FPV Transmitter and Receiver

The video transmitter (VTX) on the quad sends the image of the camera to the goggles, where a video receiver picks it up.

Bands, Channels and Output Power

Analog FPV uses the 5.8 GHz band. Its frequencies are grouped into bands of eight channels, such as A, B, E, FatShark and Raceband. Raceband spaces its channels 37 MHz apart, so that up to eight pilots can fly at the same time:

Channel	R1	R2	R3	R4	R5	R6	R7	R8
Frequency (MHz)	5658	5695	5732	5769	5806	5843	5880	5917

In the EU, and therefore in Austria, the band from 5725 to 5875 MHz can be used without a license for short-range devices with up to 25 mW e.i.r.p. (Commission Decision 2006/771/EC as amended by Decision (EU) 2019/1345, band 61). Of Raceband only R3 to R6 lie inside this band. The effective isotropic radiated power (e.i.r.p.) includes the gain of the antenna: with an antenna gain of 2.2 dBic, which is a factor of $10^{0.22} \approx 1.66$, a VTX set to 25 mW radiates about 41 mW e.i.r.p. Higher power levels need other permissions, for example for races.

Most VTX also have a pit mode, in which they transmit with very low power. It allows powering up a quad on the bench or at the start without interfering with pilots who are flying.

Control from the Flight Controller

With TBS SmartAudio the flight controller changes band, channel and power of the VTX, for example from the Betaflight OSD menu or with the Lua script on the radio. The SmartAudio wire of the VTX is connected to a TX pad of a free UART on the flight controller, and this UART is set to VTX (TBS SmartAudio) in the Ports tab of the Betaflight Configurator. Since Betaflight 4.1 the available bands, channels and power levels are defined in a VTX table.

Video Receiver

The video receiver of this build is the rapidFIRE module in the FatShark HDO goggles, see FPV System. It is a diversity receiver, which uses two antennas to receive the signal of the VTX.

VTX Used in this Build

This build uses the TBS Unify Pro HV with an SMA connector, see the assembly. Its specifications according to the TBS manual and getfpv:

Property	Value
Output power	25 mW (13 dBm), 200 mW (23 dBm), 500 mW (27 dBm), 800 mW (29 dBm)
Input	6 to 28 V (2S to 6S), directly from the battery
Control	TBS SmartAudio, pit mode
Camera supply	5 V output
Weight	7 g with SMA, without antenna

TBS Unify Pro HV with its SMA pigtail and the wiring card that comes with it.