COFDM Modulator Architecture
Modern digital video transmitters use Coded Orthogonal Frequency Division Multiplexing (COFDM) — the same modulation behind DVB-T and LTE — which divides the data stream across hundreds of narrowband subcarriers. This provides resilience against multipath fading and narrowband interference common in urban and maritime UAV operations. On the PCB, the COFDM modulator is typically an FPGA (Xilinx Artix-7) or a dedicated ASIC (HiSilicon Hi3516) that performs IFFT, cyclic prefix insertion, and digital up-conversion.
The modulator's IQ outputs drive a dual-DAC (AD9361 or LMS7002M) that generates the analog IQ baseband signals. The IQ traces from DAC to RF modulator are differential pairs with 100 Ω impedance, length-matched to within 0.5 mm, and routed without vias. Any skew between I and Q channels directly degrades the error vector magnitude (EVM) and increases bit error rate.
Power Amplifier Design & Thermal Management
The video transmitter PA delivers +27 to +33 dBm (0.5–2W) at 5.8 GHz, with a power-added efficiency (PAE) of only 30–45%. The remaining 55–70% of DC input power is dissipated as heat — up to 3W in a typical 2W transmitter. Thermal management on the PCB includes:
Copper coin: A solid copper disc (8–12 mm diameter) embedded in the PCB beneath the PA QFN package, thermally coupled to the bottom-side ground plane through an array of copper-filled vias at 0.8 mm pitch.
Thermal pad on bottom layer: A dedicated copper area on the bottom layer, free of solder mask, that interfaces with an aluminum heatsink or chassis rail through a thermal gap pad (1–2 W/mK conductivity).
Temperature sensing: An NTC thermistor placed adjacent to the PA package, monitored by the MCU ADC, provides overtemperature shutdown at 105°C junction temperature.
Harmonic Filtering & Spectral Purity
Video transmitters must comply with FCC Part 15 / ETSI EN 300 440 spectral mask requirements, which specify maximum out-of-band emissions of -30 dBm (below 1 GHz) and -20 dBm (above 1 GHz). The PA output feeds a 5th-order Chebyshev low-pass filter implemented as microstrip stubs on the Rogers laminate layer, with S-parameter simulation to verify >50 dB suppression at the second harmonic (11.6 GHz for a 5.8 GHz fundamental). The filter occupies a dedicated PCB zone with no digital traces beneath.
Antenna Diversity on Transmit
Unlike receiver diversity, transmit diversity uses multiple antennas to improve link margin through spatial combining rather than selection. The video transmitter board includes dual PA outputs with a 90° hybrid coupler feeding two orthogonally polarized antennas (horizontal + vertical), achieving circular polarization equivalent and 3 dB diversity gain. The hybrid coupler is implemented as a quarter-wave branch-line structure on the Rogers laminate, with critical dimensions controlled to ±25 µm.
Conclusion
The Video Transmission RF PCB pushes the limits of RF power, thermal density, and modulation fidelity on a single compact board. From COFDM IQ routing to PA copper-coin thermal management, every aspect of the design impacts the link budget and video quality at the ground receiver. Superb Tech fabricates video transmitter PCBs with Rogers hybrid stackups, embedded copper coins, and full harmonic filter S-parameter verification.
