The video transmitter is one of the more important pieces of equipment for FPV flying. There are many factors to consider when choosing a Video Transmitter or VTX for short. You need to consider the VTX power in mW, if you are flying indoors or outdoors, if you are flying with others and also the bandwidth your FPV goggles or monitors can tune into. More features to be aware of are how easy is it to change and read the channels and bands. In the case your FPV camera doesn't have a microphone and you prefer to have onboard audio, some VTXs have a built in microphone.
There is a new trend in the FPV drone racing industry and that is low latency HD. The Animon Connex Prosight HD system is a fairly new technology that hasn't been fully adapted yet by most racers. The System is pricey and requires the use of a ground station. The advantage of an HD system is the increase in resolution and clarity. It also doesn't get as much "noise" as an analog system. Instead it displays more of a pixilation loss when the signal becomes weak.
The majority of FPV drone racers still rely on 5.8 GHz for video transmission. If you are planning to fly long range or between many obstacles I would recommend a 2.4 GHz or a 1.3 GHz VTX. These greatly improve the long distance effectiveness at a cost of picture quality. When using 2.4 GHz video you must make sure you are not using a 2.4 GHz radio link, it would be best to use FrSky or JR transmitter with a FrSky JR module bay for use with a 433 MHz or 900 MHz output module.
25mW, 200mW, and 600mW are the usual power output choices. There are a few VTX out there where the output is adjustable like the TBS Unify Pro and the soon to be released Vantac VT241T. Many countries legally limit this output to 25mW so always know your countries laws on this before buying. 25mW is also good for flying indoors where Multi Path can affect the signal, that is when the signal gets bounced off surfaces and creates interference in the picture. 200mW is usually what is required at most outdoor FPV races and is more than enough for flying with friends as well. A 600mW VTX doesn't mean it gets 3 times the range as a 200mW, higher powered VTXs tend to generate more heat and create more interference when flying with others, it also can have more of a Multi Path effect in confined areas. If longer range on 5.8 GHz is your goal there are some 1000mW VTXs out there, that along with higher gain antennas will get you decent range out of a 5.8 GHz system. Many VTXs output 5V for an FPV camera, look for this if your camera requires one, you can also get 5V off a PDB. Some cameras require 5V and some can even take up to 22V, make sure you know your cameras requirements.
Channel and Band Switching
Whether you are FPV Racing or just flying with friends at a park, it's always good to be able to access your channel and band selection. Most VTX have either a dip switch or push button for changing channels and bands. Dip switches are more tedious to change and push buttons are easier but you must go through other channels and you may step on someone's channel mid-air. There are a few transmitters where you can turn transmission power very low or completely off to change channels in the pits. A VTX with a push button has an indicator that tells you what channel and band it is tuned to, it's either a combination of LEDs or a digital display indicator.
Increase your chances of a clearer signal by running a LC filter and using a 12V regulator to power your VTX. Mount your VTX away from your control receiver and antenna away from being obstructed by carbon fiber. Also do not let the ground from the VTX touch the carbon fiber as it can cause issues.
Keep it Working
Don't let the VTX sit on for too long on the bench, in the air it has constant airflow, left sitting for too long can burn it out. Also please do not power on your VTX without an antenna, that will surely burn out your VTX quickly. Also be aware of how your antenna is mounted because in a crash the antenna is sometimes pulled off along with the VTX or the SMA/RPSMA output. Some people like to install a extension from the VTX to relieve the stress in the event of a crash.
Choose a VTX with the features you will actually need. A more expensive VTX will not have a higher quality picture. Spend less on your VTX and more on a camera and quality antennas.